## 4 Quorum Sensing Regulation in Other Rhizobia  
S. meliloti is a model organism among the rhizobia for studying QS regulation because of the presence of a single AHL-based system (ExpR/Sin). Nevertheless, investigations on QS regulatory systems in different rhizobia have shown that they are highly diverse and can control a broad range of bacterial functions (Table 1). In fact, even within a single rhizobial species, a different range of QS systems can be found among different isolates. This diversity suggests that there may be no unifying paradigm of what is controlled by QS in these bacteria [17]. One common characteristic is that all the AHL synthases and response regulators identified in rhizobia belong to the LuxI and LuxR protein families, respectively. Besides _S. meliloti_, the QS regulatory mechanisms of _Rhizobium leguminosarum_ and _R. etli_ have been well studied. Here, the most important characteristics of QS regulation in these two species will be summarized. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Rhizobial Strain** & **QS Signal** & **Genes** & **Phenotypes Regulated** & **References** \\ \hline _Bradylhizobium japonicum_ (_B. dizzobifications_) & & & & \\ \hline USDA110 & Bradyovetin & Unknown & _mod_ gene regulation & [20; 64] \\ USDA290 and other strains & Non-characterized AHL & _bjai_/_juR\({}_{1}\)_ & Unknown. & [21] \\ \hline _Bradylhizobium_ spp. & & & & \\ \hline ORS278 & Cinnamyj-HL & _lrai_/_tranR_ & Unknown & [22] \\ \hline Peanut-nodalating strains & C6-HL, 3-oxo-C10-HL, & Unknown & Motility, biofilm formation, cell aggregation \({}^{*}\) & [66] \\ \hline SR-6 & C6-HL, 3-OH-C6-HL, & Unknown & Nodulation \({}^{\text{b}}\) & [67] \\  & C8-HL, C10-HL, & & & \\  & 3-oxo-C10-HL, & & & \\  & 3-oxo-C12-HL & & & \\ \hline _Mesotheliomaium locii_ & & & & \\ \hline NZP2213 & 3-oxo-C6-HL, C8-HL, & _mrl_12 & Unknown & [68] \\  & C10-HL & & & \\  & C12-HL & _mrl_11 & Nodulation & \\ R7A & 3-oxo-C6-HL & _lrai_/_tranR_ & Symbiosis island transfer & [69] \\  & C4-HL, 3-oxo-C12-HL & Unknown & Unknown & \\ \hline _Mesotheliomaium_ & & & & \\ _tianshumes_ & & & & \\ \hline CCBAU3306 & 3-oxo-C6-HL \({}^{*}\), & _mrl_/_mrtR_ & Nodulation (Nod’) & [70] \\  & 3-oxo-C8-HL \({}^{*}\), & & & \\ CCBAU60A & 3-oxo-C6-HL \({}^{*}\), & & & \\  & 3-oxo-C12-HL \({}^{*}\) & & & \\ \hline _Mesotheliomaium hualtii_ & & & & \\ \hline CCBAU21173 & Peptide-related signal & Putative peptidase & Growth rate, biofilm formation, nodulation (Nod’) & [72] \\ \hline _Rhizobium elti_ & & & & \\ \hline CNPAF512 & 3-OH-(slc)-HL \({}^{d}\) & _cinl/cinR_ & Nitrogen fixation, symbisome development, growth & [73; 74] \\  & & & rate, swarming & \\  & Short-chain AHLs \({}^{c}\) & _rail_/_traiR_ & Nodulation & [75] \\ CFN42 & C8-HL \({}^{*}\), 3-oxo-C6-HL \({}^{c}\), & _cinl/cinR_ & Nodulation, nitrogen fixation & [17; 76] \\  & 3-oxo-C8-HL \({}^{c}\) & _rail_/_traiR_ & & \\  & 3-OH-C8-HL \({}^{*}\) & _lrai_/_traiR_ & & \\ RT1 & 3-oxo-C8-HL, & Unknown & Swarming, biofilm formation \({}^{*}\) & [78] \\  & 3-OH-C14-HL & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Quorum-sensing systems in symbiotic rhizobia. _R. leguminosarum_bv. _viciae_ (Rlv), the symbiont of pea, vetch and lentil, possesses four different AHL-based QS systems with their LuxI-type AHL synthases (CinI, Rhil, Rail, and TraI) and cognate LuxR-type regulators (CinR, RhiR, RaiR, and TraR) [17; 80; 81] (Table 1 and Figure 2). In addition, several orphan LuxR-type regulators have been identified such as BisR and ExpR. The Tra system and BisR regulate the transfer of the symbiotic plasmid in Rlv in response to CinI-made AHL from potential recipient cells. For details, readers are referred to [83]. CinI, CinR and ExpR are the orthologs of SinI, SinR and ExpR, respectively, in _S. meliloti_. In Rlv, the Cin system is at the top of a hierarchical regulatory cascade, controlling the expression of the other QS systems and acting as an overall switch potentially influencing many aspects of rhizbial physiology [79; 82; 83; 84].