Global change has the potential to alter the symbiotic relationships between host organisms and the microbes with which they associate. Changes such as altered precipitation regimes can influence the frequency with which microbial symbionts infect their hosts, which may have important consequences for host performance. In particular, plants associate with a diversity of fungi that live asymptomatically as endophytes within plant tissue. Endophytes may be systemic or localized, and this distinction is important for understanding the drivers of their infection. In this study, we investigated the role of precipitation on endophyte communities, and specifically we addressed (1) how precipitation alters systemic endophyte infection within individuals and vertical transmission to seeds and new seedlings, and (2) how infections by localized endophytes of leaves and seeds are influenced by precipitation and by systemic endophyte infection. We conducted a field experiment in which we manipulated infection by the systemic, fungal endophyte Epichloë (E- or E+) in the beachgrass Ammophila breviligulata, and subjected plants to one of three precipitation treatments (low, ambient, or high). We measured endophyte infection using culture-based and microscopy approaches. We also conducted a germination study to investigate the influence of the systemic Epichloë endophyte on seed germination success.
Infection of the systemic endophyte Epichloë in leaves decreased in the low precipitation treatment relative to ambient. Epichloë transmission from adult to seed was 81.7%, and did not differ by precipitation treatment. Epichloë infection decreased the proportion of germinating seeds, but also decreased seed mortality compared with uninfected seeds. Infection by localized (non-Epichloë) endophytes in leaves did not differ between E- and E+ plants, indicating that Epichloë had neither facilitative nor antagonistic effect on localized endophyte infection. Localized endophyte infection in leaves was less frequent in the low precipitation treatment relative to ambient in the E- plants, but no effect of precipitation was found in the E+ plants. The presence of Epichloë decreased infection of localized endophytes in seed. Epichloë also increased infection in seed by ‘p-endophytes’, a group of endophytes that are often found co-occurring in host plants with Epichloë. Our results show that decreases in precipitation can reduce infection by systemic and localized endophytes. These findings have important implications for plant-fungal symbiosis in the face of global change.