Jean Lorquin

Photosynthetic Bradyrhizobia Are Natural Endophytes of the African Wild Rice Oryza breviligulata

ABSTRACT We investigated the presence of endophytic rhizobia within the roots of the wetland wild rice Oryza breviligulata, which is the ancestor of the African cultivated rice Oryza glaberrima. This primitive rice species grows in the same wetland sites as Aeschynomene sensitiva, an aquatic stem-nodulated legume associated with photosynthetic strains ofBradyrhizobium. Twenty endophytic and aquatic isolates were obtained at three different sites in West Africa (Senegal and Guinea) from nodal roots of O. breviligulata and surrounding water by using A. sensitiva as a trap legume. Most endophytic and aquatic isolates were photosynthetic and belonged to the same phylogenetic Bradyrhizobium/Blastobacter subgroup as the typical photosynthetic Bradyrhizobium strains previously isolated from Aeschynomene stem nodules. Nitrogen-fixing activity, measured by acetylene reduction, was detected in rice plants inoculated with endophytic isolates. A 20% increase in the shoot growth and grain yield of O. breviligulata grown in a greenhouse was also observed upon inoculation with one endophytic strain and one Aeschynomene photosynthetic strain. The photosynthetic Bradyrhizobium sp. strain ORS278 extensively colonized the root surface, followed by intercellular, and rarely intracellular, bacterial invasion of the rice roots, which was determined with a lacZ-tagged mutant of ORS278. The discovery that photosynthetic Bradyrhizobium strains, which are usually known to induce nitrogen-fixing nodules on stems of the legume Aeschynomene, are also natural true endophytes of the primitive rice O. breviligulatacould significantly enhance cultivated rice production.

Isolation and Characterization of Canthaxanthin Biosynthesis Genes from the Photosynthetic Bacterium Bradyrhizobium sp. Strain ORS278

A carotenoid biosynthesis gene cluster involved in canthaxanthin production was isolated from the photosynthetic Bradyrhizobium sp. strain ORS278. This cluster includes five genes identified as crtE, crtY, crtI, crtB, and crtW that are organized in at least two operons. The functional assignment of each open reading frame was confirmed by complementation studies.

Symbiotic and Taxonomic Diversity of Rhizobia Isolated from Acacia tortilis subsp. raddiana in Africa

A collection of rhizobia isolated from Acacia tortilis subsp. raddiana from various sites in the North and South of Sahara was analyzed for their diversity at both taxonomic and symbiotic levels. On the basis of whole cell protein (SDS-PAGE) and 16S rDNA sequence analysis, most of the strains were found to belong to the Sinorhizobium and Mesorhizobium genera where they may represent several different genospecies. Despite their chromosomal diversity, most A. tortilis Mesorhizobium and Sinorhizobium symbionts exhibited very similar symbiotic characters. Nodulation tests showed that the strains belong to the Acacia-Leucaena-Prosopis nodulation group, although mainly forming non-fixing nodules on species other than A. tortilis. Most of the strains tested responded similarly to flavonoid nod gene inducers, as estimated by using heterologous nodA-lacZ fusions. Thin layer chromatography analysis of the Nod factors synthesized by overproducing strains showed that most of the strains exhibited similar profiles. The structures of Nod factors produced by four different Sinorhizobium sp. strains were determined and found to be similar to other Acacia-Prosopis-Leucaena nodulating rhizobia of the Sinorhizobium-Mesorhizobium-Rhizobium branch. They are chitopentamers, N-methylated and N-acylated by common fatty acids at the terminal non reducing sugar. The molecules can also be 6-O sulfated at the reducing end and carbamoylated at the non reducing end. The phylogenetic analysis of available NodA sequences, including new sequences from A. tortilis strains, confirmed the clustering of the NodA sequences of members of the Acacia-Prosopis-Leucaena nodulation group.

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...

Bradyrhizobium sp. Strains That Nodulate the Leguminous Tree Acacia albida Produce Fucosylated and Partially Sulfated Nod Factors

ABSTRACT We determined the structures of Nod factors produced by six different Bradyrhizobium sp. strains nodulating the legume tree Acacia albida (syn. Faidherbia albida). Compounds from all strains were found to be similar, i.e., O-carbamoylated and substituted by an often sulfated methyl fucose and different from compounds produced byRhizobium-Mesorhizobium-Sinorhizobium strains nodulating other species of the Acaciae tribe.

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.