C. Boivin

Rhizobium meliloti Genes Encoding Catabolism of Trigonelline Are Induced under Symbiotic Conditions.

Rhizobium meliloti trc genes controlling the catabolism of trigonelline, a plant secondary metabolite often abundant in legumes, are closely linked to nif-nod genes on the symbiotic megaplasmid pSym [Boivin, C., Malpica, C., Rosenberg, C., Denarie, J., Goldman, A., Fleury, V., Maille, M., Message, B., and Tepfer, D. (1989). In Molecular Signals in the Microbe-Plant Symbiotic and Pathogenic Systems. (Berlin: Springer-Verlag), pp. 401-407]. To investigate the role of trigonelline catabolism in the Rhizobium-legume interaction, we studied the regulation of trc gene expression in free-living and in endosymbiotic bacteria using Escherichia coli lacZ as a reporter gene. Experiments performed with free-living bacteria indicated that trc genes were organized in at least four transcription units and that the substrate trigonelline was a specific inducer for three of them. Noninducing trigonelline-related compounds such as betaines appeared to antagonize the inducing effect of trigonelline. None of the general or symbiotic regulatory genes ntrA, dctB/D, or nodD seemed to be involved in trigonelline catabolism. trc fusions exhibiting a low basal and a high induced [beta]-galactosidase activity when present on pSym were used to monitor trc gene expression in alfalfa tissue under symbiotic conditions. Results showed that trc genes are induced during all the symbiotic steps, i.e., in the rhizosphere, infection threads, and bacteroids of alfalfa, suggesting that trigonelline is a nutrient source throughout the Rhizobium-legume association.

Stem Nodulation in Legumes: Diversity, Mechanisms, and Unusual Characteristics

Abstract Rhizobia can establish a nitrogen-fixing symbiosis with plants of the Leguminosae family. They elicit on their host plant the formation of new organs, called nodules, which develop on the ...

Nod factor thin-layer chromatography profiling as a tool to characterize symbiotic specificity of rhizobial strains: application to Sinorhizobium saheli, S. teranga, and Rhizobium sp. strains isolated from Acacia and Sesbania

Rhizobia isolated from Acacia or Sesbania belong to several taxonomic groups, including the newly described species Sinorhizobium sahelì, Sinorhizobium teranga, and the so-called cluster U. A collection of strains belonging to these different groups was analyzed in order to determine whether the host range of a strain could be correlated with various molecular nodulation determinants. Nodulation tests showed that, independently of their taxonomic position, all the strains isolated from the same plant genus exhibited a similar host range, which was different for Sesbania and Acacia isolates. The fact that S. teranga strains nodulate either Acacia or Sesbania led us to subdivide this species into biovars acaciae and sesbaniae. Thin-layer chromatography (TLC) analysis of the Nod factors synthesized by overproducing strains showed that (i) strains isolated from the same plant genus exhibited similar TLC profiles and (ii) profiles of Acacia and Sesbunia symbionts were easily distinguishable, Acacia strains producing, in particular, sulfated molecules. In contrast, no correlation could be established between the host range of a strain and its plasmid content, the nature of the nod gene inducers or the presence of DNA sequences homologous to specific nod genes. We thus propose that Nod factor TLC profiling may be used as an easy and powerful tool for the classification of rhizobial strains on the basis of their symbiotic properties.

Nod Factors from Sinorhizobium saheli and S. teranga bv. sesbaniae Are Both Arabinosylated : and Fucosylated, a Structural Feature Specific to Sesbania rostrata Symbionts

Sesbania spp. can establish symbiotic interactions with rhizobia from diverse taxonomical origins, including the newly described Sinorhizobium saheli and S. teranga bv. sesbaniae, in addition to the Sesbania rostrata-specific symbiont Azorhizobium caulinodans. These different rhizobia exhibit a narrow host range, which is limited mainly to Sesbania spp. Nod factors from overproducing strains of S. saheli ORS611 were purified and their structures determined. Remarkably, the terminal reducing glucosamine of most compounds was found to bear both an arabinosyl group on C-3 and a fucosyl substitution on C-6. Other structural features are as follows: Nod factors are mainly chitopentameric compounds, N-methylated, O-carbamoylated and N-substituted either by a C18:1 or a C16:0 acyl chain at their nonreducing end. Nod factors from an overproducing strain of S. teranga bv. sesbaniae ORS604 were found to be identical to those of S. saheli on the basis of high-pressure liquid chromatography separation and liquid seco...

Molecular Symbiotic Characterization of Rhizobia

Using a collection of tropical rhizobia from different species and host plants, we established a high correlation between rhizobia host range, NF TLC profiles, NF structures and nodA sequences. Both techniques based on NF and nod gene analysis could be used to develop a polyphasic approach for the symbiotic characterization of rhizobia.

A New Approach to Symbiotically Characterize Rhizobia, Based on NOD Factor Analysis

Rhizobia are currently classified using a combination of general taxonomic characters which now tend to exclude symbiotic properties as a taxonomic criterion, since symbionts from the same host plant are not always closely related, while very similar bacterial strains have different host ranges. The host range of rhizobia is however of great significance for agro-ecological studies, biodiversity analysis and plant-bacteria coevolution. Our aim is hence to develop alternative methods to nodulation tests for the symbiotic characterization of rhizobia.