Bernard Dreyfus

Bio: Bernard Dreyfus is an academic researcher from Institut national de la recherche agronomique. The author has contributed to research in topics: Bradyrhizobium & Rhizobia. The author has an hindex of 33, co-authored 99 publications receiving 5165 citations. Previous affiliations of Bernard Dreyfus include SupAgro.

Legumes Symbioses: Absence of Nod Genes in Photosynthetic Bradyrhizobia

Abstract: Leguminous plants (such as peas and soybeans) and rhizobial soil bacteria are symbiotic partners that communicate through molecular signaling pathways, resulting in the formation of nodules on legume roots and occasionally stems that house nitrogen-fixing bacteria. Nodule formation has been assumed to be exclusively initiated by the binding of bacterial, host-specific lipochito-oligosaccharidic Nod factors, encoded by the nodABC genes, to kinase-like receptors of the plant. Here we show by complete genome sequencing of two symbiotic, photosynthetic, Bradyrhizobium strains, BTAi1 and ORS278, that canonical nodABC genes and typical lipochito-oligosaccharidic Nod factors are not required for symbiosis in some legumes. Mutational analyses indicated that these unique rhizobia use an alternative pathway to initiate symbioses, where a purine derivative may play a key role in triggering nodule formation.

Methylotrophic Methylobacterium bacteria nodulate and fix nitrogen in symbiosis with legumes.

Abstract: Rhizobia described so far belong to three distinct phylogenetic branches within the alpha-2 subclass of Proteobacteria. Here we report the discovery of a fourth rhizobial branch involving bacteria of the Methylobacterium genus. Rhizobia isolated from Crotalaria legumes were assigned to a new species, "Methylobacterium nodulans," within the Methylobacterium genus on the basis of 16S ribosomal DNA analyses. We demonstrated that these rhizobia facultatively grow on methanol, which is a characteristic of Methylobacterium spp. but a unique feature among rhizobia. Genes encoding two key enzymes of methylotrophy and nodulation, the mxaF gene, encoding the alpha subunit of the methanol dehydrogenase, and the nodA gene, encoding an acyltransferase involved in Nod factor biosynthesis, were sequenced for the type strain, ORS2060. Plant tests and nodA amplification assays showed that "M. nodulans" is the only nodulating Methylobacterium sp. identified so far. Phylogenetic sequence analysis showed that "M. nodulans" NodA is closely related to Bradyrhizobium NodA, suggesting that this gene was acquired by horizontal gene transfer.

Characterization of Azorhizobium caulinodans gen. nov., sp. nov., a stem-nodulating nitrogen-fixing bacterium isolated from Sesbania rostrata

Abstract: Twenty stem- and root-nodulating bacterial strains isolated from stem nodules of Sesbania rostrata were compared by numerical analysis of 221 phenotypic features with nine strains which effectively nodulate only the roots of this plant and with representative strains from the genera Rhizobium and Bradyrhizobium. Representative organisms from the different clusters were investigated further, together with possibly related organisms, by performing comparative gel electrophoresis of whole-cell proteins and by performing deoxyribonucleic acid (DNA)-DNA and DNA-ribosomal ribonucleic acid (rRNA) hybridizations. 3H-labeled rRNA was prepared from Sesbania stem- and root-nodulating bacterial strain ORS 571T (T = type strain); [14C]rRNA from Bradyrhizobium japonicum NZP 5549T was also used. The following conclusions were drawn: (i) the Sesbania root-nodulating bacterial strains are genuine rhizobia; (ii) the Sesbania stem- and root-nodulating strains are quite different from Rhizobium and Bradyrhizobium, and thus they constitute a separate rRNA subbranch on the Rhodopseudomonas palustris rRNA branch in rRNA superfamily IV; and (iii) the closest relative of these organisms is Xanthobacter, but they are phenotypically and genotypically sufficiently different from the latter genus to deserve a separate generic rank. Because the feature of free-living nitrogen fixation is quite discriminative, a new genus, Azorhizobium, is proposed, with one species, Azorhizobium caulinodans. The type strain is strain ORS 571 (= LMG 6465).

Polyphasic Taxonomy of Rhizobia: Emendation of the Genus Sinorhizobium and Description of Sinorhizobium meliloti comb. nov., Sinorhizobium saheli sp. nov., and Sinorhizobium teranga sp. nov.

Abstract: A total of 80 bacterial strains isolated from different Sesbania and Acacia species growing in various sites in Senegal (West Africa) were compared with 35 reference strains of Rhizobium, Bradyrhizobium, Azorhizobium, and Agrobacterium species and with 33 representative strains of the different groups of Brazilian isolates described on the basis of the results of a numerical analysis of the whole-cell protein patterns obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Fifty-two strains could be placed in three protein electrophoretic clusters, two of which were different from the clusters containing various reference or representative strains, while 30 other strains could not be placed in any group. The strains belonging to the three clusters were studied by determining their nodulation host ranges and their morphological, physiological, and auxanographic characteristics. Representative strains of the three clusters were also genotypically characterized by determining their DNA base compositions, by performing DNA-DNA and DNA-rRNA hybridization experiments, and by determining their 16S rRNA gene sequences. Our results showed that two of the clusters identified on the basis of SDS-PAGE data are genotypically and phenotypically distinct groups that belong on the Rhizobium meliloti-Rhizobium fredii rRNA branch. The third cluster is localized on the Rhizobium loti rRNA branch in the vicinity of Rhizobium huakuii and contains strains isolated in Africa, in Brazil, and in New Zealand from different leguminous species. On the basis of the results of the present study, we propose to emend the genus Sinorhizobium and to reclassify R. meliloti as Sinorhizobium meliloti comb. nov. In addition, two new species, Sinorhizobium saheli and Sinorhizobium teranga, are proposed for isolates from Senegal.

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 of Bradyrhizobium. 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. breviligulata could significantly enhance cultivated rice production.

Cell biology and molecular basis of denitrification.

Abstract: Denitrification is a distinct means of energy conservation, making use of N oxides as terminal electron acceptors for cellular bioenergetics under anaerobic, microaerophilic, and occasionally aerobic conditions. The process is an essential branch of the global N cycle, reversing dinitrogen fixation, and is associated with chemolithotrophic, phototrophic, diazotrophic, or organotrophic metabolism but generally not with obligately anaerobic life. Discovered more than a century ago and believed to be exclusively a bacterial trait, denitrification has now been found in halophilic and hyperthermophilic archaea and in the mitochondria of fungi, raising evolutionarily intriguing vistas. Important advances in the biochemical characterization of denitrification and the underlying genetics have been achieved with Pseudomonas stutzeri, Pseudomonas aeruginosa, Paracoccus denitrificans, Ralstonia eutropha, and Rhodobacter sphaeroides. Pseudomonads represent one of the largest assemblies of the denitrifying bacteria within a single genus, favoring their use as model organisms. Around 50 genes are required within a single bacterium to encode the core structures of the denitrification apparatus. Much of the denitrification process of gram-negative bacteria has been found confined to the periplasm, whereas the topology and enzymology of the gram-positive bacteria are less well established. The activation and enzymatic transformation of N oxides is based on the redox chemistry of Fe, Cu, and Mo. Biochemical breakthroughs have included the X-ray structures of the two types of respiratory nitrite reductases and the isolation of the novel enzymes nitric oxide reductase and nitrous oxide reductase, as well as their structural characterization by indirect spectroscopic means. This revealed unexpected relationships among denitrification enzymes and respiratory oxygen reductases. Denitrification is intimately related to fundamental cellular processes that include primary and secondary transport, protein translocation, cytochrome c biogenesis, anaerobic gene regulation, metalloprotein assembly, and the biosynthesis of the cofactors molybdopterin and heme D1. An important class of regulators for the anaerobic expression of the denitrification apparatus are transcription factors of the greater FNR family. Nitrate and nitric oxide, in addition to being respiratory substrates, have been identified as signaling molecules for the induction of distinct N oxide-metabolizing enzymes.

Plant growth-promoting rhizobacteria (PGPR): emergence in agriculture

Abstract: Plant growth-promoting rhizobacteria (PGPR) are the rhizosphere bacteria that can enhance plant growth by a wide variety of mechanisms like phosphate solubilization, siderophore production, biological nitrogen fixation, rhizosphere engineering, production of 1-Aminocyclopropane-1-carboxylate deaminase (ACC), quorum sensing (QS) signal interference and inhibition of biofilm formation, phytohormone production, exhibiting antifungal activity, production of volatile organic compounds (VOCs), induction of systemic resistance, promoting beneficial plant-microbe symbioses, interference with pathogen toxin production etc. The potentiality of PGPR in agriculture is steadily increased as it offers an attractive way to replace the use of chemical fertilizers, pesticides and other supplements. Growth promoting substances are likely to be produced in large quantities by these rhizosphere microorganisms that influence indirectly on the overall morphology of the plants. Recent progress in our understanding on the diversity of PGPR in the rhizosphere along with their colonization ability and mechanism of action should facilitate their application as a reliable component in the management of sustainable agricultural system. The progress to date in using the rhizosphere bacteria in a variety of applications related to agricultural improvement along with their mechanism of action with special reference to plant growth-promoting traits are summarized and discussed in this review.

Soil beneficial bacteria and their role in plant growth promotion: a review

Abstract: Soil bacteria are very important in biogeochemical cycles and have been used for crop production for decades. Plant–bacterial interactions in the rhizosphere are the determinants of plant health and soil fertility. Free-living soil bacteria beneficial to plant growth, usually referred to as plant growth promoting rhizobacteria (PGPR), are capable of promoting plant growth by colonizing the plant root. PGPR are also termed plant health promoting rhizobacteria (PHPR) or nodule promoting rhizobacteria (NPR). These are associated with the rhizosphere, which is an important soil ecological environment for plant–microbe interactions. Symbiotic nitrogen-fixing bacteria include the cyanobacteria of the genera Rhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium, Sinorhizobium and Mesorhizobium. Free-living nitrogen-fixing bacteria or associative nitrogen fixers, for example bacteria belonging to the species Azospirillum, Enterobacter, Klebsiella and Pseudomonas, have been shown to attach to the root and efficiently colonize root surfaces. PGPR have the potential to contribute to sustainable plant growth promotion. Generally, PGPR function in three different ways: synthesizing particular compounds for the plants, facilitating the uptake of certain nutrients from the soil, and lessening or preventing the plants from diseases. Plant growth promotion and development can be facilitated both directly and indirectly. Indirect plant growth promotion includes the prevention of the deleterious effects of phytopathogenic organisms. This can be achieved by the production of siderophores, i.e. small metal-binding molecules. Biological control of soil-borne plant pathogens and the synthesis of antibiotics have also been reported in several bacterial species. Another mechanism by which PGPR can inhibit phytopathogens is the production of hydrogen cyanide (HCN) and/or fungal cell wall degrading enzymes, e.g., chitinase and s-1,3-glucanase. Direct plant growth promotion includes symbiotic and non-symbiotic PGPR which function through production of plant hormones such as auxins, cytokinins, gibberellins, ethylene and abscisic acid. Production of indole-3-ethanol or indole-3-acetic acid (IAA), the compounds belonging to auxins, have been reported for several bacterial genera. Some PGPR function as a sink for 1-aminocyclopropane-1-carboxylate (ACC), the immediate precursor of ethylene in higher plants, by hydrolyzing it into α-ketobutyrate and ammonia, and in this way promote root growth by lowering indigenous ethylene levels in the micro-rhizo environment. PGPR also help in solubilization of mineral phosphates and other nutrients, enhance resistance to stress, stabilize soil aggregates, and improve soil structure and organic matter content. PGPR retain more soil organic N, and other nutrients in the plant–soil system, thus reducing the need for fertilizer N and P and enhancing release of the nutrients.

Bacterial Endophytes and Their Interactions with Hosts

Abstract: Recent molecular studies on endophytic bacterial diversity have revealed a large richness of species. Endophytes promote plant growth and yield, suppress pathogens, may help to remove contaminants, solubilize phosphate, or contribute assimilable nitrogen to plants. Some endophytes are seed-borne, but others have mechanisms to colonize the plants that are being studied. Bacterial mutants unable to produce secreted proteins are impaired in the colonization process. Plant genes expressed in the presence of endophytes provide clues as to the effects of endophytes in plants. Molecular analysis showed that plant defense responses limit bacterial populations inside plants. Some human pathogens, such as Salmonella spp., have been found as endophytes, and these bacteria are not removed by disinfection procedures that eliminate superficially occurring bacteria. Delivery of endo-phytes to the environment or agricultural fields should be carefully evaluated to avoid introducing pathogens.