The productivity of plants depends on their growth and differentiation. The differentiation, expansion and polarity of the leaves are of central importance for achieving optimal photosynthesis as well as optimal use of water. Dr. Yuval Eshed of the Department of Plant Sciences studies the regulation of leaf polarity and its importance in growth initiations of leaves. Through this study Dr. Eshed has discovered novel genes and mechanisms that control this fundamental process.
Breeding of crop plants depends on a number of factors, one of which is the exchange of genes between wild progenitors of crop plants and of those that were evolved from them. Prof. Jonathan Gressel (Emeritus) and colleagues are working towards an understanding of these processes, which may assist, on one hand, with incorporation of desirable genes from the progenitors into the crop plants, and, on the other hand, may enable the estimate of the risk in transfer of genetically engineered genes from the crop plants to their progenitors.
Below we offer scientific reports on the progress of the above two projects supported by grants from the Harry and Jeanette Weinberg Center for Plant Molecular Genetics Research follow. In addition to these research grants, the Weinberg Center’s income was used to fund scientific support staff, infrastructure and supplies.
Roles of organ polarity in growth initiation
Background
Each plant species has unique leaves, both in shape and size, which characterize it, and can differ dramatically from its neighbor plant species. The upper surface, facing the sun is usually shiny and darker with cells containing the green pigment chlorophyll. The lower leaf surface is usually pale, and rich in tiny holes called stomata that allow absorption of carbon dioxide and emission of oxygen. Yet, within the species, a great uniformity exists, and leaves formed in different seasons or locations on the plant are all alike. Taken together, a very tight mechanism exists to allow the precise and repeated formation of the same leaf over and over again.
However, this tight regulation can be easily manipulated to form yet another leaf with different size and shape, while maintaining the initial uniformity. Understanding this mode of dual property - precision and variation are the primary goal of the work in our laboratory.
Leaf development is sort of a tug-of-war between gene products that dictate top and bottom cell types that results in ultimate leaf stretching. The identification of these genes helps us understand how nature can manipulate a few key genes and shape at once the tiny leaf of Arabidopsis or the giant leaf of a palm. Since leaf shape is critical to optimal use of light energy and productivity, these research discoveries can be channeled to help optimize leaf architecture.
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| Dr. Yuval Eshed (left) and Eyal Blum |
Scientific Report
Recent studies in model species such as Antirrhinum and Arabidopsis have demonstrated that specification of lateral organ polarity along the abaxial/adaxial (ab/ad) organ axis has major consequences for overall organ shape. Polar morphology results in specific adaptations of the leaf with an adaxial (top) surface specialized for light capture and an abaxial (bottom) surface specialized for gas exchange. Furthermore, the establishment of polarity is required for proper lamina development. A model of leaf blade development by Waites and Hudson (1995) proposed that juxtaposition of abaxial and adaxial domains is required for lamina outgrowth. This model was based on a) lack of lateral expansion in abaxialized leaves of phantastica mutant plants, and b) ectopic blade expansion around revertant spots of the transposon induced mutation.
The ability to determine the plane and magnitude of blade growth provides a powerful mechanism to regulate shape. Within Arabidopsis, novel abaxial/adaxial boundaries can lead to trumpet shaped leaves. For example, in phb-1d/+ mutants, such leaves appear to have an adaxialized petiole, adaxialized cells on the outer parts of the trumpet, and abaxial cells inside the trumpet. In all cases it appears that a new boundary has been formed at the distal part of the leaf primordia, perpendicular, instead of parallel to the ab/ad plane of the leaf initiation. The flexibility of ab/ad relations, and the quantitative rather than qualitative relationships between them could account for the enormous variation in lateral organ shape and size that characterizes the plant kingdom.
The formation of leaf lamina in angiosperms is similar in many respects to the development of laminar structures in metazoans (animals whose body is differentiated into cells and organs and contains a digestive cavity). For example, during Drosophila wing development, the boundary between juxtaposed dorsal and ventral cells results in the production of secreted factors that are produced at the boundary and act at distance to specify cell fates in dorsal and ventral compartments. Unfortunately, none of the characterized regulators of such boundaries in animals (such as DPP) are present in Arabidopsis genome. However, the proposed experiments will help to determine whether developmental concepts are conserved among kingdoms even when the participating molecules are different. The extent to which the boundaries between abaxial and adaxial domains, perhaps mediated by different relative levels of KANADI activity, act as organizing centers in the lateral organs of plants represents such a case study.
The described project attempts to refine our understanding of the ab/ad boundary-mediated growth regulation and its relation to the various members of the KANADI gene family. Boundary-related factors will be identified and characterized, and their relation to the KANADI mediated growth will be examined. Manipulations on the position and duration of the ab/ad boundary will be carried out, both as a way to understand the process and as a way to regulate it.
To get better insight into the mechanisms of the suppression of blade expansion by KANADI, RNA expression profile was determined on the following genotypes: wild type seedlings, pANT>>KAN2 seedlings and phb-1d seedlings. RNA was extracted from 14 days old plants grown in short days, in the following manner. Cotyledons were cut down at their base and the other aerial parts were separated from the hypocotyl. The resulting tissue comprised of the shoot apical meristem along with all leaves and leaf primordia generated at that time. While in wild type these leaves show considerable blade expansion, the two other genotypes lack lateral expansion entirely. Isolated RNA from a pool of several hundreds seedlings was used for hybridization with Affimetrix gene chips. As of today, two replicates from each genotype were examined, and very high correlations between replicates were obtained.
Putative boundary factors were selected on the following criteria: increased expression (2 fold and higher) in wild type compared with phb-1d (1121 genes), and increased expression in wild type compared with pANT>>KAN2 (2197 genes). Of these, 73 genes corresponded to the set criteria of up regulation in wild type compared with both mutant backgrounds. These candidate genes will be subjected for further evaluation in the future as described in the original proposal.
Objectives and Expected Significance of the Research
The molecular and morphological characterization of the primary determinants of lateral organs adaxial/abaxial polarity has led to some unexpected findings. Although both the adaxial promoting PHB-like factors and the abaxial promoting KANADI genes are expressed in tissues showing lateral expansion, both type of factors inhibit growth when expressed throughout lateral organ primordia. These results support early models by Ian Sussex (1955) and later by Waites and Hudson (1995) who proposed that polarity, or specifically, juxtaposition of the two leaf domains, is necessary to induce laminar expansion.
We speculated further, that a boundary, generated between KANADI expressing cells and cells not expressing KANADI, acts as a source for a growth-inducing signal(s) (Eshed, et. al. 2001). As of today, no molecular, genetic or anatomical markers are known to correspond to such a hypothetical boundary. To further evaluate and refine models of boundary formation and boundary-induced growth, we wish to identify its putative molecular components.
Relevant Publication
- Eshed et al., (2004) "Asymmetric leaf development and blade expansion in Arabidopsis are mediated by KANADI and YABBY activities." Development 131(12): 2997-3006.
Sporadic Inter–generic DNA Introgression from Wheat into a Wild Aegilops Species
Background
Breeding of transgenic plants by genetic engineering is among the most promising approaches to increase plant productivity. Yet, it is undesirable to have such genetically engineered genes flow from the crop plants into their wild relatives. Prof. Jonathan Gressel of the Department of Plant Sciences strives through the study of plant molecular genetics to develop novel approaches that will minimize the flow of the genetically engineered genes from the crop plants to their wild relatives.
Introgressive hybridization has played a crucial role in the evolution of many plant species. It is composed of three sequential processes: inter-specific or inter-generic hybridization that produces highly sterile F1 hybrids, repeated backcrossing to the recurrent parent, and natural selection of backcross derivatives in a habitat where they are superior to either of the original parents. Introgression can greatly enrich the gene pool of the recurrent species and correspondingly increases its evolutionary potential.
The introduction of cultivars from distant areas might introduce foreign genes into wild relatives that can increase their capability to adapt to agricultural environments and compete with crops. Moreover, with the expanding use of genetic engineering, transgenes may introgress from engineered crops into adjacent wild or weedy relatives, creating a danger to biodiversity and producing more competitive weeds. Several recent reports have demonstrated the occurrence of gene flow from crops to their immediate wild progenitors. However, little is known about spontaneous gene movement from crops to more distant relatives.
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| Prof. Jonathan Gressel, Sarit Weissmann and Prof. Moshe Feldman |
Scientific Report
Common (bread) wheat (Triticum aestivum) is a case of a major crop that was formed by hybridization in farmers’ fields. It is a hexaploid (2n=6x=42; genome BBAADD) produced by hybridization between a previously domesticated tetraploid wheat (2n=4x=28; genome BBAA), that was formed by hybridization between unknown diploid donor of the B genome and diploid wheat T. urartu (2n=2x=14; genome AA), and the wild diploid Aegilops tauschii (2n=2x=14; genome DD) (Feldman 2001). There are no known species presently extant having a B genome at the diploid level. The closest species with the greatest homology to the B genome is Ae. speltoides containing the SS genome.
Consequently, bread wheat has no wild form. Domestic wheat is grown on large areas worldwide, where it comes in contact with various wild members of the tribe Triticeae that grow in or near wheat fields, and may exchange genes with them. Experimental work showed that genes from domesticated wheat can stably introgress into Ae. cylindrica (2n=4x=28: genome CCDD), a tetraploid species having one of its two genomes homologous to the D genome of common wheat. Although hybrids can form between domesticated polyploid wheat and other Triticeae with genomes that are homoeologous (partially homologous) to those of domesticated polyploid wheat, the hybrids are usually self–sterile and there is no information about stabilization of crop genes in backcrosses to the wild parent, only about stabilization by backcrossing of wild species genes by breeders into wheat. Despite the potential importance of gene flow to wild species, there is no information about natural homoeologous gene flow and stabilization in the field.
We have recently identified spontaneous introgression and stabilization of a DNA sequence from domesticated polyploid wheat into distantly related wild Aegilops peregrina in and near agroecosystems, in the field, despite not having homologous chromosomes. This is the first report of a natural, spontaneous, intergeneric DNA transfer from a crop (in this case wheat) to a wild species with only partial chromosomal homology to the crop, growing in the vicinity of the crop, suggesting that similar intergeneric gene flow could occur from transgenic crops. The consequences of this study should be taken into account in breeding programs of transgenic crops.
Relevant Publications
- Weissmann S., Feldman M. and Gressel J. 2003. "Evidence for sporadic introgression of a DNA sequence from polyploidy wheat into Aegilops peregrina (Ae. variabilis)." Proc. 10th Inter. Wheat Genet. Symp. Vol. 2, pp. 539-541.
- Weissmann S., Feldman M. and Gressel J., 2005. "Sporadic inter-generic introgression from domesticated wheat into wild tetraploid Aegilops peregrina." Molec. Biol. Evol. (Submitted).
| Amount in $ US | ||
| Income | ||
| Yield at 6% of $ 4,220,462 | 253,228 | |
| Less defrayal of Institution supplies at 20% | 50,646 | |
| Operating Budget | 202,582 | |
| Expenditure | ||
| Grants for staff research | 70,000 | |
| Dr. Yuval Eshed | 45,000 | |
| Prof. Jonathan Gressel | 25,000 | |
| Salaries for Staff | ||
| Zohar Hajbi, Greenhouse Technician | 25,000 | |
| Fellowship for Ph.D. Student (Sarit Weissman) | 20,400 | |
| Instrumentation | 64,052 | |
| Fluorescence absorbance Luminescence | 38,739 | |
| Upgrade of Olympus Confocal Microscope | 25,313 | |
| Biological Services Expenses | 23,130 | |
| Plant Cell Culture | 11,130 | |
| Scientific Supplies and Disposables | 12,000 | |
| TOTAL | 202,582 | |