Joint effects of host genotype and species arrival order govern plant microbiome composition and function

There is mounting evidence that species arrival order can affect microbiome composition, a phenomenon known as priority effects. However, it is less clear whether priority effects during microbiome assembly have consequences for the host, or whether intraspecific variation in host traits can alter the trajectory of microbiome assembly. In a greenhouse inoculation experiment, using the Populus trichocarpa foliar microbiome, we manipulated host genotype and the arrival order of a synthetic community of foliar fungi. We quantified microbiome assembly outcomes using marker-gene sequencing and tested host susceptibility to a foliar rust pathogen. We found that the effect of species arrival order on microbiome composition and function (i.e., disease modification) depended on host genotype. Additionally, we found that fungal species arrival order can affect host disease susceptibility independent from microbiome composition, suggesting that the interactive effects of species arrival order and host genotype can decouple composition and function in the plant microbiome.