Contribution of the gas1 Gene of the Entomopathogenic Fungus Beauveria bassiana, Encoding a Putative Glycosylphosphatidylinositol-Anchored β-1,3-Glucanosyltransferase, to Conidial Thermotolerance and Virulence▿†

摘要

Beauveria bassiana is a mycoinsecticide alternative to chemicals for use in biological pest control. The fungus-insect interaction is also an emerging model system to examine unique aspects of the development, pathogenesis, and diversity of fungal lifestyles. The glycoside hydrolase 72 (GH72) family includes β-1,3-glucanosyltransferases that are glycosylphosphatidylinositol (GPI)-anchored cell wall-modeling enzymes affecting fungal physiology. A putative B. bassiana GPI-anchored β-1,3-glucanosyltransferase (Bbgas1) was isolated and characterized. B. bassiana targeted gene knockouts lacking Bbgas1 were affected in Congo red and salt sensitivity but displayed minor growth defects in the presence of sorbitol, SDS, or calcofluor white. Lectin and antibody mapping of surface carbohydrates revealed increased exposure of carbohydrate epitopes, including β-1,3-glucans, in the ΔBbgas1 strain. Transmission electron micrographs revealed localized destabilization of the cell wall in ΔBbgas1 conidia, in which fraying of the outer cell wall was apparent. Heat shock temperature sensitivity profiling showed that in contrast to the wild-type parent, ΔBbgas1 conidial spores displayed decreased germination after 1 to 4 h of heat shock at temperatures >40°C, and propidium iodide exclusion assays revealed decreased membrane stability in the knockout strain at temperatures >50°C. The ΔBbgas1 knockout showed reduced virulence in Galleria mellonella insect bioassays in both topical and intrahemocoel-injection assays. B. bassiana ΔBbgas1 strains complemented with the complete Bbgas1 open reading frame were indistinguishable from the wild-type parent in all phenotypes examined. The Bbgas1 gene did not complement the phenotype of a Saccharomyces cerevisiae β-1,3-glucanosyltransferase Δgas1 mutant, indicating that this family of enzymes likely posses discrete cellular functions.