A quorum-quenching approach to identify quorum-sensing-regulated functions in Azospirillum lipoferum

A quorum-quenching approach was exploited in order to identify functions regulated by quorum-sensing (QS) in the plant growth-promoting bacterium Azospirillum lipoferum. The AttM lactonase from Agrobacterium tumefaciens was shown to enzymatically inactivate N-acyl homoserine lactones (AHLs) produced by two A. lipoferum strains. The targeted analysis of several phenotypes revealed that in strain B518, a rice endophyte, AHL inactivation abolished pectinase activity, increased siderophore synthesis and reduced indoleacetic acid production (in stationary phase) but no effect was observed on cellulase activity or on swimming and swarming motilities. None of the tested phenotypes appeared to be under QS regulation in strain TVV3 isolated from the rice rhizosphere. Moreover, AHL inactivation had no deleterious effect on the phytostimulatory effect of the two strains in vitro. A global proteomic approach revealed little modification of protein patterns when comparing attM-expressing TVV3 and the wild-type strain, but numerous proteins appeared to be regulated by the AHL-mediated QS system in strain B518. Several proteins identified by MS-MS analysis were revealed to be implicated in transport (such as OmaA) and chemotaxis (ChvE). Altogether, the results indicate that in A. lipoferum, QS regulation is strain-specific and is dedicated to regulating functions linked to rhizosphere competence and adaptation to plant roots.