Primary roles of two dehydrogenases in the mannitol metabolism and multi-stress tolerance of entomopathogenic fungus Beauveria bassiana

Knockout and complement mutants of mannitol-1-phosphate dehydrogenase (MPD) and mannitol dehydrogenase (MTD) were constructed to probe the roles of both enzymes in the mannitol metabolism and multi-stress tolerances of entomopathogenic fungus Beauveria bassiana. Compared with wild-type and complement mutants, Delta BbMPD lost 99.5% MPD activity for reducing fructose-6-phosphate to mannitol-1-phosphate while Delta BbMTD lost 78.9% MTD activity for oxidizing mannitol to fructose. Consequently, mannitol contents in mycelia and conidia decreased 68% and 83% for Delta BbMPD, and 16% and 38% for Delta BbMTD, accompanied by greatly enhanced trehalose accumulations due to 81-87% decrease in their neutral trehalase expression. Mannitol as mere carbon source in a nitrate-based minimal medium suppressed the colony growth of Delta BbMTD instead of Delta BbMPD, and delayed more conidial germination of Delta BbMTD than Delta BbMPD. Based on median lethal responses, conidial tolerances to H2O2 oxidation, UV-B irradiation and heat stress at 45 degrees C decreased 38%, 39% and 22% in Delta BbMPD, and 18%, 16% and 11% in Delta BbMTD respectively. Moreover, Delta BbMPD and Delta BbMTD lost 14% and 7% of their virulence against Spodoptera litura larvae respectively. Our findings highlight the primary roles of MPD and MTD in mannitol metabolism and their significant contributions to multi-stress tolerances and virulence influential on the biocontrol potential of B. bassiana.