A putative methyltransferase, mtrA, contributes to development, spore viability, protein secretion and virulence in the entomopathogenic fungus Beauveria bassiana

The filamentous fungus, Beauveria bassiana, is a ubiquitously distributed insect pathogen, currently used as an alternative to chemical pesticides for pest control. Conidiospores are the means by which the fungus disseminates in the environment, and these cells also represent the infectious agent most commonly used in field applications. Little, however, is known concerning the molecular basis for maintenance of spore viability, a critical feature for survival and persistence. Here, we report on the role of a putative methyltransferase, BbmtrA, in conidial viability, normal fungal growth and development, and virulence, via characterization of a targeted gene knockout strain. Loss of BbmtrA resulted in pleiotropic effects including reduced germination, growth and conidiation, with growing mycelia displaying greater branching than the WT parent. Conidial viability dramatically decreased over time, with <5% of the cells remaining viable after 30 days as compared with >80% of the WT. Reduced production of extracellular proteins was also observed for the Delta BbmtrA mutant, including protease/peptidases, glycoside hydrolases and the hydl hydrophobin. The latter was further confirmed by hyd1 gene expression analysis. Insect bioassays using the greater wax moth, Galleria mellonella, further revealed that the Delta BbmtrA strain was attenuated in virulence and failed to sporulate on host cadavers. These data support a global role for mtrA in fungal physiological processes.