Inhibitory phosphorylation of a mitotic cyclin-dependent kinase regulates the morphogenesis, cell size and virulence of the smut fungus Ustilago maydis

The regulation of cyclin-dependent kinase (CDK) activity through inhibitory phosphorylation seems to play an important role in the eukaryotic cell cycle. We have investigated the influence that inhibitory phosphorylation of the catalytic subunit of mitotic CDK has on cell growth and pathogenicity of the corn smut fungus Ustilago maydis. This model pathogen is worthy of attention since it is well suited to analyze the relationships between the cell cycle, morphogenesis and pathogenicity. We set out to study these relationships by producing a cdk1 mutant allele that was refractory to inhibitory phosphorylation. The expression of this mutant in U. maydis cells dramatically altered their morphology. Since this kind of mutation makes the CDK catalytic subunit resistant to regulation by Wee1-related kinases in other organisms, we characterized the orthologous Wee1 kinase from U. maydis. We found that Wee1 is essential in U. maydis. Overexpression of wee1 produces cell cycle arrest in G2, the target of Wee1 apparently being the Cdk1/Clb2 complex, which is required specifically for the onset of mitosis. Given the connection between the cell cycle control and pathogenesis in U. maydis, we also analyzed whether cells with impaired inhibitory phosphorylation of Cdk1 were able to infect plants. We found that inhibitory phosphorylation was required for mating, a prerequisite to initiate pathogenic development. By examining plant-specific expression of the constitutively unphosphorylated cdk1[superscript AF] allele, we also found that appropriate levels of inhibitory phosphorylation were required at stages of infection subsequent to penetration by the fungus. These data reinforces the connections between cell cycle, morphogenesis and virulence in this smut fungus.