Epichloë-the obligate mutualist protecting some commercially important temperate grasses from biotic and abiotic...

The relationship between Epichloë fungal endophytes found in some temperate grasses spans the continuum from antagonistic to mutualistic. The predominant focus here is on those that form mutualistic associations with those temperate grasses of the New World due to their substantial economic impact. This review describes the evolutionary origin, discovery, lifestyle and life cycle of Epichloë as background to understand the close association between plant host and fungal endophyte. Many asexual Epichloë fungal endophytes are unique in being obligate mutualists, lacking a free-living stage in nature, and being maternally inherited through host seed. The compatibility between Epichloë and its natural host plant is so complete and specific that manually transferring an Epichloë endophyte strain to other grasses, even some that are closely related genetically, though possible, often results in endophyte and/or plant death or poorly performing plant genotypes. The stability of the association is facilitated through the biotic and abiotic protection of the host grass through the endophyte promoting endogenous defense responses in the host, or directly through the production of secondary metabolites. Epichloë endophytes are renowned for producing a diverse range of alkaloids that can negatively affect invertebrate and vertebrate herbivores, either through toxicity or feeding deterrence, thereby enhancing host survival. However, Epichloë chemistry can significantly impact the health and welfare of grazing animals resulting in large economic impacts. This includes effects known as ryegrass staggers, heat stress and fescue toxicosis. In some temperate areas of the world, notably New Zealand, Australia and USA, grasses such as perennial ryegrass and tall fescue need to have the Epichloë-derived protection to ensure persistence against invertebrate pests and drought but without the downsides of clinical and subclinical losses for grazing animals. Extensive screening has identified strains of Epichloë which have been successfully commercialised for ryegrasses and fescues that ensure persistence of the grass host without causing serious health and welfare impacts on the grazing animal. The development and deployment of selected Epichloë strains therefore represent a critical intersection of plant protection, animal welfare, and agricultural productivity, a theme explored throughout this review.