The MAP kinase Kpp2 regulates mating and pathogenic development in Ustilago maydis

In the phytopathogenic fungus Ustilago maydis, fusion of compatible haploid cells is a prerequisite for infection. This process is genetically control led by the biallelic a locus, encoding pheromone precursors and receptors. These are presumed to be coupled to a heterotrimeric G protein and a MAP ki nase cascade, leading to activation of the HMG domain transcription factor Prf1. Here, we have demonstrated that putative MAP kinase sites in Prf1 are required for its activity during mating. In addition, we have identified a gene, kpp2, which encodes a putative MAP kinase related to Pmk1 of Magnapo rthe grisea and Fus3p of Saccharomyces cerevisiae. kpp2 deletion mutants ar e attenuated in several steps of development: cell fusion, induction of phe romone-responsive genes and pathogenicity. Epistasis analysis shows that kp p2 does not affect pheromone gene expression through the cAMP signalling ca scade. Pathogenicity of kpp2 mutants can be partially restored by overexpre ssing the b genes, indicating a regulation of Prf1 by Kpp2. These data supp ort the hypothesis that the MAP kinase Kpp2 transmits the pheromone signal.