The leaf microbiome affects disease resistance/susceptibility, drought tolerance, and gas exchange. Since this leaf community changes temporally throughout the growing season, it is important to understand what are the sources of the microbial species that colonize and the stability of this community in the face of outside, natural, colonists. We established two experiments to test the effects of aerial microbial colonists on leaf microbiome development of switchgrass (Panicum virgatum). First, to determine prevalence and magnitude of natural aerial dispersal into leaf microbiomes, we germinated and grew switchgrass seedlings in pots with sterile soil then placed them near mature switchgrass field monocultures. Seedlings were left in the field for 51 days, and we collected rain and dry deposited microbes throughout the period to characterize the abundance and composition of the aerial fungal colonists. In our second experiment we tested the direct effects of single rain events on the development of the leaf microbiome of germinating seedlings. We germinated and grew seedlings for ~24 days in sterilized sealed petri dishes inoculated with either live rain, autoclave sterilized rain, or nanopure water.
Results/Conclusions
We found that the fungal community inhabiting the endosphere of seedling leaves (endophytes) showed rapid and significant shifts from the seed microbiome that were driven primarily through enrichment of species found in the aerial community. Interestingly, the fungal endophyte community of the mature switchgrass leaves hosts many species from the seed microbiome along with the aerial community. In our second experiment, we found that seedlings that germinated and grew in live rain showed significant shifts in the fungal endophyte community compared to the starting seed microbiome but seedlings grown in both sterile treatments showed little to no shift in the leaf endophyte community, suggesting an active interchange between the leaf microbiome and the aerial community. In further support, we found that the temporal turnover of the aerial fungal community strongly tracked the seasonal development of switchgrass across the year and a half of rain events under analysis. Our results suggest that the leaf microbiome of seedlings has greater sensitivity to these aerial colonists while the mature endophyte community is more difficult to invade. The exchange of the microbial species between the leaf microbiome and the aerial community depends both on seasonal patterns in the growing season and ontogenetic development.