SummaryIn Gram-negative bacteria, quorum-sensing (QS) communication is mostly mediated by N-acyl homoserine lactones (N-AHSL). The diversity of bacterial populations that produce or inactivate the N-AHSL signal in soil and tobacco rhizosphere was investigated by restriction fragment length polymorphism (RFLP) analysis of amplified 16S DNA and DNA sequencing. Such analysis indicated the occurrence of N-AHSL-producing strains among the a -, b -and gproteobacteria, including genera known to produce N-AHSL ( Rhizobium , Sinorhizobium and Pseudomonas ) and novel genera with no previously identified N-AHSL-producing isolates ( Variovorax , Sphingomonas and Massilia ). The diversity of N-AHSL signals was also investigated in relation to the genetic diversity of the isolates. However, N-AHSL-degrading strains isolated from soil samples belonged to the Bacillus genus, while strains isolated from tobacco rhizospheres belonged to both the Bacillus genus and to the a subgroup of proteobacteria, suggesting that diversity of N-AHSL-degrading strains may be modulated by the presence of the tobacco plant. Among these rhizospheric isolates, novel N-AHSL-degrading genera have been identified ( Sphingomonas and Bosea ). As the first simultaneous analysis of both N-AHSL-degrading and -producing bacterial communities in a complex environment, this study revealed the coexistence of bacterial isolates, belonging to the same genus or species that may produce or degrade N-AHSL.

Thirty Azospirillum strains were isolated from the rhizosphere of 13 maize (Zea mays L.) cultivars grown in 14 French soils, using a new specific method, which has been given the name ROSEA. Among these strains 26 were Azospirillum lipoferum and 4 were Azospirillum brasilense. Their characterization was achieved using biochemical tests and plasmid profiles. Biochemical patterns allowed clear differentiation between the two species. A large diversity in carbon source metabolism was found among the Azospirillum sp. strains regardless of their origin. The A. brasilense were much more closely related, and were found in only two of the rhizospheres studied. The 30 plasmid patterns were all different, and the plasmid-profiling technique can therefore be considered as strain specific. All the A. lipoferum harboured a 150-MDa plasmid, while all the A. brasilense harboured a 90 to 100-MDa plasmid. This result reinforces the hypothesis of the presence of such plasmids as an additional criterion for differentiating these two species. Key words: Zea mays, Azospirillum, ROSEA method, biochemical tests, plasmid profiles.