## 7 Role of the Legume Host in Quorum Sensing Regulation in Rhizobia  
Since QS regulates bacterial behaviors which are crucial in the outcome of host-microbe interactions, it seems logical that eukaryotes have developed strategies to interfere with the cell-to-cell communication system. During this process, known as quorum quenching (QQ), the host interferes with the signaling pathway or manipulates AHLs. Several studies have shown that root exudates of different plants contain low molecular weight compounds that affect QS regulation in bacteria. These molecules can act as agonists or antagonists of QS, enhancing or inhibiting, respectively, AHL-regulated phenotypes [115; 116]. The production of these AHL-mimics probably modulates the function and composition of bacterial populations in the rhizosphere, shaping the plant rhizomicrobiome [117]. [PERSON] et al. (2003) [115] found that the model legume _M. truncatula_ can produce 15 to 20 separable compounds with the capacity to affect QS regulation based on AHL. The secretion of these signals changed with the plant age. Interestingly, significant changes in the amount and kinds of signal-mimic compounds were detected when _M. truncatula_ seedlings were exposed to different AHLs, indicating that the plant has the potential to manipulate the behavior of the different bacteria it encounters actively and specifically [106]. The chemical identities of plant molecules that affect bacterial communication mediated by AHLs are mostly unknown. Although it seems logical that AHL-mimics bind to the autoinducer-binding domain of LuxR-type regulators and therefore they should be structurally close to AHLs, different studies indicate that these compounds can have a structure very different from that of AHL [118; 119; 120]. L-canavanine, an arginine analog, was found in alfalfa seed exudates as a compound that inhibits QS-regulation in _S. melidi_[118]. Specifically, this compound inhibited expression of _exp_ genes required for EPS II production. The observation that AHL-mimics produced by rice and bean plants can alter biofilm formation in plant-interacting bacteria, including rhizobia [119], suggests that these compounds could have a pivotal role during the first steps in the plant-bacterium interaction. Recently, the production of QS-like molecules by peanut plants (_Arachis hypogaea_) has been examined [121]. Peanut is a legume crop modulated by _Bradynhizobium_ spp. which synthesize long-chain AHL. Seed and root exudates of peanut plants contained QS-mimics that function similar to long-chain AHL and inhibitors of functions regulated by short-chain AHL. Although the nature of these signal molecules remains to be determined, these results could indicate that peanut plants select bacteria with QS regulation mediated by long-chain AHLs and disrupts the QS mechanisms of bacteria that communicate through short-chain AHLs. Future research will determine the nature as well as the mechanism of action of QS-mimic compounds produced by legume hosts. This knowledge will help to elucidate if these plant signals serve to stimulate crucial events in the _Rhizobium_-legume symbiosis such as infection initiation and/or bacteroid formation and at the same time block undesired bacterial phenotypes that could harm plant health. The plant host can also affect QS regulation by influencing the availability of AHLs in the rhizosphere. [PERSON] et al. (2013) [108] found that when _S. meliloti_ was co-cultivated with _Arabidopsis_ plants, the amount of AHLs was reduced. In that study, the authors did not investigate if the negative effect exerted by the plant on AHL levels was caused by inhibition of AHL production, or by altering their stability or availability. To the best of our knowledge, enzymatic activities able to modify AHL signals have not yet been identified in legumes.