|Ph.D Student||Ella Pardoviz-Kedmi|
|Subject||Development and Gene Expression in Trichoderma Virens:|
Insights into Biocontrol-Related Signaling
|Department||Department of Biology||Supervisor||Full Professor Horwitz Benjamin|
|Full Thesis text - in Hebrew|
Microorganisms that antagonize plant pathogens are an environmentally friendly way to provide defense to crop plants against disease. Combining such biological control agents with chemical control can provide protection while reducing fungicide load. Species of the genus Trichoderma interact with plant roots, and parasitize other fungi and soil-living pathogens. In addition to directly attacking pathogens, the interaction with the plant induces systemic resistance, a form of immunity that provides defense against further attack. Trichoderma cells respond to host-derived and other signals. Conserved fungal signal transduction pathways are therefore central to the three-way interaction between biocontrol fungus, plant pathogen, and plant. We isolated signal transduction- related genes of T. virens, including three G-protein alpha subunits (TgaA, TgaB,TgaC) and two MAP kinases (TmkA and TmkB). TgaA belongs to the fungal Gαi class, while TgaB and TgaC correspond to gna-2 and gna3, respectively, of the genetic model fungus Neurospora crassa. We compared the mutants to the wild type by growth, sporulation, germination of conidia (spores), the ability to overgrow colonies of Rhizoctonia solani and Sclerotium rolfsii in confrontation assays, and to colonize the sclerotia of these pathogens in soil. Ga subunit TgaA is involved in antagonism against S. rolfsii, but none of the G protein subunits is involved in antagonism against R. solani. T. virens, thus employs G-protein pathways in a host-specific manner. The functions of MAPK TmkB (ortholog of the yeast cell-integrity MAPK) were studied and compared to those of the pathogenicity MAP kinase (TmkA). Like tmkA mutants, tmkB mutants exhibited reduced radial growth and constitutive conidiation. tmkB mutants, in contrast to tmkA, had defects in cell-wall integrity. Like tmkA, tmkB mutants had attenuated ability to overgrow Sclerotium rolfsii, while retaining the ability to overgrow Rhizoctonia solani and Pythium sp. We identified a gene encoding a small cysteine-rich secreted protein of unknown function, the TmkA-repressed gene Mrsp1. We found that Mrsp1 is also up-regulated in the TmkB mutants; thus both of these MAPKs are required to repress the expression of this gene, indicating a combined role in its regulation. When this study began there was no genome sequence of any Trichoderma species. Following recent annotation of three genomes, signal transduction-related genes can now be easily identified in silico, particularly in view of the high level of conservation of some of them, providing a new way to address the questions studied here.