Host preference and invasiveness of commensals in the Lotus and Arabidopsis root microbiota

Healthy plants are colonized by microorganisms from the surrounding environment, which form stable communities and provide beneficial services to the host. Culture-independent profiling of the bacterial root microbiota shows that different plant species are colonized by distinct bacterial communities, even if they share the same habitat. It is, however, not known to what extent the host actively selects these communities and whether commensal bacteria are adapted to a specific plant species. Here, we report a sequence-indexed culture collection from roots and nodules of the model legume Lotus japonicus that contains representatives from the majority of species found in culture-independent profiles. Using taxonomically paired synthetic communities from L. japonicus and the crucifer Arabidopsis thaliana in a multi-species gnotobiotic system, we detect clear signatures of host preference among commensal bacteria in a community context, but not in mono-associations. Sequential inoculation of either host reveals strong priority effects during the assembly of the root microbiota, where established communities are resilient to invasion by late-comers. However, we found that host preference by commensal bacteria confers a competitive advantage in their native host. We reveal that host preference is prevalent in commensal bacteria from diverse taxonomic groups and that this trait is directly linked to their invasiveness into standing root-associated communities. ### Competing Interest Statement The authors have declared no competing interest.