Tripartite associations: A bacterial symbiont of fungi promotes plant growth without compromising the benefits conferred by an <i>Epichloë </i>endophyte

<p dir="ltr">Few studies have evaluated the effects of bacteria that form associations with fungi on plant-microbial symbioses. We investigated the effects of a novel symbiotic bacterium (E226) of <i>Epichloë</i>, a well-researched mutualistic fungal endophyte of grasses, on perennial ryegrass (<i>Lolium perenne</i>) also associated with <i>Epichloë</i>. We hypothesised that E226 would promote plant growth but would not interfere with the <i>in-planta</i> growth of <i>Epichloë </i>and its production of antiherbivore alkaloids. E226, identified as Micrococcus luteus, formed an ectosymbiotic relationship with <i>Epichloë </i>when cultured on artificial media (i.e., bacterial cells were located on the hyphal surface) and possessed plant-growth promoting traits identified through whole genome analysis. Seeds of <i>L. perenne</i>, already colonised by an <i>Epichloë </i>strain, were inoculated with E226. The bacterium established within the seedlings, and the abundance of <i>M. luteus</i> populations initially increased with plant age. In agreement with our hypotheses, plant inoculation with E226 enhanced the production of leaves and did not affect the growth of <i>Epichloë </i>or the concentration of <i>Epichloë</i>-derived alkaloids. The putative abilities of E226 to solubilise phosphate and produce vitamins and metabolic cofactors may explain the observed plant growth promotion.</p>