Involvement of putative response regulator genes of the rice blast fungus Magnaporthe oryzae in osmotic stress response, fungicide action, and pathogenicity.

Rice blast fungus (Magnaporthe oryzae) has ten histidine kinases (HKs), one histidine-containing phosphotransfer protein (HPt), and three response regulators (RRs) as putative components of the two-component signal transduction system (TCS). Here, we constructed knockout mutants of two putative RR genes (MoSSK1, MoSKN7) and a RR homolog gene (MoRIM15) to analyze the roles of TCS in environmental adaptation and pathogenicity. The DeltaMossk1 strain had increased sensitivity to high osmolarity and decreased sensitivity to fludioxonil. The DeltaMoskn7 strain had slightly decreased sensitivity to fludioxonil. The involvement of MoSkn7 in the osmoresponse was obvious only on the DeltaMossk1 background. These results show that MoSsk1 and MoSkn7 are major and minor contributors, respectively, in the high osmolarity response and fludioxonil action. The DeltaMossk1 strain was more osmosensitive than the predicted upstream HK gene disruptant Deltahik1, which shows sugar-specific high osmolarity sensitivity. The DeltaMossk1 and DeltaMoskn7 strains showed enhanced hyphal melanization, suggesting that RRs regulate hyphal melanization. MoSsk1 and MoRim15 are required for full virulence, because the DeltaMossk1 and DeltaMorim15 strains exhibited reduced virulence. These results suggest that the putative RRs of the rice blast fungus are involved in the osmotic stress response, fludioxonil action, and pathogenicity.