CONICET | Buscador de Institutos y Recursos Humanos

Symbiotic associations between grasses and fungal endophytes are generally regarded as mutualistic, yet benefits to host plants vary with environmental context. Previous studies emphasized how endophytes influence plant responses to single stressors. In contrast, the outcome of endophyte-grass interactions under simultaneous biotic and abiotic stresses remains poorly explored. We hypothesized that benefits of endophyte symbiosis become most apparent in ´complex´ environments where hosts experience multiple stresses. We evaluated the performance of endophyte infected (E+) vs. uninfected (E?) Lolium multiflorum plants in a factorial experiment with water supply (control vs. drought) and insect herbivory (with vs. without aphids). Endophyte infection delayed tiller production in well-watered plants, while water stress reduced tillering only in E? plants. Endophyte mediation of herbivory tolerance was contingent on water supply. Whereas aphids were detrimental to E+ plants in well-watered soils, herbivory interacted with drought stress in decreasing reproductive output of E? but not E+ plants. Moreover, endophytes decreased aphid colony growth in drought-stressed but not in control plants. Thus endophyte symbiosis enhanced plant tolerance to overlapping biotic and abiotic stresses, although infected plants failed to outgrowth their uninfected conspecifics. Our results support the view that mutualistic endophyte effects may not be apparent in low-stress environments.