| Summary: | <p>Abstract</p> <p>Background</p> <p>Swarming is a multicellular phenomenom characterized by the coordinated and rapid movement of bacteria across semisolid surfaces. In <it>Sinorhizobium meliloti </it>this type of motility has been described in a <it>fadD </it>mutant. To gain insights into the mechanisms underlying the process of swarming in rhizobia, we compared the transcriptome of a <it>S. meliloti fadD </it>mutant grown under swarming inducing conditions (semisolid medium) to those of cells grown under non-swarming conditions (broth and solid medium).</p> <p>Results</p> <p>More than a thousand genes were identified as differentially expressed in response to growth on agar surfaces including genes for several metabolic activities, iron uptake, chemotaxis, motility and stress-related genes. Under swarming-specific conditions, the most remarkable response was the up-regulation of iron-related genes. We demonstrate that the pSymA plasmid and specifically genes required for the biosynthesis of the siderophore rhizobactin 1021 are essential for swarming of a <it>S. meliloti </it>wild-type strain but not in a <it>fadD </it>mutant. Moreover, high iron conditions inhibit swarming of the wild-type strain but not in mutants lacking either the iron limitation response regulator RirA or FadD.</p> <p>Conclusions</p> <p>The present work represents the first transcriptomic study of rhizobium growth on surfaces including swarming inducing conditions. The results have revealed major changes in the physiology of <it>S. meliloti </it>cells grown on a surface relative to liquid cultures. Moreover, analysis of genes responding to swarming inducing conditions led to the demonstration that iron and genes involved in rhizobactin 1021 synthesis play a role in the surface motility shown by <it>S. meliloti </it>which can be circumvented in a <it>fadD </it>mutant. This work opens a way to the identification of new traits and regulatory networks involved in swarming by rhizobia.</p>
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