Complementation of the Magnaporthe grisea Delta cpkA mutation by the Blumeria graminis PKA-c gene: Functional genetic analysis of an obligate plant pathogen

Obligate plant-pathogenic fungi have proved extremely difficult to characte rize with molecular genetics because they cannot be cultured away from host plants and only can be manipulated experimentally in limited circumstances . Previously, in order to characterize signal transduction processes during infection-related development of the powdery mildew fungus Blumeria gramin is (syn. Erysiphe graminis) f. sp. hordei, we described a gene similar to t he catalytic subunit of cyclic AMP-dependent protein kinase A (here renamed Bka1). Functional characterization of this gene has been achieved by expre ssion in a Delta cpkA mutant of the nonobligate pathogen Magnaporthe grisea . This nonpathogenic M. grisea Delta cpkA mutant displays delayed and incom plete appressorium development, suggesting a role for PKA-c in the signal t ransduction processes that control the maturation of infection cells. Trans formation of the Delta cpkA mutant with the mildew Bkal open reading frame, controlled by the M. grisea MPG1 promoter, restored pathogenicity and appr essorium maturation kinetics. The results provide, to our knowledge, the fi rst functional genetic analysis of pathogenicity in an obligate pathogen an d highlight the remarkable conservation of signaling components regulating infection-related development in pathogenic fungi.