# Quorum Sensing and Quorum Quenching in _Agrobacteriumum_: A \"Go/No Go System\"?
## 6 Quorum Sensing and Quorum Quenching as Two Parts of an Integrated Regulation Mechanism  
The Ti plasmid conjugation is a biologically costly process. First, the Ti plasmid represents in strain C58 up to 5% of the whole genome [63; 64]. Second, the induction of Ti plasmid conjugation also favors the conjugative transfer of the At plasmid [65] that may represent up to 10% of the genome [63; 64]. Aside from the cost of DNA replication, the transfer involves the establishment of a T4 SS that consists of a large number of proteins and requires ATP to export the DNA/protein complex [6; 66]. In agreement, an _accR_ mutant, which constitutively expresses the T4 SS for Ti plasmid conjugation, is impaired in growth yield as compared to its wild-type parent [67]. Clearly, conjugation has to be regulated, and it is tightly regulated in _Agrobacteriumium_. However, control of conjugation of plasmid Ti does not only deal with metabolic cost in donors, but also to fitness advantage that is conferred by Ti plasmid (opine niche construction and exploitation) in the donors and transconjugations. Regulation of Ti plasmid conjugation mirrors the trade-offs between cost and gain related to Ti plasmid. As Ti plasmid confers a selective advantage to _Agrobacteriumium_ in plant tumor, this ecological niche is optimal for vertical and horizontal propagation of Ti plasmid: the master regulator AccR and its selective interaction with opine by-product ensures opine niche-related control. Because Ti plasmid conjugation is a costly process, donors need to be in a viable state, with enough nutrients and energy resources: the lactanase BlcC, which responds to starvation [38; 39; 55], and stress alarmone,(p)ppGpp [55], as well as plant hormone, salicylic acid [46], both contribute to reduce Ti plasmid transfer under stressful conditions. However, the Ti plasmid donor population needs time to proliferate, hence, to be selected as the fitter population in the opine niche before transferring the Ti plasmid to recipient population: delay in OOHL synthesis and accumulation is a highly controlled process, as the QQ protein TraM, the lactanase AiiB, as well as the GABA/proline regulated lactanase BlcC, all contribute to this time control during the infection process. Both QS and QQ are regulated to optimize Ti plasmid transfer from donor _Agrobacteriumum_. Other authors [68] argued that OOHL degradation by BlcC was mainly accidental. In our opinion, however, under the light of the data accumulated over the past decade, the lactanase BlcC appears as one of several QQ molecular actors (TraM, BlcC, and AiiB) contributing to QS modulation in _Agrobacteriumum_. By controlling QS-signal level, QQ could also contribute to reduce QS-signal hijacking by agrobacteria (called QS-hijackers) which do not produce any QS-signal but are able to use that produced by others for activating the transfer of their own Ti plasmid. This hypothesis was recently investigated by authors [67] who reported, for the first time, the role of QQ lactonases in policing QS-signal hijacking. They showed that the QQ-lactonase may attenuate dissemination of QS-signal hijacking Ti-plasmid in co-culture assays, in which QS-signal emitting strain and QS-signal negative strain, each carrying a Ti plasmid, and a recipient strain, were mixed. The lactonase was encoded either by the QS-producing Ti-plasmid itself, by a companion plasmid in the same QS-producing donor cells, or by one in the recipient cells. In all cases, the lactonase can serve as a mechanism for controlling QS exploitation by QS signal-negative mutants [67]. Other arguments support our opinion that QS (TraR/TraI) and QQ (TraM/AiiB/BlcC) are not systems that only sense the quorum of a population and merely accidentally impair OOHL QS signals. First, as indicated earlier, even in the absence of agrocinopines, _traR_ is expressed at a level that suffices, in the absence of TraM, to bind OOHL at subquorate bacterial concentrations [31; 32; 33].