Diverse function and regulation of CmSnf1 in entomopathogenic fungus Cordyceps militaris

SNF1/AMPK protein kinases play important roles in fungal development and activation of catabolite-repressed genes. In this study, we characterized the role of SNF1 ortholog in Cordyceps militaris (CmSnf1). The vegetative growth of a CmSnf1 deletion mutant was (ΔCmSnf1) reduced by 42.2% with arabinose as a sole carbon source. Most strikingly, the ΔCmSnf1 produced only a few conidia and exhibited delayed conidial germination. We found that CmSnf1 was necessary for mycelium to penetrate the insect cuticle to form the fruiting body on silkworm pupae, consistent with the down-regulation of chitinase- and protease-encoding genes in ΔCmSnf1. However, cordycepin content increased by more than 7 times in culture supernatants. Correspondingly, the relative expression levels of cordycepin gene cluster members were also elevated. In particular, the expression of cns4 associated with cordycepin transfer was up-regulated >10-fold. Furthermore, transcriptional analysis showed that CmSnf1 regulated the expression of genes involved in cell autophagy and oxidative stress tolerance. We speculated that under environmental stress, both the ATG and SNF1 pathways might collaborate to sustain adverse environments. Our study provides an initial framework to probe the diverse function and regulation of CmSnf1 in C. militaris, which will shed more light on the direction of molecular improvement of medicinal fungi.