Testing for loss of Epichloë and non-epichloid symbionts under altered rainfall regimes

Premise: Microbial symbionts can buffer plant hosts from environmental change. Therefore, understanding how global change factors alter the associations between hosts and their microbial symbionts may improve predictions of future changes in host population dynamics and microbial diversity. Here, we investigated how one global change factor, precipitation, affected the maintenance or loss of symbiotic fungal endophytes in a C₃ grass host. Specifically, we examined the distinct responses of Epichloë (vertically transmitted and systemic) and non-epichloid endophytes (typically horizontally transmitted and localized) by considering (1) how precipitation altered associations with Epichloë and non-epichloid endophytic taxa across host ontogeny, and (2) interactive effects of water availability and Epichloë on early seedling life history stages. Methods: We manipulated the presence of Epichloë amarillans in American beachgrass (Ammophila breviligulata) in a multiyear field experiment that imposed three precipitation regimes (ambient or ±30% rainfall). In laboratory assays, we investigated the interactive effects of water availability and Epichloë on seed viability and germination. Results: Reduced precipitation decreased the incidence of Epichloë in leaves in the final sampling period, but had no effect on associations with non-epichloid taxa. Epichloë reduced the incidence of non-epichloid endophytes, including systemic p-endophytes, in seeds. Laboratory assays suggested that association with Epichloë is likely maintained, in part, due to increased seed viability and germination regardless of water availability. Conclusions: Our study empirically demonstrates several pathways for plant symbionts to be lost or maintained across host ontogeny and suggests that reductions in precipitation can drive the loss of a plant's microbial symbionts.