Mrlac1, an extracellular laccase, is required for conidial morphogenesis as well as the well adaptability in field of Metarhizium rileyi

Abstract

Acting as an extremely promising fungal pesticide, Metarhizium rileyi exhibits robust insecticidal activity against Lepidoptera pests, particularly the larvae. Though there is a slight delay in efficacy, biopesticides offer salient advantages over traditional chemical pesticide especially in environmental safety, cyclic infection and resistant inhibition. In this study, an exterior T-DNA was randomly inserted into the genome of M. rileyi, resulting in the acquisition of a mutant strain that displayed a colour transition from green to yellow within its conidia. The disruption of Mrlac1, a laccase, has been confirmed to attribute to the epigenetic alterations. Mrlac1 is a secreted protein harboring an N-terminal signaling peptide that undergoes in vivo synthesis and accumulates on the cell wall of M. rileyi. Targeted knock-out mutant exhibited alterations not just in conidia coloration, but significantly diminished capacity to withstand external stressors, particularly non-biological factors such as high humidity, Congo red exposure, and UV radiation. The disruptant suffered a constraint on hyphal polar growth, alteration in conidial surface structure, as well as noticeable increase in adhesion forces between conidia, the core infection factors. There is a remarkable diminution in virulence of Mrlac1 deletion variant against larvae of Spodoptera litura by topical inoculation, but not hemolymph injection. Our findings suggest that Mrlac1 acts as a positive regulator in the normal morphogenesis of fungal conidia, encompassing pigment production, inter-conidia adhesion, and conidial cell wall integrity, while the preservation of these structures holds paramount importance for the survival and infection of M. rileyi in the field.