# Quorum Sensing and Quorum Quenching in _Agrobacteriumum_: A \"Go/No Go System\"?
## 5 The Quorum Quenching Lactonases as Cogs to Sense Environmental Cues  
While the AHL degrading ability of BlcC has never been argued, whether there is an impact on Ti plasmid conjugation frequency has been debated. In tomato, the transfer of the Ti plasmid occurred at comparable frequency from the wild-type donor strain C58 or its _blcC_ defective derivative [37]. A similar result was observed by other authors [42] who reported that transconjugants of a C58 _blcC_ mutant appear ca. one week earlier than those of wild-type strain C58 in in planta conjugations, but with a similar frequency in fine. In _Arabidopsis thaliana_ (Col-0 ecotype), however, the conjugation frequency was ca. 100 times higher with a _blcC_ mutant of strain C58 as a donor than with the wild-type strain C58 [47]. The above discrepancy is related to the sensing of environmental cues by the bacteria, amongst which GABA and proline play critical roles. Indeed, in agrobacteria, GABA is taken up by the ABC transporter Bra and the cognate periplasmic binding protein (PBP) Att2422 [45; 53]. The PBP Att2422 is not strictly specific for GABA. Indeed, though this uptake system does not appear to import GBL and GHB, it contributes to that of the imino/amino acids proline, alanine, and valine, as deduced from the Att2422 structure analysis and observation that proline, as well as alanine and valine, act as competitive antagonists of GABA transport [53; 54]. As a consequence, OOHL concentrations in culture supernatants of strain C58 grown in the presence of GABA are ca. 10 times lower than those measured in culture supernatant of strain C58 grown in the presence of GABA and proline, alanine or valine due to, respectively, the full or reduced activation of the _blcABC_ operon [54]. The presented data indicate that GABA and proline ratios play important roles in the modulation of the concentration of OOHL in agrobacteria, hence, possibly on Ti plasmid conjugation. This assertion was evaluated by in planta conjugations using _A. thaliana_ Col-0 plants and a mutant plant line overproducing GABA. Under those conditions, the GABA to proline ratios were 1:4 in wild type plant tumors, and 5:1 in the GABA overproducing plant tumors. Concomitantly, the Ti plasmid transfer frequencies of strain C58 Ti plasmid were ca. 100 times higher in wild-type plant tumors than in the GABA overproducing plant tumor [47]. The GABA to proline ratio may therefore be a mean for strain C58 to sense whether the bacteria is in healthy plant tissues or in a tumor. Indeed, the GABA to proline ratios in _A. thaliana_ healthy and tumor tissues shifted from 3:1 in control tissues to 1:4 in tumors [47]. The above findings strongly suggest that the _blc_ Lectanase of _Agrobacteriumium_ may possibly play a role in the sensing of environmental clues other than the only GABA and proline concentrations. This view is strengthened by the following observations. First, the production of the BlcC lactanase activity is growth phase-dependent. The activity is only moderate throughout the log phase and becomes 8 to 10 times higher during the stationary phase. As a corollary, OOHL accumulates at the highest concentration in the late exponential growth phase [38]. Second, further investigation indicated that the BlcC lactanase activity responds to both carbon and nitrogen starvation. The underlying mechanism involves the _relA_ gene that is responsible for the synthesis and degradation of the alarmones (p)ppGpp [55]. The alarmones (p)ppGpp are secondary messengers responsible for pleiotropic adaptations of bacteria (and plant chloroplasts) in response to starvation or stress, in relation with changes in RNA polymerase activity (for reviews on (p)ppGpp: [56; 57]). The activation of the _blcABC_ operon of _Agrobacteriumium_ under starvation may permit the bacteria to sense whether resources are fading as this could be the case in decaying tumors (after the death of the host plant or the fall of the tumor on the soil) and consequently to prevent Ti plasmid conjugation.