Geographic variation in the structure and function of fungal leaf endophyte communities
Fungal leaf endophytes are nonpathogenic microbes found within the tissues of nearly all land plants. Endophyte communities are hyperdiverse and vary with environmental conditions. Given this variability, it is challenging to determine how endophyte communities affect host plant fitness. One way endophytes may benefit host plants is by supressing the negative effects of disease causing microbes. Inoculation studies have demonstrated a range of disease outcomes when endophytes interact with pathogens, but whether such functional effects are common across a host species’ geographic range in the face of significant turnover of endophyte communities is not known. To examine host-endophyte interactions across a range of environments we used high-throughput DNA sequencing and traditional culturing to characterize the structure of fungal leaf endophyte communities of Populus trichocarpa throughout the core of its geographic range. Next, we used greenhouse inoculation experiments to test interactions between endophyte isolates captured by culturing and Melampsora rust, the major foliar pathogen of Populus taxa worldwide.
Environmental factors at spatial scales both large and small structured endophyte community composition and diversity. Despite community differences among study sites, for most communities we found significant interactions between individual endophytes species and Melampsora. These interactions had profound consequences: endophytes facilitated or constrained disease by up to 30 times (in either direction). Our findings suggest that fungal leaf endophyte interactions with pathogens are widespread and play a key role in plant disease.