| Summary: | <p>Motility and chemotaxis are crucial processes for soil bacteria and plant–microbe interactions. This applies to the symbiotic bacterium <em>Rhizobium leguminosarum</em>, where motility is driven by flagella rotation controlled by two chemotaxis systems, Che1 and Che2. The Che1 cluster is particularly important in free-living motility prior to the establishment of the symbiosis, with a <em>che1</em> mutant delayed in nodulation and reduced in nodulation competitiveness. The Che2 system alters bacteroid development and nodule maturation. In this work, we also identified 27 putative chemoreceptors encoded in the <em>R. leguminosarum bv. viciae</em> 3841 genome and characterized its motility in different growth conditions. We describe a metabolism-based taxis system in rhizobia that acts at high concentrations of dicarboxylates to halt motility independent of chemotaxis. Finally, we show how PTS<sup>Ntr</sup> influences cell motility, with PTS<sup>Ntr</sup> mutants exhibiting reduced swimming in different media. Motility is restored by the active forms of the PTS<sup>Ntr</sup> output regulatory proteins, unphosphorylated ManX and phosphorylated PtsN. Overall, this work shows how rhizobia typify soil bacteria by having a high number of chemoreceptors and highlights the importance of the motility and chemotaxis mechanisms in a free-living cell in the rhizosphere, and at different stages of the symbiosis.</p>
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