Rhizobial motility preference in root colonization of <i>Medicago truncatula</i>

Tunnel-like infection thread (IT) structures support root colonization by symbiotic nitrogen-fixing rhizobia bacteria in most legume species. These tip-grown structures are key to directing rhizobia from root hairs to developing nodules, where they are hosted to fix nitrogen. Rhizobia likely progress inside ITs by combining growth and motility by modes not yet defined. Here, we tackled this question by combining mathematical modeling, live cell imaging, and bacterial mutant phenotyping in Medicago truncatula. Modeling the motion of fluorescently-labeled Sinorhizobium meliloti inside root hair IT compartments estimated slow movement (2-6 μm h^-1), compatible with passive rather than active motility. Consistent with this model, flagella-less S. meliloti mutants were impaired in active swimming motility in vitro, yet could colonize host roots and nodules in planta. By contrast, mutation in the rhizobactin 1021 siderophore rhbE biosynthesis gene affected surface motility in vitro and host root and nodule colonization. This mutation also promoted the formation of branched ITs in root hairs, which ultimately resulted in impaired nodule development and infection. In line with the slow motion of S. meliloti inside ITs estimated by modeling, our findings suggest that rhizobia favor flagella-independent surface translocation to reach developing nodules in M. truncatula.