Foliar endophytic fungi (FEF) have been isolated from every plant species sampled to date. FEF can confer health benefits, such as pathogen resistance, to hosts, and receive nutrients in return. However, mechanisms that maintain these aboveground symbioses have not been investigated. In belowground symbioses (e.g., with mycorrhizae and rhizobia), there is evidence that plants can preferentially allocate nutrients to relatively more beneficial symbionts, consistent with theoretical predictions for mutualism stability. Here we examined if similar processes were also present in plants as part of their interactions with foliar symbionts. We inoculated leaves of Theobroma cacao seedlings with beneficial, neutral, and pathogenic FEF, as well as sterile water as a control, to make pairwise comparisons of plant nutrient allocation to differentially treated leaves. We pulsed the soil with 15N and traced allocation of 15N to leaves using isotope-ratio mass spectrometry. We predicted plants would preferentially allocate nutrients toward 1) endophyte-infected leaves compared to endophyte-free leaves, 2) leaves infected by more beneficial endophytes compared to less beneficial endophytes, and 3) leaves co-infected by a beneficial endophyte and a pathogen, compared to the beneficial endophyte or pathogen alone. Additionally, we conducted an in vitro assay of endophyte competition.
Results/Conclusions
Our results indicate that the presence and identity of endophytic fungi influences host plant nutrient allocation. Specifically, preferential allocation of of 15N varied with inoculation by beneficial, neutral, and pathogenic FEF. Complementary in vitro tests demonstrated that in addition to competing for resources, FEF also directly compete with one another. The ability of plants to preferentially allocate nutrients to FEF has strong implications for community assembly and maintenance of endophytic symbioses. Identifying mechanisms that maintain this symbiosis not only advances our understanding of the maintenance of mutualism between plants and aboveground fungal symbionts, but also further elucidates the factors that drive endophyte community assembly, and the mechanisms by which plants employ endophytes to mount a defense against their enemies.