Research on the genomic basis of plant microbiome composition has generally revealed only moderate associations between plant genotypic variation and microbial community composition. Progress towards more generalizable and predictable links between plant genes and microbiome composition may therefore require a comprehensive approach that encompasses both the process and outcome of microbial community assembly. For example, plant traits that affect the relative rate of colonization or the effective size of microbial communities could determine the importance of stochastic variation in species arrival order during community assembly (i.e. priority effects). To determine whether priority effects in the foliar microbiome are influenced by genotypic variation in a plant host species, we conducted a greenhouse experiment using 12 genotypes of the model tree, Populus trichocarpa, and a synthetic community of 8 endophytic fungi isolated from natural P. trichocarpa populations. We manipulated assembly history in the leaf microbiome by inoculating plants with 1 endophyte 2 weeks prior to inoculation with the full synthetic community. At 2 time-points, endophyte communities were profiled using Illumina metabarcoding. To assess the functional consequences of priority effects in the P. trichocarpa leaf microbiome, we also inoculated plants with a Melampsora rust pathogen and quantified disease severity.
Results/Conclusions
We found that P. trichocarpa rust disease severity depended on host genotype, the colonization order of fungal endophytes, and their interaction. Relative to uninoculated control plants, the presence of foliar endophytes either reduced or had no effect on disease severity. However, the interaction between host genotype and endophyte colonization timing on disease modification was not simply due to a lack of an effect of endophytes in more rust-resistant plant genotypes; endophyte arrival order influenced disease severity in both rust susceptible and rust resistant genotypes. These results suggests that the functional consequences of priority effects in the leaf microbiome depend on complex interactions between the host, pathogen, and the microbial community. Ongoing analysis of the fungal community metabarcoding data will allow us to determine whether host genotype also affected community assembly outcomes under different assembly history regimes and to determine if differences in endophyte community assembly outcomes can explain the observed variation in disease severity.