The objectives of this study were to characterize the genetic diversity and evaluate the ability to tolerate stress as well as to assess the symbiotic efficiency of bacteria from cowpea nodules in agricultural soils with different uses in the semiarid region of Bahia state (Brazil). Soil samples were collected from six crop lands and one from the pristine Caatinga biome. After a trap-host experiment, the bacteria were isolated and culturally characterized. Isolates with typical characteristics of Bradyrhizobium were subjected to the nodC symbiotic gene amplification and those positive were evaluated by 16S-23S IGS-RFLP. Twenty-seven isolates belonging to different genetic clusters were selected for 16S-23S IGS sequencing. In additions, the selected bacteria were characterized biochemically and symbiotically. Among 420 characterized isolates, approximately 60% (251 isolates) displayed typical Bradyrhizobium cultural features. A total of 161, out of 251 isolates, showed positive amplification of the nodC gene fragment. The IGS-RFLP profiles analysis generated 33 groups and 27 were selected for further analysis. The fertility of the soils influenced the distribution of the isolates in the IGS-RFLP clusters. The bacteria were assigned to two genera, Bradyrhizobium and Microvirga, with 26 and 1 representative bacteria, respectively. Some isolates were able to tolerate NaCl as well as acidic and alkaline pH. In addition, isolates showed the abilities to produce biofilm under stress and to produce indole compounds, as well as efficient nodulation and nitrogen fixation. The isolates displayed great genetic, biochemical, and symbiotic variability with promising biotechnological potential.

The objective of this study was to evaluate the influence of biochar amendment on the bacterial genetic diversity and symbiosis in cowpea-nodulating rhizobial communities in two agricultural soils of Brazilian drylands. Vertisol and Ultisol surface layer soil samples were collected in the Brazilian semiarid region. The soils were mixed with biochar and cowpea seeds (BRS Marataoã) and sown in pots. Forty-five days after plant emergence, the nodule bacteria were isolated. The genetic variability of the nodule bacteria was evaluated by means of restriction fragment length polymorphism of 16S-23S rRNA intergenic spacer region (IGS-RFLP), and those that shared identical IGS-RFLP profiles were fingerprinted by BOX-PCR. By selecting of representative isolates in the genetic clusters, 27 bacteria were identified by 16S-23S IGS sequencing and symbiotically assessed. The IGS-RFLP dendrogram of the 73 isolates had 20 clusters; the multivariate correspondence analysis grouped both Vertisol treatments, and the Ultisol treatments were placed far away in the biplot. Ultisol showed higher bradyrhizobial diversity than the Vertisol. The fingerprinting indicated the presence of only three clonal colonies, showing high intraspecific diversity. The 16S-23S IGS sequences indicated the prevalence of bacteria related to Bradyrhizobium guangxiense in the Vertisol and Bradyrhizobium zhanjiangense in the Ultisol, in addition to the presence of three putative Microvirga spp. in the Ultisol. The symbiotic efficiency of 22 out of 27 bacteria was comparable with that of the inoculant strain Bradyrhizobium yuanmingense BR 3267. The genetic diversity of the cowpea-nodulating bradyrhizobia was not primarily affected by the biochar application in two agricultural soils of Brazilian drylands. Both soils harbored symbiotically efficient bacteria, without the influence of biochar amendment .

The co-inoculation of Bradyrhizobium with other non-bradyrhizobial strains was already assessed on cowpea, but the co-inoculation of two Bradyrhizobium strains was not tested up to now. This study aimed to evaluate the cowpea growth, N accumulation, and Bradyrhizobium competitiveness of the elite strain B. pachyrhizi BR 3262 when co-inoculated with other efficient Bradyrhizobium from the Brazilian semiarid region. Three potted-plant experiments were carried out. In the first assay, 35 efficient Bradyrhizobium isolates obtained from the semiarid region of Brazil were co-inoculated with the elite strains B. pachyrhizi BR 3262. The experiment was conducted in gnotobiotic conditions. The plant growth, nodulation, N nutritional variables, and nodular occupation were assessed. Under gnotobiotic and non-sterile soil conditions, ten selected bacteria plus the elite strain B. yuanmingense BR 3267 were used at the second and third experiments, respectively. The cowpea was inoculated with the 11 bacteria individually or co-inoculated with BR 3262. The plant growth and N nutritional variables were assessed. A double-layer medium spot method experiment was conducted to evaluate the interaction among the co-inoculated strains in standard and diluted YMA media. The co-inoculation treatments showed the best efficiency when compared to the treatments inoculated solely with BR 3262. This strain occupied a low amount of cowpea nodules ranging from 5 to 67.5%. The treatments with lower BR 3262 nodule occupancy showed the best results for the shoot nitrogen accumulation. The culture experiment showed that four bacteria inhibited the growth of BR 3262. In contrast, seven strains from the soils of Brazilian semiarid region were benefited by the previous inoculation of this strain. In the second and third experiments, the results indicated that all 11 co-inoculated treatments were more efficient than the single inoculation, proofing the best performance of the dual inoculation of Bradyrhizobium on cowpea.

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