Bradyrhizobium Strains Research Articles

Cowpea Genotypic Variations in N2 Fixation, Water Use Efficiency (δ13C), and Grain Yield in Response to Bradyrhizobium Inoculation in the Field, Measured Using Xylem N Solutes, 15N, and 13C Natural Abundance

To evaluate cowpea genotypic differences in response to inoculation with Bradyrhizobium sp. strains BR 3267 and CB756 in the field, 15 genotypes were assessed for N2 fixation using 15N natural abundance and assays of xylem N solutes (ureides, nitrates, and amino-N). Carbon isotope discrimination (δ13C) was used as a surrogate for water use efficiency and grain yield determined at maturity. Within each location, the tested cowpeas elicited contrasting responses to inoculation with either Bradyrhizobium strains. For example, inoculating Apagbaala and IT90K-277-2 with strain BR 3267 doubled N-fixed and grain yield at Nyankpala when compared to inoculation of the same genotypes with strain CB756, whereas the inoculation of Padituya with strain CB756 increased those same parameters relative to non-inoculation. Similar contrasting results were also obtained at Savelugu and Gbalahi, where bacterial inoculation with each strain resulted in up to a five-fold increase in N-fixed and a four-fold increase in grain yield depending on the cowpea/inoculant combinations used and the planting location. The main effect of genotype showed that the percent N derived from N2 fixation (%Ndfa) by the test cowpeas ranged between 74.4 and 89.8%, 11.7 and 50.5%, and 71.9 and 90.3% at Nyankpala, Savelugu, and Gbalahi, respectively. The genotypes grown at Savelugu generally exhibited low %Ndfa which was augmented by greater soil N uptake leading to grain yield increases, a finding supported by significant positive correlations when N-fixed and N content were each plotted against grain yield within locations. The inverse relationship between symbiosis and soil N uptake was also supported by negative correlations between xylem nitrate concentration and relative ureide N abundance (RU-N), and between xylem nitrate concentration and %Ndfa measured from 15N natural abundance. Some high yielding genotypes also elicited greater water use efficiency (δ13C), a trait that could be exploited for increased cowpea production in water limiting environments. This study demonstrates the presence of genotypic differences in cowpea response to inoculation in the field and contributes to the literature regarding the factors influencing legume inoculation response in the tropics.

Nod factor-independent 'crack-entry' symbiosis in dalbergoid legume Arachis hypogaea.

Dalbergoids are typified by crack-entry symbiosis which is evidenced to be Nod Factor (NF)-independent in several Aeschynomene legumes. Natural symbionts of the dalbergoid legume Arachis hypogaea are always NF-producing, prompting us to check whether symbiosis in this legume could also be NF-independent. For this, we followed the symbiosis with two NF-containing bradyrhizobial strains - SEMIA6144, a natural symbiont of Arachis and ORS285, a versatile nodulator of Aeschynomene legumes, along with their corresponding nodulation (nod) mutants. Additionally, we investigated NF-deficient bradyrhizobia like BTAi1, a natural symbiont of Aeschynomene indica and the WBOS strains that were natural endophytes of Oryza sativa, collected from an Arachis-Oryza intercropped field. While SEMIA6144ΔnodC was non-nodulating, both ORS285 and ORS285ΔnodB could induce functional nodulation, although with lower efficiency than SEMIA6144. On the other hand, all the NF-deficient strains - BTAi1, WBOS2 and WBOS4 showed comparable nodulation with ORS285 indicating Arachis to harbour an NF-independent mechanism of symbiosis. Intriguingly, symbiosis in Arachis, irrespective of whether it was NF-dependent or independent, was always associated with the curling or branching of the rosette root hairs at the lateral root bases. Thus, despite being predominantly described as an NF-dependent legume, Arachis does retain a vestigial, less-efficient form of NF-independent symbiosis.

Effect of induction moment on Bradyrhizobium-soybean (Glycine max) interaction

Introduction. Legume nodulation is regulated by the exchange of molecular signals between the plant and rhizobia. The bacterial nodulation genes are activated by flavonoids present in root exudates. As a result of this activation, the Nod factors are synthesized, which in turn participate in the morphogenesis and function of the nodules. It is then possible to regulate the induction of Nod factors and other signals related to the nodulation process, during the inoculants production. Objective. To determine if the moment to induce the nodulation genes in Bradyrhizobium inoculants affects the synthesis of compounds related to the nodulation process and its interaction with soybean plants. Materials and methods. The research was developed in Rizobacter S.A., Argentina and Fyteko S.A. laboratories, Belgium, and in a plant growth chamber of the first one, during May-June of 2017. Two Bradyrhizobium strains were induced with daidzein in three moments. The signals produced by the strains and the biological effect in soybean plants were studied. Results. Differences were detected in the molecules produced by the bacteria, but it did not affect the growing and plants nodulation related to the different moments of induction. Conclusion. The metabolites excreted by Bradyrhizobium changed with strain and induction moment, although the latter did not affect nodulation or soybean plants growth.

Evaluation of nitrous oxide emission by soybean inoculated with Bradyrhizobium strains commonly used as inoculants in South America

Evaluation of nitrous oxide emission by soybean inoculated with Bradyrhizobium strains commonly used as inoculants in South America

Enhanced Soybean Productivity by Inoculation With Indigenous Bradyrhizobium Strains in Agroecological Conditions of Northeast Germany.

Commercial inoculants are often used to inoculate field-grown soybean in Europe. However, nodulation efficiencies in these areas are often low. To enhance biological nitrogen (N) fixation and increase domestic legume production, indigenous strains that are adapted to local conditions could be used to develop more effective inoculants. The objective of this study was to assess the ability of locally isolated Bradyrhizobium strains to enhance soybean productivity in different growing conditions of Northeast Germany. Three indigenous Bradyrhizobium isolates (GMF14, GMM36, and GEM96) were tested in combination with different soybean cultivars of different maturity groups and quality characteristics in one field trial and two greenhouse studies. The results showed a highly significant strain × cultivar interactions on nodulation response. Independent of the Bradyrhizobium strain, inoculated plants in the greenhouse showed higher nodulation, which corresponded with an increased N uptake than that in field conditions. There were significantly higher nodule numbers and nodule dry weights following GMF14 and GMM36 inoculation in well-watered soil, but only minor differences under drought conditions. Inoculation of the soybean cultivar Merlin with the strain GEM96 enhanced nodulation but did not correspond to an increased grain yield under field conditions. USDA110 was consistent in improving the grain yield of soybean cultivars Sultana and Siroca. On the other hand, GMM36 inoculation to Sultana and GEM96 inoculation to Siroca resulted in similar yields. Our results demonstrate that inoculation of locally adapted soybean cultivars with the indigenous isolates improves nodulation and yield attributes. Thus, to attain optimal symbiotic performance, the strains need to be matched with specific cultivars.

Effects of Bradyrhizobium Japonicum on Nitrogen Concentration in Soybean Leaves and Seeds Cultivated on Acidic Soils

The legume-rhizobium symbiosis plays an important role in Nitrogen (N) uptake by plants, more particularly in the humid tropical region where soils are deeply weathered and have poor electrochemical properties. However, the influence of Bradyrhizobium on N allocation in soybean organs is not clearly understood. Here, we assessed the effect of Bradyrhizobium japonicum on N concentration of soybean leaves and seeds cultivated on acidic soils. We conducted two experiments using a randomized full device with three replicates in two sites (i.e., Kasapa and Kanyameshi). The main plots included three strains of Bradyrhizobium and the uninoculated control while the subplots included soybean variety. We found that different strains of Bradyrhizobium did not induce significant effects on total N concentration in soybean leaves and seeds in the Kasapa site. In contrast, Bradyrhizobium strains affected significantly N concentration in soybean leaves in the Kanyameshi site. We demonstrated that soil N concentration, which varies between investigated sites, positively influences yield and nodulation. We conclude that the efficiency of the soybean-Bradyrhizobium symbiosis and its influence on the allocation of N through soybean organs strongly depend on the chemical characteristics of soil and particularly on the initial N concentrations in soils.

Phosphate–solubilizing fungi co–inoculated with Bradyrhizobium promote cowpea growth under varying N and P fertilization conditions

We evaluated the compatibility between two nitrogen–fixing Bradyrhizobium inoculant strains and phosphate–solubilizing fungal strains and the effect of co–inoculation of these bacterial and fungal strains on cowpea growth under different N and P conditions. First, the compatibility between Bradyrhizobium strains UFLA03–84 and INPA03–11B and fungi Haematonectria ipomoeae FSA381, Eleutherascus lectardii FSA257a, Pochonia chlamydosporia var. catenulata FSA109, and Acremonium polychromum FSA115 was tested in both solid and liquid media. Cowpea growth and nodulation promotion under two mineral N doses and two P conditions (a low dose of soluble P plus a high dose of Ca3(PO4)2 and another condition with a high dose of soluble P) were tested with two N2 fixing Bradyrhizobium strains co–inoculated with each of the P–solubilizing fungal strains FSA109, FSA115, and FSA381. There was compatibility between each fungal strain and the two Bradyrhizobium strains, except for FSA257a with either of the bacterial strains in liquid medium. When both mineral N and P were limiting, plants were able to grow and accumulate N and P based on biological N2 fixation and solubilization of calcium phosphate in the same amount as the mineral N and soluble phosphate. Even when both nutrients were fully available, the type of co–inoculation promoted plant growth and nutrient accumulation. The responses varied in accordance with the co–inoculated strains, the N source, and the P source, reflecting the enormous complexity of the biological interactions between plants and microorganisms, and the nutrient conditions provided by the environment.

The intensity of drought-induced oxidative processes in soybeans depends on symbiosis with Bradyrhizobium strains

The intensity of drought-induced oxidative processes in soybeans depends on symbiosis with Bradyrhizobium strains

Symbiosis Contribution of Non-nodulating Bradyrhizobium cosmicum S23321 after Transferal of the Symbiotic Plasmid pDOA9.

The symbiotic properties of rhizobial bacteria are driven by the horizontal gene transfer of symbiotic genes, which are located in symbiosis islands or on plasmids. The symbiotic megaplasmid pDOA9 of Bradyrhizobium sp. DOA9, carrying the nod, nif, fix, and type three secretion system (T3SS) genes, has been conjugatively transferred to different Bradyrhizobium strains. In the present study, non-nodulating B. cosmicum S23321, which shows a close phylogenetic relationship with Bradyrhizobium sp. DOA9, but lacks symbiotic properties, was used to carry pDOA9 (annotated as chimeric S2:pDOA9). The results obtained showed that pDOA9 conferred symbiotic properties on S23321; however, nodulation phenotypes varied among the DOA9, chimeric ORS278:pDOA9, and S2:pDOA9 strains even though they all carried symbiotic pDOA9 plasmid. S23321 appeared to gain symbiotic nodulation from pDOA9 by processing nodulation genes and broadening the host range. The present results also showed the successful formation of active nodules in Arachis hypogaea (Dalbergoid) and Vigna radiata (Millitoid) by chimeric S2:pDOA9, while Crotalaria juncea (Genistoid) and Macroptilium atropurpureum (Millitoid) formed nodule-like structures. The formation of nodules and nodule-like structures occurred in a nod factor-dependent manner because the nod factor-lacking strain (S2:pDOA9ΩnodB) completely abolished nodulation in all legumes tested. Moreover, T3SS carried by S2:pDOA9 exerted negative effects on symbiosis with Crotalaria juncea, which was consistent with the results obtained on DOA9. T3SS exhibited symbiotic compatibility with V. radiata when nodulated by S23321. These outcomes implied that pDOA9 underwent changes during legume evolution that broadened host specificity and the compatibility of nodulation in a manner that was dependent on the chromosomal background of the recipient as well as legume host restrictions.

Leaf growth, gas exchange and assimilation performance of cowpea varieties in response to Bradyrhizobium inoculation

Supplying nitrogen to crops through selecting high N fixing legumes and effective inoculant is one of the key strategies to improve crop productivity. However, studies related to the effect of Bradyrhizobial inoculation on leaf growth, its functioning in relation to photosynthesis, and transpiration efficiency (WUE) of cowpea [Vigna unguiculata (L.) Walp] varieties in the tropics were inadequate. A two-year field experiment was conducted at three sites to evaluate the effect of inoculation on leaf growth, gas exchanges and photosynthetic efficiency of cowpea varieties. The study treatments were composed of four varieties, Keti (IT99K-1122), TVU, Black eye bean, and White wonderer trailing and three levels of inoculation (non-inoculated or inoculated with Bradyrhizobium strains CP-24 or CP-37). Gas exchange was measured on live plants at 67–77 days after sowing, between 8:00 to 11:00 a.m. and 14:00 to 16:00 p.m. Leaf growth parameters (leaf number and leaf area) were measured by destructive sampling, and the yield data was determined by harvesting plants in the three central rows at physiological maturity. Variety TVU performed best in terms of leaf number, photosynthesis rate, and WUE. Whereas, Black eye bean revealed superior performances for leaf area, leaf area index, and stomatal conductance compared with the rest two varieties. The effect of inoculation was significant with 14.0, 23.8, 13.7, and 11.0% advantage in leaf area, leaf area index, net photosynthesis, and WUE, respectively. Moreover, the performance of cowpea of the 2018 cropping season showed a relative advantage over 2019 in terms of leaf number, leaf area, leaf area index, net photosynthesis, and stomatal conductance. Therefore, inoculating cowpea varieties with effective Bradyrhizobium strain can be a viable alternative to enhance growth, gas exchange, photosynthetic efficiency, and grain yield.

Genetic and symbiotic characterization of rhizobia nodulating legumes in a mining area in southeast Brazil

Identification and selection of nitrogen-fixing bacterial strains for inoculation into native leguminous tree species can assist in the recovery of degraded areas. Additionally, native strains from these areas are genetic resources adapted to these conditions and are thus suitable for selection. The aim of this study was to symbiotically and genetically characterize 18 bacterial strains from the Rhizobium and Bradyrhizobium genera isolated from Machaerium nyctitans, Platypodium elegans, and Ormosia arborea grown in a nursery in an iron mining area. Three experiments were conducted under axenic conditions in a greenhouse. The nodulation capacity of the strains was evaluated by the number (NN) and dry matter (NDM) of nodules. Symbiotic efficiency was evaluated based on the following parameters: SPAD index (SPAD), shoot dry matter (SDM), root dry matter (RDM), and total dry matter (TDM) of the plants, relative efficiency (RE), shoot nitrogen content (SNC), and total nitrogen content in the plant (TNC). The atpD and gyrB housekeeping genes and the nifH gene were sequenced for phylogenetic analysis, and the nodC and nodD symbiotic genes of the strains were amplified. Out of the 18 strains, 16 were authenticated by nodulation capacity in the species of origin. The SPAD variable allowed for the detection of differences between treatments before the SDM. Additionally, the SPAD index showed correlation with TNC, and the strain Bradyrhizobium sp., UFLA01-839, which may represent a new species, was outstanding in Machaerium nyctitans. The nifH, nodD, and nodC genes were detected in UFLA01-839.

Características agronômicas de cultivares de soja com aplicação tardia de nitrogênio em suplementação à inoculação de Bradyrhizobium spp.

ABSTRACT Late season nitrogen (N) applications may serve as cultural practices to increase soybean yield and grain protein concentration in modern high-yielding cultivars. A study to evaluate the response of soybean cultivars to late N application in supplementation to Bradyrhizobium spp. inoculation was conducted in two Brazilian Cerrado agricultural soils with medium and high fertility during the 2019/2020 growing season. Treatments were arranged in a randomized block design in a 2×2×5 factorial scheme: two agricultural production environments [UFMS 1 (medium fertility) and UFMS 2 (high fertility)], two soybean cultivars (TMG 7067 IPRO and BMX Bônus IPRO), and five late N application rates (0, 50, 100, 150 and 200 kg N ha-1), with four replicates. Canonical correlation analysis (CCA) was used to investigate the interrelationships between the groups of independent (agricultural production environments, soybean cultivars, and N application rates) and dependent (soybean agronomic traits) variables. Nitrogen rates were applied in topdressing at the R5.3 soybean growth stage. Late N applications did not affect soybean agronomic traits (plant height, first pod height, pod number per plant, grain number per pod, and 1,000-grain mass) and did not increase the grain protein concentration or yield of the soybean cultivars, regardless of the fertility level of the agricultural area. We conclude that when efficient strains of Bradyrhizobium spp. are used in soybean cropping in medium- and high-fertility Cerrado soils, there is no need to apply late rates of N fertilizer.

Co-inoculation of Rhizobium and Bradyrhizobium promotes growth and yield of common beans

Some strains of Bradyrhizobium benefit the biological N2 fixation (BNF) and growth of common beans (Phaseolus vulgaris L.) when co-inoculated with Rhizobium tropici under greenhouse conditions. Here, we evaluated the benefit of Bradyrhizobium diazoefficiens strain CPAC 7 co-inoculated with Rhizobium tropici strain CIAT 899 on common bean in the field. Three Bradyrhizobium strains were co-inoculated with R. tropici CIAT 899 in one experiment in the greenhouse and two in the field. The greenhouse experiment combined each of three Bradyrhizobium strains (29w, USDA 110, and CPAC 7) in co-inoculation with CIAT 899 in Leonard jars. Then, two of these strains, 29w and CPAC 7, were co-inoculated with Rhizobium on-seeds (1.2 × 106 CFU seed−1) or both on-seed and in-furrow (1.08 × 108 CFU m−1) in a field experiment. Finally, a second field experiment evaluated strain CPAC 7 at doses ranging from 1.5 × 105 to 1.2 × 106 CFU seed−1. Co-inoculation with strain CPAC 7 improved shoot biomass accumulation in Leonard jars. In the field, co-inoculation of CPAC 7 with CIAT 899 on-seed enhanced grain yield by 20% compared with inoculation of CIAT 899 only (p < 0.10). The percentage of N derived from the air (%Ndfa) was between 50 and 60%, with only a tendency towards co-inoculation benefit. Given that the %Ndfa of the inoculated plants was not statistically different between treatments, the increased grain yield was not entirely due to BNF but also due to an increase of soil N uptake. In the second field trial, larger doses of inoculum increased nodulation and root biomass, but significant differences in grain yield were not observed. Our results show that the Rhizobium-Bradyrhizobium co-inoculation can enhance growth and grain yield of common beans and indicate that a plant growth-promoting mechanism besides BNF may be involved in the beneficial co-inoculation effect. However, further adjustments are needed to ensure a better response under field conditions.

Insights into the bacterial community composition of farmed Caulerpa lentillifera: A comparison between contrasting health states.

The bacterial communities of Caulerpa lentillifera were studied during an outbreak of an unknown disease in a sea grape farm from Vietnam. Clear differences between healthy and diseased cases were observed at the order, genus, and Operational Taxonomic Unit (OTU) level. A richer diversity was detected in the diseased thalli of C. lentillifera, as well as the dominance of the orders Flavobacteriales (phylum Bacteroidetes) and Phycisphaerales (Planctomycetes). Aquibacter, Winogradskyella, and other OTUs of the family Flavobacteriaceae were hypothesized as detrimental bacteria, this family comprises some well‐known seaweed pathogens. Phycisphaera together with other Planctomycetes and Woeseia were probably saprophytes of C. lentillifera. The Rhodobacteraceae and Rhodovulum dominated the bacterial community composition of healthy C. lentillifera. The likely beneficial role of Bradyrhizobium, Paracoccus, and Brevundimonas strains on nutrient cycling and phytohormone production was discussed. The bleaching of diseased C. lentillifera might not only be associated with pathogens but also with an oxidative response. This study offers pioneering insights on the co‐occurrence of C. lentillifera‐attached bacteria, potential detrimental or beneficial microbes, and a baseline for understanding the C. lentillifera holobiont. Further applied and basic research is urgently needed on C. lentillifera microbiome, shotgun metagenomic, metatranscriptomic, and metabolomic studies as well as bioactivity assays are recommended.

Using amplicon sequencing of rpoB for identification of inoculant rhizobia from peanut nodules

To improve the nitrogen fixation, legume crops are often inoculated with selected effective rhizobia. However, there is large variation in how well the inoculant strains compete with the indigenous microflora in soil. To assess the success of the inoculant, it is necessary to distinguish it from other, closely related strains. Methods used until now have generally been based either on fingerprinting methods or on the use of reporter genes. Nevertheless, these methods have their shortcomings, either because they do not provide sufficiently specific information on the identity of the inoculant strain, or because they use genetically modified organisms that need prior authorization to be applied in the field or other uncontained environments. Another possibility is to target a gene that is naturally present in the bacterial genomes. Here we have developed a method that is based on amplicon sequencing of the bacterial housekeeping gene rpoB, encoding the beta-subunit of the RNA polymerase, which has been proposed as an alternative to the 16S rRNA gene to study the diversity of rhizobial populations in soils. We evaluated the method under laboratory and field conditions. Peanut seeds were inoculated with various Bradyrhizobium strains. After nodule development, DNA was extracted from selected nodules and the nodulating rhizobia were analysed by amplicon sequencing of the rpoB gene. The analyses of the sequence data showed that the method reliably identified bradyrhizobial strains in nodules, at least at the species level, and could be used to assess the competitiveness of the inoculant compared to other bradyrhizobia.

BIOLOGICAL FEATURES OF GROUNDNUT MICROSYMBIONTS WIDESPREAD IN THE SOILS OF UKRAINE

Objective. Study the presence of nodule bacteria in the soils of Ukraine, capable of nodulatinggroundnut, isolate new strains of rhizobia from peanut nodules, study their morphological, culturaland serological properties and the ability to form symbiosis with different legumes. Methods. Microbiological (isolation of nodule bacteria from nodules and cultivation of microorganisms, studying biological properties of strains), serological (producing antisera to Bradyrhizobium lupini367a, studying rhizobia diversity in groundnut nodule populations, studying serological identity ofnew strains), vegetation experiment (studying formation and functioning of groundnut symbioticsystems with nodule bacteria, studying host specificity of new strains of groundnut rhizobia), gaschromatography (determining nitrogen-fixing activity of rhizobia in symbiosis with groundnut),mathematical and statistical. Results. Representatives of two species, B. lupini and B. japonicum,were found in nodule populations of rhizobia during the cultivation of groundnut plants on sodpodzolic soil and leached chernozem. The dominant microsymbionts of groundnut were lupine nodule bacteria of serogroup 367a (54.2 % and 45.8 % according to soils). Fewer nodules wereformed by intensive growing soybean rhizobia of serogroup КВ11 (16.7 % and 12.5 %). The shareof nodule bacteria not classified in the studied serogroups was 21.9 % and 41.7 %. Fifteen newstrains of nodule bacteria were isolated from groundnut nodules, which were morphologically, culturally and serologically identified as B. lupini serogroup 367a (7 of them), B. japonicumserogroup KB11 (4 of them) and Bradyrhizobium sp. (4 of them). New strains of B. lupini fromgroundnut nodules are able to infect white and yellow lupine but do not nodulate soybeans. Strainsidentified as B. japonicum form nodules on soybean roots but do not infect lupine. Serologically unidentified strain Bradyrhizobium sp. AR3, which is able to form a symbiosis with both lupine andsoybean (phenotypes Nod+Fix+) was obtained. Conclusion. For the first time it was established thatgroups of nodule bacteria capable of nodulating groundnut are present in the agrocenoses ofUkraine. Fifteen new strains of groundnut rhizobia were obtained, which were identified as B. lupini, B. japonicum and Bradyrhizobium sp.

The Type III Effectome of the Symbiotic Bradyrhizobiumvignae Strain ORS3257.

Many Bradyrhizobium strains are able to establish a Nod factor-independent symbiosis with the leguminous plant Aeschynomene indica by the use of a type III secretion system (T3SS). Recently, an important advance in the understanding of the molecular factors supporting this symbiosis has been achieved by the in silico identification and functional characterization of 27 putative T3SS effectors (T3Es) of Bradyrhizobium vignae ORS3257. In the present study, we experimentally extend this catalog of T3Es by using a multi-omics approach. Transcriptome analysis under non-inducing and inducing conditions in the ORS3257 wild-type strain and the ttsI mutant revealed that the expression of 18 out of the 27 putative effectors previously identified, is under the control of TtsI, the global transcriptional regulator of T3SS and T3Es. Quantitative shotgun proteome analysis of culture supernatant in the wild type and T3SS mutant strains confirmed that 15 of the previously determined candidate T3Es are secreted by the T3SS. Moreover, the combined approaches identified nine additional putative T3Es and one of them was experimentally validated as a novel effector. Our study underscores the power of combined proteome and transcriptome analyses to complement in silico predictions and produce nearly complete effector catalogs. The establishment of the ORS3257 effectome will form the basis for a full appraisal of the symbiotic properties of this strain during its interaction with various host legumes via different processes.

Bradyrhizobium Inoculation of Field-Grown Kersting's Groundnut [Macrotyloma geocarpum (Harms) Marechal &amp; Baudet] Increased Grain Yield and N2 Fixation, Measured Using the Ureide, and 15N Natural Abundance Techniques

Here, we report on the symbiotic N2 fixation and water use efficiency (δ13C) of Kersting's groundnut, an important but underutilized grain legume in Africa, in response to inoculation with Bradyrhizobium strains 3267 and CB756. The 15N natural abundance and xylem ureide techniques were used to quantify N2 fixation. The landraces in this study derived variable levels of their N requirements from symbiosis, which translated into marked differences in the amounts of N-fixed by the landrace–inoculant combinations across the test locations. In most instances, the landrace-strain combinations that elicited higher shoot biomass also recorded greater N-fixed and/or ureide-N in xylem sap as well as grain yield. Although some landraces coupled increased grain yield with higher water use efficiency (shoot δ13C), a trait that could be tapped for crop improvement, others recorded lower yields despite eliciting relatively high shoot δ13C values, indicating genotypic variations in adaptation to the different environments. Grain yield of the test landraces showed marked variation and ranged from 131 to 1349.8 kg ha−1 depending on the landrace–strain combination used and the planting location. The high symbiotic dependence by landraces in this study could explain why Kersting's groundnut thrives in the low nutrient soils that are prevalent in its cultivation areas. These results provide more insights into the literature regarding the Kersting's groundnut–rhizobia symbiosis as well as the crop's water use efficiency.

Identification of effective Rhizobium isolate(s) for efficient symbiosis with soybean under moisture stress conditions

The objective of the paper is to observe the effectiveness of Bradyrhizobial species on symbiosis with soybean under moisture stress conditions. In physiological tests, treatment T3: B. daqingense (under irrigated condition) showed significant amount of leghaemoglobin content (1.82 mg). Proline content was recorded highest (2.8 mg g-1 seed) in T8 B. liaoningense in stress. Highest chlorophyll content (1.84 mg/LFW) was recorded in T4 B. liaoningense under irrigated condition. The highest N-uptake in plant shoot, root and grain was obtained in B. daqingense under irrigated condition. Similarly, highest was obtained in in treatment T3 B. daqingense under irrigated condition. At R8 stage, grain yield was significantly highest (6.48 g) in treatment T3 (B. daqingense under irrigated condition). In both irrigated and moisture stress conditions bradyrhizobial strains B. daqingense (treatment T3 and Treatment T7) were most effective in terms of nitrogen fixation, nitrogen accumulation, phosphorus accumulation, shoot dry weight, root dry weight and grain yield as compared to the other treatments.

The Bradyrhizobium Sp. LmicA16 Type VI Secretion System Is Required for Efficient Nodulation of Lupinus Spp.

Many bacteria of the genus Bradyrhizobium are capable of inducing nodules in legumes. In this work, the importance of a type VI secretion system (T6SS) in a symbiotic strain of the genus Bradyrhizobium is described. T6SS of Bradyrhizobium sp. LmicA16 (A16) is necessary for efficient nodulation with Lupinus micranthus and Lupinus angustifolius. A mutant in the gene vgrG, coding for a component of the T6SS nanostructure, induced less nodules and smaller plants than the wild-type (wt) strain and was less competitive when co-inoculated with the wt strain. A16 T6SS genes are organized in a 26-kb DNA region in two divergent gene clusters of nine genes each. One of these genes codes for a protein (Tsb1) of unknown function but containing a methyltransferase domain. A tsb1 mutant showed an intermediate symbiotic phenotype regarding vgrG mutant and higher mucoidity than the wt strain in free-living conditions. T6SS promoter fusions to the lacZ reporter indicate expression in nodules but not in free-living cells grown in different media and conditions. The analysis of nodule structure revealed that the level of nodule colonization was significantly reduced in the mutants with respect to the wt strain.