Transcriptome profiling of a Sinorhizobium meliloti fadD mutant reveals the role of rhizobactin 1021 biosynthesis and regulation genes in the control of swarming

Abstract

BackgroundSwarming is a multicellular phenomenom characterized by the coordinated and rapid movement of bacteria across semisolid surfaces. In Sinorhizobium meliloti this type of motility has been described in a fadD mutant. To gain insights into the mechanisms underlying the process of swarming in rhizobia, we compared the transcriptome of a S. meliloti fadD mutant grown under swarming inducing conditions (semisolid medium) to those of cells grown under non-swarming conditions (broth and solid medium).ResultsMore 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 S. meliloti wild-type strain but not in a fadD 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.ConclusionsThe 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 S. meliloti 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 S. meliloti which can be circumvented in a fadD mutant. This work opens a way to the identification of new traits and regulatory networks involved in swarming by rhizobia.

ExpR Is Not Required for Swarming but Promotes Sliding in Sinorhizobium meliloti

“…On semisolid MM, surface translocation of these two strains was severely affected compared to that of their parental strains although not completely abolished, as is the case for 2011rhbA62 and 1021rhbA, mutant strains unable to produce the siderophore rhizobactin 1021 that are derived from Rm2011 and Rm1021, respectively (Fig. 1C) (32). This result demonstrates that ExpR-deficient S. meliloti strains are able to show flagellum-driven surface translocation (i.e., swarming motility) on semisolid MM, and therefore, we can conclude that ExpR is not required for swarming.…”

“…However, our recent data are in disagreement with these findings. We have reported that the commonly used S. meliloti laboratory strain Rm1021 and the closely related strain Rm2011, both known to have a disrupted copy of expR, are able to swarm on semisolid minimal medium (32). To solve the discrepancies between these reports, in this work, we have reexamined the role played by the expR gene in swarming of S. meliloti.…”

“…Swimming was examined on plates prepared with BM containing 0.3% Bacto agar and inoculated with 3-l droplets of rhizobial cultures grown in TY (optical density at 600 nm ϭ 1). Surface motility was analyzed using two different methodologies: (i) the motility assay described by Bahlawane et al (2) in which 3 l of overnight TY rhizobial cultures was inoculated onto the surface of BM containing 0.6% Bacto agar and (ii) the motility test described in our previous work (32,42) in which 2 l of washed, 10-fold-concentrated of cultures grown in TY broth to the late exponential phase was inoculated onto semisolid MM plates. For swimming and surface motility tests performed on BM, the migration zone was determined as the colony diameter in millimeters.…”

Legume seed exudates and Physcomitrella patens extracts influence swarming behavior in Rhizobium leguminosarum

“…In R. etli, swarming is regulated by the CinIRS quorum-sensing system, and the long-chain N-acyl homoserine lactone molecules produced by CinI have biosurfactant-like activity and are present in the extracellular matrix secreted by swarmer cells (Daniels et al 2006). Swarming motility has also been demonstrated in Bradyrhizobium japonicum (Covelli et al 2013) and in Sinorhizobium meliloti (Soto et al 2002;Nogales et al 2010;Gao et al 2012). Similar to R. leguminosarum, swarmer cells formed by S. meliloti QS77 are hyperflagellated.…”

“…Swarming behaviour is prevalent within the rhizobia (Soto et al 2002;Daniels et al 2006;Braeken et al 2008;Nogales et al 2010;Tambalo et al 2010c;Covelli et al 2013), but little is known regarding the signals and signalling pathways that induce swarmer cell differentiation. Therefore, we initiated this study to investigate the possible effects of host plant seed exudates on the swarming behaviour of R. leguminosarum.…”