Characterization of the Hog1 MAPK pathway in the entomopathogenic fungus Beauveria bassiana

High-osmolarity glycerol (HOG) pathway required for yeast osmoregulation relies upon the mitogen-activated protein kinase (MAPK) Hog1 cascade that comprise the MAPKKKs Ssk2/Ssk22 and Ste11 converging on the MAPKK Pbs2. Here we show a Hog1 cascade with the unique MAPKKK Ssk2 acting in Beauveria bassiana. Hypersensitivity to high osmolarity and high resistance to fludioxonil fungicide appeared in ssk2, pbs2 and hog1 mutants whereas the two hallmark phenotypes were reversed in ste11. Increased sensitivity to heat shock and decreased sensitivity to cell wall perturbation also occurred in the three mutants but not in ste11 although antioxidant phenotypes were different in all deletion mutants. Intriguingly, signals of Hog1 phosphorylation induced by osmotic, oxidative and thermal cues were present in ste11 but absent in ssk2 and pbs2. Moreover, vegetative growth on minimal media with different carbon/nitrogen sources was much more suppressed in ste11 and ssk2 than in pbs2 and hog1 although all mutants suffered similar, but severe, conidiation defects on a standard medium. Normal host infection was abolished in ste11 while virulence was differentially attenuated in other mutants. Our findings exclude Ste11 from the Hog1 cascade that regulates multiple stress responses and environmental adaptation of B. bassiana and perhaps other filamentous fungi.