Molecular basis of pathogenicity by soilborne fungal plant pathogen, Verticillium dahliae

Verticillium dahliae Klebahn is a soil-borne plant pathogenic fungus that causes vascular wilt in a variety of economically important crops worldwide. Despite the economic importance of Verticillium wilt in agriculture, there have been limited studies on the mechanisms of pathogenicity by V. dahliae. The main objective of my thesis project is to enhance our understanding of the molecular basis of Verticillium wilt by determining the role of V. dahliae genes that are homologous to known or suspected pathogenicity genes in other fungal pathogens in pathogenicity. Target genes that have been cloned and characterized in my study included two genes (VdPMK1 and VdMPS1) encoding MAP (Mitogen-Activated Protein) kinases, the catabolite-repression regulator SNF1 homologue (VdSNF1) three genes encoding ABC (ATP-Binding Cassette) drug transporters, a gene ( VdCPKA) for cyclic-AMP dependent protein kinase (cPKA), and a gene VdrTHNR involved in melanin biosynthesis. To test the role of these genes in pathogenicity and growth, I have mutagenized them using a technique based on ATMT (A&barbelow;grobacterium t&barbelow;umefaciens-m&barbelow;ediated t&barbelow;ransformation). Mutations in VdPMK1 (homologous to the rice blast fungus Magnaporthe grisea PMK1 gene) completely abolished virulence regardless of the hosts used, suggesting that the gene is essential to pathogenicity. The mutations also affected colony morphology and microsclerotia formation, suggesting that the gene is involved in controlling multiple cellular processes. The VdABC1 homologue was essential for pathogenicity in M. grisea. In contrast, there was no noticeable difference in pathogenicity between wild-type strains and their mutants in VdABC1, suggesting that the role of this ABC transporter in pathogenicity is pathogen (or host) specific. Vdabc1 mutants also exhibited normal colony morphology, conidiation and microsclerotia formation. Mutations in VdSNF1 significantly reduced virulence. Experiments to determine the role of the remaining V. dahliae genes in pathogenicity are in progress and are summarized in Chapter 5.