Showing papers on "Rhizobia published in 2003"

Diversity of Rhizobium-Phaseolus vulgaris symbiosis: overview and perspectives

Abstract: Common bean (Phaseolus vulgaris) has become a cosmopolitan crop, but was originally domesticated in the Americas and has been grown in Latin America for several thousand years. Consequently an enormous diversity of bean nodulating bacteria have developed and in the centers of origin the predominant species in bean nodules is R. etli. In some areas of Latin America, inoculation, which normally promotes nodulation and nitrogen fixation is hampered by the prevalence of native strains. Many other species in addition to R. etli have been found in bean nodules in regions where bean has been introduced. Some of these species such as R. leguminosarum bv. phaseoli, R. gallicum bv. phaseoli and R. giardinii bv. phaseoli might have arisen by acquiring the phaseoli plasmid from R. etli. Others, like R. tropici, are well adapted to acid soils and high temperatures and are good inoculants for bean under these conditions. The large number of rhizobia species capable of nodulating bean supports that bean is a promiscuous host and a diversity of bean-rhizobia interactions exists. Large ranges of dinitrogen fixing capabilities have been documented among bean cultivars and commercial beans have the lowest values among legume crops. Knowledge on bean symbiosis is still incipient but could help to improve bean biological nitrogen fixation.

Legumes: Importance and Constraints to Greater Use

Abstract: Legumes, broadly defined by their unusual flower structure, podded fruit, and the ability of 88% of the species examined to date to form nodules with rhizobia ([de Faria et al., 1989][1]), are second only to the Graminiae in their importance to humans. The 670 to 750 genera and 18,000 to 19,000

Plant recognition of symbiotic bacteria requires two LysM receptor-like kinases

Abstract: Although most higher plants establish a symbiosis with arbuscular mycorrhizal fungi, symbiotic nitrogen fixation with rhizobia is a salient feature of legumes. Despite this host range difference, mycorrhizal and rhizobial invasion shares a common plant-specified genetic programme controlling the early host interaction. One feature distinguishing legumes is their ability to perceive rhizobial-specific signal molecules. We describe here two LysM-type serine/threonine receptor kinase genes, NFR1 and NFR5, enabling the model legume Lotus japonicus to recognize its bacterial microsymbiont Mesorhizobium loti. The extracellular domains of the two transmembrane kinases resemble LysM domains of peptidoglycan- and chitin-binding proteins, suggesting that they may be involved directly in perception of the rhizobial lipochitin-oligosaccharide signal. We show that NFR1 and NFR5 are required for the earliest physiological and cellular responses to this lipochitin-oligosaccharide signal, and demonstrate their role in the mechanism establishing susceptibility of the legume root for bacterial infection.

Host sanctions and the legume–rhizobium mutualism

Abstract: Explaining mutualistic cooperation between species remains one of the greatest problems for evolutionary biology. Why do symbionts provide costly services to a host, indirectly benefiting competitors sharing the same individual host? Host monitoring of symbiont performance and the imposition of sanctions on 'cheats' could stabilize mutualism. Here we show that soybeans penalize rhizobia that fail to fix N(2) inside their root nodules. We prevented a normally mutualistic rhizobium strain from cooperating (fixing N(2)) by replacing air with an N(2)-free atmosphere (Ar:O(2)). A series of experiments at three spatial scales (whole plants, half root systems and individual nodules) demonstrated that forcing non-cooperation (analogous to cheating) decreased the reproductive success of rhizobia by about 50%. Non-invasive monitoring implicated decreased O(2) supply as a possible mechanism for sanctions against cheating rhizobia. More generally, such sanctions by one or both partners may be important in stabilizing a wide range of mutualistic symbioses.

Amino-acid cycling drives nitrogen fixation in the legume– Rhizobium symbiosis

Abstract: The biological reduction of atmospheric N2 to ammonium (nitrogen fixation) provides about 65% of the biosphere's available nitrogen. Most of this ammonium is contributed by legume–rhizobia symbioses1, which are initiated by the infection of legume hosts by bacteria (rhizobia), resulting in formation of root nodules. Within the nodules, rhizobia are found as bacteroids, which perform the nitrogen fixation: to do this, they obtain sources of carbon and energy from the plant, in the form of dicarboxylic acids2,3. It has been thought that, in return, bacteroids simply provide the plant with ammonium. But here we show that a more complex amino-acid cycle is essential for symbiotic nitrogen fixation by Rhizobium in pea nodules. The plant provides amino acids to the bacteroids, enabling them to shut down their ammonium assimilation. In return, bacteroids act like plant organelles to cycle amino acids back to the plant for asparagine synthesis. The mutual dependence of this exchange prevents the symbiosis being dominated by the plant, and provides a selective pressure for the evolution of mutualism.

Nitrogen fixation in perennial forage legumes in the field

Abstract: Nitrogen acquisition is one of the most important factors for plant production, and N contribution from biological N2 fixation can reduce the need for industrial N fertilizers. Perennial forages are widespread in temperate and boreal areas, where much of the agriculture is based on livestock production. Due to the symbiosis with N2-fixing rhizobia, perennial forage legumes have great potential to increase sustainability in such grassland farming systems. The present work is a summary of a large number of studies investigating N2 fixation in three perennial forage legumes primarily relating to ungrazed northern temperate/boreal areas. Reported rates of N2 fixation in above-ground plant tissues were in the range of up to 373 kg N ha−1 year−1 in red clover (Trifolium pratense L.), 545 kg N ha−1 year−1 in white clover (T. repens L.) and 350 kg N ha−1 year−1 in alfalfa (Medicago sativa L.). When grown in mixtures with grasses, these species took a large fraction of their nitrogen from N2 fixation (average around 80%), regardless of management, dry matter yield and location. There was a large variation in N2 fixation data and part of this variation was ascribed to differences in plant production between years. Studies with experiments at more than one site showed that also geographic location was an important source of variation. On the other hand, when all data were plotted against latitude, there was no simple correlation. Climatic conditions seem therefore to give as high N2 fixation per ha and year in northern areas (around 60°N) as in areas with a milder climate (around 40°N). Analyzing whole plants or just above-ground plant parts influenced the estimate of N2 fixation, and most reported values were underestimated since roots were not included. Despite large differences in environmental conditions, such as N fertilization and geographic location, N2 fixation (Nfix; kg N per ha and year) was significantly (P<0.001) correlated to legume dry matter yield (DM; kg per ha and year). Very rough, but nevertheless valuable estimations of Nfix in legume/grass mixtures (roots not considered) are given by Nfix = 0.026ċDM + 7 for T. pratense, Nfix = 0.031ċDM + 24 for T. repens, and Nfix = 0.021ċDM + 17 for M. sativa.

Legume symbiotic nitrogen fixation by beta-proteobacteria is widespread in nature.

Abstract: Following the initial discovery of two legume-nodulating Burkholderia strains (L. Moulin, A. Munive, B. Dreyfus, and C. Boivin-Masson, Nature 411:948–950, 2001), we identified as nitrogen-fixing legume symbionts at least 50 different strains of Burkholderia caribensis and Ralstonia taiwanensis, all belonging to the-subclass of proteobacteria, thus extending the phylogenetic diversity of the rhizobia. R. taiwanensis was found to represent 93% of the Mimosa isolates in Taiwan, indicating that-proteobacteria can be the specific symbionts of a legume. The nod genes of rhizobial-proteobacteria (-rhizobia) are very similar to those of rhizobia from the-subclass (-rhizobia), strongly supporting the hypothesis of the unique origin of common nod genes. The-rhizobial nod genes are located on a 0.5-Mb plasmid, together with the nifH gene, in R. taiwanensis and Burkholderia phymatum. Phylogenetic analysis of available nodA gene sequences clustered-rhizobial sequences in two nodA lineages intertwined with-rhizobial sequences. On the other hand, the-rhizobia were grouped with free-living nitrogen-fixing-proteobacteria on the basis of the nifH phylogenetic tree. These findings suggest that-rhizobia evolved from diazotrophs through multiple lateral nod gene transfers

Surface polysaccharide involvement in establishing the rhizobium-legume symbiosis.

Abstract: When the rhizosphere is nitrogen-starved, legumes and rhizobia (soil bacteria) enter into a symbiosis that enables the fixation of atmospheric dinitrogen. This implies a complex chemical dialogue between partners and drastic changes on both plant roots and bacteria. Several recent works pointed out the importance of rhizobial surface polysaccharides in the establishing of the highly specific symbiosis between symbionts. Exopolysaccharides appear to be essential for the early infection process. Lipopolysaccharides exhibit specific roles in the later stages of the nodulation processes such as the penetration of the infection thread into the cortical cells or the setting up of the nitrogen-fixing phenotype. More generally, even if active at different steps of the establishing of the symbiosis, all the polysaccharide classes seem to be involved in complex processes of plant defense inhibition that allow plant root invasion. Their chemistry is important for structural recognition as well as for physico-chemical properties.

Endophytic colonization of plant roots by nitrogen-fixing bacteria

Abstract: Nitrogen-fixing bacteria are able to enter into roots from the rhizosphere, particularly at the base of emerging lateral roots, between epidermal cells and through root hairs. In the rhizosphere growing root hairs play an important role in symbiotic recognition in legume crops. Nodulated legumes in endosymbiosis with rhizobia are amongst the most prominent nitrogen-fixing systems in agriculture. The inoculation of non-legumes, especially cereals, with various non-rhizobial diazotrophic bacteria has been undertaken with the expectation that they would establish themselves intercellularly within the root system, fixing nitrogen endophytically and providing combined nitrogen for enhanced crop production. However, in most instances bacteria colonize only the surface of the roots and remain vulnerable to competition from other rhizosphere micro-organisms, even when the nitrogen-fixing bacteria are endophytic, benefits to the plant may result from better uptake of soil nutrients rather than from endophytic nitrogen fixation. Azorhizobium caulinodans is known to enter the root system of cereals, other non-legume crops and Arabidopsis, by intercellular invasion between epidermal cells and to internally colonize the plant intercellularly, including the xylem. This raises the possibility that xylem colonization might provide a non-nodular niche for endosymbiotic nitrogen fixation in rice, wheat, maize, sorghum and other non-legume crops. A particularly interesting, naturally occurring, non-nodular xylem colonising endophytic diazotrophic interaction with evidence for endophytic nitrogen fixation is that of Gluconacetobacter diazotrophicus in sugarcane. Could this beneficial endophytic colonization of sugarcane by G. diazotrophicus be extended to other members of the Gramineae, including the major cereals, and to other major non-legume crops of the World?

Quorum Sensing in Nitrogen-Fixing Rhizobia

Abstract: Members of the rhizobia are distinguished for their ability to establish a nitrogen-fixing symbiosis with leguminous plants. While many details of this relationship remain a mystery, much effort has gone into elucidating the mechanisms governing bacterium-host recognition and the events leading to symbiosis. Several signal molecules, including plant-produced flavonoids and bacterially produced nodulation factors and exopolysaccharides, are known to function in the molecular conversation between the host and the symbiont. Work by several laboratories has shown that an additional mode of regulation, quorum sensing, intercedes in the signal exchange process and perhaps plays a major role in preparing and coordinating the nitrogen-fixing rhizobia during the establishment of the symbiosis. Rhizobium leguminosarum, for example, carries a multitiered quorum-sensing system that represents one of the most complex regulatory networks identified for this form of gene regulation. This review focuses on the recent stream of information regarding quorum sensing in the nitrogen-fixing rhizobia. Seminal work on the quorum-sensing systems of R. leguminosarum bv. viciae, R. etli, Rhizobium sp. strain NGR234, Sinorhizobium meliloti, and Bradyrhizobium japonicum is presented and discussed. The latest work shows that quorum sensing can be linked to various symbiotic phenomena including nodulation efficiency, symbiosome development, exopolysaccharide production, and nitrogen fixation, all of which are important for the establishment of a successful symbiosis. Many questions remain to be answered, but the knowledge obtained so far provides a firm foundation for future studies on the role of quorum-sensing mediated gene regulation in host-bacterium interactions.

Biochemistry and Molecular Biology of Antioxidants in the Rhizobia-Legume Symbiosis

Abstract: The complete reduction of molecular oxygen to water requires four electrons and is catalyzed by cytochrome oxidase in aerobic bacteria and mitochondria. However, 1% to 3% of all oxygen consumed by respiration is inevitably reduced to superoxide radicals and hydrogen peroxide (H2O2). These and other

Defining new roles for plant and rhizobial molecules in sole and mixed plant cultures involving symbiotic legumes

Abstract: Summary The view that symbiotic legumes benefit companion and subsequent plant species in intercrop and rotation systems is well accepted. However, the major contributions made separately by legumes and their microsymbionts that do not relate to root-nodule N2 fixation have been largely ignored. Rhizobia (species of Rhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium, Sinorhizobium and Mesorhizobium) produce chemical molecules that can influence plant development, including phytohormones, lipo-chito-oligosaccharide Nod factors, lumichrome, riboflavin and H2 evolved by nitrogenase. When present in soil, Nod factors can stimulate seed germination, promote plant growth and increase grain yields of legume and nonlegume crops, as well as stimulate increased photosynthetic rates following plant leaf spraying. Very low concentrations of lumichrome and H2 released by bacteroids also promote plant growth and increase biomass in a number of plant species grown under field and glasshouse conditions. Rhizobia are known to suppress the population of soil pathogens in agricultural and natural ecosystems and, in addition to forming nodule symbioses with rhizobia, the legume itself releases phenolics that can suppress pathogens and herbivores, solubilize nutrients, and promote growth of mutualistic microbes. Phytosiderophores and organic acid anions exuded by the host plant can further enhance mineral nutrition in the system. This review explores new insights into sole and mixed plant cultures with the aim of identifying novel roles for molecules of legume and microbial origin in natural and agricultural ecosystems.

Contribution of biological nitrogen fixation to cowpea: a strategy for improving grain yield in the semi-arid region of Brazil

Abstract: Nodulating bacteria from the family Rhizobiaceae are common in the semi-arid tropics around the world. The Brazilian semi-arid region extends over 95 million hectares of which only 3% is suitable for irrigation, therefore leaving an immense dryland area to be exploited by peasant farmers, who often lack appropriate technologies for sustainable management. Cowpea is an important crop in this area, representing the staple protein source for human nutrition. This work aimed to identify rhizobial strains capable of guaranteeing sufficient nitrogen derived from biological fixation for cowpea cultivated in dryland areas, evaluating not just efficiency but also the ecological parameters of competitiveness and survival in the soil. Grain yield and nodulation parameters showed that strain BR 3267 is capable of establishing efficient nodulation, improving both yield and total N accumulated in grain. Cowpea inoculated with strain BR 3267 showed grain productivity similar to plants receiving 50 kg of N per hectare, which is the amount of fertilizer commonly used in the north-east region. These characteristics associated with previously determined ecological properties makes strain BR 3267 an important resource for the optimization of biological nitrogen fixation in cowpea in the dryland areas of the semi-arid tropics. Data on the dynamics of rhizobial populations in such areas have shown that (1) the naturalized rhizobium population is very small and, by themselves, do not promote proper nodulation and, (2) the inoculant rhizobia do not persist between crops. Such characteristics represent an opportunity for the introduction of superior rhizobia strains, such as BR 3267, during the cowpea crop.

Benefits of inoculation of the common bean (Phaseolus vulgaris) crop with efficient and competitive Rhizobium tropici strains

Abstract: Cropping in low fertility soils, especially those poor in N, contributes greatly to the low common bean (Phaseolus vulgaris L.) yield, and therefore the benefits of biological nitrogen fixation must be intensively explored to increase yields at a low cost. Six field experiments were performed in oxisols of Parana State, southern Brazil, with a high population of indigenous common bean rhizobia, estimated at a minimum of 103 cells g–1 soil. Despite the high population, inoculation allowed an increase in rhizobial population and in nodule occupancy, and further increases were obtained with reinoculation in the following seasons. Thus, considering the treatments inoculated with the most effective strains (H 12, H 20, PRF 81 and CIAT 899), nodule occupancy increased from an average of 28% in the first experiment to 56% after four inoculation procedures. The establishment of the selected strains increased nodulation, N2 fixation rates (evaluated by total N and N-ureide) and on average for the six experiments the strains H 12 and H 20 showed increases of 437 and 465 kg ha–1, respectively,in relation to the indigenous rhizobial population. A synergistic effect between low levels of N fertilizer and inoculation with superior strains was also observed, resulting in yield increases in two other experiments. The soil rhizobial population decreased 1 year after the last cropping, but remained high in the plots that had been inoculated. DGGE analysis of soil extracts showed that the massive inoculation apparently did not affect the composition of the bacterial community.

The mosaic structure of the symbiotic plasmid of Rhizobium etli CFN42 and its relation to other symbiotic genome compartments

Abstract: Background: Symbiotic bacteria known as rhizobia interact with the roots of legumes and induce the formation of nitrogen-fixing nodules. In rhizobia, essential genes for symbiosis are compartmentalized either in symbiotic plasmids or in chromosomal symbiotic islands. To understand the structure and evolution of the symbiotic genome compartments (SGCs), it is necessary to analyze their common genetic content and organization as well as to study their differences. To date, five SGCs belonging to distinct species of rhizobia have been entirely sequenced. We report the complete sequence of the symbiotic plasmid of Rhizobium etli CFN42, a microsymbiont of beans, and a comparison with other SGC sequences available. Results: The symbiotic plasmid is a circular molecule of 371,255 base-pairs containing 359 coding sequences. Nodulation and nitrogen-fixation genes common to other rhizobia are clustered in a region of 125 kilobases. Numerous sequences related to mobile elements are scattered throughout. In some cases the mobile elements flank blocks of functionally related sequences, thereby suggesting a role in transposition. The plasmid contains 12 reiterated DNA families that are likely to participate in genomic rearrangements. Comparisons between this plasmid and complete rhizobial genomes and symbiotic compartments already sequenced show a general lack of synteny and colinearity, with the exception of some transcriptional units. There are only 20 symbiotic genes that are shared by all SGCs. Conclusions: Our data support the notion that the symbiotic compartments of rhizobia genomes are mosaic structures that have been frequently tailored by recombination, horizontal transfer and

Compatibility of rhizobial genotypes within natural populations of Rhizobium leguminosarum biovar viciae for nodulation of host legumes

Abstract: Populations of Rhizobium leguminosarum biovar viciae were sampled from two bulk soils, rhizosphere, and nodules of host legumes, fava bean (Vicia faba) and pea (Pisum sativum) grown in the same soils. Additional populations nodulating peas, fava beans, and vetches (Vicia sativa) grown in other soils and fava bean-nodulating strains from various geographic sites were also analyzed. The rhizobia were characterized by repetitive extragenomic palindromic-PCR fingerprinting and/or PCR-restriction fragment length polymorphism (RFLP) of 16S-23S ribosomal DNA intergenic spacers as markers of the genomic background and PCR-RFLP of a nodulation gene region, nodD, as a marker of the symbiotic component of the genome. Pairwise comparisons showed differences among the genetic structures of the bulk soil, rhizosphere, and nodule populations and in the degree of host specificity within the Vicieae cross-inoculation group. With fava bean, the symbiotic genotype appeared to be the preponderant determinant of the success in nodule occupancy of rhizobial genotypes independently of the associated genomic background, the plant genotype, and the soil sampled. The interaction between one particular rhizobial symbiotic genotype and fava bean seems to be highly specific for nodulation and linked to the efficiency of nitrogen fixation. By contrast with bulk soil and fava bean-nodulating populations, the analysis of pea-nodulating populations showed preferential associations between genomic backgrounds and symbiotic genotypes. Both components of the rhizobial genome may influence competitiveness for nodulation of pea, and rhizosphere colonization may be a decisive step in competition for nodule occupancy.

Transcriptome analysis of Sinorhizobium meliloti during symbiosis.

Abstract: Background: Rhizobia induce the formation on specific legumes of new organs, the root nodules, as a result of an elaborated developmental program involving the two partners. In order to contribute to a more global view of the genetics underlying this plant-microbe symbiosis, we have mined the recently determined Sinorhizobium meliloti genome sequence for genes potentially relevant to symbiosis. We describe here the construction and use of dedicated nylon macroarrays to study simultaneously the expression of 200 of these genes in a variety of environmental conditions, pertinent to symbiosis. Results: The expression of 214 S. meliloti genes was monitored under ten environmental conditions, including free-living aerobic and microaerobic conditions, addition of the plant symbiotic elicitor luteolin, and a variety of symbiotic conditions. Five new genes induced by luteolin have been identified as well as nine new genes induced in mature nitrogen-fixing bacteroids. A bacterial and a plant symbiotic mutant affected in nodule development have been found of particular interest to decipher gene expression at the intermediate stage of the symbiotic interaction. S. meliloti gene expression in the cultivated legume Medicago sativa (alfalfa) and the model plant M. truncatula were compared and a small number of differences was found. Conclusions: In addition to exploring conditions for a genome-wide transcriptome analysis of the model rhizobium S. meliloti, the present work has highlighted the differential expression of several classes of genes during symbiosis. These genes are related to invasion, oxidative stress protection, iron mobilization, and signaling, thus emphasizing possible common mechanisms between symbiosis and pathogenesis.

Legume seed flavonoids and nitrogenous metabolites as signals and protectants in early seedling development.

Abstract: Flavonoids and nitrogenous metabolites such as alkaloids, terpenoids, peptides and amino acids are major components of plant seeds. Conjugated forms of these compounds are soluble in water, and therefore, are easily released as chemical signals following imbibition. Once in the soil, these metabolites are first in line to serve as eco-sensing signals for suitable rhizobia and arbuscular mycorrhizal (AM) fungal partners required for the establishment of symbiotic mutualisms. They may also serve as defence molecules against pathogens and insect pests, as well as playing a role in the control of parasitic members of the family Scrophulariaceae, especially Striga, a major plant pest of cereal crops in Africa. Seed metabolites such as flavonoids, alkaloids, terpenoids, peptides and amino acids define seedling growth and, ultimately, crop yields. Thus, an improvement in our understanding of seed chemistry would permit manipulation of these molecules for effective control of pathogens, insect pests, Striga and destructive weeds, as well as for enhanced acquisition of N and P via symbioses with soil rhizobia and AM fungi.

Suppression of arbuscular mycorrhizal colonization and nodulation in split-root systems of alfalfa after pre-inoculation and treatment with Nod factors.

Abstract: Roots of legumes establish symbiosis with arbuscular mycorrhizal fungi (AMF) and nodule-inducing rhizobia. The existing nodules systemically suppress subsequent nodule formation in other parts of the root, a phenomenon termed autoregulation. Similarly, mycorrhizal roots reduce further AMF colonization on other parts of the root system. In this work, splitroot systems of alfalfa (Medicago sativa) were used to study the autoregulation of symbiosis with Sinorhizobium meliloti and the mycorrhizal fungus Glomus mosseae. It is shown that nodulation systemically influences AMF root colonization and vice versa. Nodules on one half of the split-root system suppressed subsequent AMF colonization on the other half. Conversely, root systems pre-colonized on one side by AMF exhibited reduced nodule formation on the other side. An inhibition effect was also observed with Nod factors (lipo-chito-oligosaccharides). NodSm-IV(C16:2, S) purified from S. meliloti systemically suppressed both nodule formation and AMF colonization. The application of Nod factors, however, did not influence the allocation of 14 C within the split-root system, excluding competition for carbohydrates as the regulatory mechanism. These results indicate a systemic regulatory mechanism in the rhizobial and the arbuscular mycorrhizal association, which is similar in both symbioses.

Genetic resources for improving nitrogen fixation in legume-rhizobia symbiosis

Abstract: Leguminous crops are genetically polymorphous for the balance between symbiotrophic and combined types of nitrogen nutrition. In pea, polebean, alfalfa and fenugreek the wild-growing populations and local varieties exceed the agronomically advanced cultivars in the activity of N2 fixation that occurs in symbiosis with nodule bacteria (rhizobia). Combined nitrogen nutrition ensures higher productivity than symbiotrophic one in the “old” leguminous crops (pea, alfalfa, common vetch, polebean, soybean), while the symbiotrophic type dominates in some “young” crops (hairy vetch, kura clover, goat's rue). An importance is emphasized of using the symbiotically active wild-growing genotypes as the initial material for breeding the legume cultivars. The data on high heritability (broad sense, narrow sense, realized) of the legume symbiotic activity demonstrate that the plant selection for this activity may be highly effective. A range of methods to select the legumes for an improved symbiotic activity is available including plant growth in N-depleted substrates, analysis of nodulation scores, direct (“isotopic”) and indirect (acetylene reduction) estimation of nitrogenase activity. Analysis of the specificity of interactions between different plant genotypes and bacterial strains (via two-factor analysis of variance) demonstrates the strain-specific plant polygenes are of a special importance in controlling the intensity of nitrogen fixation. Therefore, a coordinated plant-bacteria breeding is required to create the optimal combinations of partners' genotypes. Selection and genetic construction of the commercially attractive rhizobia strains should involve improvement of nitrogen fixing, nodulation and competitive abilities expressed in combination with the symbiotically active plant genotypes, Breeding of the leguminous crops for the preferential nodulation by highly active rhizobia strains, for the ability to support N2 fixation under moderate N fertilization levels and to ensure a sufficient energy supply of symbiotrophic nitrogen nutrition is required

Endophytic nifH gene diversity in African sweet potato.

Abstract: A cultivation-independent approach was used to identify potentially nitrogen-fixing endophytes in seven sweet potato varieties collected in Uganda and Kenya. Nitrogenase reductase genes (nifH) were amplified by PCR, and amplicons were cloned in Escherichia coli. Clones were grouped by restriction fragment length polymorphism analysis, and representative nifH genes were sequenced. The resulting sequences had high homologies to nitrogenase reductases from alpha-, beta-, and gamma-Proteobacteria and low G+C Gram positives, however, about 50% of the sequences derived from rhizobia. Several highly similar or even identical nitrogenase reductase sequences clustering with different bacterial genera and species, including Sinorhizobium meliloti, Rhizobium sp. NGR234, Rhizobium etli, Klebsiella pneumoniae, and Paenibacillus odorifer, could be detected in different plants grown in distinct geographic locations. This suggests that these bacterial species preferentially colonize African sweet potato as endophytes and that the diazotrophic, endophytic microflora is determined only to a low degree by the plant genotype or the soil microflora.

Distribution and diversity of rhizobia nodulating agroforestry legumes in soils from three continents in the tropics.

Abstract: The natural rhizobial populations of Calliandra calothyrsus, Gliricidia sepium, Leucaena leucocephala and Sesbania sesban were assessed in soils from nine sites across tropical areas of three continents. The rhizobial population size varied from undetectable numbers to 1.8 x 104 cells/g of soil depending on the trap host and the soil. Calliandra calothyrsus was the most promiscuous legume, nodulating in eight soils, while S. sesban nodulated in only one of the soils. Polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) analyses of the 16S rRNA gene and the internally transcribed spacer (ITS) region between the 16S and 23S rRNA genes were used to assess the diversity and relative abundance of rhizobia trapped from seven of the soils by C. calothyrsus, G. sepium and L. leucocephala. Representatives of the 16S rRNA RFLP groups were also subjected to sequence analysis of the first 950 base pairs of the 16S rRNA gene. Eighty ITS groups were obtained, with none of the ITS types being sampled in more than one soil. RFLP analysis of the 16S rRNA yielded 23 'species' groups distributed among the Rhizobium, Mesorhizobium, Sinorhizobium and Agrobacterium branches of the rhizobial phylogenetic tree. The phylogeny of the isolates was independent of the site or host of isolation, with different rhizobial groups associated with each host across the soils from widely separated geographical regions. Although rhizobial populations in soils sampled from the centre of diversity of the host legumes were the most genetically diverse, soil acidity was highly correlated with the diversity of ITS types. Our results support the hypothesis that the success of these tree legumes in soils throughout the tropics is the result of their relative promiscuity (permissiveness) allowing nodulation with diverse indigenous rhizobial types.

Signals exchanged between legumes and Rhizobium: agricultural uses and perspectives

Abstract: Legumes and rhizobia exchange at least three different, but sometimes complementary sets of signals. Amongst the variety of substances normally and continuously secreted into the rhizosphere by plants are phenolic compounds. Flavonoid components of these mixtures are especially active in inducing rhizobial nodulation genes. Many nod-genes exist. Some (e.g., nodD) serve as regulators of transcription, but most code for enzymes involved in the synthesis of a family of lipo-chito-oligosaccharides (LCOs) called Nod-factors. Nod-factors possess hormone-like properties, are key determinants in nodulation, and allow rhizobia to enter the plant. As Nod-factors also stimulate the synthesis and release of flavonoids from legume roots, the response to inoculation is amplified. Once the bacteria enter the plant, other sets of signals are exchanged between the symbionts. These include extra-cellular polysaccharides (EPSs) as well as proteins externalised via type-three secretion systems. These carbohydrates/proteins may be active in invasion of the root. At the time of writing, only flavonoids and Nod-factors have been chemically synthesised and of these only the former are available in large quantities. Field trials in North America show that seed application of flavonoids stimulates nodulation and nitrogen fixation in soybeans grown at low soil temperatures. The biological basis to these responses is discussed.

Rhizobia as a biological control agent against soil borne plant pathogenic fungi.

Abstract: Rhizobia promote the growth of plants either directly through N2 fixation, supply of nutrients, synthesis of phytohormones and solubilization of minerals, or indirectly as a biocontrol agent by inhibiting the growth of pathogens. The biocontrol effect of rhizobia is due to the secretion of secondary metabolites such as antibiotics and HCN. Siderophore production in iron stress conditions provides rhizobia an added advantage, resulting in exclusion of pathogens due to iron starvation.

Analysis of the legume-rhizobia symbiosis in shrubs from central western Spain.

Abstract: S. R ODRIGUEZ-ECHEVERRIA, M.A. P EREZ-FERNAN D E Z , S. V L A A R A N D T. F I N N A N. 2003. Aims: This work analyses the diversity of rhizobia associated with some of the predominant shrubby legumes in central-western Spain. Symbiotic promiscuity and effectiveness were studied using cross-inoculation experiments with shrubby species. Material and Results: Six new bradyrhizobia strains were isolated from nodules collected from wild plants of six leguminous species, Cytisus balansae, C. multiflorus, C. scoparius, C. striatus, Genista hystrix and Retama sphaerocarpa. These isolates were genetically characterized by 16S rDNA partial sequencing and random amplification of polymorphic DNA-PCR fingerprinting. The phylogenetic analysis revealed that these isolates could represent three new Bradyrhizobium species. Shrubby legumes and bradyrhizobia displayed a high symbiotic promiscuity both for infectivity and effectiveness. Symbioses were effective in more than 70% of the associations established by four of the six plant species. Conclusions: Native woody legumes in western Spain are nodulated by Bradyrhizobium strains. The high degree of symbiotic promiscuity and effectiveness highlights the complex dynamics of these communities in wild ecosystems under a Mediterranean-type climate. Furthermore, the results from this study suggest a potential importance of inoculation for these legume species in soil-restoration projects. Significance and Impact of the Study: This is the first study, to our knowledge, that combines both molecular analysis and pot trials to study the rhizobia-legume symbiosis for wild legumes.

Symbiosis, Inventiveness by Recruitment?

Abstract: With one notable exception, namely the genus Parasponia in the elm family, the ability to form nitrogen-fixing symbiosis with gram-negative soil bacteria known as rhizobia is restricted to the legume family, Leguminosae. It has been well established that initiation of successful nodular symbiosis

Conservation and divergence of signalling pathways between roots and soil microbes — the Rhizobium-legume symbiosis compared to the development of lateral roots, mycorrhizal interactions and nematode-induced galls

Abstract: This review compares endophytic symbiotic and pathogenic root—microbe interactions and examines how the development of root structures elicited by various micro-organisms could have evolved by recruitment of existing plant developmental pathways. Plants are exposed to a multitude of soil micro-organisms which affect root development and performance. Their interactions can be of symbiotic and pathogenic nature, both of which can result in the formation of new root structures — how does the plant regulate the different outcomes of interactions with microbes? The idea that pathways activated in plant by micro-organisms could have been ‘hijacked’ from plant developmental pathways is not new, it was essentially proposed by P. S. Nutman in 1948, but at that time, the molecular evidence to support that hypothesis was missing. Genetic evidence for overlaps between different plant—microbe interactions have previously been examined. This review compares the physiological and molecular plant responses to symbiotic rhizobia with those to arbuscular mycorrhizal fungi, pathogenic nematodes and the development of lateral roots and summarises evidence from both molecular and cellular studies for substantial overlaps in the signalling pathways underlying root—micro-organism interactions. A more difficult question has been why plant responses to micro-organisms are so similar, even though the outcomes are very different. Possible hypotheses for divergence of signalling pathways and future approaches to test these ideas are presented.

Can introduced and indigenous rhizobial strains compete for nodule formation by promiscuous soybean in the moist savanna agroecological zone of Nigeria

Abstract: Promiscuous soybean lines have been bred on the basis that they would nodulate freely without artificial inoculation. However, our recent studies have demonstrated that the indigenous rhizobia are not able to meet their full nitrogen (N) requirement. Rhizobia inoculation might be necessary. We examined the competition for nodule formation among native Rhizobia spp. and two inoculated Bradyrhizobia strains (R25B indigenous strain and a mixture of R25B+IRj 2180A indigenous strain from soybean lines in the savanna of northern Nigeria), their effect on N fixation, and their contribution to the yield of four soybean cultivars, grown in the field in three different agroecological zones in the moist savanna of Nigeria. About 34% of nodules were formed by the mixture of introduced R25B+IRj 2180A, while R25B formed only about 24% of the nodules but did not influence biomass and grain yield production. The indigenous rhizobia strains that nodulated the soybean varieties fixed up to 70% of their accumulated total N, confirming the promiscuous nature of these soybean varieties. Even though these varieties fixed about 75 kg N ha−1; this was not enough to sustain their optimum grain yield, as earlier reported. However, the grain yield from inoculated soybean was not significantly higher than that from the uninoculated soybean, showing a degree of competitiveness among the introduced rhizobial strains and the native rhizobia population.

Contribution of pilA to Competitive Colonization of the Squid Euprymna scolopes by Vibrio fischeri

Abstract: All animals and plants are hosts to a native microbiota. These microbial symbionts often contribute to the normal health and development of their respective hosts in exchange for a relatively privileged niche. Many plant and animal symbionts are transmitted horizontally, through the environment after embryogenesis, and are not directly inherited through germplasm from the previous generation. Therefore, with each host generation, environmental bacteria must compete to colonize the empty niches within new host individuals. In perhaps the best-studied example of such competition, mixed communities of rhizobia compete for access to the roots and nodules of leguminous plants. Many complex traits, including antibiotic production, motility, and specific cell surface attributes contribute to the nodulation competitiveness of rhizobial strains (3, 9, 41, 46). Less is known about the competition between environmental bacteria for colonization of animal hosts, especially the natural modes of infection by the native, nonpathogenic microbiota. The light organ symbiosis between the luminescent bacterium Vibrio fischeri and the nocturnal Hawaiian squid Euprymna scolopes serves as a model chronic, mutualistic association between extracellular bacteria and animal epithelia (32, 43). At the time of hatching, the light organs of juvenile squid are not colonized by V. fischeri but they rapidly acquire these symbionts from the surrounding seawater (28). The nascent light organ possesses elaborate ciliated appendages that entrain and concentrate planktonic V. fischeri in a mucus matrix, facilitating initial colonization (31). During this process, motile V. fischeri cells move from pores on the surface of the light organ, through ducts, into epithelium-lined crypt spaces (12, 31). Within hours after infection by V. fischeri, the squid also display a diurnal behavior in which they expel approximately 90% of their light organ symbionts each morning and supply the remaining bacteria with sufficient nutrients to repopulate the light organ by nightfall (6, 13). Therefore, V. fischeri strains may compete during initial entry into the light organ, as well as during subsequent daily expulsion and regrowth. Increased populations of V. fischeri in squid habitats appear to be a direct result of occupation of this symbiotic niche (24), illustrating an ecological advantage for strains that compete effectively for light organ colonization. Although most, if not all, wild-type V. fischeri isolates can colonize E. scolopes, isolates native to this host outcompete isolates from other sources for light organ colonization in mixed inoculations. For example, when presented to E. scolopes juveniles in equal ratios, strain ES114, which was isolated from E. scolopes, was more than 10-fold more competitive than strains EM17 and ET101, which were isolated from light organs of Euprymna morsei and Euprymna tasmanica, respectively (30). V. fischeri isolates not associated with squid may be even less competitive for colonization of E. scolopes (22). It is likely that several factors contribute to colonization competitiveness, and mutant analyses provide a means by which to reveal them. For example, Visick and Ruby demonstrated that a katA mutant, deficient in the production of a periplasmic catalase, was outcompeted by the parent strain (44). McFall-Ngai and colleagues found that addition of a mannose analog to seawater, or treatment of V. fischeri with extracellular protease, could partially block infection of E. scolopes and suggested that a V. fischeri mannose-binding cell surface protein contributes to the initiation of the V. fischeri-E. scolopes light organ symbiosis (16, 27). We therefore became interested in testing the role of cell surface molecules in colonization and colonization competitiveness. In this paper, we report the cloning and characterization of V. fischeri pilA, a gene that encodes a protein with similarity to type IV-A pilins and that contributes to the competitiveness of V. fischeri during colonization of the E. scolopes light organ.