MripacC regulates blastosphere budding and influences virulence of the pathogenic fungus Metarhizium rileyi.

Fungal dimorphism is the ability of certain fungi to switch between two different cellular forms, yeast and mycelial forms, in response to external environmental factors. The pacC/Pal signal transduction pathway responds to neutral and alkaline environments and is also involved in the fungal dimorphic transition. In this study, we investigated the function of the pacC homolog, MripacC, which regulates the dimorphic transition and modulates virulence of the insect pathogenic fungus Metarhizium rileyi. MripacC expression was upregulated under alkaline condition, with increased number of yeast-like cells compared to the number of hyphae cells. A MripacC deletion mutant (ΔMripacC) was obtained by homologous replacement and exhibited decreased blastospore budding, with direct development of conidia into hyphae without entering the yeast-like stage when cultured on alkaline medium. Observation of host hemolymph morphology and analysis of samples to detect the main immune factors revealed a decreased ability of ΔMripacC to evade the host immune system. The results of insect bioassays showed that ΔMripacC had decreased virulence with extended median lethality time. Together, the results suggested that MripacC not only regulated adaptation to acidic and alkaline environments, but also influenced virulence by budding blastospores. This elucidation of the function of MripacC adds to our understanding of blastospore budding and virulence of this fungal pathogen.