Endophyte traits relevant to stress tolerance, resource use and habitat of origin predict effects on host plants.

All terrestrial plants are colonized by foliar endophytic fungi that can affect plant growth and physiology, but the prediction of these effects on the plant host remains a challenge. Here, we examined three paradigms that potentially control how endophytes affect plant hosts: habitat adaptation, evolutionary history and functional traits. We screened 35 plant-endophyte pairings in a microcosm experiment under well-watered and drought conditions with Panicum virgatum as the host. We related the measured plant responses to fungal phylogenetic relatedness, characteristics of fungal habitats across a rainfall gradient and functional traits of the fungi related to stress tolerance and resource use. The functional traits and habitat characteristics of the fungi predicted 26-53% of endophyte-mediated effects on measures of plant growth, physiology and survival. Overall, survival was higher for plants grown with more stress-tolerant fungi, and aboveground biomass was enhanced by fungi from warmer and drier habitats. Plant growth and physiology were also dependent on fungal resource use indicators; however, specific predictors were dependent on water availability. Simple ecological traits of foliar endophytic fungi observed in culture can translate to symbiotic lifestyles. These findings offer new insights and key testable predictions for likely pathways by which endophytes benefit the plant host.