MaMHR Contributes to Conidiation, Stress Adaptation, and Virulence in Metarhizium acridum

Metarhizium acridum is an important entomopathogenic fungus widely used for biological control of locust pests. Although Zn2Cys6 transcription factors regulate diverse fungal developmental and stress-responsive processes, the functions of MHR-containing Zn2Cys6 transcription factors remain poorly understood, particularly in entomopathogenic fungi. Here, we characterized MaMHR, a previously uncharacterized Zn2Cys6 transcription factor containing a fungal transcription factor middle homology region (MHR), through targeted gene deletion and complementation in M. acridum. Loss of MaMHR delayed conidial germination, reduced conidial production by approximately 17.6% and 28.3% on 1/4 SDAY and SYA media, respectively, and shifted conidiation from the microcycle to the normal mode. The ΔMaMHR mutant also showed impaired tolerance to UV-B irradiation and wet heat, with UV-B and heat IT50 values reduced by approximately 26.5% and 22.6%, respectively, as well as reduced tolerance to selected chemical stresses. Furthermore, MaMHR deletion reduced melanin accumulation by approximately 47.0%, disturbed reactive oxygen species (ROS) homeostasis, altered antioxidant enzyme activities, and affected the expression of nucleotide excision repair-related genes following UV-B stress, whereas trehalose accumulation was unaffected. Disruption of MaMHR also reduced fungal virulence during topical infection as reflected by an approximately 14.6% increase in LT50, and reduced cuticle penetration-associated fungal outgrowth, whereas virulence following intra-hemocoel injection was not significantly affected. Together, these findings demonstrate that MaMHR contributes to fungal development, environmental stress adaptation, and host infection, potentially through regulation of oxidative stress, melanin biosynthesis, and DNA repair-associated responses.