The autophagy gene BbATG5, involved in the formation of the autophagosome, contributes to cell differentiation and growth but is dispensable for pathogenesis in the entomopathogenic fungus Beauveria bassiana

Abstract

Autophagy is a highly conserved process, representing the major eukaryotic degradative pathway of cellular components. Autophagy-mediated recycling of cellular materials contributes to cell differentiation, tissue remodelling and proper development. In fungi, autophagy is required for normal growth and cell differentiation. The entomopathogenic fungus Beauveria bassiana and its invertebrate targets represent a unique model system with which to examine host-pathogen interactions. The ATG5 gene is one of 17 involved in autophagosome formation, and the B. bassiana homologue (BbATG5) was identified. The role of autophagy in B. bassiana growth and virulence was investigated via construction of a targeted gene knockout of BbATG5. The mutant strain displayed increased sensitivity to nutrient limitation, with decreased germination and growth as compared with the wild-type parent. Conidiation was severely compromised and conidia derived from the ΔBbATG5 strain were altered in morphology. Cell differentiation into blastospores was also greatly reduced. Despite the significant growth and developmental defects, insect bioassays using the oriental leafworm moth, Spodoptera litura, indicated a modest (~40 %) decrease in virulence in the ΔBbATG5 strain. The phenotypic defects of the ΔBbATG5 strain could be restored by introduction of an intact copy of BbATG5. These data suggest that unlike several plant and animal pathogenic fungi, where ATG5 is required for infection, in B. bassiana it is dispensable for pathogenesis.