Activation of the cAMP pathway in Ustilago maydis reduces fungal proliferation and teliospore formation in plant tumors

In the corn smut fungus Ustilago maydis, mating of two haploid sporidia is a prerequisite for subsequent colonization of the host. Cyclic AMP (cAMP) a nd pheromone signals have been implicated in this developmental program. Th e cAMP pathway is also needed for subsequent fungal development in planta, as null mutants in any component of the pathway fail to form tumors. Here w e show that moderate activation of the pathway conferred either by mutation in the Ga subunit or by mutation in the regulatory subunit of the protein kinase A influences tumor morphology. In the resulting tumors, the amount o f fungal material is drastically reduced and fungal development is arrested at the stage of sporogenic hyphae, We conclude that tight regulation of th e cAMP pathway is crucial for fungal development within the plant but does not interfere with the tumor induction process.