Bacterial symbionts of Epichloë fungal endophytes of grasses

Abstract

Bacteria often form symbiotic associations with plant beneficial fungi, and they can increase the fitness of either their fungal and/or plant hosts. We characterised the bacterial microbiota associated with Epichloë fungal endophytes of perennial ryegrass (Lolium perenne) and investigated the effects of the bacterial symbionts on the performance of Epichloë or L. perenne (e.g., growth, Epichloë-derived alkaloid concentrations). We hypothesised that the bacterial microbiota associated with Epichloë would not be detrimental towards the fungal or plant host and instead promote plant growth without affecting either fungal growth or production of Epichloë-derived alkaloids. The bacterial microbiota associated with Epichloë was dominated by members of the genus Paenibacillus, although bacteria within genera Delftia, Micrococcus and Bradyrhizobium were also present. Two bacterial strains, Paenibacillus E222 and Micrococcus E226, were isolated from Epichloë mycelia and characterised further. Both E222 and E226 formed ectosymbiotic relationships with the fungal endophyte (i.e., bacterial cells were located on the hyphal surface) and possessed plant-growth promoting traits identified by whole genome analysis. Further experiments, enriching the abundance of E222 and E226 by inoculating these bacteria onto perennial ryegrass seed colonised by Epichloë, confirmed their plant growth promoting abilities in planta. E226 did not affect the growth of Epichloë or the concentration of Epichloë-derived alkaloids while E222 did the opposite. The mechanism of plant growth promotion by E222 and E226 is likely related to the production of growth promoting hormones/volatiles, abilities to solubilise phosphorus, and/or synthesise vitamins/metabolic cofactors.