Visualization of N-acylhomoserine lactone-mediated cell-cell communicationbetween bacteria colonizing the tomato rhizosphere

Given that a large proportion of the bacteria colonizing the roots of plant s is capable of producing N-acyl-L-homoserine lactone (AHL) molecules, it a ppears likely that these bacterial pheromones may serve as signals for comm unication between cells of different species. In this study, we have develo ped and characterized novel Gfp-based monitor strains that allow in situ vi sualization of AHL-mediated communication between individual cells in the p lant rhizosphere. For this purpose, three Gfp-based AHL sensor plasmids tha t respond to different spectra of AHL molecules were transferred into AHL-n egative derivatives of Pseudomonas putida IsoF and Serratia liquefaciens MG 1, two strains that are capable of colonizing tomato roots. These AHL monit or strains were used to visualize communication between defined bacterial p opulations in the rhizosphere of axenically grown tomato plants. Furthermor e, we integrated into the chromosome of AHL-negative P. putida strain F117 an AHL sensor cassette that responds to the presence of long-chain AHLs wit h the expression of Gfp. This monitor strain was used to demonstrate that t he indigenous bacterial community colonizing the roots of tomato plants gro wing in nonsterile soil produces AHL molecules. The results strongly suppor t the view that AHL signal molecules serve as a universal language for comm unication between the different bacterial populations of the rhizosphere co nsortium.