All plants in natural ecosystems appear to be symbiotic with fungal endophytes. This
highly diverse group of fungi can have profound impacts on plant communities
through increasing fitness by conferring abiotic and biotic stress tolerance, increasing
biomass and decreasing water consumption, or decreasing fitness by altering
resource allocation. Despite more than 100 yr of research resulting in thousands of
journal articles, the ecological significance of these fungi remains poorly characterized.
Historically, two endophytic groups (clavicipitaceous (C) and nonclavicipitaceous
(NC)) have been discriminated based on phylogeny and life history traits. Here, we
show that NC-endophytes represent three distinct functional groups based on host
colonization and transmission, in planta biodiversity and fitness benefits conferred
to hosts. Using this framework, we contrast the life histories, interactions with hosts
and potential roles in plant ecophysiology of C- and NC-endophytes, and highlight
several key questions for future work in endophyte biology.