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

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 Epichloë, a well-researched mutualistic fungal endophyte of grasses, on perennial ryegrass (Lolium perenne) also associated with Epichloë. We hypothesised that E226 would promote plant growth but would not interfere with the in-planta growth of Epichloë and its production of antiherbivore alkaloids. E226, identified as Micrococcus luteus, formed an ectosymbiotic relationship with Epichloë 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 L. perenne, already colonised by an Epichloë strain, were inoculated with E226. The bacterium established within the seedlings, and the abundance of M. luteus 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 Epichloë or the concentration of Epichloë-derived alkaloids. The putative abilities of E226 to solubilise phosphate and produce vitamins and metabolic cofactors may explain the observed plant growth promotion.