Foliar endophytic fungi are environmentally-acquired symbionts, whose diverse communities reside cryptically within the healthy photosynthetic tissue of all plant species sampled to date. Endophyte colonization can benefit host plants by enhancing defense against pathogens and herbivores, but with demonstrated costs including latent pathogenicity and lowered host photosynthetic rate. A critical question in both endophyte ecology and broader ecological theory is to what extent do abiotic and biotic factors affect community composition? Moreover, does community assembly of endophytes carry functional consequences for plant hosts? We hypothesized that canopy height as well as leaf litter presence and identity would influence endophyte community assembly, and that hosts with different endophyte communities would vary in their resistance to pathogen infection. We manipulated litter exposure and canopy height of endophyte-free Theobroma cacao (cacao tree) seedlings on Barro Colorado Island, Panama. After two weeks, we used fungal culturing and sequencing techniques to compare the composition of early endophyte communities among treatments. Subsequently, we inoculated seedlings with a common pathogen of T. cacao (Phytophthora sp.) and compared pathogen damage among treatments.
Both litter and canopy height significantly affected the community structure of endophytes inhabiting plant tissue. Plants in the upper canopy were the least colonized by endophytes. Plants without litter were colonized by both a greater number and diversity of endophytes than seedlings treated with conspecific (T. cacao) or heterospecific (mixed tree species) litter. Further, in our analysis of pathogen infection following endophyte colonization, we found that seedlings treated with conspecific litter were more resistant to infection by Phytophthora than seedlings treated with heterospecific or no litter. Control, endophyte-free seedlings were the most susceptible to pathogen infection. These results are consistent with recent studies showing enhancement of host defense by dominant endophytes of healthy conspecific tissues. We suggest that that leaf litter, particularly from healthy conspecifics, results in a priority effect in the early stages of endophyte community assembly. This would promote colonization by the endophyte species that are the most effective at inhibiting further endophyte colonization as well as reduce pathogen infection. This phenomenon has both ecological and applied implications, especially in a global cash crop such as cacao.