Foliar fungal endophytes in herbaceous plants: A marriage of convenience?

Abstract

Foliar fungal endophytes are widespread in herbaceous plants, although their interactions and ecological roles are little understood. They are phylogenetically and ecologically diverse, with the potential to be influential members of the biotic community. Compositionally, the endophyte community within a plant is determined by both the fungi (genotype, competitive ability, tissue specificity, infection location) and the host (genotype, variations in plant defences, geographical location). The plant-endophyte relationship is dynamic, as fungal composition varies temporally across months and seasons, with subsequent infections occurring after initial colonisation. Transmission generally occurs horizontally via air- or waterborne spores, with hyphae entering the host through stomata or through direct penetration. Contrasting to extensive mycorrhizal fungal colonisation in roots, infection by any one endophyte in aerial parts appears to be limited, due to plant defences, intra- or interspecific competition between endophytes and other factors governing niche occupancy. Fungal endophytes colonise host tissues for at least part of their life cycle, with no apparent outward pathology. Simultaneously, they can benefit their hosts through improved tolerance to biotic stress such as drought, enhanced photosynthesis and transpiration, protection against pathogens through induced plant systemic resistance and the deterrence of phytophagous invertebrates (depending on their feeding guild and degree of specialism). These benefits arise directly from endophyte metabolism or indirectly through the production of compounds that alter the host's physiology. Thus, the influence of fungal endophytes may pervade beyond their host plant, potentially affecting the nature of plant communities and that of higher tropic levels.