ABSTRACT Microbial colonization of plant roots involves strong selective pressures that shape the structure and function of root-associated communities. In particular, the endosphere represents a highly selective environment requiring host entry and in planta persistence. However, strain-specific microbial traits that enable endosphere colonization remain poorly understood. Here, we use a defined, genome-resolved community of 28 Variovorax strains isolated from the roots of Populus deltoides and Populus trichocarpa (poplar trees) to determine which strains partition between rhizosphere and endosphere compartments and to identify the genomic traits associated with endosphere specialization. By combining strain-resolved metagenomic profiling, comparative genomics, and functional assays, we demonstrate that dominant endosphere colonizers are enriched in genes related to nutrient metabolism, redox balance, transcriptional regulation, and a conserved L-fucose utilization pathway experimentally shown to enhance root colonization. Not all strains succeed through the same strategy. Community-wide functional profiling revealed a distinct and reduced set of traits in the endosphere, including orthogroups associated with low-abundance strains that were overlooked in strain-level analyses. These findings reveal that multiple ecological strategies, such as metabolic competition, regulatory adaptation, and niche specialization, can support endosphere colonization. Our results advance the understanding of how bacterial colonization traits are distributed and deployed within a plant microbiome and suggest that host filtering selects for distinct, and sometimes complementary, microbial strategies. This work supports a shift toward mechanistic, genome-resolved models of microbiome assembly and offers a framework for linking microbial function to host colonization success. IMPORTANCE Plants often depend on diverse microbial partners to support their growth, resilience, and adaptation to changing environments. Among these microbes, some bacteria inhabit the rhizosphere (the narrow zone around roots where microbes interact with the plant) while others are able to enter and persist within root tissues. The traits that distinguish these two lifestyles remain poorly understood. In this study, we examined a group of related Variovorax strains from poplar tree root microbiomes to ask why some rhizosphere-associated strains also become successful endosphere colonizers. We found that strains appear to succeed through different strategies: some may benefit from rapid growth on plant-derived carbon sources, while others may rely on stress tolerance or fine-tuned regulation. These results suggest that there is no single path from the rhizosphere into the root interior, but rather multiple strategies shaped by the host environment. Understanding this diversity can inform efforts to design resilient plant-microbe communities.
Beals et al. (Fri,) studied this question.