Publications
2023
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(2023) Plants People Planet. Abstract[All authors]
Societal Impact Statement: Given the rapidly increasing drought and temperature stresses associated with climate change, innovative approaches for food security are imperative. One understudied opportunity is using feral crops—plants that have escaped and persisted without cultivation—as a source of genetic diversity, which could build resilience in domesticated conspecifics. In some cases, however, feral plants vigorously compete with crops as weeds, challenging food security. By bridging historically siloed ecological, agronomic, and evolutionary lines of inquiry into feral crops, there is the opportunity to improve food security and understand this relatively understudied anthropogenic phenomenon. Summary: The phenomenon of feral crops, that is, free-living populations that have established outside cultivation, is understudied. Some researchers focus on the negative consequences of domestication, whereas others assert that feral populations may serve as useful pools of genetic diversity for future crop improvement. Although research on feral crops and the process of feralization has advanced rapidly in the last two decades, generalizable insights have been limited by a lack of comparative research across crop species and other factors. To improve international coordination of research on this topic, we summarize the current state of feralization research and chart a course for future study by consolidating outstanding questions in the field. These questions, which emerged from the colloquium “Darwins' reversals: What we now know about Feralization and Crop Wild Relatives” at the BOTANY 2021 conference, fall into seven categories that span both basic and applied research: (1) definitions and drivers of ferality, (2) genetic architecture and pathway, (3) evolutionary history and biogeography, (4) agronomy and breeding, (5) fundamental and applied ecology, (6) collecting and conservation, and (7) taxonomy and best practices. These questions serve as a basis for ferality researchers to coordinate research in these areas, potentially resulting in major contributions to food security in the face of climate change.
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(2023) Pest Management Science. Abstract
Despite biocontrol conceptually being a useful way to control specific pests, there are very few products that are used beyond the glasshouse level, into the field. Only if organisms meet four criteria (four pillars) will they be widely used in the field to replace or augment conventional agrichemicals. (i) The virulence of the biocontrol agent must be enhanced to overcome evolutionary barriers either by mixing with synergistic chemicals or with one or more organisms, and/or by mutagenic or transgenic enhancing of virulence of the biocontrol fungus. (ii) Inoculum production must be cost-effective; many inocula are produced by expensive, labour-intensive solid-phase fermentation. (iii) Inocula must be formulated both to have long shelf life of inocula as well as being formulated to establish on, and control the target pest. Usually spores are formulated, while chopped mycelia from liquid culture are cheaper to produce and are immediately active upon application. (iv) After fulfilling these three criteria, the product must be biosafe: not produce mammalian toxins that affect users and consumers, and have a host range that does not include crops and beneficial organisms, and in most cases that it will not spread from application sites or have environmental residues beyond those needed to control the target pest.
2022
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(2022) Pest Management Science. 78, 6, p. 2108-2112 Abstract
Evolved resistance and regulatory deregistration have severely limited farmers' pesticide options. Many potential new pesticide target sites have been elucidated using targeted gene suppression and mutational tools, but few small molecules could be found that inhibit the target enzymes; the targets were considered 'undruggable'. Some organisms from all biological kingdoms use toxic peptides to ward off or kill enemies, and the agrochemical industry has used a few peptide analogs (glufosinate and bialophos) for field application. Conversely, pharmaceutical scientists have been using three-dimensional target protein structure to discover and synthesize short peptides that bind tightly to the surfaces of, and inhibit previously undruggable targets. New computational tools to quickly elucidate 3-D protein structure from amino acid sequence have just emerged. They replace crystallizing target proteins and performing X-ray crystallography to elucidate 3-D structure. These new tools allow prediction of peptides that will bind to the target proteins. They have further modified such peptides to enhance penetration, translocation and temperature stability. There is reason to assume that the same pioneering techniques can be used to develop peptide pesticides as well as pesticide synergists that act against undruggable targets and have excellent environmental and toxicological profiles.
2020
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(2020) Pest Management Science. 76, 2, p. 421-425 Abstract
For many years, the emphasis of industry discovery programs has been on finding new target sites of pesticides and finding pesticides that inhibit single targets. There had been an emphasis on genomics in finding single targets for potential pesticides. There is also the claim that registration of single target inhibiting pesticides is simpler if the mode of action is known. Conversely, if one looks at the evolution of resistance from an epidemiological perspective to ascertain which pesticides have been the most recalcitrant to evolutionary forces, it is those that have multiple target sites of action. Non-target-site resistances can evolve to multi-target-site inhibitors, but these resistances can often be overcome by structural modification of the pesticide. Industry has looked at pest-toxic natural products as pesticide leads, but seems to have abandoned those where they can find no single target of action. Perhaps nature has been intelligent and evolved many natural products that are synergistic multi-target-site inhibitors, and that is why natural compounds have been active for millennia? We should be learning from nature while combining new chemistry technologies with vast accrued databases and computer aided design allowing fragment-based discovery and scaffold hopping to produce multi-target site inhibitors instead of single target pesticides.
2018
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(2018) Pest Management Science. 74, 3, p. 511-515 Abstract
Microbiome organisms can degrade environmental xenobiotics including pesticides, conferring resistance to most types of pests. Some cases of pesticide resistance in insects, nematodes and weeds are now documented to be due to microbiome detoxification, and is a demonstrated possibility with rodents. Some cases of metabolic resistance may have been misattributed to pest metabolism, and not to organisms in the microbiome, because few researchers use axenic pests in studying pesticide metabolism. Instances of microbiomes evolving pesticide resistance contributing to resistance of their hosts may become more common due the erratic nature of climate change, as microbiome populations typically increase and evolve faster in stressful conditions. Conversely, microbiome organisms can be engineered to provide crops and beneficial insects with needed resistance to herbicides and insecticides, respectively, but there has not been sufficient efficacy to achieve commercial products useful at the field level, even with genetically engineered microbiome organisms.
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(2018) Pest Management Science. 74, 1, p. 17-21 Abstract
Liver-affecting, carcinogenic aflatoxins produced by Aspergillus spp. are a major problem, especially in the humid developing world where storage conditions are often optimal for the fungi. Peanuts and maize have been transformed with RNAi constructs targeting Aspergillus flavus polyketide-synthase, an early key enzyme in aflatoxin biosynthesis. Aflatoxin biosynthesis was suppressed in developing immature grain, less so in late maturing grain, and it is doubtful that the technology will be effective in near dry mature grain. The infected grain was still mouldy. As Aspergillus that infects grain preharvest can continue to grow and produce aflatoxin in poorly stored grain, and grain storage insects vector further infections, this technology seems to have little potential utility in the humid tropics. The biotechnological approaches of RNAi directly targeting Aspergillus, coupled with transgenic insecticidal proteins should be far more effective. These biotechnological approaches can be used in tandem with the RNAi against polyketide-synthase, as well as with irradiation, biocontrol and better grain drying and hermetic dry storage in a controlled atmosphere.
2017
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(2017) Pesticide Dose. Solomon K., Kudsk P. & Duke S. O.(eds.). Vol. 1249. p. 61-72 (trueACS Symposium Series). Abstract
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(2017) Pest Management Science. 73, 1, p. 22-34 Abstract
Resistance has evolved to single transgenic traits engineered into crops for arthropod and herbicide resistances, and can be expected to evolve to the more recently introduced pathogen resistances. Combining transgenes against the same target pest is being promoted as the solution to the problem. This solution will work if used pre-emptively, but where resistance has evolved to one member of a stack, resistance should easily evolve for the second gene in most cases. We propose and elaborate criteria that could be used to evaluate the value of stacked traits for pest resistance management. Stacked partners must: target the same pest species; be in a tandem construct to preclude segregation; be synchronously expressed in the same tissues; have similar tissue persistence; target pest species that are still susceptible to at least two stacked partners. Additionally, transgene products must not be degraded in the same manner, and there should be a lack of cross-resistance to stacked transgenes or to their products. With stacked herbicide resistance transgenes, both herbicides must be used and have the same persistence. If these criteria are followed, and integrated with other pest management practices, resistance may be considerably delayed. (c) 2016 Society of Chemical Industry
2015
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(2015) Pest Management Science. 71, 5, p. 658-667 Abstract
Genes regularly move within species, to/from crops, as well as to their con- specific progenitors, feral and weedy forms (vertical' gene flow). Genes occasionally move to/from crops and their distantly related, hardly sexually interbreeding relatives, within a genus or among closely related genera (diagonal gene flow). Regulators have singled out transgene flow as an issue, yet non-transgenic herbicide resistance traits pose equal problems, which cannot be mitigated. The risks are quite different from genes flowing to natural (wild) ecosystems versus ruderal and agroecosystems. Transgenic herbicide resistance poses a major risk if introgressed into weedy relatives; disease and insect resistance less so. Technologies have been proposed to contain genes within crops (chloroplast transformation, male sterility) that imperfectly prevent gene flow by pollen to the wild. Containment does not prevent related weeds from pollinating crops. Repeated backcrossing with weeds as pollen parents results in gene establishment in the weeds. Transgenic mitigation relies on coupling crop protection traits in a tandem construct with traits that lower the fitness of the related weeds. Mitigation traits can be morphological (dwarfing, no seed shatter) or chemical (sensitivity to a chemical used later in a rotation). Tandem mitigation traits are genetically linked and will move together. Mitigation traits can also be spread by inserting them in multicopy transposons which disperse faster than the crop protection genes in related weeds. Thus, there are gene flow risks mainly to weeds from some crop protection traits; risks that can and should be dealt with. (c) 2014 Society of Chemical Industry
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(2015) Advances in Agronomy. 129, p. 181-228 3. Abstract[All authors]
Ongoing increases in the human population necessitate that rice will continue to be a critical aspect of food security for the twenty-first century. While production must increase in the coming decades to meet demand, such increases will be accompanied by diminished natural resources and rising production costs that will alter how rice is grown and managed. Such resource constraints are the impetus for the ongoing transition from traditional flooding and transplanting to direct-seeded rice (DSR). However, such a transition can result in an increase in pest pressures, especially weeds. Rice production can be particularly vulnerable to weed competition, with significant yield losses (i.e., >50%) occurring. Among pernicious weeds, weedy (red) rice (Oryza sativa L.) is increasingly recognized as a major constraint in achieving maximum yield in DSR. Weedy rice is congeneric to crop rice with phenotypic similarity; hence, its ability to negatively influence qualitative and quantitative aspects of production is substantial. As rice will continue to serve as a cornerstone for future food security and sustainability, a comprehensive assessment of weedy rice impacts associated with increasing adoption of DSR is both timely and critical. In this chapter, we examine the biological basis for the competitive ability of weedy rice, including its evolution, ecophysiology, and genetics; quantify spatial-temporal shifts in its distribution and spread; and emphasize and outline a number of regional and global management strategies for its detection and control. Lastly, a number of critical research areas are suggested that deserve additional scrutiny with respect to weedy rice management.
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(2015) Outlooks on Pest Management. 26, 5, p. 220-222 Abstract
When an organism that had previously been controlled by a treatment is no longer controlled by that treatment, the grower considers the organism to be resistant. If equally susceptible pests were controlled in the same field, with the same treatment, I would consider that grower is correct, but this may not be the case due to the present accepted definition of resistance. This uncontrolled pest may have evolved resistance or became resistant by other means. Crops can be rendered resistant to insects, pathogens or herbicides by human intervention, without the resistance being inherited. The professional scientific societies that deal with crop protection have all generated criteria for defining resistance, and inheritance is a required criterion in all the definitions. Inheritance was included in the definitions to assure that experiments were repeated such that the phenomenon is reproducible, as many cases of lack of control in the field are presumed to be due to inappropriate treatments and not actual resistance. Information about non-inherited yet clear-cut cases of non-control where there had been effective control, continue to accumulate, and it is expected that they will become more common in the future. These cases of non-control where there had been control occur both in weed, pathogen and arthropod pests as well as in crops. In some of these cases it has not been possible to show classical Mendelian or maternal inheritance, yet the resistance is reproducible. Many such cases have been reported over the years with microbial and cancer cell resistance to drugs. Thus, the time has come to consider removing this inheritance restriction from the definition of resistance, as outlined in the article. The growers definition of resistance is used in the examples below, because the scientific societies definition does not apply and seems outdated. 2015, Research Information Ltd. All rights reserved.
2014
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(2014) Plant Physiology. 166, 3, p. 1221-1231 Abstract
We speculate that multicopy transposons, carrying both fitness and unfitness genes, can provide new positive and negative selection options to intractable weed problems. Multicopy transposons rapidly disseminate through populations, appearing in approximately 100% of progeny, unlike nuclear transgenes, which appear in a proportion of segregating populations. Different unfitness transgenes and modes of propagation will be appropriate for different cases: (1) outcrossing Amaranthus spp. (that evolved resistances to major herbicides); (2) Lolium spp., important pasture grasses, yet herbicide-resistant weeds in crops; (3) rice (Oryza sativa), often infested with feral weedy rice, which interbreeds with the crop; and (4) self-compatible sorghum (Sorghum bicolor), which readily crosses with conspecific shattercane and with allotetraploid johnsongrass (Sorghum halepense). The speculated outcome of these scenarios is to generate weed populations that contain the unfitness gene and thus are easily controllable. Unfitness genes can be under chemically or environmentally inducible promoters, activated after gene dissemination, or under constitutive promoters where the gene function is utilized only at special times (e. g. sensitivity to an herbicide). The transposons can be vectored to the weeds by introgression from the crop (in rice, sorghum, and Lolium spp.) or from planted engineered weed (Amaranthus spp.) using a gene conferring the degradation of a no longer widely used herbicide, especially in tandem with an herbicide-resistant gene that kills all nonhybrids, facilitating the rapid dissemination of the multicopy transposons in a weedy population.
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(2014) Trends in Biotechnology. 32, 2, p. 65-69 Abstract
Cultivating algae on a large scale will inevitably lead to spills into natural ecosystems. Most risk analyses have dealt only with transgenic algae, without considering the risks of cultivating the corresponding non-transgenic wild type species. This is despite the long-studied 'paradox of the plankton', which describes the unsuitability of laboratory experimentation or modeling to predict the outcome of introducing non-native algae into a new ecosystem. Risk analyses of transgenic strains of native algae can be based on whether they. are more fit or less fit than their wild type, but these are not possible with non-native species. Risks from spills can be minimized by mutagenically or transgenically deleting genes that are unnecessary in culture but obligatory in nature.
2013
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(2013) Plant Science. 213, p. 123-127 Abstract
Sub-toxic doses of many toxicants have positive, beneficial effects on productivity, or stress resistance (hormesis). Transcriptomic, proteomic, and metabolomic responses to a disparate variety hormetic agents, coupled with bioinformatic analyses, can be used to identify consensus genes, their controlling elements, and their metabolites related to stimulation of growth and/or health. This information can then be used as a method for generating healthier and higher yielding crops using transgenic or other biotechnological techniques. The same bioinformatic information can be used to develop knowledgebased, transcriptomic, proteomic and metabolomic high throughput pre-screens using young plants to identify hormetic chemicals that are potentially useful for enhancement of crop health and yield. Such pre-screens preclude the need to use whole plants through maturity. While the hormetic effectors themselves have to date been of limited direct utility, it is clear that they can be used to help pinpoint genes and chemicals that are potentially useful. This is superior to the presently used random screening or even "educated guess" screening of genes and chemicals. (C) 2013 Elsevier Ireland Ltd. All rights reserved.
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(2013) Algal Research-Biomass Biofuels And Bioproducts. 2, 3, p. 286-298 Abstract
As there are industrial and societal interests in cultivating algae on a large scale, there inevitably will be spills of cultured algae into natural ecosystems. The assessment of environmental risks of such spills is especially hard for cultured, non-native microalgae species due to the "paradox of the plankton", the paradox that large numbers of species with varying degrees of fitness co-exist in natural ecosystems in an unpredictable, fluctuating species balance. The risk analysis may be more straightforward for special cases, e. g. transgenic or mutated strains of common, indigenous species because their behavior can be compared to their wild types. Risk assessment can be based on presently used Good Industrial Large Scale Practice considerations, particularly the use of mitigating traits that severely decrease fitness. Some desirable introduced genes may have some unfitness in natural ecosystems and can be coupled with antisense or RNAi suppressed genes to mitigate genes that increase fitness. The most stringent mitigation systems are needed especially for non-native species and can utilize deletion mutations (e. g. in carbon capture or nitrate utilization) that allow cultivation only in artificial systems and are lethal to the algae in nature. (C) 2013 Elsevier B. V. All rights reserved.
2012
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(2012) Biotechnology Advances. 30, 5, p. 1023-1030 Abstract
There are two major energy and cost constraints to bulk production of single cell microalgae for biofuels or feed: expensive culture systems with high capital costs and high energy requirements for mixing and gas exchange; and the cost of harvesting using high-speed continuous centrifugation for dewatering. This report deals with the latter; harvesting by flocculation where theory states that alkaline flocculants neutralize the repelling surface charge of algal cells, allowing them to coalesce into a floc. It had been assumed that with such electrostatic flocculation, the more cells to be flocculated, the more flocculant needed, in a linear stoichiometric fashion, rendering flocculation overly expensive. Counter to theory of electrostatic flocculation, we find that the amount of alkaline flocculant needed is a function of the logarithm of cell density, with dense cultures requiring an order of magnitude less base than dilute suspensions, with flocculation occurring at a lower pH. Various other theories abound that flocculation can be due to multi-valent cross-linking, or co-precipitation with phosphate or with magnesium and calcium, but are clearly not relevant with the flocculants we used. Monovalent bases that cannot cross-link or precipitate phosphate work with the same log-linear stoichiometry as the divalent bases, obviating those theories, leaving electrostatic flocculation as the only tenable theory of flocculation with the materials used. The cost of flocculation of dense cultures with this procedure should be below $1.00/T algae for mixed calcium:magnesium hydroxides. (C) 2012 Elsevier Inc. All rights reserved.
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(2012) Weed Science. 60, 2, p. 283-289 Abstract
Strip is a major constraint to food production in Africa. Most technologies developed for the eradication of Striga asiatica from the United States are not adaptable to Africa. Imazapyr and pyrithiobac coated imidazolinone-resistant (IR)-resistant maize seed prior to planting at rates of 30 to 45 g ha(-1) provide near season long control of Striga and can increase maize yields three- to fourfold if supplied with fertilizer. Slow release seed coatings reduce maize injury when post-planting rains are sparse and improve Striga control when there is excessive rainfall early in the season. Models suggest that herbicide resistance may not be a significant threat in short season maize, but vigilance in removing flowering Striga plants that are not controlled is recommended due to the known risk of evolution of resistance to these herbicides. Stacking the IR gene with glyphosate resistance and using imazapyr treated seed and applying glyphosate mid-season would provide season long Strip control and delay the evolution of resistance to both herbicides. To date, adoption of this technology has been limited by a number of factors. However, it should be included as one component of a multi-factor approach to increasing maize productivity in areas of Africa where Striga is problematic. Nomenclature: imazapyr; pyrithiobac; Strip, Striga hermonthica (Del.) Benth. STRHE; Zea mays L.
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2011
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(2011) Pest Management Science. 67, 3, p. 253-257 Abstract
At very low pesticide rates, a certain low proportion of pests may receive a sublethal dose, are highly stressed by the pesticide and yet survive. Stress is a general enhancer of mutation rates. Thus, the survivors are likely to have more than normal mutations, which might include mutations leading to pesticide resistance, both for multifactorial (polygenic, gene amplification, sequential allelic mutations) and for major gene resistance. Management strategies should consider how to eliminate the subpopulation of pests with the high mutation rates, but the best strategy is probably to avoid too low application rates of pesticides from the outset. (C) 2010 Society of Chemical Industry
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(2011) Agrobiodiversity Management For Food Security: A Critical Review. p. 99-110 AbstractTransgenics Can Enhance Crop Diversity - Under Certain Circumstances
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(2011) Journal of Agricultural Science. 149, p. 47-53 Abstract
Weeds have been controlled so successfully by herbicides, subsequently supplemented by transgenic herbicide-resistant crops, that past intractable problems have been ignored and new ones are evolving. However, industry-led discovery of new herbicide targets is now lacking and weed science as a discipline is contracting globally. This paper describes novel technologies for dealing with major problems, including: better understanding of weed biology coupled with genomics; novel herbicide-resistant crops as well as engineered weed-competing crops; multi-target herbicides; and enhanced biocontrol agents. Together, these approaches may comprise the components of future integrated packages to slow down the evolution of new weed problems.
2010
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(2010) Plant Science. 179, 6, p. 630-634 Abstract
There are a few instances where seed-expressed traits may pose a biosafety risk, depending on the species, the trait, and where it is cultivated. The fewest ecological risks of pollen flow are posed to the natural environment, as there are few crops growing in or near pristine environments that have interbreeding relatives, or bear traits that could disrupt ecological balances. Some seed-expressed traits might pose an agro-ecological risk if introgressed into related weeds. Only pharmaceutical or industrial seed-expressed traits are likely to introgress into other varieties of a crop at regulatory red flag levels due to zero tolerance at the most sensitive levels of detection when grown at normal separation distances. Other traits would be below regulatory thresholds if required separations distances are met. Two of the three transgenic containment methods described (plastome transformation, attenuated viruses) are unlikely to work with seed-expressed traits. Transgenic mitigation could be used where the seed-expressed trait is linked with a trait that is positive or neutral to a crop but deleterious to its wild or weedy relative. Typical mitigation traits are dwarfing, no seed shattering, and lack of secondary dormancy. There are special mitigator genes, such as transgenically dominant sugary endosperm that might be appropriate for pharmaceutical traits. Biosafety gene flow protection mechanisms are also needed to prevent wild type poisonous seed crops (e.g. castor and Jatropha) from introgressing toxin genes into seeds of varieties where these toxins have been transgenically eliminated. (C) 2010 Elsevier Ireland Ltd. All rights reserved.
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(2010) Journal of Biotechnology. 150, p. S16-S16 Abstract
Keywords: Algae; Biofuel; Fishmeal substitute; Transgenic mitigation
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(2010) New Biotechnology. 27, 5, p. 522-527 Abstract
The developing world has many unique constraints to crop production and, lacking inputs, they are best overcome if solutions are seed borne. Classical breeding cannot overcome many of these constraints because the species have attained a 'genetic glass ceiling', the genes are not available within the species. Transgenics can supply the genes, but typically not as 'hand me down genes' from the developed world because of the unique problems: mainly parasitic weeds, and weedy rice, stem borers and post-harvest insects, viral diseases, tropical mycotoxins, anti-feedants, toxic heavy metals and mineral deficiencies. Public sector involvement is imperative for genetically engineering against these constraints, as the private biotechnology sector does not see the developing world as a viable market in most instances. Rice, sorghum, barley, wheat and millets have related weeds, and in certain cases, transgenic gene containment and/or mitigation is necessary to prevent establishment of transgenes in the weedy relatives.
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(2010) Weed Science. 58, 3, p. 324-328 Abstract
Except for a small number of cases in which biocontrol agents were introduced from the site of origin of a weed (classical biocontrol), there have been few cases where a pathogen was virulent enough to perform cost effectively in the field as a mycoherbicide. Mycoherbicides are typically weed species specific, so compatibility with herbicides used to control other weeds is often studied. There can be a synergy between mycoherbicides and herbicides at the field level due to overlapping weed spectra (such synergies are not discussed in depth herein). Two approaches have been used to ascertain whether there is synergy in controlling the target weed: (1) random screening with herbicides; (2) using herbicides as antimetabolites to inhibit specific pathways, enhancing virulence. Glyphosate is the most common herbicide to synergize mycoherbicides, possibly due to its dual function as an inhibitor of biosynthesis of phenylpropanoid phytoalexins by suppressing enolphosphate-shikimate phosphate synthase, or by suppressing callose production (by inhibiting callose synthase) as well as inhibiting other calcium-dependent pathways due to the calcium-chelating properties of glyphosate.
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(2010) Abiotic Stress Adaptation In Plants: Physiological, Molecular And Genomic Foundation. p. 471-483 Abstract
Organisms need genetic mechanisms to rapidly adapt to changing, stressful environments. Having a high mutation frequency would have a drag on a population due to the deleterious nature of mutations, but having a sub-population with high mutation rate due to the presence of mutator genes seems to be nature's solution. Far more is known about mutator genes in bacteria than in higher organisms. Mutator effects can be genetic, through mutations in genes that affect genome stability or it can be epigenetic through up- or down-regulation of these genes. The mutator genes can be genes with partially lost function, which deal with DNA replication or repair, or with detoxification of DNA-damaging cellular components. Transposons, which are sensitive to environmental stress, can also act as imitators in plants. Mutators can be constitutive or stress-induced. Most evidence for mutator-assisted evolution of stress resistance in plants is circumstantial, except for the evolution of atrazine herbicide resistance due to a nuclearly-inherited plastome mutator, which was repeated experimentally. An important feature of the mutator effect is that it is transient and is followed by reversion to the stable wild type, and can be counter-selected following outcrossing with the wild type. Similarly, "remembered" epigenetic stress-induced mutator effects were shown to last for a few generations. In summary, mutator genes could be playing an important role in the evolution of resistance to stress in plants, as it does in other systems, but to an extent that is yet unclear.
2009
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(2009) Food Security. 1, 4, p. 463-478 Abstract
The greatest cost to the farmer in time and variable costs is control of weeds (Highly Invasive Vegetation = HIV). Despite farmer efforts, weeds still result in the greatest crop losses of all biotic constraints. The costs due to this HIV are greatest in the parts of the developing world where manual labor (more typically "femanual") is used, and the populations are becoming more aged (due to youth abandoning agriculture), or more feeble due to debilitating diseases such as malaria and HIV-AIDS. Two case studies are presented: the previously intractable problems with Striga (witchweed) species in Africa; and rice cultivation and the emerging problems with the labor-saving shift to direct seeding, which has resulted in outbreaks of a weedy/feral form of rice, as well as herbicide-resistant Echinochloa spp. Biotechnology has much to offer as part of the solution to these major HIV problems, whether as transgenic herbicide resistant crops, or as weed resistant crops or through transgenically enhanced weed-specific biocontrol agents. Where necessary, transgenic tricks will also be needed in many cases to prevent transgene flow from crop to related weedy relatives, and possible technologies are described. Ever since the advent of agriculture, solutions to crop protection problems have been effective but ephemeral, and new solutions will be needed in the future. Using mixed solutions leads to solutions lasting synergistically longer, so efforts should be made to use mixed technologies to extend the lifetime of the next generation of sorely needed solutions to the problems of HIV.
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(2009) Pest Management Science. 65, 11, p. 1164-1173 Abstract
A greater number of, and more varied, modes of resistance have evolved in weeds than in other pests because the usage of herbicides is far more extensive than the usage of other pesticides, and because weed seed output is so great. The discovery and development of selective herbicides are more problematic than those of insecticides and fungicides, as these must only differentiate between plant and insect or pathogen. Herbicides are typically selective between plants, meaning that before deployment there are already some crops possessing natural herbicide resistance that weeds could evolve. The concepts of the evolution of resistance and the mechanisms of delaying resistance have evolved as nature has continually evolved new types of resistance. Major gene target-site mutations were the first types to evolve, with initial consideration devoted mainly to them, but slowly 'creeping' resistance, gradually accruing increasing levels of resistance, has become a major force owing to an incremental accumulation of genetic changes in weed populations. Weeds have evolved mechanisms unknown even in antibiotic as well as other drug and pesticide resistances. It is even possible that cases of epigenetic 'remembered' resistances may have appeared. (C) 2009 Society of Chemical Industry
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(2009) Crop Protection. 28, 10, p. 845-853 Abstract
Imidazolinone resistant maize seed dressed with imazapyr has been successfully used to control the parasitic weed Striga in many locations, and has begun to be commercially successful in Africa. Despite this, occasionally poor effectiveness of control had been documented in some sites that required explanation. Analysis of the data against rainfall patterns suggested that: 1. poor maize emergence occurred when there was limited rainfall at germination, possibly due to inhibition by a very high herbicide concentration too near the maize seed; 2. there was poor Striga control in seasons of very high rainfall, possibly due to herbicide washout away from the maize root zone where Striga germinates and attaches. These field assessments were matched by experiments suggesting that slow release formulations might alleviate the problems. A series of slow release formulations were synthesized based on binding imazapyr to high capacity anion exchangers and using them to coat maize seed. The best seems to be a polyethyleneimine gel. Epidemiological field data from a multitude of sites support the conclusion that the slow release formulations increased stand establishment across sites and seasons compared to the control where there was low rainfall. (C) 2009 Elsevier Ltd. All rights reserved.
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(2009) BMC Biotechnology. 9, Abstract
Background: One theoretical explanation for the relatively poor performance of Brassica rapa (weed) x Brassica napus (crop) transgenic hybrids suggests that hybridization imparts a negative genetic load. Consequently, in hybrids genetic load could overshadow any benefits of fitness enhancing transgenes and become the limiting factor in transgenic hybrid persistence. Two types of genetic load were analyzed in this study: random/linkage-derived genetic load, and directly incorporated genetic load using a transgenic mitigation (TM) strategy. In order to measure the effects of random genetic load, hybrid productivity (seed yield and biomass) was correlated with crop-and weed-specific AFLP genomic markers. This portion of the study was designed to answer whether or not weed x transgenic crop hybrids possessing more crop genes were less competitive than hybrids containing fewer crop genes. The effects of directly incorporated genetic load (TM) were analyzed through transgene persistence data. TM strategies are proposed to decrease transgene persistence if gene flow and subsequent transgene introgression to a wild host were to occur. Results: In the absence of interspecific competition, transgenic weed x crop hybrids benefited from having more crop-specific alleles. There was a positive correlation between performance and number of B. napus crop-specific AFLP markers [seed yield vs. marker number (r = 0.54, P = 0.0003) and vegetative dry biomass vs. marker number (r = 0.44, P = 0.005)]. However under interspecific competition with wheat or more weed-like conditions (i.e. representing a situation where hybrid plants emerge as volunteer weeds in subsequent cropping systems), there was a positive correlation between the number of B. rapa weed-specific AFLP markers and seed yield (r = 0.70, P = 0.0001), although no such correlation was detected for vegetative biomass. When genetic load was directly incorporated into the hybrid genome, by inserting a fitness-mitigating dwarf
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(2009) Crop Protection. 28, 9, p. 749-755 Abstract
Abutilon theophrasti (Malvaceae) is a widely distributed weed that infests a variety of agricultural crops. Cotton is in the same botanical family, precluding the use of selective chemical herbicides. An Abutilon-specific pathovar of Colletotrichum coccodes has been considered as a potential bioherbicide for the selective control of A. theophrasti. An evolutionary balance between microorganism and weed maintains a relatively low virulence of the microorganism, such that it does not destroy its food supply by rapidly killing its host. We overexpressed the oahA (oxaloacetate acetylhydrolase) gene in C. coccodes to increase oxalate production, as oxalate inhibits plant-defensive callose synthase by chelating calcium, an obligate co-factor of this enzyme. C. coccodes oahA transformants acidified the surrounding medium, an indication of acid production by hydrolysis of oxaloacetate. Less callose was seen visually around lesions caused by C. coccodes oahA transformants than the wild type in preliminary experiments, but this was not biochemically quantifiable. Further optimization might be obtained through gene stacking, or the addition of multiple virulence genes to this potential biocontrol agent. (C) 2009 Elsevier Ltd. All rights reserved.
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(2009) Pest Management Science. 65, 7, p. 723-731 Abstract
Transgenic herbicide-resistant rice is needed to control weeds that have evolved herbicide resistance, as well as for the weedy (feral, red) rice problem, which has been exacerbated by shifting to direct seeding throughout the world-firstly in Europe and the Americas, and now in Asia, as well as in parts of Africa. Transplanting had been the major method of weedy rice control. Experience with imidazolinone-resistant rice shows that gene flow to weedy rice is rapid, negating the utility of the technology. Transgenic technologies are available that can contain herbicide resistance within the crop (cleistogamy, male sterility, targeting to chloroplast genome, etc.), but such technologies are leaky. Mitigation technologies tandemly couple (genetically link) the gene of choice (herbicide resistance) with mitigation genes that are neutral or good for the crop, but render hybrids with weedy rice and their offspring unfit to compete. Mitigation genes confer traits such as non-shattering, dwarfism, no secondary dormancy and herbicide sensitivity. It is proposed to use glyphosate and glufosinate resistances separately as genes of choice, and glufosinate, glyphosate and bentazone susceptibilities as mitigating genes, with a six-season rotation where each stage kills transgenic crop volunteers and transgenic crop x weed hybrids from the previous season. (C) 2009 Society of Chemical Industry
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(2009) Pest Management Science. 65, 5, p. 560-565 Abstract
It is necessary to control root parasitic weeds before or as they attach to the crop. This can only be easily achieved chemically with herbicides that are systemic, or with herbicides that are active in soil. Long-term control can only be attained if the crops do not metabolise the herbicide, i.e. have target-site resistance. Such target-site resistances have allowed foliar applications of herbicides inhibiting enol-pyruvylshikimate phosphate synthase (EPSPS) (glyphosate), acetolactate synthase (ALS) (e.g. chlorsulfuron, imazapyr) and dihydropteroate synthase (asulam) for Orobanche control in experimental conditions with various crops. Large-scale use of imazapyr as a seed dressing of imidazolinone-resistant maize has been commercialised for Striga control. Crops with two target-site resistances will be more resilient to the evolution of resistance in the parasite, if well managed. (C) 2009 Society of Chemical Industry
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(2009) Pest Management Science. 65, 5, p. 588-595 Abstract
BACKGROUND: The NEP1 gene encoding a fungal toxin that successfully conferred hypervirulence when transformed into Colletotrichum coccodes (Wallr.) Hughes attacking Abutilon theophrosti (L.) Medic. was tested to ascertain if it would enhance pathogenicity of Fusarium species to Orobanche aegyptiaca Pers. parasitising crops. RESULTS: None of the Fusarium oxysporum [#CNCM I-1622) NEP1 transformants was hypervirullent. NEP1 transformants of a new but unnamed Fusarium sp. (#CNCM I-1621 - previously identified as F. arthrosporioides) killed Orobanche more rapidly than the wild type. Transformed lines of both species were NEP1 PCR positive, as was the wild type of F. oxysporum #CNCM I-1622 and five other formae speciales of F. oxysporum. All six wild-type formae speciales of F.oxysporum tested excrete minute amounts of immunologically and bioassay-detectable NEP1-like protein. NEP1 expression of most F. oxysporum transformants was suppressed, suggesting that the native gene and the transgene silence each other. The sequence of the putative NEP1 gene in Fusarium oxysporum #CNCM I-1622 differs from the sequence in the toxin-overproducing strain of F. oxysporum f. sp. erythroxyli in four or five amino acids in the first exon. CONCLUSION: Wild-type Fusarium sp. #CNCM I-1621 does not contain a NEP1-like gene, explaining why it seemed amenable to transformation with high expression, and its virulence was probably enhanced by not cosuppressing the endogenous gene as occurred with Fusarium oxysporum #CNCM I-1622. (C) 2009 Society of Chemical Industry
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2008
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(2008) Plant Science. 175, 3, p. 410-414 Abstract
Durum and bread wheat need transgenic traits such as herbicide and disease resistance due to recent evolution of herbicide resistant grass weeds and an intractable new strain of stein rust. Transgenic wheat varieties have not been commercialized partly due to potential transgene movement to wild/weedy relatives, which occurs naturally to closely related Aegilops and other spp. Recombination does not occur in the F-1 hybrid between wheat and its relatives due to the presence of the Phi gene on wheat chromosome arm 5BL, which acts as a chaperone, preventing promiscuous homoeologous pairing to similar, but not homologous chromosomes of the wild/weedy species. Thus recombination must occur during backcrossing after the wheat Pill gene has been eliminated. Based on these findings, we speculate that Phi could be used to prevent gene introgression into weedy relatives. We propose two methods to prevent such transgene establishment: (I) link the transgene in proximity to the wheat Phi gene and (2) insert the transgene in tandem with the lethal barnase on any chromosome arm other than 5BL, and insert barstar, which suppresses barnase on chromosome arm 5BL in proximity to Phi. The presence of Phi in backcross plants containing 5BL will prevent the homoeologous establishment of barnase coupled to the desired transgene in the wild population. 5BL itself will be eliminated during repeated backcrossing to the wild parent, and progeny bearing the desired transgene in tandem with barnase but without the Ph1-barstar complex will die. (C) 2008 Elsevier Ireland Ltd. All rights reserved.
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(2008) Plant Science. 174, 3, p. 246-263 Abstract
Petroleum dependency is a challenge that can potentially be partly offset by agricultural production of biofuels, while decreasing net, nonrenewable carbon dioxide output. Plants have not been domesticated for modern biofuel production, and the quickest, most efficient, and often, the only way to convert plants to biofuel feedstocks is biotechnologically. First generation biofuel feedstock sources: sugarcane and cereal grains to produce bioethanol and biobutanol and oilseeds to produce biodiesel compete directly with needs for world food security. The heavy use of oilseed rape releases quantities of methyl bromide to the atmosphere, which can be prevented by gene suppression. Second generation bioethanolic/biobutanolic biofuels will come from cultivated lignocellulosic crops or straw wastes. These presently require heat and acid to remove lignin, which could be partially replaced by transgenically reducing or modifying lignin content and upregulating cellulose biosynthesis. Non-precipitable silicon emissions from burning could be reduced by transgenically modulating silicon content. The shrubby Jatropha and castor beans should have highly toxic protein components transgenically removed from their meal, cancer potentiating diterpenes removed from the oils, and allergens from the pollen, before extensive cultivation. Algae and cyanobacteria for third generation biodiesel need transgenic manipulation to deal with "weeds", light penetration, photoinhibition, carbon assimilation, etc. The possibilities of producing fourth generation biohydrogen and bioelectricity using photosynthetic mechanisms are being explored. There seem to be no health or environmental impact study requirements when the undomesticated biofuel crops are grown, yet there are illogically stringent requirements should they transgenically be rendered less toxic and more efficient as biofuel crops. (c) 2007 Elsevier Ireland Ltd. All rights reserved.
2007
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(2007) Patent No. WO2007015238A2, 01 Aug 2005, Priority No. 60/703,878 Abstract
Methods of reducing stable transgene introgression from cultivated transgenic plants to non-cultivated plants are provided. Specifically, the present invention provides methods of producing transgenic plants having an exogenous genomically integrated sequence that is incapable of stably introgressing into related crops or wild or weedy species.
2006
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(2006) Proceedings of the National Academy of Sciences of the United States of America. 103, 33, p. 12215-12216 Abstract
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(2006) Plant Science. 170, 5, p. 953-961 Abstract
Genes may introgress from transgenic crops into sexually compatible wild relatives via pollen flow. This could potentially enhance the ecological expansion of the introgressed hybrids and their progeny at the cost of other plant species, or affect health of humans and animals, depending on the novel trait engineered into the crop. To prevent generating such competitive transgenic progeny, we previously used tobacco (Nicotiana tabacum L.) as a model for validating a transgenic mitigation (TM) mechanism using tandem constructs where a gene of choice is linked to mitigating genes that are positive or neutral to the crop, but deleterious to a recipient when in competition with the wild type. In the present study, attempts were made to achieve interspecific sexual hybridization between transgenic TM allotetraploid N. tabacum (pollen donor, representing a crop bearing novel traits) into one of its progenitors, diploid wild type Nicotiana sylvestris (representing a wild relative as well as a progenitor). N. sylvestris plants were manually pollinated by transgenic tobacco. The F, interspecific sexual hybrids had > 75% pollen sterility and produced no seeds. When the F, was backcrossed as the pollen donor to N. sylvestris, the progeny produced almost no germinable seeds. With such low risk of gene flow, transgenic tobacco bearing novel traits could be Cultivated with minimal concern where N. sylvestris is a native or ornamental species. (c) 2006 Elsevier Ireland Ltd. All rights reserved.
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(2006) Plant Biotechnology Journal. 4, 1, p. 7-21 Abstract
Transgenic oilseed rape (Brassica napus) plants may remain as 'volunteer' weeds in following crops, complicating cultivation and contaminating crop yield. Volunteers can become feral as well as act as a genetic bridge for the transfer of transgenes to weedy relatives. Transgenic mitigation using genes that are positive or neutral to the crop, but deleterious to weeds, should prevent volunteer establishment, as previously intimated using a tobacco (Nicotiana tabacum) model. A transgenically mitigated (TM), dwarf, herbicide-resistant construct using a gibberellic acid-insensitive (Delta gai) gene in the B. napus crop was effective in offsetting the risks of transgene establishment in volunteer populations of B. napus. This may be useful in the absence of herbicide, e.g. when wheat is rotated with oilseed rape. The TM dwarf B. napus plants grown alone had a much higher yield than the non-transgenics, but were exceedingly unfit in competition with non-transgenic tall cohorts. The reproductive fitness of TM B. napus was 0% at 2.5-cm and 4% at 5-cm spacing between glasshouse-grown plants relative to non-transgenic B. napus. Under screen-house conditions, the reproductive fitness of TM B. napus relative to non-transgenic B. napus was less than 12%, and the harvest index of the TM plants was less than 40% of that of the non-transgenic competitors. The data clearly indicate that the Delta gai gene greatly enhances the yield in a weed-free transgenic crop, but the dwarf plants can be eliminated when competing with non-transgenic cohorts (and presumably other species) when the selective herbicide is not used.
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(2006) Plant Biotechnology Journal. 4, 1, p. 23-33 Abstract
Transgenic oilseed rape (Brassica napus) plants can interbreed with nearby weedy Brassica rapa, potentially enhancing the weediness and/or invasiveness of subsequent hybrid offspring. We have previously demonstrated that transgenic mitigation effectively reduces the fitness of the transgenic dwarf and herbicide-resistant B. napus volunteers. We now report the efficacy of such a tandem construct, including a primary herbicide-resistant gene and a dwarfing mitigator gene, to preclude the risks of gene establishment in the related weed B. rapa and its backcross progeny. The transgenically mitigated and non-transgenic B. rapa x B. napus interspecific hybrids and the backcrosses (BC1) with B. rapa were grown alone and in competition with B. rapa weed. The reproductive fitness of hybrid offspring progressively decreased with increased B. rapa genes in the offspring, illustrating the efficacy of the concept. The fitness of F-2 interspecific non-transgenic hybrids was between 50% and 80% of the competing weedy B. rapa, whereas the fitness of the comparable T-2 interspecific transgenic hybrids was never more than 2%. The reproductive fitness of the transgenic T-2 BC1 mixed with B. rapa was further severely suppressed to 0.9% of that of the competing weed due to dwarfism. Clearly, the mitigation technology works efficiently in a rapeseed crop-weed system under biocontainment-controlled environments, but field studies should further validate its utility for minimizing the risks of gene flow.
2005
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(2005) Journal of Agricultural and Food Chemistry. 53, 25, p. 9708-9712 Abstract
yNeohespericlin dihydrochalcone (NHDC) is a serninatural, safe, low-calorie sweetener, bitterness blocker, and flavor enhancer with unique properties and applications for the food, beverage, pharmaceutical, and animal feed industries. Current production is limited by the availability of the substrate neohesperidin, a flavonoid that accumulates to significant levels only in the inedible bitter citrus species. We propose a process to convert hesperidin, a tasteless flavonoid extracted from orange peels that are abundant byproducts of the vast orange juice industry, into neohesperidin using metabolic engineering and biotransformation via three steps: (i) extraction of hesperidin from orange peels, (ii) hydrolysis of sugar moieties, and (iii) biotransformation of hesperidin hydrolysis products into neohesperidin. We overcame the current technological bottleneck in biotransformation of hesperidin hydrolysis products into neohesperidin using metabolically engineered plant cell cultures expressing a recombinant flavanone-7-O-Lglucoside-2-O-Lrhamnosyltransferase. A small-scale production experiment established the feasibility of the proposed process.
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(2005) Crop Protection. 24, 11, p. 1007-1015 Abstract
Models related to plant protection can give an appreciation of a phenomenon, and provide ideas for priorities in research, as well as suggest management strategies. The problem with models is that mistakes can be huge when inaccurate assumptions are made about key parameters, as described with three sets of models: (1) our own model predicting that five herbicide-resistant Striga plants would appear per hectare per season was based on an inaccurate assumption that heterozygotes would be selected, and a heterozygous mutation frequency was used, while a recessive mutant frequency should have been used. A revised model with a recessive mutation would predict five resistant plants per million hectares per season; (2) the model predicting that Bt resistant insects would quickly evolve in transgenic cotton and maize unless massive refuges were instituted, assuming a single binding site for the toxin and minor unfitness of resistant individuals, not realizing that resistant individuals may be extremely unfit or that Bt may have multiple targets; (3) a model that claims that unfit transgenes from crops would decimate wild relatives by swamping. The model assumed animal-type low progeny numbers, and did not consider competition for replacement, nor the infrequency that crop pollen could reach wild relatives. Models must be retrospectively critiqued, based on field data and new knowledge, and not be allowed to become accepted as being axiomatic. (c) 2005 Elsevier Ltd. All rights reserved.
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(2005) Molecular Biology and Evolution. 22, 10, p. 2055-2062 Abstract
Introgressive hybridization has played a crucial role in the evolution of many plant species, especially polyploids. The duplicated genetic material and wide geographical distribution facilitate hybridization and introgression among polyploid species having either homologous or homoeologous genomes. Such introgression may lead to the production of recombinant genomes that are more difficult to form at the diploid level. Crop genes that have introgressed into wild relatives can increase the capability of the wild relatives to adapt to agricultural environments and compete with crops or to compete with other wild species. Although the transfer of genes from crops into their conspecific immediate wild progenitors has been reported, little is known about spontaneous gene movement from crops to more distantly related species. We describe recent spontaneous DNA introgression from domesticated polyploid wheat into distantly related, wild tetraploid Aegilops peregrina ( syn. Aegilops variabilis) and the stabilization of this sequence in wild populations despite not having homologous chromosomes. Our results show that DNA can spontaneously introgress between homoeologous genomes of species of the tribe Triticeae and, in the case of crop- wild relatives, possibly enrich the wild population. These results also emphasize the need for fail- safe mechanisms in transgenic crops to prevent gene flow where there may be ecological risks.
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(2005) Planta. 222, 2, p. 372-385 Abstract
Transgenic crops can interbreed with other crop cultivars or with related weeds, increasing the potential of the hybrid progeny for competition. To prevent generating competitive hybrids, we previously tested tobacco (Nicotiana tabacum L.) as a model for validating the transgenic mitigation (TM) concept using tandem constructs where a gene of choice is linked to mitigating genes that are positive or neutral to the crop, but deleterious to a recipient under competition. Here, we examine the efficacy of the TM concept at various ratios of transgenically mitigated tobacco in competition with the wild type tobacco in an ecological replacement series. The dwarf/herbicide-resistant TM transgenic plants cultivated alone under self-competition grew well and formed many more flowers than the tall wild type, which is an indication of greater reproductivity. In contrast to the wild type, TM flowering was almost completely suppressed in mixed cultures at most TM/wild type ratios up to 75% transgenic, as the TM plants were extremely unfit to reproduce. In addition, homozygous TM progeny had an even lower competitive fitness against the wild type than hemizygous/homozygous TM segregants. Thus, the TM technology was effective in reducing the risk of transgene establishment of intraspecific transgenic hybrids at different competitive levels, at the close spacing typical of weed populations.
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(2005) Journal of Plant Growth Regulation. 24, 1, p. 19-27 Abstract
Mechanisms are needed to prevent gene flow from transgenic crops, and the later establishment of these transgenes in populations of other varieties, weeds, or wild relatives. Such prevention can be achieved by containing the transgene within a crop, and then mitigating the effects of the inherent leakage and unidirectionality of containment systems. Mitigation lowers the fitness of recipients below that of the wild-type so that transgenes cannot spread. Transplastomic and male-sterility systems suppress transgene outflow, but not the influx of pollen from relatives, requiring mitigation. The Arabidopsis thaliana Delta gai (gibberellic acid-insensitive) gene, driven by its own promoter, induced male sterility in transgenic tobacco (Nicotiana tabacum), which is chemically reversible by kinetin applications. Female reproduction was not affected. Kinetin-treated sterile hemizygous and homozygous dwarf tobacco produced viable pollen, becoming self-fertile with copious viable seed, restoring the small amount of seed production needed for such a crop. Thus, Delta gai, under its endogenous promoter, can be used as a containment mechanism to prevent transgene outflow. This application is in addition to the previously described highly effective role of Delta gai as a dwarfing mitigator gene, which renders the rare transgenic tobacco hybrids unfit and unable to compete with the wild-type in the mixed cultures. Delta gai is unique in that it can be used both to prevent transgene outflow and to mitigate the flow should containment fail or should gene influx occur, a dual role for the gene, not previously reported.
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(2005) Zeitschrift Fur Naturforschung Section C-A Journal Of Biosciences. 60, 4-Mar, p. 154-165 AbstractAssessing and managing biological risks of plants used for bioremediation, including risks of ransgene flow
The plants used for phytoremediation pose special biological risks, whether transgenic or not, as most of the species: (a) are semi-domesticated; (b) are introduced from other habitats; (c) can become established in the contaminated site; (d) can spread and displace native species, and/or; (e) may introgress transgenes into related species. The addition of transgenes can reduce the risks, e.g. to sterilize or render the species and hybrid offspring hypersensitive to environmental effects (heat, cold), or to a chemical that will cull the species. Various measures can contain transgenes used in phytoremediation species to prevent gene flow, but most containment technologies are both uni-directional (prevent either outflow or influx), and are inherently leaky, even a concept specifically utilizable for phytoremediation - grafting non-transgenic scions on bioremediating transgenic rootstocks. Containment mechanisms should be either stacked with each other or with "mitigator" genes. Transgenic mitigation (TM) has mitigator genes added in tandem to the desired primary transgene (genetically linked) and the mitigator genes confer traits that are positive or neutral to the desired species but are deleterious to hybrids, keeping them at very low frequencies. The concept was demonstrated in tobacco and oilseed rape with a dwarfing mitigator gene that enhanced the reproductive productivity (harvest index) when cultured alone, but eliminated it from mixed populations. Besides the mitigator genes previously proposed for crop species (sterility, no seed shattering, dwarfing, no secondary dormancy) there are genes especially appropriate for phytoremediation, e.g. overexpression of cytokinin oxidase (reduces cytokinin levels) conferring reduced shoot systems (unfitness to compete) with a more extensive root system that is better for extracting toxic wastes as well as no-flowering for vegetatively propagated species. Thus, biotechnology can be harnessed to reduce risks from both
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(2005) Crop Ferality And Volunteerism. p. 1-7 AbstractIntroduction - The challenges of ferality
Keywords: Agronomy; Plant Sciences
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(2005) Crop Ferality And Volunteerism. p. 371-388 AbstractMolecular containment and mitigation of genes within crops - Prevention of gene establishment in volunteer offspring and feral strains
Keywords: HIGHER-PLANTS; PLASTID TRANSFORMATION; PATERNAL INHERITANCE; TANDEM CONSTRUCTS; TRANSGENIC PLANTS; WILD RELATIVES; INDIAN MUSTARD; EXPRESSION; MAIZE; PHYTOREMEDIATION
2004
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(2004) Plant Journal. 40, 1, p. 88-100 Abstract[All authors]
Species of the genus Citrus accumulate large quantities of flavanones that affect fruit flavor and have been documented to benefit human health. Bitter species, such as grapefruit and pummelo, accumulate bitter flavanone-7-O-neohesperidosides responsible, in part, for their characteristic taste. Non-bitter species, such as mandarin and orange, accumulate only tasteless flavanone-7-O-rutinosides. The key flavor-determining step of citrus flavanone-glycoside biosynthesis is catalyzed by rhamnosyltransferases; 1,2 rhamnosyltransferases (1,2RhaT) catalyze biosynthesis of the bitter neohesperidosides, while 1,6 rhamnosyltransferases (1,6RhaT) catalyze biosynthesis of the tasteless rutinosides. We report on the isolation and functional characterization of the gene Cm1,2RhaT from pummelo which encodes a citrus 1,2RhaT. Functional analysis of Cm1,2RhaT recombinant enzyme was conducted by biotransformation of the substrates using transgenic plant cell culture. Flavanones and flavones, but not flavonols, were biotransformed into 7-O-neohesperidosides by the transgenic BY2 tobacco cells expressing recombinant Cm1,2RhaT. Immunoblot analysis established that 1,2RhaT protein was expressed only in the bitter citrus species and that 1,6RhaT enzyme, whose activity was previously documented in non-bitter species, was not cross-reactive. Expression of Cm1,2RhaT at the RNA level was prominent in young fruit and leaves, but low in the corresponding mature tissue, thus correlating well with the developmental pattern of accumulation of flavanone-neohesperidosides previously established. Phylogenetic analysis of the flavonoid glycosyltransferase gene family places Cm1,2RhaT on a separate gene cluster together with the only other functionally characterized flavonoid-glucoside rhamnosyltransferase gene, suggesting a common evolutionary origin for rhamnosyltransferases specializing in glycosylation of the sugar moieties of flavonoid glucosides.
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(2004) Crop Protection. 23, 8, p. 661-689 Abstract
The input costs of pesticides to control biotic constraints are often prohibitive to the subsistence farmers of Africa and seed based solutions to biotic stresses are more appropriate. Plant breeding has been highly successful in dealing with many pest problems in Africa, especially diseases, but its limited to the genes available within the crop genome. Years of breeding and studying cultural practices have not always been successful in alleviating many problems that biotechnology may be able to solve. We pinpoint the major intractable regional problems as: (1) weeds: parasitic weeds (Striga and Orobanche spp.) throughout Africa; grass weeds of wheat (Bromus and Lolium) intractable to herbicides in North Africa; (2) insect and diseases: stem borers and post-harvest grain weevils in sub-Saharan Africa; Bemesia tabaci (white fly) as the vector of the tomato leaf curl virus complex on vegetable crops in North Africa; and (3) the mycotoxins: fumonisins and aflatoxins in stored grains. Abiotic stresses may exacerbate many of these problems, and biotechnological alleviations of abiotic stress could partially allay some predicaments. Some of these constraints are already under study using biotechnological procedures, but others may require longer-term research and development to alleviate the problems. Despite the huge impacts of post-harvest weevils and of mycotoxins in grains, these issues had not been given high priority in national biotechnological programs, possibly due to a lack of knowledge of their immensity. The need for public sector involvement is accentuated for cases where immediate profits are not perceived (e.g. lowering mycotoxin levels in farmer utilized grain, which does not increase yield) but where the public weal will gain, and will be invaluable, especially where the private sector supplies genes already isolated. (C) 2004 Elsevier Ltd. All rights reserved.
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(2004) Molecular Ecology. 13, 3, p. 697-710 Abstract
Some transgenic crops can introgress genes into other varieties of the crop, to related weeds or themselves remain as 'volunteer' weeds, potentially enhancing the invasiveness or weediness of the resulting offspring. The presently suggested mechanisms for transgene containment allow low frequency of gene release (leakage), requiring the mitigation of continued spread. Transgenic mitigation (TM), where a desired primary gene is tandemly coupled with mitigating genes that are positive or neutral to the crop but deleterious to hybrids and their progeny, was tested as a mechanism to mitigate transgene introgression. Dwarfism, which typically increases crop yield while decreasing the ability to compete, was used as a mitigator. A construct of a dominant ahas(R) (acetohydroxy acid synthase) gene conferring herbicide resistance in tandem with the semidominant mitigator dwarfing Deltagai (gibberellic acid-insensitive) gene was transformed into tobacco (Nicotiana tabacum). The integration and the phenotypic stability of the tandemly linked ahas(R) and Deltagai genomic inserts in later generations were confirmed by polymerase chain reaction. The hemizygous semidwarf imazapyr-resistant TM T-1 (= BC1) transgenic plants were weak competitors when cocultivated with wild type segregants under greenhouse conditions and without using the herbicide. The competition was most intense at close spacings typical of weed offspring. Most dwarf plants interspersed with wild type died at 1-cm, > 70% at 2.5-cm and 45% at 5-cm spacing, and the dwarf survivors formed no flowers. At 10-cm spacing, where few TM plants died, only those TM plants growing at the periphery of the large cultivation containers formed flowers, after the wild type plants terminated growth. The highest reproductive TM fitness relative to the wild type was 17%. The results demonstrate the suppression of crop-weed hybrids when competing with wild type weeds, or such crops as volunteer weeds, in seasons when the selector (
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(2004) Theoretical And Applied Genetics. 108, 2, p. 315-320 Abstract
It has been claimed that engineering traits into the chloroplast will prevent transgene transmission by pollen, precluding transgene flow from crops. A Setaria italica (foxtail or birdseed millet) with chloroplast-inherited atrazine resistance (bearing a nuclear dominant red-leaf base marker) was crossed with five male-sterile yellow- or green-leafed herbicide susceptible lines. Chloroplast-inherited resistance was consistently pollen transmitted at a 3x10(-4) frequency in >780,000 hybrid offspring. The nuclear marker segregated in the F-2, but resistance did not segregate, as expected. Pollen transmission of plastome traits can only be detected using both large samples and selectable genetic markers. The risk of pollen transmission at this frequency would be several orders of magnitude greater than spontaneous nuclear-genome mutation-rates. Chloroplast transformation may be an unacceptable means of preventing transgene outflow, unless stacked with additional mechanisms such as mitigating genes and/or male sterility.
2003
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(2003) Trends in Biotechnology. 21, 12, p. 525-530 Abstract
Approximately half of aboveground biomass of grain crops (straw) is wasted, with negative ecological and economic impact. Straw use could increase animal production by at least one third if lignin could be transgenically decreased or modified, rendering more carbohydrate available to ruminant animals. Such digestible straw can be upgraded by ammonification, which further separates lignin and serves as a nitrogen source for ruminant bacteria, or biotreatment with ligninolytic fungi and/or physical treatments. Such technologies could supply animal protein to an increasing population, while releasing more grain for direct human consumption without the need to put more land under plough.
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(2003) Microbe. 69, 10, p. 498-502 AbstractEnhancing microbiocontrol of weeds
Future biocontrol preparations will likely carry specific hypervirulence and other genes to keep the agents on target, overcome host defenses.
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(2003) Crop Protection. 22, 5, p. 697-706 Abstract
Plant parasitic Striga (witchweed) species have not been controlled in susceptible host crops prior to exerting damage. High dose, localized herbicide levels can be applied on or near maize seed bearing acetolactate synthase (ALS) target-site resistance. Such seed coating was cost-effective in preventing damage from parasitic witchweeds Striga hermonthica and S. asiatica in Kenya, Malawi, Tanzania, and Zimbabwe. Imazapyr at 30-45 g ha(-1) and pyrithiobac at 11-21 g ha(-1) were used at 3 experiment stations and in 93 farmers' fields over six seasons to further evaluate the effectiveness of this technology. Seed coating with imazapyr and pyrithiobac gave season-long Striga control in most cases resulting in a 3-4-fold increased maize yield when Striga density was high. Once herbicide resistant maize has been produced using locally adapted varieties, this technology should greatly benefit small-scale farmers in sub-Sahara Africa. (C) 2003 Elsevier Science Ltd. All rights reserved.
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(2003) Reactive & Functional Polymers. 54, 3-Jan, p. 17-24 Abstract
Three different methodologies have been evaluated to estimate the octanol-water partition coefficient (P-ow) of amphiphilic dithiocarbamates (DTC) (sodium salts of dithiocarbamic acids) with different substituents. The conventional shake-flask method has been used to measure P-ow coefficients. This method gave good results for dithiocarbamates with short aliphatic substituents, but it presents difficulties for measuring P-ow of DTC with long-chain substituents. The fragment contribution method PROLOGP, which uses theoretical calculations, has the advantage of its speed but it is unable to distinguish between isomeric compounds. Indirect methods show some advantages for screening purposes. This is the case of chromatographic and electrophoretic methods. Preliminary HPLC experiments showed that it was not useful for the determination of P-ow for DTC. So, micellar electrokinetic capillary chromatography (MECC) was used to estimate P-ow. Two different micellar systems, sodium dodecyl sulphate (SDS) and sodium deoxycholate (NaDCh) were tested. NaDCh was the most suitable surfactant for the analysis of DTCs. Retention measurements, at the same experimental conditions, for some compounds with known P-ow values were used to describe the correlation between P-ow and the MECC capacity factor (k'). Results showed a linear relation, which allowed the estimation of the P-ow for dithiocarbamates. The obtained data were compared with the measured P-ow coefficients obtained by the conventional shake-flask method and those calculated with the fragment contribution method PROLOGP. (C) 2002 Elsevier Science B.V. All rights reserved.
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(2003) Bcpc International Congress Crop Science & Technology 2003, Vol 1 And 2, Congress Proceedings. p. 1175-1180 AbstractContainment and mitigation of transgene flow from crops
There are many ways to prevent transgene introgression from crops to other varieties, or to related weeds or wild species (containment strategies), as well as to preclude the impact should containment fail (mitigation strategies). The needs are most acute with rice and sunflowers, which have con-specific weeds, and with oilseed rape, sorghum, barley, which have closely related weeds. Containment and mitigation are critical for pharmaceutical crops, where gene flow from the crop to edible varieties must be precluded. Some gene flow (leakage) is inevitable with all containment mechanisms and once leaked, could then move throughout populations of undesired species, unless their spread is mitigated. Leakage even occurs with chloroplast-encoded genes, a >0.03% pollen transmission was found in the field. We focused on mitigation, which should be coupled with containment as a last resort. A mechanism for mitigation was proposed where the primary transgene (herbicide resistance, etc.) is tandemly coupled with flanking genes that could be desirable or neutral to the crop, but unfit for the rare weed into which the gene introgresses. Mitigator traits include dwarfing, non-bolting, no secondary dormancy, no seed shattering, and poor seed viability, depending on the instance. We demonstrated the potential utility of the concept using tobacco as a model, and dwarfing as the mitigator with herbicide resistance as the primary gene. Hybrids with the tandem construct were unable to reach maturity when grown interspersed with the wild type. Such mitigation should greatly decrease risk of transgene movement especially when coupled with containment technologies, allowing cultivation of transgenic crops having related weeds. As the number of transgenic plants being released is increasing, and the problems of monitoring such genes increases geometrically, we suggest that a uniform biobarcode(TM) system be used, where a small piece of non-coding DNA carrying an assigned variable region
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(2003) p. 539-541 AbstractEvidence for sporadic introgression of a DNA sequence from polyploidy wheat into Aegilops peregrina (Ae. variabilis)
2002
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(2002) Pest Management Science. 58, 12, p. 1187-1193 Abstract
Parasitic Orobanche spp are major constraints to vegetable crop production in the Mediterranean basin (to eastern Europe) and in localized places in India, China and the USA. Transgenic target-site herbicide resistance (eg, to acetolactate synthase inhibitors) allows for movement of unmetabolized herbicide through the crop to the photosynthate sink in the parasite, as well as through the soil. We report the successful engineering of a mutant acetolactate synthase (ALS) gene into carrot, allowing control of broomrape already in heterozygotes of the first back-crossed generation, by imazapyr, an imidazolinone ALS inhibitor. It is expected that homozygotes will have higher levels of resistance. (C) 2002 Society of Chemical Industry.
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(2002) Trends in Plant Science. 7, 12, p. 542-544 Abstract
The needs for recognition of novel conventional or transgenic organisms include protection of patented or Identity Preserved lines, detecting transgenics and tracing dispersal. We propose simple 'Biobarcodes(TM)' using universal PCR primers to recognize the universal 'nonsense' recognition site of all biobarcodes, followed by a variable nonsense sequence. The proposed sequences are long enough to allow recognition in spite of mutations, have stop codons to prevent coding, and will not self anneal. Sequences of PCR-amplifted biobarcodes can be compared to a universal database.
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(2002) Annals of Botany. 90, 5, p. 567-578 AbstractInfection of tubercles of the parasitic weed Orobanche aegyptiaca by mycoherbicidal Fusarium species
Progression of the infection by host-specific strains of Fusarium oxysporum and Fusarium arthrosporioides of Orobanche aegyptiaca (Egyptian broomrape) tubercles attached to tomato roots was tracked using light, confocal and electron microscopy. Mycelia transformed with the gene for green fluorescent protein were viewed using a confocal microscope. Fungal penetration was preceded by a rapid loss of starch, with approx. 10 % remaining at 9 h and no measurable starch at 24 h. Penetration into the Orobanche tubercles began by 12 h after inoculation. Hyphae penetrated the outer six cell layers by 24 h, reaching the centre of the tubercles by 48 h and infecting nearly all cells by 72 h. Most of the infected tubercles were dead by 96 h. Breakdown of cell walls and the disintegration of cytoplasm in and around the infected cells occurred between 48 and 96 h. Lignin-like material increased in tubercle cells of infected tissues over time, but did not appear to be effective in limiting fungal penetration or spread. Callose, suberin, constitutive toxins and phytoalexins were not detected in infected tubercles, suggesting that there are no obvious defence mechanisms to overcome. Both Fusarium spp. pathogenic on Orobanche produced fumonisin-like ceramide synthase inhibitors, while fusaric acid was produced only by F. oxysporum in liquid culture. The organisms do not have sufficient virulence for field use (based on glasshouse testing), suggesting that virulence should be transgenically enhanced or additional isolates sought. (C) 2002 Annals of Botany Company.
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(2002) Nature Biotechnology. 20, 10, p. 1035-1039 Abstract
Agents proposed for biocontrol of major weeds in arable row-crop agriculture have not met expectations because an evolutionary balance has developed between microorganism and weed, even when the mycoherbicide is used inundatively at very high levels (> 10(4) spores/cm(2)). Sufficient virulence can be achieved by transferring genes to the microorganism, tipping the evolutionary balance. Virulence was increased ninefold and was more rapidly effected; furthermore, the requirement for a long duration at high humidity was decreased by introducing NEP1 encoding a phytotoxic protein, to an Abutilon theophrasti-specific, weakly mycoherbicidal strain of Colletotrichum coccodes. The parent strain was at best infective on juvenile cotyledons of this intransigent weed. The transgenic strain was lethal through the three-leaf stage, a sufficient time window to control this asynchronously germinating weed. Strategies of coupling virulence genes with fail-safe mechanisms to prevent spread (due to broadened host range) and to mitigate transgene introgression into crop pathogens could be very useful in the biocontrol of major weeds in row crops.
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(2002) Journal of Agricultural and Food Chemistry. 50, 22, p. 6353-6360 Abstract
Infection by many fungi activates a variety of calcium dependent defenses in the hosts, slowing or suppressing the attacker and limiting the efficacy of mycoherbicides. The calcium requirement for fungal growth is so low that it could only be implied based on fungi containing calcium-dependent signaling enzymes. Analytical grade media contain
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(2002) Crop Protection. 21, 8, p. 611-619 Abstract
Imazapyr and pyrithiobac dressed to seeds of imidazolinone-resistant maize effectively control Striga hermonthica (witchweed). The effects of the movement of these herbicides in the soil and in maize plants was measured on Striga germination and on legumes intercropped with maize. Striga seeds were killed when high rates of either herbicide percolated through simulated soil columns; almost no viable Striga seeds remained in the upper 10 cm, and >80% were killed at 30 cm. The herbicides applied to maize leaf whorls moved systemically out of roots, killing attached and germinating Striga. Sensitive crops (beans, cowpea, and yellow gram) were unaffected when planted at >15 cm. from maize coated with 0.4 mg a.i. pyrithiobac or 0.84 mg a.i. imazapyr seed(-1), but were severely inhibited when planted within 12 cm. Simple herbicide seed coatings are thus compatible with commonly used African intercropping systems, while facilitating maize growth and depleting the Striga seed bank. (C) 2002 Published by Elsevier Science Ltd.
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(2002) Phytopathology. 92, 6, p. 590-596 AbstractTransgenically enhanced expression of indole-3-acetic acid confers hypervirulence to plant pathogens
Fusarium oxysporum and F arthrosporioides, pathogenic on Orobanche aegyptiaca, were transformed with two genes of the indole-3-acetamide (IAM) pathway leading to indole-3-acetic acid (IAA) to attempt to enhance virulence. Transgenic F oxysporum lines containing both the tryptophan-2-monooxyngenase (iaaM and indole-3-acetamide hydrolase (iaaH) genes produced significantly more IAA than the wild type. IAM accumulated in culture extracts of F oxysporum containing iaaM alone. F arthrosporioides containing only iaaM accumulated IAM and an unidentified indole. Some transformants of F oxysporian expressing only the iaaM gene also produced more IAA than the wild type. Sub-threshold levels (that barely infect Orobanche) of transgenic F oxysponan expressing both genes and of F arthrosporioides expressing iaaM were more effective in suppressing the number and size of Orobanche shoots than the wild type on tomato plants grown in soil mixed with Orobanche seed. Stimulating an auxin imbalance enhanced pathogen virulence by affecting the host in a manner similar to low doses of auxin herbicides such as 2,4-dichlorophenoxy acetic acid.
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2001
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(2001) Crop Protection. 20, 10, p. 885-895 Abstract
High herbicide levels can be localized on or near seed of acetolactate synthase (ALS) resistant maize (Zea mays). The magnesium salt of imazapyr was optimal for seed dressings (drenching, priming, and coating), for preventing field damage from parasitic Striga hermonthica (witchweed) in three seasons. Striga emerged on untreated maize from 6 to 12 weeks after planting. There was almost no Striga emergence for 3 months on imazapyr-dressed homozygous ALS-R 3245IR maize seeds. Occasional Striga that emerged and flowered formed no seed by harvest. Dust or polyvinylpyrrolidone adhesive coatings were safer in maize and as effective in Striga control as priming. Seed dressing coupled with pulling Striga escapes reduces infestation and can be used to deplete the Striga seed bank until genetic crop resistance becomes available. (C) 2001 Elsevier Science Ltd. All rights reserved.
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(2001) Langmuir. 17, 18, p. 5621-5635 Abstract
Controlled inhibition of the enzyme Cu/Zn superoxide dismutase (SOD), one of the most important enzymes of the antioxidant defense system of aerobic organisms, is the subject of this paper. It is desirable in several noted medical and agricultural applications. This paper describes the synthesis of a very limited "sublibrary" of bifunctional metal chelators. The first function is the metal chelator DTC (sodium diethyldithiocarbamate (Et2DTC)), which has the sole chemical function of specific metal binding. An amphiphile (more precisely, an oligoether) is the second function and has a sole physical function: changing the physical properties of the molecule. The paper discusses novel copper chelators with variable amphiphilic properties: disubstituted dithiocarbamates (DTCs, R-1(CH2CH2O)(2)(NRCS2Na)-C-2) with one alkyl (R-2 = hexyl, octyl, decyl, or dodecyl) and one oligoether (R-1(CH2CH2O)(2), where RI = Et or Bu) substituent. The octanol-water distribution ratios of the dithiocarbamates (partition measurements by the shake-flask method) and their penetration through the liposome bilayer were measured to predict their transport behavior through biological membranes. The comparative copper binding constants and the stepwise transformation of Fe(DTC)(3) to Cu(DTC)(2) were measured and show the selectivity of the ligands for copper over iron. Differential effects of dithiocarbamates on the rates of copper removal from SOD are shown. The influence of DTCs on SOD superoxide dismutation activity was measured by the cytochrome C/xanthine/xanthine oxidase assay. The SOD dismutation activity was recovered after incubation of inactivated SOD with copper. Inhibition of peroxidase activity of SOD by different DTCs was determined using electron paramagnetic resonance spectra of the DMPO-(OH)-O-. adduct formed in solutions containing H2O2 and CuZnSOD and in the presence of the spin trap compound. The conclusions are that the addition of an oligoether chain of up to eight alipha
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(2001) Biological Control. 21, 3, p. 274-284 Abstract
Although there are reports of isolation of mycoherbicidal pathogens attacking the widespread broom-rapes (Orobanche spp,) that parasitize legumes and vegetables, none is in use or available. This is despite there being no good method of controlling broomrapes in most crops other than by preplant fumigation with methyl bromide. Two highly parasitic fungi, Fusarium arthrosporioides strain E4a (CNCM I-164) and F. oxysporum strain E1d (CNCM 1-1622), were isolated from nearly 100 organisms found on diseased, juvenile, emerging Orobanche flower stalks. A near-axenic polyethylene envelope system for culturing broomrape on tomato roots was used to ascertain pathogenicity of these strains. Both organisms fulfilled Koch's postulates for being primary pathogens. Their DNAs were analyzed and fingerprinted by restriction fragment length polymorphism and random amplified polymorphic DNA, showing that they are indeed different from each other and from many other Fusarium spp, and other formae speciales of F, oxyspomm including a strain that attacks O, cumana on sunflowers. Both strains infect O. aegyptiaca, O. cernua, and O. ramosa, but not O. cumana. They did not infect any of the vegetable and legume crops tested and thus seem specific to Orobanche. Tomato plant roots dipped into a fungal spore and mycelial suspension and planted in broomrape-infested soil were protected for 6 weeks, as were tomato transplants in pot experiments. About 90% control was also achieved by posttransplant soil drench with fungal suspensions in pot experiments. These pathogens may be effective as seed, transplant, or soil-drench treatments of high-value vegetable and other crops. (C) 2001 Academic Press.
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(2001) BioControl. 46, 2, p. 211-228 Abstract[All authors]
Parasitic broomrapes (Orobanche spp.) are major uncontrolled weeds in the Mediterranean regions of Europe and the Near East causing major losses to vegetable, grain legume, and sunflower crops. Selective herbicides alone cannot provide persistent, season-long control of these parasites, and much methyl bromide is used for their control, where affordable. Thus they are excellent targets for biocontrol. The recent progress by the COST 816 Orobanche working group in this area is reviewed herein. Natural infestation by the fly Phytomyza orobanchia of seed capsules of Orobanche crenata parasitising faba bean halved Orobanche seed production while inundative releases of adults reduced it to 5% of viable seeds. The fungi Fusarium arthrosporioides E4a and F. oxysporum E1d, as well as strains of bacteria were isolated from diseased, juvenile, Orobanche flower stalks. They are pathogenic to O. aegyptiaca, O. crenata and O. ramosa on most vegetable crops. A F. oxysporum f. sp. orthoceras was specifically pathogenic to O. cumana on sunflowers. All were used in various experiments with a modicum of success. Methods were developed to formulate isolated mycelia, which could eventually allow the use of transgenic hypervirulent pathogens in asporogenic (deletion) mutants (as a failsafe against spread). Mycotoxins were also isolated from different Fusarium and other fungal species that kill Orobanche, and are being considered for direct use, or to augment other strategies. All three Fusarium spp. used have been transformed with gus and/or gfp genes allowing tracing their movement in the environment, and opening the way to future transformations to hypervirulence.
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(2001) Trends in Biotechnology. 19, 4, p. 149-154 Abstract
Microbial biocontrol agents are typically inefficient owing to the evolutionary necessity to be in balance with their hosts to survive. If transgenetically rendered hypervirulent, however, they could be competitive alternatives to pesticides. Potential means are delineated to prevent, contain or mitigate uncontrollable spread of hypervirulent biocontrol organisms, mutations that increase their host range, and the sexual or asexual introgression of hypervirulence genes into pathogens of other organisms. The use of asporogenic deletion mutants as a platform for generating transgenic hypervirulent biopesticides would prevent such spread. Hypervirulence genes flanked with available 'transgenetic mitigator' (TM) genes (genes that are neutral or positive to the biocontrol agent but deleterious to recombinants) would decrease virulence to non-target species.
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(2001) Enhancing Biocontrol Agents And Handling Risks. 339, p. 167-176 AbstractMitigating the spread and introgression of native and transgenic biocontrol agents
Means are needed to prevent, or contain uncontrolled spread of biocontrol organisms, or mitigate mutations in their host range, and mitigate the sexual or asexual introgression of their virulence genes into pathogens of crops and other organisms. Possible means are described, using terminator genes,,switch on' activating promoters or asporogenic deletion mutants as a platform to preclude their spread. The virulence genes could be flanked with "transgenetic mitigator" (TM) genes, genes neutral or positive to the biocontrol agent but deleterious to any recombinant. TM genes could suppress sporulation of the recombinant, weaken its spores, or otherwise decrease its virulence to non-target species. Such genes are available. The TM could also contain a "recoverable block of function" construct that would self-destruct any recombination without the presence of an exogenous chemical switch.
2000
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(2000) Planta. 211, 1, p. 50-61 Abstract
The elucidation of mechanisms plants use to overcome oxidative stress is facilitated where there is intra-specific genetic variability. The differential induction of higher levels of mRNAs, cytosol and chloroplast antioxidant enzyme activities, and proteins occurred after sub-lethal paraquat treatment of the oxidant-resistant biotype of Conyza bonariensis (L.) Cronq. By 6 h after sub-lethal paraquat treatment the activities of superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11), dehydroascorbate reductase (EC 1.8.5), monodehydroascorbate reductase (EC 1.6.5.4), and glutathione peroxidase (EC 1.11.1.9) had increased, peaking at 24 h and then slowly reverting back to the basal level. Similarly, the levels of mRNAs encoding these enzymes were enhanced by 12 h and peaked at 18-24 h after sub-lethal paraquat treatment. The time courses of the transient elevation of both transcript and antioxidant enzyme levels correlated with a further transient 2.5- to 3.0-fold increase of paraquat resistance. which occurred only in the constitutively resistant ozone, biotype. The individual enzymes seem to be part of a stresses. coordinately controlled oxidant tolerance in the resistant Plant biotype, utilizing oxidant-induced, increasingly abundant transcript levels, upon which more antioxidant enzymes were synthesized.
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(2000) Weed Science. 48, 2, p. 176-180 Abstract
Whole-plant, negative cross-resistance was studied in Conyza canadensis and Echinochloa crus-galli, important global weeds. Negative cross-resistance can be a most useful preemptive, cost-effective tool for delaying the evolution of resistance, as well as for resistance management, after resistant populations evolve. Seeds of triazine-resistant and -susceptible biotypes were collected in or near orchards that had been continuously treated with atrazine for more than 10 yr. Plants grown from the seeds were treated, in a greenhouse, with herbicides from the following chemical families: triazine, benzothiadiazole, phenyl-pyridazine, arylophenoxy-propionate, cyclohexanedione, phenoxycarboxylic acid, pyridine carboxylic acid, phosphinic acid, glycine phosphate, chloroacetamide, sulfonylurea, and bipyridylium. Eleven of the 18 herbicides tested exerted significant negative cross-resistance against atrazine-resistant weeds, ranging from 0.03 to 0.67 of the concentration required to affect the triazine-sensitive type. No synergism was found between bentazon and fluroxypyr in mixture on Conyza, even though both separately exerted negative cross-resistance. Using a mixture with half the amount of each component lowers the environmental effect of each component while controlling a broader spectrum of other weeds.
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(2000) Pesticide Biochemistry and Physiology. 66, 3, p. 182-194 Abstract
Glutathione peroxidase is involved in scavenging free radicals in many biological systems. its presence in plants has been implicated by identification of genes with nucleotide sequences similar to those of mammals, as well as by indirect, coupled enzyme assays. We directly quantified glutathione peroxidase activity in crude plant extracts using an organic hydroperoxide substrate and measuring GSSG, the direct reaction product, by adapting a sensitive florometric method. The constitutive levels of glutathione peroxidase in a paraquat-resistant biotype of Conyza bonariensis (resistant to many oxidative stresses) were almost double those of the sensitive biotype. providing correlative evidence that glutathione peroxidase is part of the system preventing oxidative damage. Glutathione peroxidase activity was separated from both glutathione transferase and glutathione reductase activities by specific antibody immunoprecipitation and by substrate-affinity chromatography. Conyza glutathione peroxidase cross-reacted with citrus polyclonal anti-phospholipid hydroperoxide glutathione peroxidase. The data suggest that this glutathione peroxidase is a component of the systems dealing with oxidative damage protection in plants and is biochemically different from glutathione peroxidases that also have glutathione transferase activity. (C) 2000 Academic Press.
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(2000) Transgenic Research. 9, 5-Apr, p. 355-382 Abstract
The vast commercial effort to utilize chemical and molecular tools to solve weed control problems has had a major impact on the basic biological sciences as well as benefits to agriculture, and the first generation of transgenic products has been successful, while somewhat crude. More sophisticated products are envisaged and expected. Biotechnologically-derived herbicide-resistant crops have been a considerable benefit, yet in some cases there is a risk that the same useful transgenes may introgress into related weeds, specifically the weeds that are hardest to control without such transgenic crops. Biotechnology can also be used to mitigate the risks. Molecular tools should be considered for weed control without the use of, or with less chemicals, whether by enhancing crop competitiveness with weeds for light, nutrients and water, or via allelochemicals. Biocontrol agents may become more effective as well as more safe when rendered hypervirulent yet non-spreading by biotechnology. There might be ways to disperse deleterious transposons throughout weed populations, obviating the need to modify the crops.
1999
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(1999) Reactive & Functional Polymers. 42, 3, p. 243-254 Abstract
Amphiphilic dithiocarbamate chelators were synthesized to afford the ability to traverse biological membranes. Comparison of copper (II) stability constants for dithiocarbamates was accomplished by a competition reaction between copper oxinates and copper dithiocarbamates using UV/VIS spectrophotometry. The stability constants of mixed copper hydroxyquinoline-dithiocarbamate complexes were calculated by a continuous variation and subtractive spectrophotometric methods. Using these techniques, the competitive stability constants log K-c were determined as 1.53, 1.95 and 2.03 for amphiphilic dithiocarbamates and 3.52 for dibutyldithiocarbamate. (C) 1999 Elsevier Science B.V. All rights reserved.
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(1999) Crop Protection. 18, 10, p. 643-649 Abstract
The production of large amounts of fungal spores for preservation and formulation are considered constraints to effective use of fungal biocontrol agents. Few successful attempts have been made to store fungal mycelia alone. Late log-phase liquid fermenter cultured isolated mycelia of two Fusarium spp. specific to the parasitic broomrapes (Orobanche spp.) from fermenters were formulated in alginate beads or in 'Stabileze' (starch, sucrose, corn oil, and silica) and air-dried. 'Stabileze' formulations exhibited 9 months and retained pathogenicity to the weed for over a year, while mycelia harvested earlier, and conidia from liquid culture exhibited >40% loss of viability. Mycelia from liquid culture yielded >20 times more colony forming units (cfu) of F. arthrosporioides and >2 times more cfu of F. oxysporum than spores at late log phase. Efficient formulation of mycelia should significantly change the economics of biocontrol. (C) 2000 Elsevier Science Ltd. All rights reserved.
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(1999) Trends in Biotechnology. 17, 9, p. 361-366 Abstract
Transgenic crops may interbreed with nearby weeds, increasing their competitiveness, and may themselves become a 'volunteer' weed in the following crop. The desired transgene can be coupled in tandem with genes that would render hybrid offspring or volunteer weeds less able to compete with crops, weeds and wild species. Genes that prevent seed shatter or secondary dormancy, or that dwarf the recipient could all be useful for mitigation and may have value to the crop, Many such genes have been isolated in the past few years.
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(1999) 1999 Brighton Conference: Weeds, Vols 1-3. p. 637-645 AbstractHerbicide resistant tropical maize and rice: Needs and biosafety considerations
African tropical maize is plagued by the root parasitic witchweeds (Striga spp.), which cannot be controlled by selective herbicides while underground. Seed dressings of imazapyr and pyrothiobac provide season long control on imidazolinone-resistant maize, which allow traditional interplanting of legumes. It is also proposed to control Striga hermonthica directly by deleterious transposons. The acute weed problems of direct seeded rice, especially feral and weedy Oryza spp and herbicide-resistant Echinochloa spp could be controlled by herbicides in transgenic resistant rice. The inevitable introgression of the resistance genes into feral and weedy rices can theoretically be mitigated by using tandem constructs with genes deleterious to weedy rices spliced to the resistance genes.
1998
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(1998) Pesticide Biochemistry and Physiology. 60, 3, p. 133-145 Abstract
The effect of herbicides that act by generating active oxygen species is often mitigated by the enzymes of the Halliwell-Asada oxygen detoxification pathway. Use of compounds interfering with this pathway synergizes herbicides, allowing the killing of weeds at lower herbicide doses. We have used different dithiocarbamate chelators (Dtcs) capable of removing copper from superoxide dismutase, the first enzyme of the pathway, suppressing its action. Dtcs with different hydrophilic-lipophilic properties (sodium salts of ethyl (2-(2-ethoxyethoxy)ethyl)-, butyl (2-(2-ethoxyethoxy)ethyl) -, hexyl (2-(2-ethoxyethoxy)ethyl)-, and dibutyldithiocarbamic acids) were synthesized so that they might penetrate plant cuticles better than sodium diethyldithiocarbamate. Octanol-water distribution ratios and the stabilities of their copper complexes were determined. All Dtcs tested had a high affinity for copper. The Dtcs did not prevent iron-catalyzed Fenton reaction leading to hydroxyl radical production in vitro. Therefore they would not protect plants from the oxidative damage caused by herbicide, as do compounds that complex iron and thereby prevent the phytotoxic iron-catalyzed Fenton reaction. These amphiphilic compounds show some synergistic activity in vivo, with paraquat as well as lactofen, herbicides that generate active oxygen species by different mechanisms. The results demonstrate that dithiocarbamates have potential as synergists for oxidant-generating herbicides. (C) 1998 Academic Press.
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(1998) Weed Science. 46, 4, p. 459-466 Abstract
Parasitic witchweeds inflict most of their damage while still underground and attached to crop roots. Most selective translocated herbicides are detoxified by crops such as corn and thus cannot reach the attached parasites. Corn with target site resistance to acetolactate synthase (ALS)-inhibiting herbicides was tested to ascertain whether these herbicides could control witchweeds, assuming that witchweeds do not obtain amino acids from the crop. Postemergence directed sprays of 27 g ae ha(-1) imazapyr 54 d after planting (DAP) delayed Striga asiatica emergence on corn in South Carolina from 3 wk (control) to 7 wk and to 11 wk when mixed with 45 g ae ha(-1) AC 263 222. Treatments with up to 71 g ae ha(-1) imazamox, and up to 71 g ae ha(-1) AC 263 222 only delayed Striga emergence by 1 wk, and 71 g ae ha(-1) imazethapyr was ineffective. ALS-inhibiting herbicides were far more effective when applied in 1-ml drenches above the seed at planting. Chlorsulfuron (10 g ai ha(-1)) and sulfometuron (50 g ai ha(-1)) were somewhat phytotoxic to Pioneer 3245IR. Rimsulfuron (30 g ai ha(-1)), metsulfuron (10 g ai ha(-1)), halosulfuron (120 g ai ha(-1)), and imazethapyr (140 g ae ha(-1)) were marginally active in Kenya, with some mature Striga hermonthica seed.-bearing capsules appearing at harvest (12 wk). Imazapyr at 15 g ae ha(-1) gave 70 to 95% suppression of capsule formation, whereas no capsules appeared at 30 g ae ha(-1). The use of imazapyr in Kenya increased the harvest index by 17% when corn plants in Striga-infested soils were kept insect and disease free by using insecticides and fungicides. Thus, complete control can be achieved at affordable cost by farmers in subsistence conditions.
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(1998) Theoretical And Applied Genetics. 96, 1, p. 132-137 Abstract
Root-attaching parasitic flowering broomrapes (Orobanche spp.) are major constraints to vegetable, legume and sunflower production around the Mediterranean and elsewhere, with banned methylbromide fumigation or land abandonment of these affected crops as the major "solutions" to the problem. We report the specific generation of transgenic asulam-resistant potatoes as a way to eradicate this pest. The target-site resistance in the crop allows the herbicide to be translocated from treated leaves to the parasite via crop roots. This inhibitor of dihydropteroate synthase lethally then prevents folic acid biosynthesis in the parasite. Additionally, we demonstrate that asulam can be used directly in selecting resistant transformants, without the need for another selectable marker.
1997
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(1997) Plant Physiology. 115, 4, p. 1443-1451 AbstractConstitutively elevated levels of putrescine and putrescine-generating enzymes correlated with oxidant stress resistance in Conyza bonariensis and wheat
Oxidant stress resistance in Conyza bonariensis and wheat (Triticum aestivum) has been correlated with high levels of antioxidant enzyme activities. Additionally, external oxidant stresses can increase a plant's levels of the enzymes of polyamine biosynthesis and polyamines, especially putrescine. We investigated the constitutive relationships between putrescine, putrescine-generating enzymes, and oxidant stress resistance in wheat and C. bonariensis. Putrescine was constitutively elevated (2.5- to 5.7-fold) in 2-week-old-resistant wheat and C. bonariensis biotypes, which correlated with a 10- to 15-fold increase in paraquat oxidant resistance. Arginine and ornithine decarboxylase activities doubled, along with higher putrescine levels in resistant C. bonariensis. The variations in levels of putrescine and arginine and ornithine decarboxylase activities paralleled the constitutive variation of antioxidant enzymes, as well as oxidant resistance. Higher levels of both putrescine and antioxidant enzyme activities occurred during a peak of oxidant resistance at 10 weeks, when paraquat resistance in C. bonariensis plants is >50-fold greater than in the sensitive biotype. Application of 100 mu M putrescine can double oxidant-stress resistance in the resistant C. bonariensis. Putrescine may play an important role in contributing to the base level of oxidant resistance found at the nonpeak period.
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(1997) Analytical Biochemistry. 246, 2, p. 159-165 Abstract
A new method is described that allows the selective staining and quantification of glutathione reductases (EC 1.6.4.2) in cell extracts following acrylamide gel electrophoresis. The method is based on modifications of two previous procedures; it uses DTNB [5,5'-dithiobis(2-nitrobenzoic acid)] to develop a yellow color on reaction with GSH formed from the NADPH-dependent reduction of oxidized glutathione. This is followed by specific counterstaining of glutathione reductase with dichlorophenolindophenol/nitroblue tetrazolium. The use of DTNB in the initial staining step inhibits enzymes other than glutathione reductase that could be stained with the dichlorphenolindophenol/nitroblue tetrazolium counterstain. Enzymes such as thioredoxin reductase, which can directly reduce DTNB with NADPH, may be nonselectively stained by this new procedure. Plant ferredoxin-thioredoxin reductase is not reduced by NADPH and therefore does not appear. Glutathione reductase stains much quicker with DTNB in the presence of GSSG than with thioredoxin reductase, allowing them to be distinguished, if parallel gels are run without GSSG, where the two enzymes react at the same rate. The sequential use of two staining procedures results in distinct, sharp permanent bands that can be used to quantify the activity of glutathione reductase while precluding artifacts generated by the previous methods. (C) 1997 Academic Press.
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(1997) Weed And Crop Resistance To Herbicides. p. 3-14 AbstractBurgeoning resistance requires new strategies
Keywords: Agronomy; Plant Sciences
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(1997) Proceedings Of The Fiftieth New Zealand Plant Protection Conference. p. 298-306 AbstractGenetic engineering can either exacerbate or alleviate herbicide resistance
Sweeping generalizations promulgating dangers of herbicide-resistant crops and ''superweeds'' rarely meet scrutiny. Engineering resistance to little used, and especially multi-site herbicides can relieve selection pressures for evolution of resistance to the currently sometimes overused herbicide groups (e.g. inhibitors of ALS and ACCase). Engineering resistance to widely-used, single site herbicides into crops, increases evolutionary pressure for resistance. Such transfers could be justified for the control of parasitic weeds, where the alternatives are worse. Engineering resistance into crops that easily interbreed with weeds can sometimes be unwise. The rates of movements of such genes to weeds are unknown but gene transfer could be learnt using innocuous cases. There could still be advantages to resistant crops, without weed danger to other cropping situations (rice/red rice). Rational, case by case risk/cost/benefit analysis must replace irrational fear-striking generalizations, as agriculture can gain from many herbicide-resistant crops.
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(1997) Weed And Crop Resistance To Herbicides. p. 267-274 AbstractSynergized mycoherbicides for resistance management
Keywords: Agronomy; Plant Sciences
1996
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(1996) Scientist. 10, 19, p. 10-10 AbstractPlant biotechnology can quickly offer solutions to hunger in Africa
Weeding out hunger: Biotechnology offers the means to end hunger in Africa by destroying parasitic weeds that infest field crops, yet large-scale implementation efforts lack industry support, contends Jonathan Gressel, who holds the Gilbert de Botton Chair of Plant Sciences at the Weizmann Institute of Science in Rehovot, Israel.
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(1996) International Journal of Pest Management. 42, 2, p. 113-129 Abstract
Acetolactate synthase (ALS)-inhibiting imidazolinone and sulphonylurea herbicides have been found to be effective in selectively controlling the pernicious parasitic weeds Orobanche, Striga, and Alectra spp. in some crops. This control could be effected both as whole field applications and as seed dressings. Weeds rapidly evolve resistance to this single-target, high mutation frequency group of herbicides, which usually exert heavy selection pressure. This type of rapid evolution of resistant populations was previously predicted by models, and later validated in the field in other weed and cropping situations. The selection pressure of this herbicide group may be exceedingly strong with parasitic weeds, as they are controlled by very low dose rates and the doses used are in the 'overkill' range. A good management strategy with non-parasitic weeds was to lower selection pressure, but this may be less effective with parasitic weeds. Many of the areas of the world where parasitic weeds are a problem do not use mechanical harvesters that rapidly spread weed seed. The inhomogeneous seed spread with hand harvesting necessitated developing new models. With mechanical-harvester spread of parasitic weed seeds, the best strategy is to treat the resistant crop seed with high herbicide levels (that are low per hectare) and ensure the immediate removal of the rare resistant plants that appears, before they set seed, as well as to ensure that crop seed is free of resistant seed of the parasitic weeds. Modelling suggests that resistant parasitic weed populations will evolve within two to six seasons of use without resistance management, using mechanical crop harvesting. With hand harvesting of crop, delays of about eight seasons of effective herbicide use can be expected, much longer ii the mitigating strategies of roguing resistant stalks and using parasite-free crop seed are implemented.
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(1996) Journal Of Environmental Science And Health Part B-Pesticides Food Contaminants And Agricultural Wastes. 31, 3, p. 399-405 Abstract
Inundatively used mycoherbicides have been uneconomical and ineffectual, mainly because a large inoculum is required to overcome weeds' innate defenses, as well as the need for long dew periods to become established. Weed defenses can be suppressed by specific chemical agents, or overcome biologically (using mixed organisms and/or engineering enhanced pathogenicity), and better formulants replace the long dew periods.
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(1996) Engineering Plants For Commercial Products And Applications. 792, p. 140-153 Abstract
Keywords: C-P BONDS; PLANT-METABOLISM; GLYPHOSATE; GENE; TOLERANCE; AFRICA
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(1996) Second International Weed Control Congress - Proceedings, Vols 1-4. p. 1211-1221 AbstractSynergizing weed control
There is a dire, sometimes conflicting need to lower both the economic and the chemical inputs used in weed control. This can be attained by combining rotations, cultural practices, different chemicals, and biologicals, such that an increase cost-effectiveness is achieved; i.e. the combinations lead to $ynergy. $ynergy can be attained by mixing herbicides with each other or with adjuvants that lower inputs, overcome resistance, and/or broaden the spectrum of weeds controlled. Mycoherbicides can be $ynergized by formulants that assist in establishment or in overcoming plant defenses, by engineering genes enhancing pathogenicity and/or by mixing with facilitory organisms.
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(1996) Molecular Genetics And Evolution Of Pesticide Resistance. 645, p. 169-186 AbstractPrevention versus remediation in resistance management
''After the fact'' remedial strategies are often ineffective, especially where resistance is widespread and/or refuges are large. Good pesticides are too often lost. The ''it won't happen here'' view accounts for the rarity of area-wide management strategies. The successful national example of abolishing agricultural use of DDT in Sri Lanka in favor of its use only in mosquito control precluded resistance until now. Preventive strategies must be immediately cost-effective, as well as useful in delaying resistance, or they will not be implemented. The tendency to cut dose rates is increasing resistances due to multiple-cumulative events (polygenic, amplifications, or sequential mutations within a gene). We have modeled alternating low with intermediate dose rates to delay both major gene and multiple cumulative-resistances as part of IPM. Such novel. strategies must be verified with economic and pest control data to convince farmers that they can work.
1995
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(1995) Brighton Crop Protection Conference, Weeds - 1995, Vols 1-3. p. 587-590 AbstractCreeping resistances: The outcome of using marginally-effective or reduced rates of herbicides
Keywords: Agricultural Economics & Policy; Agronomy; Biochemistry & Molecular Biology; Plant Sciences
1994
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(1994) Crop Protection. 13, 8, p. 563-566 Abstract
A purported drawback to the use of transgenic herbicide-resistant crops is the fear that the crop or interbreeding wild relatives will become weedy. It has been posited that a change even in a single trait can confer weediness. This hypothesis was tested with Brachypodium distachyon. This innocuous species came into contact with herbicides through the use of crushed rock from its habitat for road foundations. It evolved s-triazine resistance and developed as a monoculture. When true weeds later evolved simazine resistance, B. distachyon was partially competed from the ecosystem and then disappeared upon the use of non-triazine herbicides. Thus, this wild species remained a weed only until true weeds evolved resistance or until other herbicides were used. One gene mutation did not convert it into a weed, which implies that this will be equally improbable in other cases, when the gene codes for an otherwise neutral trait such as herbicide resistance.
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(1994) Plant Science. 102, 2, p. 147-151 Abstract
Paraquat resistance in a biotype of Conyza at about 10 weeks of age is genetically and biochemically correlated with constitutively elevated enzyme activities of superoxide dismutase, ascorbate peroxidase and glutathione reductase relative to the sensitive biotype. This high activity of superoxide dismutase and glutathione reductase in total cell extracts was consistent with changes to higher levels of resistance to paraquat and photoinhibition at this stage of plant development, during bolting and flowering, but not at other vegetative stages. We have now measured the constitutive ascorbate peroxidase activity at different ages and compared it with our previous results with superoxide dismutase and glutathione reductase during development (which were also repeated in part). Between the ages of about 9 and 12 weeks, the paraquat-resistant biotype always has higher constitutive ascorbate peroxidase activity than sensitive plants. This parallels and extends our previous findings.
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(1994) Biology And Management Of Orobanche: Proceedings Of The Third International Workshop On Orobanche And Related Striga Research. p. 406-418 AbstractGENETIC ENGINEERING CAN HELP CONTROL PARASITIC WEEDS
Keywords: Agronomy; Plant Sciences
1993
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(1993) Plant Physiology. 103, 4, p. 1377-1384 AbstractJUVENILE-SPECIFIC LOCALIZATION AND ACCUMULATION OF A RHAMNOSYLTRANSFERASE AND ITS BITTER FLAVONOID IN FOLIAGE, FLOWERS, AND YOUNG CITRUS-FRUITS
1-2-Rhamnosyltransferase catalyzes the production of disaccharide-flavonoids that accumulate to 75% of dry weight. Vast energy is expended in a short time span to produce these flavonoids. The highest rhamnosyltransferase activities and immunodetected concentrations were observed in early development of Citrus grandis (pummelo), coinciding with up to 13% of fresh weight as naringin. The concentration of naringin in leaves, petals, receptacles, filaments, albedo, and flavedo drops drastically during development and correlates directly with a decrease in the activity and amounts of 1-2-rhamnosyltransferase. Anthers had minute rhamnosyltransferase activities and low concentrations of naringin. Conversely, high 1-2-rhamnosyltransferase activity and naringin concentrations appeared in both young and mature ovaries, as well as in young fruits. The total amounts of naringin in mature leaves decreased without detectable in vitro degradation of naringin in leaves. There was still a net accumulation of naringin in the albedo and flavedo of older fruit even though these tissues had only traces of 1-2-rhamnosyltransferase. Traces of enzyme synthesis in fruits, or import of the product from leaves, may explain the net accumulation of naringin in growing fruits. Unlike the late-expressed genes for glycosyltransferases in anthocyanin biosynthesis, the rhamnosyltransferases from Citrus are active only in juvenile stages of development.
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(1993) Plant Physiology. 103, 4, p. 1097-1106 AbstractDEVELOPMENTAL VARIABILITY OF PHOTOOXIDATIVE STRESS TOLERANCE IN PARAQUAT-RESISTANT CONYZA
Paraquat-resistant hairy fleabane (Conyza bonariensis L. Cronq.) has been extensively studied, with some contention. A single, dominant gene pleiotropically controls levels of oxidant-detoxifying enzymes and tolerance to many photooxidants, to photoinhibition, and possibly to other stresses. The weed forms a rosette on humid short days and flowers in dry long days and, thus, needs plasticity to photooxidant stresses. In a series of four experiments over 20 months, the resistant and susceptible biotypes were cultured in constant 10-h low-light short days at 25-degrees-C. Resistance was measured as recovery from paraquat. The concentration required to achieve 50% inhibition of the resistant biotype was about 30 times that of the susceptible one just after germination, increased to >300 times that of the susceptibles at 10 weeks of growth, and then decreased to 20-fold, remaining constant except for a brief increase while bolting. Resistance increased when plants were induced to flower by long days. The levels of plastid superoxide dismutase and of glutathione reductase were generally highest in resistant plants compared to those of the susceptibles at the times of highest paraquat resistance, but they were imperceptibly different from the susceptible type at the times of lower paraquat resistance. Photoinhibition tolerance measured as quantum yield of oxygen evolution at ambient temperatures was highest when the relative amounts of enzymes were highest in the resistant biotype. Resistance to photoinhibition was not detected by chlorophyll a fluorescence. Enzyme levels, photoinhibition tolerance, and paraquat resistance all increased during flowering in both biotypes. Imperceptibly small increases in enzyme levels would be needed for 20-fold resistance, based on the moderate enzyme increases correlated with 300-fold resistance. Thus, it is feasible that either these enzymes play a role in the first line of defense against photooxidants, or another, yet unknown mecha
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(1993) Journal Of Phytopathology-Phytopathologische Zeitschrift. 138, 3, p. 233-243 Abstract
The severity of fungal infection is usually estimated visually as a rating of injury. A more quantitative method was needed to ascertain the effect of additives used to enhance the virulence of mycoherbicidal preparations. A sensitive and accurate serological method is presented for quantitative measurement of Alternaria cassiae infection of Cassia obtusifolia. An antiserum was prepared against a homogenate of mycelium of A. cassiae. A fast and simple procedure of leaf immuno-autoradiography was developed for the visualization of A. cassiae mycelium on inoculated leaves. A radioimmunosorbent assay (RISA) was developed for quantification of the extent of fungal infection. As little as 1.6 ng/ml dry weight equivalent of mycelium could be detected by RISA. There was a linear relationship between the logarithm of the RISA values and the logarithm of fungus concentration up to 300 ng/ml. Reduced sensitivity of the assay was pronounced in extracts containing 400 mug/ml or more fresh weight of leaves. There was 30-40 % cross-reactivity of the antiserum with two other species of Alternaria, as well as with Monilinia fructicola, and almost no reactivity was found with three other fungi tested.
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(1993) Pest Control with Enhanced Environmental Safety. Vol. 524. p. 48-61 (trueACS Symposium Series). Abstract
There is great need to reduce pesticide inputs to save costs, reduce environmental loads, and lower the selection pressures resulting in rapid evolution of resistance. This can often be done by judiciously chosen mixtures; with synergism given the economic definition of a mixture that gives more cost-effective pest control than the sum of the separate components. Pesticides are often used alone at high rates to control the most recalcitrant pests in the infestation spectrum, where a low rate of a second pesticide can be complimentary and even synergistic. Herbicide mixtures based on field weed spectra matched to the best available compounds are more prevalent in Europe than in the U.S.A. High use rates are often required to overcome endogenous degradation pathways in certain pests, and inhibitors of the offending pathways can be biorationally chosen to synergistically lower rates. This requires a better knowledge of the reasons for the lack-of-action of pesticides in target pests, an area insufficiently researched. There are already compounds known that enhance pesticide penetration, that block pesticide degradation by monooxygenases, glutathione transferases, as well as compounds that block detoxification of the active oxygen species generated by many herbicides.
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(1993) Opportunities For Molecular Biology In Crop Production. 55, p. 245-255 AbstractIMPROVING WEED-CONTROL BY BIOTECHNOLOGICALLY CONFERRING HERBICIDE RESISTANCE ON CROPS - A PROGRESS REPORT
Keywords: Agronomy; Biochemistry & Molecular Biology; Biotechnology & Applied Microbiology
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(1993) International Crop Science I. p. 121-127 AbstractMANAGEMENT OF HERBICIDE-RESISTANT WEEDS IN CROP PRODUCTION
Keywords: Agronomy; Environmental Sciences
1992
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(1992) Weed Technology. 6, 3, p. 509-525 Abstract
Weed science is responsible for reducing weed competition with crops, within constraints, allowing food production for a growing world population. We have been successful, but with an increasing reliance on the fantastic arsenal of herbicides. Heavy herbicide usage and the concomitant loss of control due to changing weed species spectra, as well as evolved herbicide resistance, along with cancellations of registrations, limit chemical options. Old "innovations" such as well-conceived rotations and various mechanical procedures partly alleviate chemical dependency. We must try innovative, meaningful herbicide mixtures, whether synergistic or additive, mixtures of crop varieties, more competitive varieties, including allelopathic varieties, which could all help to lower chemical dependency. Three cases where innovative genetic-engineering coupled with chemistry seem to be needed are: (a) for the control of parasitic higher plant weeds such as broomrapes (Orobanche spp.), dodders (Cuscuta spp.), witchweeds (Striga spp.) that cannot be controlled by mechanical or selective chemical means with a sufficient margin of user error; (b) for the control of weeds in wheat (Triticum spp.) that are evolving multiple and cross resistances to all wheat-selective herbicides; and (c) to replace the major s-triazine and chloroacetamide herbicides in com (Zea mays L.) that are under attack and where many local restrictions and cancellations have been unposed. We need better replacements than the resistance-prone inhibitors of acetolactate-synthase and acetyl-CoA-carboxylase. Much of the long-term innovative planning and research must come from the public sector as no one else seems willing to fill the needs.
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(1992) Plant Physiology. 98, 2, p. 654-659 Abstract
The major effort in developing pathogenic fungi into potential mycoherbicides is aimed at increasing fungal virulence to weeds without affecting crop selectivity. Specific suppression of biosynthesis of a phytoalexin derived from the shikimate pathway in Cassia obtusifolia L. by a sublethal dose (50 micromolar) of glyphosate increased susceptibility to the mycoherbicide Alternaria cassiae Jurair & Khan. Glyphosate applied with conidia suppressed phytoalexin synthesis beginning at 12 hours, but not an earlier period 8 to 10 hours after inoculation. The phytoalexin synthesis elicited by fungal inoculation was also suppressed by darkness. The magnitudes of virulence of the mycoheribicide in the dark or with glyphosate in the light were both higher than after inoculation in the light with the same concentration of conidia in the absence of glyphosate. Five times less inoculum was needed to cause disease symptoms when applied with glyphosate than without. Glyphosate did not render A. cassiae virulent on soybean (Glycine max), a crop related to the host. These results suggest that a specific inhibition of a weed's elicited defense response can be a safe way to enhance virulence and improve the efficacy of the mycoherbicide.
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(1992) Plant Physiology. 98, 1, p. 303-308 Abstract
A single phytoalexin was isolated and purified from 12- to 14-day-old leaves of Cassia obtusifolla L., inoculated with Alternaria cassiae Jurair & Khan. The structure was elucidated by H-1 and C-13-nuclear magnetic resonance and mass spectrometry as 2-(phydroxyphenoxy)-5,7-dihydroxychromone. The compound was shown to be derived in part from phenylalanine. Radial growth of A. cassiae was inhibited 50% by the compound at 0.3 millimolar. This moderate toxicity is compensated for by the relatively high levels (3 millimolar) accumulated. Phenoxychromones have been previously reported only as constitutive secondary metabolites in Artemisla capillaris Thunb, in which their function is unknown.
1991
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(1991) Journal of Biological Chemistry. 266, 31, p. 20953-20959 Abstract
The rhamnosyltransferase catalyzing the production of the bitter flavanone-glucosides, naringin and neohesperidin, was purified to homogeneity. The enzyme catalyzes the transfer of rhamnose from UDP-rhamnose to the C-2 hydroxyl group of glucose attached via C-7-O- of naringenin or hesperetin. To our knowledge this is the first complete purification of a rhamnosyltransferase. The enzyme from young pummelo leaves was purified > 2,700-fold to a specific activity of over 600 pmol/min/mg of protein by sequential column chromatographies on Sephacryl S-200, reactive green 19-agarose, and Mono-Q. The enzyme was selectively eluted from the green dye column with only three other proteins by a pulse of the substrate hesperetin-7-O-glucoside followed by UDP. The rhamnosyltransferase is monomeric (approximately 52 kDa) by gel filtration and electrophoresis. The enzyme rhamnosylates only with UDP-rhamnose. Flavonoid-7-O-glucosides are usable acceptors but 5-O-glucosides or aglycones are not. It is inhibited by 10-mu-M UDP, its end product, but not by naringin or neohesperidin. Several flavonoid-aglycones at 100-mu-m inhibited the rhamnosyltransferase; UDP-sugars did not. The K(m) for UDP-rhamnose was similar with prunin (1.3-mu-M) and hesperetin-7-O-glucoside (1.1-mu-M) as substrate. The affinity for the natural acceptor prunin (K(m) = 2.4-mu-M) was much higher than for hesperetin-7-O-glucoside (K(m) = 41.5-mu-M). The isolation of the gene may enable its use in genetic engineering directed to modifying grapefruit bitterness.
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(1991) Phytopathology. 81, 9, p. 985-988 Abstract
Mycoherbicidal preparations to selectively control weeds typically require high inoculum levels and long dew periods for establishment. The dew requirement was obviated and the inoculum threshold was reduced to one conidium per droplet when an invert emulsion was used as an adjuvant. Invert emulsion also abolished the selectivity of Alternaria cassiae and A. crassa. A. cassiae attacked eight plant species, including soybean and members of other families. The emulsion also facilitated cross-infectivity by A. crassa, a mycoherbicide specific to Datura spp., Cassia obtusifolia, and other species. Nonpathogenic fungi, such as Aspergillus nidulans and Trichoderma harzianum, colonized soybean when applied in the emulsion. Besides retaining water for spore germination, the invert emulsion may cause cuticular damage, allowing leaf penetration by the fungi. The emulsion could also suppress the plants' elicited responses to infection.
1990
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(1990) Zeitschrift Fur Naturforschung C-A Journal Of Biosciences. 45, 5, p. 470-473 AbstractNEGATIVE CROSS RESISTANCE - A POSSIBLE KEY TO ATRAZINE RESISTANCE MANAGEMENT - A CALL FOR WHOLE PLANT-DATA
Keywords: Biochemistry & Molecular Biology; Pharmacology & Pharmacy
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(1990) Zeitschrift Fur Naturforschung C-A Journal Of Biosciences. 45, 5, p. 463-469 AbstractMODE OF EVOLVED PHOTOOXIDANT RESISTANCE TO HERBICIDES AND XENOBIOTICS
Keywords: Biochemistry & Molecular Biology; Pharmacology & Pharmacy
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(1990) Weed Technology. 4, 1, p. 186-198 AbstractMODELING THE EFFECTIVENESS OF HERBICIDE ROTATIONS AND MIXTURES AS STRATEGIES TO DELAY OR PRECLUDE RESISTANCE
Keywords: Agronomy; Plant Sciences
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(1990) Managing Resistance to Agrochemicals. Vol. 421. p. 430-458 (trueACS Symposium Series). Abstract
Herbicide resistant populations have evolved only in monoculture and/or mono-herbicide conditions at the rates we have previously predicted. Contrary to our original model, no populations of atrazine-resistant weeds have appeared in corn where rotations of crops and herbicides or herbicide mixtures were used. This is due to the previous lack of information about the greatly reduced fitness of the resistant individuals, which could be expressed only during rotational cycles, and also to the greater sensitivity of resistant individuals to other herbicides, pests and control practices. Our new model, presented here, describes how these factors reduce the resistant individuals to extremely low frequencies during rotation.It is not yet clear whether such factors will delay the rate of appearance of other types of resistance; e.g. the multiple resistances to grass-killing herbicides in wheat, or to inhibitors of acetolactate synthase, where the fitness of resistant biotypes may be higher.
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1989
1988
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(1988) Theoretical And Applied Genetics. 75, 6, p. 850-856 AbstractDOMINANT PLEIOTROPY CONTROLS ENZYMES CO-SEGREGATING WITH PARAQUAT RESISTANCE IN CONYZA-BONARIENSIS
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(1988) Microbios. 55, 222, p. 7-16 AbstractCATABOLISM OF SINGLE RING AROMATIC-ACIDS BY 4 ASPERGILLUS SPECIES
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(1988) Biotechnology for Crop Protection. Vol. 379. p. 4-24 (trueACS Symposium Series). AbstractBiorational Herbicide Synergists
Compounds added to herbicides in order to rationally suppress the plants' tolerance mechanisms can synergize the herbicides. Tridiphane suppresses glutathione-S-transferase and thus prevents detoxification of chloro-s-triazine herbicides as well as some other pesticides, that are degraded by glutathione conjugation. Monoxygenase inhibitors can prevent many oxidative herbicide detoxifications. Our own work on biochemical, physiological and genetic studies of paraquat resistance led to a rationale for synergists. Paraquat rapidly, but transiently, inhibited photosynthesis of a resistant weed while permanently inhibiting the wild type. Chloroplasts of the resistant biotype dominantly and pleiotropically inherited constitutively elevated levels of superoxide dismutase, ascorbate peroxidase and glutathione reductase, enzymes engaged in detoxifying the active oxygen species generated with paraquat. Inhibition of these enzymes could lower the required threshold for phytotoxicity, synergizing the herbicides. The first two enzymes contain copper and the first also zinc. Copper and zinc chelators inhibited the enzymes, synergizing paraquat and other active oxygen generating herbicides, in all weed species tested.
1987
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(1987) Pesticide Biochemistry and Physiology. 28, 1, p. 75-84 AbstractSYNTHESIS AND CHARACTERIZATION OF A GROUP OF DIHYDROPYRIMIDO-BENZIMIDAZOLE PHOTOSYSTEM-II HERBICIDES
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(1987) Journal of Phycology. 23, 1, p. 176-181 AbstractMASSIVE ACCUMULATION OF PHYTOENE INDUCED BY NORFLURAZON IN DUNALIELLA-BARDAWIL (CHLOROPHYCEAE) PREVENTS RECOVERY FROM PHOTOINHIBITION
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(1987) Biotechnology in Agricultural Chemistry. Vol. 334. p. 41-52 (trueACS Symposium Series). Abstract
Cell cultures are ideal axenic physiological systems to study herbicide action without problems of cuticular transfer or complications of translocation. Still, not all metabolic systems function in all cells at all times in the cell cycle. Inhibitors of photosynthesis are often inactive in non green cells, and root-active herbicides may be degraded in green cells. Nutritional components in the medium may interfere with herbicide action. The criteria needed to develop universal pre-screens which show activity for all herbicides are discussed and evaluated. It is far easier to develop "dedicated" pre-screens which are used to measure single types of activity or single chemical types of herbicide. All results with in vitro systems should be validated with whole plants.
1986
1985
1984
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(1984) Origins and Development of Adaptation. p. 132-137 Abstract
Clarke: Is it possible to make an anti-ecdysone i.e. an anti-moulting hormone? [first paragraph]
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(1984) Origins and Development of Adaptation. p. 200-203 Abstract
Gressel: In the laboratory, the triazine-resistant biotypes are more sensitive to another group of photosystem 11-inhibiting herbicides (Arntzen et a1 1982), which isn’t surprising. They are also more susceptible to thiocarbamates (Laval-Martin et a1 1983), and the reason for this is unknown. We are likely to see cases that are just the opposite of the huge cross-resistances that have been described for insecticides. Does cross-resistance occur with fungicides?
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(1984) Zeitschrift Fur Pflanzenphysiologie. 114, 2, p. 97-107 AbstractEARLY MORPHOGENETIC CHANGES DURING PHYTOCHROME-INDUCED FERN SPORE GERMINATION .1. THE EXISTENCE OF A PRE-PHOTOINDUCTION PHASE AND THE ACCUMULATION OF CHLOROPHYLL
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(1984) Origins and Development of Adaptation. p. 14-19 Abstract
Clarke: A.J. Cain (1964), in his essay on ‘The perfection of animals’, said that evolution could more or less do what was necessary, when required. Perhaps you would disagree about this. Among the grasses, is there any ecological or taxonomic sense in your list (Table 1) of species that have or do not have available variation in tolerance?
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(1984) Zeitschrift Fur Pflanzenphysiologie. 114, 2, p. 109-122 AbstractEARLY MORPHOGENETIC CHANGES DURING PHYTOCHROME-INDUCED FERN SPORE GERMINATION .2. TRANSCRIPTIONAL AND TRANSLATIONAL EVENTS
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(1984) Origins and Development of Adaptation. p. 246-252 Abstract
Chemostat studies of bacteria that harbour the prokaryotic transposable elements Tn-5 and TnlO and the temperate phages A, Mu, P1 and P2 have shown that these accessory DNA elements confer a selective advantage on their hosts. We propose that similar selective effects provided the initial impetus for the evolution of nascent accessory DNA elements in primitive bacterial populations. In subsequent evolution the elements acquired or perfected the 'selfish' characteristics of over-replication and horizontal transmission. Such selfish traits led to the dissemination of accessory DNAs among commensal strains, species and genera, genetically interconnecting them to create a 'commonwealth' of species that potentially share a common gene pool. The involvement of accessory DNAs in genetic exchange provides selection at the population level for refinement and diversification of the elements and for regulation of their replication, transposition and transfer among cells. The diversity of intracellular environments encountered by the elements imposes constraints on their evolution while at the same time altering the selection pressures operating on conventional chromosomal genes. This process of coevolution of accessory DNAs with the genomes of their diverse hosts has led to a unique population structure and mechanism of genetic exchange among bacteria, which constitutes the most effective adaptive strategy yet devised by selection.
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(1984) Origins and Development of Adaptation. p. 73-87 Abstract
The first generation of species-selective phenoxy herbicides went into use at the same time as modern antibiotics, chlorinated hydrocarbon insecticides and new fungicides and rodenticides. No resistance has appeared to the still widely used phenoxy herbicides. However, resistance has developed. in many isolated areas around the world, to s-triazine herbicides and, in a few small areas, to bipyridillium herbicides. The parameters of population genetics that govern herbicide resistance and those that govern resistance of microorganisms and fungi to other xenobiotics are the same: generations, selection pressure and fitness. Even though generation times are longer with plants. a sufficient number has passed for resistance to be apparent. The only special factor that controls the development of herbicide resistance in weeds is the spaced germination of seed throughout many seasons. The reason that resistance has not developed to the phenoxy herbicides is probably because of a low effective selection pressure; germination of susceptible seeds occurs late in the season after the herbicide is biodegraded. The highly persistent triazines. and the monthly used, highly ephemeral bipyridillium, paraquat, have exerted much stronger selection pressures. Different modes of tolerance and resistance seem to have evolved in the same species. Crop and herbicide rotation can considerably delay the possibility of resistance development until it is effectively precluded.
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(1984) Origins and Development of Adaptation. p. 228-232 Abstract
Clarke: I was fascinated by the use of codons in Streptomyces fradiae, whose DNA has an extremely high guanosine and cytosine (GC) content, and leaves no room for synonymy in the genetic code. This reminded me of some early work by Sueoka (1965) on the amino acid compositions of organisms differing greatly in their GC content. In these organisms the change from one extreme of GC content to the other actually seems to have moved the amino acid compositions of the proteins. The correlations between GC content and the proportion of amino acids with high GC codons were very strong but not sufficient for the changes in amino acid composition to explain the changes in GC. Whatever is driving the DNA and pushing the GC content in one direction or another is so strong that it even moves the protein. So what is it? [first paragraph]
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(1984) Origins and Development of Adaptation. p. 32-39 Abstract
Clarke: The soils of interest here are ones that have not been physically standardized. How much variation in the soils occurs over short distances? For example if a soil has patches with low as well as high aluminium concentrations, would one need more than one variety of a crop to produce a profitable maximum growth in such a field? [first paragraph]
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(1984) Origins and Development of Adaptation. p. 162-166 Abstract
Clarke: I agree that ‘normal’ is only a relative term and that one shouldnever regard organisms as static. What is considered as ‘normal’ will changewith time. We should always consider ‘resistant’ and “susceptible’ as relativeterms. [first paragraph]
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(1984) Origins and Development of Adaptation. p. 67-72 Abstract
Clarke: Have the bare rocks at the Sudbury smelters in Canada always been bare or did they result from erosion after the vegetation had gone? [first paragraph]
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(1984) Origins and Development of Adaptation. p. 113-118 Abstract
Clarke: If the attacking fungus primes the plant so that it is ready to produce the phytoalexin response when it is attacked again, why does the plant not get itself into that ‘ready’ state all the time? What is the evolutionary explanation of the need to be primed? [first paragraph]
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(1984) Applied Biochemistry and Biotechnology. 9, 4, p. 393-394 Abstract
Keywords: Biochemistry & Molecular Biology; Biotechnology & Applied Microbiology
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(1984) Origins and Development of Adaptation. p. 182-189 Abstract
Wood: On the question of genes that give resistance to different insecticides being linked, I have a few comments. In Drosophila, Tsukamoto & Ogaki (1954) and Kikkawa (1964b) found a single locus (RZ) that gave a much wider spectrum of cross-resistance than had been found in other insects. They may have been dealing with a cluster of linked genes, although irradiation experiments suggested a single mutation site (Kikkawa 1964a). [first paragraph]
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(1984) Origins and Development of Adaptation. p. 94-99 Abstract
Wood: In insecticide resistance, as far as heterozygous advantage is concerned (see also p 87-93), linked and balanced lethal genes can be found in association with resistance genes as, for example, in the spotted root maggot. Hooper & Brown (1965) found, when selecting a resistant population of this insect with dieldrin or DDT, that they ended up with only heterozygotes. [first paragraph]
1983
1982
1981
1980
1979
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(1979) Zeitschrift Fur Naturforschung C-A Journal Of Biosciences. 34, 11, p. 905-913 AbstractREVIEW OF THE PLACE OF INVITRO-CELL CULTURE SYSTEMS IN STUDIES OF ACTION, METABOLISM AND RESISTANCE OF BIOCIDES AFFECTING PHOTOSYNTHESIS
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1978
1977
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(1977) Plant and Cell Physiology. 18, 3, p. 657-670 AbstractCELL-CULTURES VS WHOLE PLANTS FOR MEASURING PHYTOTOXICITY .2. CORRELATIONS BETWEEN PHYTOTOXICITY IN SEEDLINGS AND CALLI
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(1977) Plant and Cell Physiology. 18, 1, p. 255-259 AbstractEFFECTS OF DIKEGULAC, A NEW GROWTH-REGULATOR, ON RNA-SYNTHESES IN SPIRODELA
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(1977) Plant and Cell Physiology. 18, 3, p. 641-655 AbstractCELL-CULTURES VS WHOLE PLANTS FOR MEASURING PHYTOTOXICITY .1. ESTABLISHMENT AND GROWTH OF CALLUS AND SUSPENSION CULTURES - DEFINITION OF FACTORS AFFECTING TOXICITY ON CALLI
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(1977) Plant and Cell Physiology. 18, 4, p. 815-820 AbstractCELL-CULTURES VS WHOLE PLANTS FOR MEASURING PHYTOTOXICITY .3. CORRELATIONS BETWEEN PHYTO-TOXICITIES IN CELL-SUSPENSION CULTURES, CALLI AND SEEDLINGS
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1975
1974
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(1974) Plant and Cell Physiology. 15, 5, p. 807-811 AbstractDIRECT EVIDENCE FOR LACK OF METHYLATION OF 2 PULSE LABELED PLANT RNAS
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(1974) Plant and Cell Physiology. 15, 5, p. 891-902 AbstractDISTRIBUTION OF PLASTID RIBOSOMES AND INTEGRITY OF PLASTID RIBOSOMAL-RNA DURING GREENING AND MATURATION OF SPIRODELA FRONDS