## 5 Functions Regulated by Quorum Sensing in Rhizobia
### Bacterial Motility  
Genome-wide transcriptomic analyses have shown that QS-deficient mutants exhibit increased expression of chemotaxis and motility genes [29; 30; 88]. This process has been well investigated in _S. meliloti_. In this bacterium, at high population densities, motility genes are repressed by binding of AHL-activated ExpR within the promoter region of the _visNR_ operon, down-regulating its expression. Because VisNR are the master regulators of the flagellar regulon, this results in the decreased expression of this set of genes [43]. However, at low cell densities, the outcome of ExpR and AHL regulation is different, since ExpR activates the expression of genes involved in motility. It was found that high concentrations of AHLs were not sufficient to repress flagellar gene expression at low cell density, while they did when cells were at the late-log phase. These results indicate that, besides AHLs, additional phase-related factors are required to down-regulate motility genes [30].  
By controlling the production of EPS, the QS system impacts the surface motility behavior of _S. meliloti_ strains. Production of copious amounts of EPS II associated with cells harboring a functional ExpR/Sin system allows for a flagellum-independent surface spreading or sliding [96]. This type of motility is characterized by the passive movement of cells across the agar surface driven by the expansive forces of bacterial growth and facilitated by the production of EPS that reduces friction between cells and the surface. QS regulation of EPS II synthesis is also responsible for surfing, a type of bacterial translocation that requires flagella and which is mainly driven by physical or chemical effects created by the secreted EPS II [97; 98]. Only the high molecular weight form of EPS II facilitates this surface spreading. Moreover, among the multiple AHLs produced by _S. meliloti_, only two AHLs species, C16:1- and 3-oxo-C16:1-HLs, affected this mode of surface translocation by up-regulating the expression of the positive regulator for EPS II synthesis WggR (ExpG) [97]. In contrast to EPS II-mediated sliding and surfing, ExpR is not required for swarming motility, a specialized mode of surface translocation, dependent on rotating flagella and characterized by the rapid and coordinated movement of multicellular groups of bacteria [99]. ExpR-deficient _S. meliloti_ strains such as Rm1021 and GR4 can translocate over semisolid surfaces using swarming motility [96; 100]. Although this finding demonstrates that a fully functional QS system is not necessary for swarming, it does not rule out a possible role for AHLs and/or population density in the regulation of swarming motility, as it has been shown for many bacteria. In fact, AHLs carrying a long-chain fatty acid moiety producedby the bean symbiont _R. etli_ have a dual role in the swarming motility exhibited by this bacterium: as QS signals and as biosurfactants which promote surface translocation [74].  
In different bacteria, type IV pili have been involved in twitching motility, a mode of surface translocation powered by the extension and retraction of pili. In _S. meliloti_, AHL-activated ExpR represses expression of _pilA1_, a gene belonging to a chromosomal cluster encoding proteins for type IVb pili of the Flp (fimbrial low-molecular-weight protein) subfamily. This cluster is responsible for the formation of unilateral and polar bundles in _S. meliloti_[35]. Twitching motility has not been reported in rhizobia. Therefore, a putative role of the ExpR/Sin system in controlling this behavior could not be established in _S. meliloti_. Nevertheless, the Flp-type pili were found to affect competitive ability for nodulation in alfalfa plants, perhaps by enhancing attachment to plant roots, before the onset of EPS production [35]. Interestingly, in _S. fredii_ NGR234, chromosomal type IV genes were also down-regulated in a QS-dependent manner [88].