Unusual natural products mediated root hairs-endophyte stacking (RHESt) that traps and kills pathogens

The ancient African crop, finger millet, has broad resistance to pathogens including the toxigenic fungus Fusarium graminearum [1]. Here we report the discovery of a novel plant defence mechanism, resulting from an unusual symbiosis between finger millet and a root-inhabiting bacterial endophyte, M6 (Enterobacter sp.) [2]. Upon sensing the pathogen, M6 swarms towards Fusarium attempting to penetrate root epidermis, induces growth of root hairs through production of growth hormones, which then bend parallel to the root axis, then forms biofilm-mediated microcolonies, resulting in a remarkable, multi-layer root hair-endophyte stack (RHESt). The RHESt consists of two lines of defence, a dense layer of intercalated root hairs and endophyte microcolonies followed by a long, continuous endophyte barrier layer on the root epidermal surface (see A and B). M6 was visualized to specifically attach to Fusarium hyphae (see C) and kill it (see D). RHESt results in a physical barrier that prevents entry and/or traps F. graminearum which is then killed inside. Thus M6 creates its own specialized killing microhabitat. M6 killing requires c-di-GMP-dependent signalling, diverse fungicides (phenazine metabolites, colicin V peptide antibiotic, chitinase enzyme, etc.) [3, 4]. We show that a pre-requisite of M6 killing is expression of a fusaric acid resistance operon that effluxes the Fusarium-derived mycotoxin, fusaric acid, outside the cell (see E). Furthermore, our results show a novel epistatic regulatory interaction between the fusaric acid resistance and phenazine pathways. We propose that the phenazine-fusaric acid arms race provides a molecular and biochemical paleontological record that M6 and Fusarium co-evolved. The end-result of this remarkable symbiosis and tripartite co-evolution is reduced DON mycotoxin [5], potentially benefiting millions of subsistence farmers over thousands of years. RHESt demonstrates the value of exploring ancient, orphan crop endophytic microbiomes.

Acknowledgements: We thank Dr. Michaela Strueder-Kypke for technical assistance. We thank the graphical designer, Lisa Smith for the graphical illustration.

Keywords: endophytes, finger millet, F. graminearum, RHESt, antifungal natural products.

References:

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