Protein kinase A regulates production of virulence determinants by the entomopathogenic fungus, Metarhizium anisopliae

Metarhizium anisopliae is a model system for studying insect fungal pathogenesis. The role of cAMP signal transduction in virulence was studied by disrupting the class I PKA catalytic subunit gene (MaPKA1). The PKA mutant (ΔMaPKA1) showed reduced growth and greatly reduced virulence. PKA was dispensable for differentiation of infection structures (appressoria), but differentiation was delayed and the appressoria were defective because of reduced turgor pressure. ΔMaPKA1 germinated at similar rates as the wild type in glucose and glycerol, but germination was delayed on alanine. Conidial adhesion and appressorium formation by ΔMaPKA1 against a plastic surface was fully inhibited with glucose as sole nutrient source. Adhesion to plastic was not inhibited with glycerol or alanine, but appressorium formation was delayed. ΔMaPKA1 showed reduced tolerance to the oxidative agent diamide, but not to H2O2 and methyl-viologen. Comparative transcriptome analysis of ΔMaPKA1 and the wild type strain showed that PKA is responsible for up-regulating approximately one-third of the genes induced by insect cuticle, including subsets of those responsible for differentiation of appressoria and penetration pegs, cuticle degradation, nutrient acquisition, pH regulation, lipid synthesis, cell cycle control and the cytoskeleton. PKA was not however required for expression of toxin-producing genes. We conclude therefore that MaPKA1 is required for sensing host-related stimuli and transduction of these signals to regulate many infection processes.