Highly efficient early colonization by diverse Actinobacteria and Proteobacteria in Arabidopsis thaliana leaves poises them to exert priority effects.. Arabidopsis thaliana

Plant organs are colonized by diverse microbial organisms. Soil is thought to be the main source of the microbial community of plants, however since microbial colonization is controlled by various filtering processes only a subset of soil microorganisms can be found in lant leaves. One of those are priority effects, with early arriving phyllosphere colonizers persisting in the plant and influencing community outcome. With a natural microbial soil inoculum, we tested the importance of early community assembly by disrupting the natural colonization process. We found that early colonization is required for the development of a variable bacterial community and hypothesized that who randomly colonizes the plant first influences the outcome of the community through priority effects. Additionally, Pseudomonas was the most prominent early colonizer which was later replaced by other bacteria, especially Rhodococcus. In a new in-planta isolation approach we identified more efficient early colonizers and tested in a second community assembly experiment if we could reduce stochasticity in early colonization by giving one of the efficient colonizers a head start and with that reduce the variation between final bacterial communities. Surprisingly, if the variation was affected by the treatments, it increased. At the same time the observed colonization patterns regarding Pseudomonas and Rhodococcus where mostly unaffected by the different treatments. We hypothesize that processes leading to variation are robust and that especially Pseudomonas plays a major role in early colonization processes.