Fungal Mst3-proteins are involved in fungal innate immunity needed for the recognition of bacteria surrounding the hyphae, as well as for plant pathogenicity

Fungal innate immunity resembles mammalian innate immunity. It does not employ toll-like receptors (TLRs), but should employ endocytosis of non-fungal molecular patterns recognized by nuclear-localizing receptors (NLR). Downstream, both types of receptors are Mammalian Ste20 kinases (MSTs). We identified an MST3 ortholog in the plant pathogens Fusarium graminearum ( FgMST3 ) and Magnaporthe oryzae ( MoMST3 ). We knocked out both genes and investigated mutants using a standard panel of tests for growth, development, and pathogenicity for the respective fungi. Both ΔFgMST3 and ΔMoMST3 strains showed reduced pathogenicity. The deletions negatively affected conidia production and conidia germination but had little effect on growth rate. However, the two mutants reacted differently to various stress treatments, especially to Zn ²⁺ and gentamicin. In addition, we constructed an innate immunity reporter system for F. graminearum to detect less than 4-hour responses to non-self-molecular patterns (NSMP) like bacterial outer membrane vesicles (OMVs) and trace levels of sucrose, indicating plant. The reporter gene responses to OMVs of MST3 mutant strains are severely reduced. Our results indicate that both MoMst3 and FgMst3 are involved in fungal innate immunity downstream of unknown NLR proteins, motivating studies to identify genes for the NLR receptors. Finding such and investigating how they work and vary between fungal species and strains should be essential for understanding fungal biotic interactions with viruses, bacteria, plants, and animals.