# Quorum Sensing and Quorum Quenching in _Agrobacteriumum_: A \"Go/No Go System\"?
## 4 Two Quorum Quenching Latonases Modulating QS and Ti Plasmid Transfer  
To address the above question and fully understand the QS regulation of Ti plasmid transfer in _A. fabrum_ strain C58, it is necessary to pay attention to the various QQ systems found in these bacteria. In addition to TraM QQ regulator, strain C58 exhibits two lactonases that can degrade the AHL signal OOHL. A first one is a metallo-lactonase termed AiiB [35; 36]. It is encoded by the eponym gene located on the Ti plasmid. The expression of _aiiB_ is not induced by short- or long-chain AHLs, nor is it by various lactones. Remarkably agrocinopines induce the expression of _aiiB_[37]. This feature remains however independent of the master regulator AccR but nevertheless requires an active agrocinopines transport system. The production of AiiB leads to a marked decreased of the OOHL that accumulates in culture supernatants of strain C58. In relation, Ti transfer frequencies measured in plant tumors are ca 100 times higher with an _aiiB_ mutant as a donor than with the wild type strain. In vitro, transconjugants of an _aiiB_ defective strain appear earlier than those of the wild type strain [37]. All these data clearly demonstrate that AiiB modulates the QS-regulated transfer of the Ti plasmid.  
In strain C58, a second metallo-lactonase BlcC (formely AttM) has also been detected [35; 38]. In strain C58 and several other agrobacterial isolates, the _blcC_ gene is part of the _blcABC (attKLM)_ operon, often located on a large megapalmid, the At plasmid [35; 39]. The _blcABC_ operon is involved in the degradation of gamma-butyrolactone (GBL) to gamma-hydroxybutyrate (GHB) and semi-succinic aldehyde (SSA) [39; 40]. The Blc lactonase is able to cleave GBL and numerous AHLs, a feature related the structural similarity of these molecules. The expression of the _blcABC_ operon is regulated by the repressor BlcR (AttJ) [41]; it is not affected by the presence of AHLs or opines, including agrocinopines, but it is stimulated by GBL, GHB, and SSA [40; 42], though the true inducer may be SSA only [43], or SSA and GHB [41; 42; 43]. Plant extracts also induce the expression of BlcC [44].  
Regarding phenolics, acetosyringone does not induce the transcription of the _blcABC_ operon [45], but salicylic acid does [46]. The expression of the _blcABC_ operon is also induced by gamma-aminobutyric acid (GABA) [47]. GABA is a naturally occurring non-proteins amino acid that modulates plant growth, development, reproduction, and stress response (for reviews: [48; 49; 50; 51]). The concentration of GABA drastically varies in plants and plant organs and tissues, especially when wounded [52]. In tomato for instance, GABA concentration ranges from 0.16 \(\upmu\)mol/g fresh weight (FW) in tomato stem tissues to 0.57 \(\upmu\)mol/g FW in stem tumors, and increased rapidly after wounding to reach 0.68 and 2.69 \(\upmu\)mol/g FW in stem and tumor tissues, respectively [47]. The presence of GABA in the bacterial environment drastically increases the ability of strain C58 to inactivate OOHL. In cultures of _traR_-overexpressing mutants of strain C58 (that express _tral_ in the absence of agrocinopines), OOHL reaches a concentration of 20 nM in culture supernatants, as compared with 0.5 nM in those of the wild-type, parent strain C58. In the presence of 0.5 and 1.0 mM GABA in the culture media, OOHL is not detected anymore in the supernatants of both the mutant and wild-type strain C58 [45]. In these experiments, the ability to modulate OOHL concentration appears to be clearly dependent on the presence of both a functional _blcABC_ operon and the GABA transporter BraDEFG [45].