A Subtle Balance between Host Selection and Microbial Size Effect Mediates Plant Microbiome Assembly in Mulberry

Breeding toward improved ecological plant-microbiome interactions requires improved knowledge of ecological processes/principles underlying microecological patterns, but these remain elusive. Here, we systematically investigated in-planta biogeographic patterns of plant-associated bacteriome and mycobiome along soil- plant and episphere-endosphere continuum in Morus alba. Microecological patterns in diversity, structure, co-occurrence network, species turnover, and assembly process were revealed and varying between bacteriome and mycobiome, possibly driven by multiple factors including host selection, community cohesion, and particularly size-dependent dispersal limitation. Based on these findings, we hypothesized that host selection historically recruits microbiotas, whereas microbial size affects the subsequent turnovers due to the limited dispersal of larger-size microbes. This hypothesis was supported by datasets from other plant species and confirmed by stochastic dispersal experiments showing that smaller-size microbes are more likely to escape/disperse from endosphere niches, contributing to fleeting niches occupied by larger-size microbes. These findings may open new avenues toward an improved understanding of the dynamics of plant microbiome assembly.