## 5 Functions Regulated by Quorum Sensing in Rhizobia
### Role of Quorum Sensing in the Rhizobium-Legume Symbiosis  
In general, the role of QS regulation in the _Rhizobium_-legume symbiosis remains puzzling. Although it is clear that population density-dependent control of gene expression affects important behaviors in rhizobia, AHL-mediated regulation is not always essential for symbiosis (Table 1). It is especially surprising that mutations in similar genes of different rhizbial species have different effects on the interaction with their legume hosts. The most severe effects in symbiosis have been described for QS mutants in _R. etli_ and _Mesorhizobium tianshanense_ (Table 1). In _R. etli_ CNPAF512 mutations in _cinI_ or _cinR_ caused abnormal development of bacteroids in bean nodules, as well as decreased symbiotic nitrogen fixation [73]. Similarly, in _R. etli_ CFN42 _cinR_, _raiR_, and _tral_ mutants showed decreased nitrogen fixation activity [76]. Mutants of _M. tianshanense_ in genes highly similar to _cinI_ and _cinR_ were defective in nodulation on _Glycyrrhizia valensis_, showing decreased nodule formation efficiency or even a Nod' phenotype, depending on the isolate [70; 71]. In contrast, mutation in _cinI_ or _cinR_ in Rlv did not significantly affect symbiosis with pea [80]. Likewise, whereas _rail_ inactivation in _R. etli_ led to increased nodulation on beans, no symbiotic defect was detected in a _rail_ mutant of Rlv [75; 84]. Interestingly, in the _R. etli_-bean symbiosis the _cin_ and _rai_ systems are expressed _in planta_, whereas QS systems are repressed in Rlv or _S. meliloti_ bacteroids [30; 101], indicating that QS does not play a regulatory role in the latter bacteria once the _Rhizobium_-legume symbiosis is established. It was suggested that QS genes could be influenced by the host plant, playing a role in nodules of legumes in which terminal bacteroid differentiation does not occur [4]. Conversely, in symbiosis with the galegoid legumes (e.g. alfalfa and pea), where the bacteroids differentiate into a terminal form, QS regulation could not be relevant. In _S. meliloti_, a functional QS system is required for efficient symbiosis with its host plant. Inoculation of _Medicago sativa_ plants with a _sinI_ mutant, unable to produce AHLs, led to a significant reduction in the total number of nodules per plant compared to that in the wild type, as well as a delay in the appearance of nitrogen-fixing nodules. In contrast, an _expR_ deficient strain, which produces comparable levels of AHL, establishes symbiosis as efficiently as the wild type [24; 40]. The reason for the symbiotic impairment exhibited by the _sinI_ strain might be caused by the mutant's inability to both repress the synthesis of flagella and produce AHLs [30]. Interestingly, a _sinI_ mutant incapable of producing flagella regained the ability to establish efficient symbiosis. The presence of flagella might interfere with proper progression of infection threads or activate plant defenses that hamper the invasion process. By coordinating expression of important phenotypes such as EPS production and motility, QS regulation in rhizobia may facilitate the transition from free-living to symbiotic lifestyles. In the soil, when the number of cells is low, activation of functions such as motility might be advantageous in search for nutrients or the appropriate host. Likewise, expression of type IV pili when the cell density is low might help tight attachment to roots and colonization. It is known that QS influences biofilm formation in different rhizobia [66; 86; 91; 95; 102]. QS-mediated promotion of biofilm formation contributes to optimal root colonization which in turn could influence nodule formation efficiency and competitiveness [86; 103]. As the population density increases at the expense of root exudates, QS regulation takes place and coordinated repression of flagellar and pili production and activation of EPS synthesis allows efficient plant invasion. At this stage, repression of flagella and pili might be important to avoid recognition and alerting plant defenses.