Roles of the catalytic subunit of cAMP-dependent protein kinase A in virulence and development of the soilborne plant pathogen Verticillium dahliae

Verticillium dahliae is a soilborne fungus that causes vascular wilt disease in a broad range of hosts and survives for many years in the soil in the form of microsclerotia. Although the role of cAMP-dependent protein kinase A (PKA) has been extensively studied in foliar pathogens, there is limited information about its role in soilborne fungal pathogens that infect through the root system. Genome database search revealed the presence of two PKA catalytic subunit genes in V. dahliae, named VdPKAC1 and VdPKAC2. A phylogenetic analysis showed that VdPKAC2 groups with fungal PKA catalytic subunits that appear to play a minor role in PKA activity. This gene was expressed considerably lower than that of VdPKAC1. Although disruption of VdPKAC1 did not affect the ability of V. dahliae to infect through the roots of tomato and eggplant, disease severity was significantly reduced. Since pathogen-derived ethylene is presumed to play a major role in symptom induction in vascular wilt diseases, ethylene generation was measured in fungal culture. The mutants defective in VdPKAC1 produced less ethylene than the corresponding wild type strains, suggesting a regulatory role of PKA in ethylene biosynthesis. Growth rates of these mutants were similar to those of wild type strains, while the rate of spore germination was slightly elevated and conidia production was significantly reduced. When grown on minimal media, the mutants showed greater microsclerotia production compared with the wild type strains. These results suggest multiple roles of VdPKAC1, including virulence, conidiation, microsclerotia formation, and ethylene biosynthesis, in the soilborne fungus V. dahliae.