Molecular, Physiological, and Symbiotic Characterization of Cowpea Rhizobia from Soils Under Different Agricultural Systems in the Semiarid Region of Brazil

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.