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

Abstract

Fungal innate immunity overall responses are as fast as and resemble mammalian innate immunity. It does not, however, employ membrane toll-like receptors (TLRs), but should employ endocytosis of non-fungal molecular patterns recognized by nuclear-localizing receptors (NLR). Downstream, both types of receptors in mammals are Mammalian Ste20 kinases (MSTs). We identified single MST3 orthologs 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 each respective fungus. Both{Delta} FgMST3 and{Delta} MoMST3 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 some stress treatments, such as Zn2+ and gentamicin. Furthermore, we constructed an innate immunity reporter system for F. graminearum based on a gene with fast responses to bacterial MAMPs to detect less than 4-hour responses to non-self-molecular patterns (NSMP) like bacterial outer membrane vesicles (OMVs) as well as trace levels of sucrose, as a plant NSMP. The reporter gene responses to OMVs of MST3 mutant strains were severely reduced. Our results suggest that both MoMst3 and FgMst3 are involved in fungal innate immunity downstream of unknown NLR proteins, motivating studies to try to identify genes for the NLR-like receptors. Finding such and investigating how they work and vary between fungal species and strains should be useful for understanding fungal biotic interactions with viruses, bacteria, plants, and animals.