Characterization of the autophagy-related gene BmATG8 in Bipolaris maydis

Autophagy is involved in cellular development and the maintenance of viability under nutrient deprivation in a wide range of eukaryotes. A filamentous ascomycete Bipolaris maydis, responsible for southern corn leaf blight, is also studied as a model fungus for sexual reproduction in filamentous ascomycetes that form filiform ascospores. In order to clarify the roles of autophagy in various stages of the life cycle of B. maydis, we constructed null mutants of BmATG8, an orthologue of the Saccharomyces cerevisiae autophagy gene ATG8 in B. maydis. Deletion of BmATG8 impaired localization of cytosolic components to the vacuole under nitrogen starvation, suggesting that autophagy was deficient in the null mutants. Additionally, fluorescent microscopic observations on a eGFP-fused BmATG8 expressing strain showed that BmATG8 is associated with autophagy-related structures. In vegetative growth, ΔBmATG8 strains showed a reduction in conidiation and aerial mycelial growth. Interestingly, the mutant conidia indicated loss of the germination rate under starvation conditions and affected longevity. However, germinated mutant conidia were still capable of infecting the host plant via appressoria. In sexual reproduction, ascospores with ΔBmATG8 genetic background were aborted. Our results revealed that autophagy plays a crucial role in the function of conidia, not in host infection via appressoria in B. maydis. In addition, conservation of the importance of autophagy in ascospore development is suggested among ascomycetes including species that form bitunicate ascus.