Ecological significance of quorum quenching in the rhizosphere and plant environments – ECORUM

The proposed project aims at investigating the ecological significance of the quorum quenching (QQ) phenomenon that is by definition opposed to quorum sensing (QS). QS is a regulatory mode that couples in bacteria gene expression with cell density. It relies upon the production and sensing of signal molecules by the relevant bacterial population. Two QQ phenomena are known. The first one, of enzymatic nature, leads to signal degradation ; the second, involves molecules known as quorum sensing inhibitors (QSI) which prevent signal recognition by bacterial sensor proteins.

The proposed project is articulated around 3 axes. The first one deals with the ability of some plants of the legume clad to degrade some of the QS signals which harbor a lactone group. Such signals are being produced by their bacterial symbionts that belong to the Rhizobiaceaea family and are degraded by the legumes by a lactonase-like activity. Two of the goals of the project are to confirm that a candidate enzyme identified by purification is indeed the QS signal lactonase activity, and to precisely identify the exact ecological role. To this end, this activity would be searched for in a broad range of legume plants. Mutants or plant lines expressing RNAi affected in the production of the candidate enzyme will be used.

The second axis deals with another Rhizobiaceae, namely Agrobacterium, in which the transfer of the Ti plasmid, the main pathogenic element of the bacteria, is regulated by QS. The project aims at evaluating to which extend the conjugal transfer of the Ti plasmid might be affected in plants degrading QS signals or in the presence in the environment of isolated bacteria or bacterial consortia that degrade QS signals as these occur in natural environments colonized by Agrobacterium. With respect to the duration of the project and the likely delivery of plant mutant or RNAi legume lines at the end of the proposed project, only model plants expressing on not a lactonase activity directed at Agrobacterium QS signals will be used. The plants are already available in the laboratory.

The third axis aims at searching for QSI molecules within libraries of synthetic or semi-purified chemicals, mostly of plant origin. Screening of libraries will be performed in the presents of purified QS signals, using already available bio-indicator bacteria. These are unable to produce the only QS signals but they respond to the presence of a exogenous signals by the production of chromogenic compounds or that of easily detectable enzymatic activities. The identification of “positive hits” amongst the semi-purified compounds will be followed by purification of the active molecule. The QS inhibitory capacity of these molecules will be investigated in more depth using bioassays, such as the conjugal transfer of the Ti plasmid of Agrobacterium, or the Pectobacterium virulence assay involving potato tubers. This last series of experiments may lead to the identification of novel molecules, of which several may have a biological origin (hence be biodegradable) and be active against plant pathogenic bacteria. These experiments may therefore have valuable applied interests.