The high osmotic response and cell wall integrity pathways cooperate to regulate morphology, microsclerotia development, and virulence in Metarhizium rileyi.

Microsclerotia (MS) formation was successfully induced in Metarhizium rileyi under changing liquid culture conditions. Mitogen-activated protein kinases (MAPKs) play important roles in fungal development and in coordinating many stress responses. To investigate how M. rileyi transduces growth stress and regulates MS differentiation, we characterized the roles of two MAPKs, Hog1- and Slt2-type orthologues, in M. rileyi. Compared with the wild-type strain, the deletion mutants of Mrhog1 (ΔMrhog1) and Mrslt2 (ΔMrslt2) delayed germination and vegetative growth, displayed sensitivities to various stress, and produced morphologically abnormal clones. The ΔMrhog1 and ΔMrslt2 mutants significantly reduced conidial (42-99%) and MS (96-99%) yields. A transcriptional analysis showed that the two MAPKs regulate MS development in a cooperative manner. Insect bioassays revealed that ΔMrhog1 and ΔMrslt2 had decreased virulence levels in topical (36-56%) and injection (78-93%) bioassays. Our results confirmed the roles of MrHog1 and MrSlt2 in sensing growth-related stress and in regulating MS differentiation.