THE USTILAGO-MAYDIS REGULATORY SUBUNIT OF A CAMP-DEPENDENT PROTEIN-KINASE IS REQUIRED FOR GALL FORMATION IN MAIZE

In the plant, filamentous growth is required for pathogenicity of the corn smut pathogen Ustilago maydis. Earlier, we identified a role for the cAMP signal transduction pathway in the switch between budding and filamentous growth for this fungus. A gene designated ubc1 (for Ustil ago bypass of cyclase) was found to be required for filamentous growth and to encode the regulatory subunit of a cAMP-dependent protein kina se (PKA). Here, we show that ubc1 is important for the virulence of th e pathogen. Specifically, ubc1 mutants are able to colonize maize plan ts and, like the wild-type pathogen, cause localized symptoms in assoc iation with the presence of hyphae. However, in contrast to plants inf ected with wild-type cells that often developed galls from initially c hlorotic tissue, plants infected with the ubc1 mutant did not produce galls. These data suggest that PKA regulation is critical for the tran sition from saprophytic to pathogenic growth and from vegetative to re productive development. Plate mating assays in which exogenous cAMP wa s applied suggested that the cAMP and b mating-type morphogenetic path ways may be coordinated.