PsMPK1, an SLT2-type mitogen-activated protein kinase, is required for hyphal growth, zoosporogenesis, cell wall integrity, and pathogenicity in Phytophthora sojae.

Mitogen-activated protein kinases (MAPKs) play important roles in the regulation of vegetative and pathogenic growth in plant pathogens. Here, we identified an SLT2-type MAP kinase in Phytophthora sojae, PsMPK1, which was transcriptionally induced in sporulating hyphae and the early stages of infection. Silencing of PsMPK1 caused defects in growth and zoosporogenesis, and increased hyphal swellings after the induction of sporangia formation, along with increasing hypersensitivity to cell wall-degrading enzymes. Transmission electron microscopy showed that the cell wall of PsMPK1-silenced mutants was also deleteriously affected. A dark outermost layer in the cell walls disappeared in the mutants, and an additional layer of the mutant cell wall that was deposited abnormally inside an inner bright layer appeared nonhomogeneous and rough compared to the wild type. Pathogenicity assays showed that PsMPK1-silenced transformants lost their pathogenicity on susceptible soybean host plants and triggered stronger cell death. Overall, PsMPK1 is involved in growth, differentiation, cell wall integrity, and pathogenicity in P. sojae.