The transcription factor MrbHLH2 significantly affects stress resistance and insect virulence in Metarhizium robertsii

Basic helix-loop-helix (bHLH) transcription factors are essential regulators of various biological processes, including growth, development, and stress responses in eukaryotes. Despite their importance, the specific roles of bHLH factors in entomopathogenic fungi remain inadequately understood. In this study, we identified and characterized the bHLH transcription factor MrbHLH2 in the entomopathogenic fungus Metarhizium robertsii, which is widely used in biological control. Subcellular localization studies confirmed that MrbHLH2 is predominantly located in the nuclei of conidia. The deletion of MrbHLH2 (ΔMrbHLH2) resulted in enhanced tolerance to osmotic and heat stress, while simultaneously decreasing tolerance to ultraviolet (UV) radiation. Notably, ΔMrbHLH2 exhibited significantly reduced virulence, as indicated by a 0.83-day increase in LT₅₀ compared to the wild-type strain, along with delayed appressorium formation, impaired cuticle penetration and downregulation of several virulence-related genes. This study contributes to the understanding of bHLH transcription factors in entomopathogenic fungi and underscores the potential of targeting these factors to enhance the biocontrol efficacy of fungal pathogens.