Ilan Chet, Ada Viterbo and Yariv Brotman
Department of Biological Chemistry
Tel.+972 8 934 4466 Fax.+972 8 934 4112
1 Trichoderma-Induced systemic resistance in plants
2 Trichoderma mycoparasitism
Coiling of Trichoderma hyphae around the plant pathogen Rhizoctonia solani
Biological control, the use of specific microorganisms that interfere with plant pathogens and pests, is a nature-friendly, ecological approach to overcome the problems caused by standard chemical methods of plant protection.
Our main research interest focuses on a novel approach for induction of local and induced systemic resistance (ISR) towards plant pathogens, using a "mycorrhiza-like" saprophytic fungus, usually used as a biocontrol agent. The main objectives of our research are on one hand to isolate molecules secreted by the fungus involved in different stages of the process, from the root colonization to the recognition signaling between the plant and the fungus, and on the other to monitor changes in the plant due to the presence of the colonizing fungus.
By understanding the basis of action by Trichoderma spp. we should be able to manipulate the plant, the fungal agent and their interactions to achieve more effective plant resistance to various biotic stresses.
Trichoderma spp. are fungi that are present in nearly all agricultural soils and in other environments such as decaying wood. The antifungal abilities of these beneficial microbes have been known since the 1930s, and there have been extensive efforts to use them for plant disease control since then. These fungi grow tropically toward hyphae of other fungi, coil about them in a lectin-mediated reaction, and degrade cell walls of the target fungi by the secretion of different lytic enzymes. This process (mycoparasitism) limits growth and activity of plant pathogenic fungi.
In fact, we do not know whether most of the benefits of Trichoderma occur because they directly attack and control disease-causing fungi, as has long been believed, or because they have direct effects upon plants. Many recent findings suggest that plant development and biochemistry are strongly affected by Trichoderma strains.
Specific strains of fungi in the genus Trichoderma colonize and penetrate plant root tissues and initiate a series of morphological and biochemical changes in the plant, considered to be part of the plant defense response, which in the end leads to induced systemic resistance (ISR) in the entire plant. The capability of T. harzianum to promote increased growth response was verified both in greenhouse experiments and in the hydroponic system. A 30% increase in seedling emergence was observed and these plants exhibited a 95% increase in root area. Similarly an increase in P and Fe concentration was observed in Trichoderma inoculated plants.
Using a controlled hydroponic system we could identify at the protein level, an increase in the activity of chitinases, b-1,3-glucanases, cellulases and peroxidases in cucumber roots previously inoculated with the biocontrol agent T. harzianum T-203 as early as 48 h post inoculation. Such plants showed reduced infection levels by Pseudomonas syringae pv. lachrymans in leaves. RT-PCR of selected genes involved in the plant defence response revealed their up-regulation in both roots and leaves of primed plants. HPLC analysis of phenolic compounds extracted from leaves of plants treated with T. harzianum showed a marked change in their profile. Purification of crude phenolic extracts enabled the separation of compounds with high specific antimicrobial activity as assessed in in vitro bioassays against bacteria, yeast and fungi.
The systemic response in plants is induced through the JA/ethylene signaling pathway. The expression of several plant defense related genes is potentiated, enabling Trichoderma treated plants to be more resistant to subsequent pathogen infection. The MAPK signal transduction pathways, both of the plant and Trichoderma, are important for the induction of systemic resistance.
In order to identify molecular factors directly involved in the Trichoderma-plant interaction, we are conducting PCR differential screening of T. asperellum genes and proteomic analyses of Trichoderma cell wall proteins induced during root colonization. Among several clones we identified a Class I hydrophobin. Genetic functional analysis demonstrates that this protein has a role in plant root colonization and infection.
We are interested in the molecular regulation of the recognition and coiling process of the Trichoderma hyphae around its host.
Trichoderma spp. attach to the host hyphae via coiling, hooks and appressorium like bodies, and penetrate the host cell wall by secreting lytic enzymes. The interaction is specific and is not merely a contact response. Trichoderma recognizes signals from the host fungus, triggering coiling and host penetration. A biomimetic system consisting of lectin-coated nylon fibers was used to study the role of lectins in mycoparasitism. Using this system we could also identify specific coiling-inducing molecules.
Last updated January 19, 2006