Benefits of a fungal endophyte in Elymus virginicus decline under drought stress

In most natural systems, it remains unclear whether microbial symbionts can help plants adapt to rapidly changing environments. Here, we tested the hypothesis that fungal endophyte symbiosis in a widespread, native grass species, Elymus virginicus, would promote host tolerance of drought. In a growth chamber experiment, we manipulated water availability to plants naturally infected with the endophyte Epichloë elymi and to plants that were experimentally disinfected. The drought treatment imposed a ∼67% reduction in water, corresponding to the very high end of drought predictions for the Midwestern US, where E. virginicus is common. In contrast to expectations, E. virginicus benefited more from the endophyte under daily watering than under drought. Disinfected plants had 45% less aboveground biomass than naturally infected plants under daily watering and only 23% less under drought. Both the number of tillers produced and root biomass were reduced by disinfection, with a greater difference between endophyte treatments under daily watering than under drought. There were no effects of either the endophyte or drought on aphid colonization of the plants. To put these results into a broader geographic context, we surveyed endophyte infection frequencies across 43 populations, including wild populations, commercial stock and germplasm resources. Endophyte frequencies varied from 0% to 100%, and frequencies were much higher for wild populations than germplasm resources. Natural variation in endophyte frequencies combined with the heritable transmission of endophytes from plants to seeds provides opportunities for climate change to affect selection on the maintenance of endophyte symbiosis in E. virginicus. Bei den meisten natürlichen Systemen ist weiterhin unklar, ob mikrobielle Symbionten Pflanzen helfen können sich an schnell wandelnde Umweltbedingungen anzupassen. Wir testeten hier die Hypothese, dass die Symbiose mit einem endophytischen Pilz bei einer weit verbreiteten, indigenen Grasart, Elymus virginicus, die Trockenheitstoleranz des Wirtes stärken sollte. In einem Zuchtkammer-Experiment manipulierten wir die Wasserverfügbarkeit für natürlich mit dem Endophyten Epichloë elymi infizierte und experimentell desinfizierte Pflanzen. Die Trockenvariante bedeutete eine Reduktion der Wasserverfügbarkeit um ca. 67%; dies entspricht dem oberen Ende der Vorhersagen aus Klimamodellen für den mittleren Westen der USA, wo E. virginicus verbreitet vorkommt. Im Gegensatz z