Indigenous Rhizobial Populations Research Articles

Evaluation of Rhizobium along with Multifunctional Rhizobacteria for Improving the Growth and Yield of Black Gram [Vigna mungo (L.) Hepper

Background: Indigenous rhizobial populations may be unable to achieve successful symbiosis in the field due to rare population and unfavourable soil environment. Introduced plant growth promoting rhizobacteria (PGPR) are frequently linked to faster growth, development, and yield of plant with synergetic effect on indigenous Rhizobium bacteria. Furthermore, co-inoculating legumes with both Rhizobium and PGPR is proven more effective for improving nodulation as well as growth and development of legumes. Methods: The experiment was conducted at research farm of TCA, Dholi Muzaffarpur a campus of Dr. Rajendra Prasad Central Agriculture University, Pusa, Bihar during kharif season of 2021 to investigate the “Evaluation of Rhizobium along with multifunctional Rhizobacteria for improving the growth and yield of black gram [Vigna mungo (L.) Hepper]”. There were eight number of treatments and replicated thrice in a randomized block design. The treatment comprised as T1- control, T2- Rhizobium (Bradyrhizobium spp.), T3- Rhizobium + PUK-171 (Stenotrophomonas rhizophila), T4- Rhizobium + NE 10 (Bacillus cereus), T5-Rhizobium + Pseudomonas spp., T6-Rhizobium + LMSR 45 (Pantoea agglomerans), T7-Rhizobium + Methylobacterium, T8-N as per RDF. Result: The result revealed that co-inoculation of Rhizobium + PUK-171 (Stenotrophomonas rhizophila) significantly increase the plant height, number of leaves plant-1, number of branches plant‑1, plant dry weight, crop growth rate, and in nodule studies number of nodules plant-1, dry weight of nodules recoded significantly higher result as compare to T1, T2, and T8 and found at par with rest of treatment. In yield attributing character number of pods plant-1 recorded significantly higher result in co-inoculation of Rhizobium + PUK-171 (Stenotrophomonas rhizophila) and rest of character did not produce significant effect, in case of seed yield similar trends was followed, while in case of stover yield co-inoculation of Rhizobium along with Rhizobium + NE 10 (Bacillus cereus) recorded significantly higher result as compare to T1 and T8 and at par with rest of treatments. Harvest index did not produce significant effect.

Isolation, characterization and selection of indigenous Bradyrhizobium strains with outstanding symbiotic performance to increase soybean yields in Mozambique.

Soybean inoculation with effective rhizobial strains makes unnecessary the use of N-fertilizers in the tropics. A frequently reported problem is the failure of the inoculant strains to overcome the competition imposed by indigenous rhizobial populations. The screening of indigenous rhizobia, already adapted to local conditions, searching for highly effective strains for use as inoculants represents a promising strategy in overcoming inoculation failure. The objective of this study was to isolate and characterize indigenous rhizobia and to identify strains that hold potential to be included in inoculant formulations for soybean production, with both promiscuous and non-promiscuous soybean cultivars, in Mozambican agro-climatic conditions. A total of 105 isolates obtained from nodules of promiscuous soybean grown at 15 sites were screened for N2-fixation effectiveness in the greenhouse along with five commercial strains. Eighty-seven isolates confirmed the ability to form effective nodules on soybean and were used for genetic characterization by rep-PCR (BOX) and sequencing of the 16S rRNA gene, and also for symbiotic effectiveness. BOX-PCR fingerprinting revealed remarkable genetic diversity, with 41 clusters formed, considering a similarity level of 65%. The 16S rRNA analysis assigned the isolates to the genera Bradyrhizobium (75%) and Agrobacterium/Rhizobium (25%). Great variability in symbiotic effectiveness was detected among the indigenous rhizobia from Mozambique, with ten isolates performing better than the commercial strain B. diazoefficiens USDA 110, the best reference strain, and 51 isolates with lower performance than all reference strains. Thirteen of the best isolates from the first greenhouse trial were evaluated, along with the five commercial strains, in two promiscuous (TGx 1963-3F and TGx 1835-10E) and one non-promiscuous (BRS 284) soybean cultivars in a second greenhouse trial. In general the promiscous soybeans responded better to inoculation. The 13 isolates were also characterized for tolerance to acidity and alkalinity (pH 3.5 and 9.0, respectively), salinity (0.1, 0.3 and 0.5molL-1 of NaCl) and high temperatures (35, 40 and 45°C) in vitro. Five isolates, three (Moz 4, Moz 19 and Moz 22) belonging to the superclade B. elkanii and two (Moz 27 and Moz 61) assigned to the superclade B. japonicum, consistently showed high symbiotic effectiveness, suggesting that the inoculation with indigenous rhizobia adapted to local conditions represents a possible strategy for increasing soybean yields in Mozambique. Phylogenetic position of the five elite isolates was confirmed by the MLSA with four protein-coding housekeeping genes, dnaK, glnII, gyrB and recA.

Inorganic nitrogen application improves the yield and yield traits of common bean (Phaseolus vulgaris L.) irrespective of the indigenous rhizobial population

The effect of number and efficiency of indigenous rhizobia on the need for external inorganic nitrogen (N) application to boost common bean production is unknown. Hence, field experiments were conducted at four locations with soil populations of <100, 100–1 000 and >1 000 rhizobia to evaluate the effect of the indigenous rhizobial population on N requirement of common bean in eastern Ethiopia. The treatments comprised six levels of N application arranged in a randomised complete block design with three replications. The results showed decreases in nodule number (NN) and nodule dry weight (NDW) with increasing rate of inorganic N application in all soil types. Application of 20 kg N ha−1 resulted in a significant increase in the NN and NDW in soil with a low and moderate rhizobial population but not in soils with a high rhizobial population. The highest yield and yield traits in soil containing a high and low rhizobial population was recorded with <40 and >60 kg N ha−1 application, respectively. The association between grain yield and N application was negative in soils with a high and moderate rhizobial population, and positive in soil containing a low rhizobial population. In general, the results revealed that the number of indigenous rhizobia could affect the quantity of N applied.

Evaluation of early maturing cowpea (Vigna unguiculata) germplasm for variation in phosphorus use efficiency and biological nitrogen fixation potential with indigenous rhizobial populations

SUMMARYCowpea genotypes that efficiently utilize phosphorus (P) with high potential for biological nitrogen (N) fixation (BNF) are vital to sustainable cropping systems in West Africa. A total of 175 early maturing cowpea genotypes were evaluated in 2010 and 2011 for P use efficiency (PUE) and BNF with an indigenous rhizobial population at Shika in the Northern Guinea savanna of Nigeria. There were significant genotypic variations for all 11 variables measured. The P utilization index, percentage N derived from the atmosphere and total N fixed ranged between 2·10–4·67, 31·3–61·86% and 11·86–50 kg/ha, respectively. The 175 early maturing cowpea genotypes were divided into five categories using principal component analysis (PCA), whereby total N fixed was associated with N and P uptake and plant biomass yield. Complete linkage cluster analysis revealed a total of three distinctive clusters having remarkable correspondence with the PCA. Some genotypes were identified as potential candidates for further breeding programmes using high PUE genotypes with relatively high capacity for BNF and indigenous rhizobial populations.

Indigenous rhizobia population influences the effectiveness of Rhizobium inoculation and need of inorganic N for common bean (Phaseolus vulgaris L.) production in eastern Ethiopia

Supplement with inorganic N application is essential to improve the common bean production in sub-Saharan Africa. However, the influence of indigenous rhizobial population on the inorganic N requirement with Rhizobium inoculation to secure sustainable way of common bean production system is not well known. The effect of different rates of N application either alone or in combination with Rhizobium inoculation on the nodulation, yield and yield traits of common bean cultivated in soils with different rhizobial population were conducted. Twelve treatments were produced by factorially combined six levels of N fertilizer (0, 20, 40, 60, 80 and 100 kg N ha−1) and two Rhizobium inoculation treatments (inoculated and uninoculated). The treatments were laid out in randomized completely block design and all treatments were replicated three times. Regardless of soil types, nodule number and nodule dry weight decreased with increasing rates of N application. 20 kg N ha−1 both alone and in combination with Rhizobium inoculation resulted in the largest nodulation in all soil types. The largest nodulation were induced in soil with large rhizobial population. Rhizobium inoculation significantly (P < 0.05) improved yield and yield traits of common bean. Moreover, our result revealed that the largest values of investigated traits were observed in inoculated treatment, as compared to the corresponding N rates of uninoculated treatments. The 20, 100 and 40 kg N ha−1 treatments resulted in significantly greater plant total tissue N at soil types with small, medium and large rhizobial population, respectively, as compared to unfertilized control. The highest total biomass yield (TBY) and grain yield (GY) at soil types with small and medium rhizobial population were obtained by the 100 kg N ha−1 treatment in combination with Rhizobium inoculation, while 20 and 40 kg N ha−1 applications produced the greatest TBY and GY, respectively, in soil with large rhizobial population. These results indicate that N requirement is varied based on rhizobial population and effectiveness of native rhizobia in N2 fixation.

Effects of elevated copper levels on biological nitrogen fixation and occurrence of rhizobia in a Tanzanian coffee-cropped soil

A study was conducted to investigate the effects of increasing copper concentration in soil on rhizobial occurrence and the process of biological nitrogen fixation. Two slow-growing bradyrhizobial strains CP1 and GM8 and two fast-growing rhizobial strains PV1 and PV2 were studied by comparing their performance under increasing copper concentrations in greenhouse-based assays involving modified Leonard jar assemblies and potted-soil experiments. Additionally, field samples from soils grown to coffee and subjected to long-term use of copper-based fungicides were analyzed for their total indigenous rhizobial populations using the most probable number-plant infection technique. Results indicated that elevated copper levels in the growth medium had inhibitory effects on nodulation, biological N2 fixation and overall rhizobial numbers in soil. Significant (p= 0.05%) reductions in fresh nodule mass, fresh nodule volume and total shoot nitrogen were recorded when copper concentration was increased from 0 to 100 ppm in both modified Leonard jar assemblies and potted-soil trials. Effective decrements in all the three parameters of fresh nodule mass, fresh nodule volume and total shoot nitrogen were more pronounced with the slow-growing bradyrhizobial strains of CP1 and GM8 than with the fast-growing PV1 and PV2. The MPN-plant infection technique results showed a non-significant (p=0.05) but substantial decrement in rhizobial and bradyrhizobial numbers when the copper-contaminated field soil (82.5 mgCu/g soil) was compared to a control soil (1.8 mgCu/g soil). The study concludes, therefore, that elevated levels of copper in soil could be harmful to free-living rhizobia and their abilities to fix N2 in respective symbiotic associations with legume species. Such negative effects were more pronounced in the slow-growing bradyrhizobial than rhizobial species used.

Diversity and stress tolerance in rhizobia from Parque Chaqueño region of Argentina nodulating Prosopis alba

The aim of this work was to investigate the genetic diversity, symbiotic effectiveness, drought tolerance, and indole acetic acid production of indigenous rhizobial populations in the Parque Chaqueño of Argentina able to nodulate Prosopis alba, the dominant forest tree of this region. The populations were sampled at five locations from the Arid, Semi-arid, and Humid Chaco in the Parque Chaqueño region. A set of rhizobial strains able to nodulate P. alba was obtained and selected based on their molecular diversity. Data obtained by BOX-PCR indicated that the highest molecular variability was observed in rhizobial isolates from Semi-arid Chaco. High level of indolic compound production and tolerance to osmotic treatment were significantly (p ≤ 0.05) correlated with water restrictions of the environments where the strains belonged. A small set of rhizobial strains that stimulate P. alba growth was selected from a large group of strains. The strains were identified by 16S rDNA sequencing as belonging to the genera Mesorhizobium, Bradyrhizobium, and Ensifer. To our knowledge, this is the first report of P. alba nodulation by strains other than Mesorhizobium chacoense, which was already described for the Parque Chaqueño.

Co-inoculation effects of 'Bradyrhizobium japonicum' and 'Azospirillum' sp. on competitive nodulation and rhizosphere eubacterial community structures of soybean under rhizobia-established soil conditions

Bradyrhizobial inoculants used for soybean seed inoculation to maximize the benefit of N2-fixation should include bradyrhizobial strain with high N2-fixation rates and ability to compete with the indigenous rhizobial populations. In this study, co-inoculation of plant growth promoting rhizobacteria (PGPR) Azospirillum sp. with either of Bradyrhizobium japonicum CB 1809 or USDA 110 increased shoot and root dry weight of soybean over non-inoculated control under pot condition with no indigenous soybean nodulating bradyrhizobia. Moreover, competition for nodulation and the effects on rhizosphere soil eubacterial community structures by using single or co-inoculation of B. japonicum and Azospirillum sp. under rhizobia-established Myanmar and Thailand soils were investigated. By inoculation of gus-marked USDA 110 singly or its co-inoculation gave 93.21 to 94.75% and 74.21 to 100% in nodule occupancy, and 23.50 to 41.95% and 50.37 to 73.24% promotion in biomass dry weight over non-inoculated control in Myanmar and Thailand soil samples, respectively. Each of all the tested inoculum levels, that is 106, 107 and 108 cfu/ml of Azospirillum sp. enhanced nodulation in combination with USDA 110 with a corresponding increase in 73.8, 62.25 and 95.34%; and 51.52, 62.38 and 79.46% over non-inoculated control, respectively in Myanmar and Thailand soil, respectively. In addition, soybean rhizosphere soil eubacterial community structures were not shifted by bacterial inoculation. Therefore, Azospirillum sp. could be used in co-inoculant production with B. japonicum for soybean. Keywords : Bradyrhizobium , plant growth promoting rhizobacteria (PGPR), soybean, co-inoculation, competition, rhizosphere eubacterial community structure African Journal of Biotechnology Vol. 12(20), pp. 2850-2862

Proteomic profiling of Rhizobium tropiciPRF 81: identification of conserved and specific responses to heat stress

BackgroundRhizobium tropici strain PRF 81 (= SEMIA 4080) has been used in commercial inoculants for application to common-bean crops in Brazil since 1998, due to its high efficiency in fixing nitrogen, competitiveness against indigenous rhizobial populations and capacity to adapt to stressful tropical conditions, representing a key alternative to application of N-fertilizers. The objective of our study was to obtain an overview of adaptive responses to heat stress of strain PRF 81, by analyzing differentially expressed proteins when the bacterium is grown at 28°C and 35°C.ResultsTwo-dimensional gel electrophoresis (2DE) revealed up-regulation of fifty-nine spots that were identified by MALDI-TOF/TOF-TOF. Differentially expressed proteins were associated with the functional COG categories of metabolism, cellular processes and signaling, information storage and processing. Among the up-regulated proteins, we found some related to conserved heat responses, such as molecular chaperones DnaK and GroEL, and other related proteins, such as translation factors EF-Tu, EF-G, EF-Ts and IF2. Interestingly, several oxidative stress-responsive proteins were also up-regulated, and these results reveal the diversity of adaptation mechanisms presented by this thermotolerant strain, suggesting a cross-talk between heat and oxidative stresses.ConclusionsOur data provide valuable protein-expression information relevant to the ongoing genome sequencing of strain PRF 81, and contributes to our still-poor knowledge of the molecular determinants of the thermotolerance exhibited by R. tropici species.

Molecular diversity of peanut‐nodulating rhizobia in soils of Argentina

RSalpha sequencing is a valuable tool for identification of bacterial strains, and for evaluating the genetic structure of indigenous rhizobial populations. The purpose of this study was to evaluate, qualitatively, the presence or absence of RSalpha fragment in peanut-nodulating strains isolated from plants grown at four sites in central Argentina. RSalpha fragment was found in only three of 26 indigenous strains, and in one of three inoculant strains analyzed. In contrast to results from studies of other symbiotic nitrogen-fixing bacteria, such as soybean-nodulating strains, no correlation was found between generation time and presence of RSalpha sequence. Phylogenetic analysis of the 16S rRNA gene sequence grouped peanut-nodulating strains into two clusters, Bradyrhizobium japonicum vs. B. elkanii, and showed divergence among strains positive for RSalpha sequence. Our results confirm the genetic diversity previously reported for various peanut-nodulating rhizobial strains, and indicate that the RSalpha fragment is not applicable as a marker or tool for competition assays at the field or ecological level.

Pretreatment of Clover Seeds with Nod Factors Improves Growth and Nodulation of Trifolium pratense

The increase in legume production in sustainable agriculture depends not only on the effectiveness of the selected nitrogen-fixing inoculants but also on their competitiveness in a soil environment containing an indigenous rhizobial population. In this study, we investigated the effect of pretreatment of red clover seeds with specific Nod factor (LCOs) on germination, growth, and nodulation of clover growing under sterile conditions and in the soil. We demonstrated that, although the symbiotic ability and competitiveness of the inoculant strain RtKO17 was not improved under competitive soil conditions, LCOs treatment of clover seeds significantly enhanced clover nodulation and growth of plants.

Molecular diversity, effectiveness and competitiveness of indigenous rhizobial population infecting mungbean Vigna radiata (L. Wilczek) under semi-arid conditions

Nodules from mungbean crop raised for the first time at Ram Dhan Singh (RDS) farm of Chaudhary Charan Singh (CCS) Haryana Agricultural University, Hisar were collected from 17 different locations. Twenty-five mungbean rhizobia were isolated and authenticated by plant infection test. DNA of all these rhizobia was extracted purified and amplified using enterobacterial repetitive intergenic consensus (ERIC) primers. All the mungbean rhizobial isolates were clustered into 4 groups at 65% of similarity and were further divided into 17 subclusters at 80% of similarity. All the 4 types of rhizobia were not present at any of the location and group 2 or 4 rhizobia were invariably present. Efficacy of these rhizobia in terms of nodulation, nitrogen uptake and chlorophyll a fluorescence was determined under pot culture conditions. Strain MB 307 showed maximum nitrogen uptake of 31.9 mg N plant(-1) followed by strain MB 1205, MB 1206(2), MB 308, MB 1524 and strain MB 1521 was found to be the least efficient in terms of N 2 fixation. Nodule occupancy by different rhizobia ranged from 5.5 to 40.3%. Most of the strains belonging to the 2nd group which clustered maximum number of strains were comparatively better competitors and formed 19.5-40.3% of the nodules and were also effective. Isolate MB 307, the most efficient strain, was found to have nodule occupancy of 31.5%. Such type of predominant, efficient and better competitor strains should be selected for enhancing nodule competitiveness.

Rhizobium leguminosarum bv. viciae genotypes interact with pea plants in developmental responses of nodules, roots and shoots.

The variability of the developmental responses of two contrasting cultivars of pea (Pisum sativum) was studied in relation to the genetic diversity of their nitrogen-fixing symbiont Rhizobium leguminosarum bv. viciae. A sample of 42 strains of pea rhizobia was chosen to represent 17 genotypes predominating in indigenous rhizobial populations, the genotypes being defined by the combination of haplotypes characterized with rDNA intergenic spacer and nodD gene regions as markers. We found contrasting effects of the bacterial genotype, especially the nod gene type, on the development of nodules, roots and shoots. A bacterial nod gene type was identified that induced very large, branched nodules, smaller nodule numbers, high nodule biomass, but reduced root and aerial part development. The plants associated with this genotype accumulated less N in shoots, but N concentration in leaves was not affected. The results suggest that the plant could not control nodule development sustaining the energy demand for nodule functioning and its optimal growth. The molecular and physiological mechanisms that may be involved are discussed.

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

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.

Construction and environmental release of a Sinorhizobium meliloti strain genetically modified to be more competitive for alfalfa nodulation.

Highly efficient nitrogen-fixing strains selected in the laboratory often fail to increase legume production in agricultural soils containing indigenous rhizobial populations because they cannot compete against these populations for nodule formation. We have previously demonstrated, with a Sinorhizobium meliloti PutA- mutant strain, that proline dehydrogenase activity is required for colonization and therefore for the nodulation efficiency and competitiveness of S. meliloti on alfalfa roots (J. I. Jiménez-Zurdo, P. van Dillewijn, M. J. Soto, M. R. de Felipe, J. Olivares, and N. Toro, Mol. Plant-Microbe Interact. 8:492-498, 1995). In this work, we investigated whether the putA gene could be used as a means of increasing the competitiveness of S. meliloti strains. We produced a construct in which a constitutive promoter was placed 190 nucleotides upstream from the start codon of the putA gene. This resulted in an increase in the basal expression of this gene, with this increase being even greater in the presence of the substrate proline. We found that the presence of multicopy plasmids containing this putA gene construct increased the competitiveness of S. meliloti in microcosm experiments in nonsterile soil planted with alfalfa plants subjected to drought stress only during the first month. We investigated whether this construct also increased the competitiveness of S. meliloti strains under agricultural conditions by using it as the inoculum in a contained field experiment at León, Spain. We found that the frequency of nodule occupancy was higher with inoculum containing the modified putA gene for samples that were analyzed after 34 days but not for samples that were analyzed later.

Response of field-grown bean ( Phaseolus vulgaris L.) to Rhizobium inoculation and nitrogen fertilization in two Cerrados soils

Most soils sown with field beans (Phaseolus vulgaris L.) contain indigenous rhizobia which might interfere with the establishment of inoculated strains. As a consequence, the benefits of bean inoculation are usually questioned, and the use of N fertilizer is gradually becoming a common practice. The present study had the objective of evaluating the effectiveness of inoculation and N fertilization in field soil with (site 1) and without (site 2) a previous bean-cropping history. At site 1, which had a rhizobial population of 7×102 cells g–1 soil, inoculation had no effect on nodulation or yield, whereas at site 2 (<10 cells g–1 soil) inoculation increased nodulation, nodule occupancy by the inoculated strain and grain yield. N fertilizer decreased nodulation at both sites, but increased grain yield at site 1 but not at site 2, indicating that the response to inoculation and N fertilization depends on the cropping history. When bean was cultivated for the first time, indigenous populations of rhizobia were low and high yields were accomplished solely with seed inoculation, with no further response to N fertilizer. In contrast, previous cultivation of bean increases soil rhizobia, preventing nodule formation by inoculated strains, and N fertilizer may be necessary for maximum yields. A significant interaction effect between N fertilizer and inoculation was detected for serogroup distribution only at site 2, with N fertilizer decreasing nodule occupancy by the inoculated strain and increasing the occurrence of indigenous strains. Consequently, although no benefits were obtained by the combination of inoculation and N fertilizer, this practice may be feasible with the selection of appropriate N-tolerant strains from the indigenous rhizobial population.

Field evaluation of lentil cultivars inoculated with Rhizobium leguminosarum bv. viciae strains for nitrogen fixation using nitrogen-15 isotope dilution

A 15N isotope dilution technique was applied to quantify the extent of N2 fixation in lentil (Lens culinaris Medik.) cultivars as influenced by Rhizobium leguminosarum bv. viciae strains in a field experiment in Pakistan. The experiment was conducted on a soil with a very small indigenous rhizobial population and where N was a limiting factor for crop production. Significant variations in number of nodules, dry weight of nodules, biomass yield, grain yield, total N yield, proportion of plant N derived from N2 fixation (Pfix) and amount of N derived from the atmosphere (Ndfa) were observed among combined treatments of four rhizobial strains and six lentil varieties. In a field previously labelled with 15N, to which a basal dose of 75 kg P2O5 ha–1 was applied as single super phosphate, Ndfa ranged from 15 to 24 kg N ha–1 when calculated according to rhizobial strain and from 4 to 38 kg N ha–1 when calculated according to lentil variety. Lc 26 was the most effective strain and fixed 243% more N than the indigenous population in the uninoculated control. In treatments with the lentil variety PL-406, Ndfa was 38 kg N ha–1, which was 850% higher than with the lentil variety Precoz/F6-20-1×M-85. Generally, the varieties with greater Pfix produced a higher dry matter yield.

Use of the gusA gene marker in a competition study of the Rhizobium strains nodulating the common bean (Phaseolus vulgaris) in Senegal soils.

Indigenous rhizobial population is among the factors which influence increased crop yield through inoculation with elite strains. In this study, we compared in greenhouse conditions the competitiveness of Rhizobium strain ISRA 355 for nodulation of the common bean (Phaseolus vulgaris) cultivated in different unsterile Senegal soils in terms of pH, N and C contents. The strain ISRA 355 produced a stable GUS+ transconjugant which was used for competition with indigenous soil rhizobia in six localities. At Bayakh, the transconjugant ISRA 355gusA was less competitive than the indigenous rhizobial strains, whereas in the other localities, it was more competitive since it occupied more than 90% of the nodules. Thus the Rhizobium strain ISRA 355 should be used for successfully inoculating the common bean in Senegal soils.

Prevalence of the Rhizobium etli-like allele in genes coding for 16S rRNA among the indigenous rhizobial populations found associated with wild beans from the Southern Andes in Argentina.

A collection of rhizobial isolates from nodules of wild beans, Phaseolus vulgaris var. aborigineus, found growing in virgin lands in 17 geographically separate sites in northwest Argentina was characterized on the basis of host range, growth, hybridization to a nifH probe, analysis of genes coding for 16S rRNA (16S rDNA), DNA fingerprinting, and plasmid profiles. Nodules in field-collected wild bean plants were largely dominated by rhizobia carrying the 16S rDNA allele of Rhizobium etli. A similar prevalence of the R. etli allele was observed among rhizobia trapped from nearby soil. Intragroup diversity of wild bean isolates with either R. etli-like or Rhizobium leguminosarum bv. phaseoli-like alleles was generally found across northwest Argentina. The predominance of the R. etli allele suggests that in this center of origin of P. vulgaris the coevolution of Rhizobium spp. and primitive beans has resulted in this preferential symbiotic association.

Improvement of biological nitrogen fixation in Egyptian winter legumes through better management of Rhizobium

The diversity of rhizobia nodulating common bean ( Phaseolus vulgaris), berseem clover (Trifolium alexanderinum) and lentil (Lens culinaris) was assessed using several characterization techniques, including nitrogen fixation efficiency, intrinsic antibiotic-resistance patterns (IAR), plasmid profiles, serological markers and rep-PCR fingerprinting. Wide diversity among indigenous rhizobial populations of the isolates from lentil, bean and clover was found. Strikingly, a large percentage of the indigenous rhizobial population was extremely poor at fixing nitrogen. This emphasizes the need to increase the balance of highly efficient strains within the rhizobial population. Use of high-quality inocula strains that survive and compete with other less-desired and less-efficient N2-fixing rhizobia represents the best approach to increase biological nitrogen fixation of the target legume. In field-grown lentils, the inoculant strains were not able to outcompete the indigenous rhizobia and the native lentil rhizobia occupied 76–88% of the total nodules formed on inoculated plants. Nitrogen fixation by lentils, estimated using the 15N isotope dilution technique, ranged between 127 to 139 kg ha-1 in both inoculated and un-inoculated plants. With berseem clover, the inoculant strains were highly competitive against indigenous rhizobia and occupied 52–79% of all nodules. Inoculation with selected inocula improved N2 fixation by clover from 162 to 205 kg ha-1 in the three cuts as compared with 118 kg ha-1 in the un-inoculated treatment. The results also indicated the potential for improvement of N2 fixation by beans through the application of efficient N2-fixing rhizobia.