| Forty Azospirillum strains were tested for their ability to synthesize N-acyl-homoserine lactones (AHLs). AHL production was detected for four strains belonging to the lipoferum species and isolated from a rice rhizosphere. AHLs molecules were structurally identified for two strains: A. lipoferum TVV3 produces 3O,C8-HSL (N-3-oxo-octanoyl-homoserine-lactone), C8-HSL (N-3-octanoyl-homoserine-lactone), 3O,C10-HSL (N-3-oxo-decanoyl-homoserine-lactone), 3OH,C10-HSL (N-3-hydroxy-decanoyl-homoserine-lactone) and C10-HSL (N-3-decanoyl-homoserine-lactone) whereas A. lipoferum B518 produces 3O,C6-HSL (N-3-oxo-hexanoyl-homoserine-lactone), C6-HSL (N-3-hexanoyl-homoserine-lactone), 3O,C8-HSL, 3OH,C8-HSL and C8-HSL. Genes involved in AHL production were characterized for A. lipoferum TVV3 by generating a genomic library and complementing an AHL-deficient strain with sensor capabilities. Those genes, designated alpI and alpR, were found to belong to the luxI and luxR families, respectively. When cloned in a suitable heterologous host, alpI and alpR could direct the synthesis of the five cognate AHLs present in A. lipoferum TVV3. These two adjacent genes were found to be located on a 85 kb plasmid. Southern hybridization experiments with probes alpI/R indicated that genes involved in AHL production in the three other AHL-producing strains were not closely related to alpI and alpR and confirmed the absence of those genes in the non-AHL producing strains. This study demonstrates that AHL-based quorum-sensing is not widespread among the Azospirillum genus and could be found only in some A. lipoferum strains. |
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