ABSTRACT To mitigate the deleterious effects of abiotic stress, the use of plant growth-promoting bacteria along with diazotrophic bacteria has been increasing. The objectives of this study were to investigate the key enzymes related to nitrogen and carbon metabolism in the biological nitrogen fixation process and to elucidate the activities of these enzymes by the synergistic interaction between Bradyrhizobium and plant growth-promoting bacteria in the absence and presence of salt stress. Cowpea plants were cultivated under axenic conditions, inoculated with Bradyrhizobium and co-inoculated with Bradyrhizobium sp. and Actinomadura sp., Bradyrhizobium sp. and Bacillus sp., Bradyrhizobium sp. and Paenibacillus graminis, and Bradyrhizobium sp. and Streptomycessp.; the plants were also maintained in the absence (control) and presence of salt stress (50 mmolL-1 NaCl). Salinity reduced the amino acids, free ammonia, ureides, proteins and total nitrogen content in nodules and increased the levels of sucrose and soluble sugars. The co-inoculations responded differently to the activity of glutamine synthetase enzymes under salt stress, as well as glutamate synthase, glutamate dehydrogenase aminating, and acid invertase in the control and salt stress. Considering the development conditions of this experiment, co-inoculation with Bradyrhizobium sp. and Bacillus sp. in cowpea provided better symbiotic performance, mitigating the deleterious effects of salt stress.

Abstract Soil salinity is an important abiotic stress worldwide, and salt-induced oxidative stress can have detrimental effects on the biological nitrogen fixation. We hypothesized that co-inoculation of cowpea plants with Bradyrhizobium and plant growth-promoting bacteria would minimize the deleterious effects of salt stress via the induction of enzymatic and non-enzymatic antioxidative protection. To test our hypothesis, cowpea seeds were inoculated with Bradyrhizobium or co-inoculated with Bradyrhizobium and plant growth-promoting bacteria and then submitted to salt stress. Afterward, the cowpea nodules were collected, and the levels of hydrogen peroxide; lipid peroxidation; total, reduced and oxidized forms of ascorbate and glutathione; and superoxide dismutase, catalase and phenol peroxidase activities were evaluated. The sodium and potassium ion concentrations were measured in shoot samples. Cowpea plants did not present significant differences in sodium and potassium levels when grown under non-saline conditions, but sodium content was strongly increased under salt stress conditions. Under non-saline and salt stress conditions, plants co-inoculated with Bradyrhizobium and Actinomadura or co-inoculated with Bradyrhizobium and Paenibacillus graminis showed lower hydrogen peroxide content in their nodules, whereas lipid peroxidation was increased by 31% in plants that were subjected to salt stress. Furthermore, cowpea nodules co-inoculated with Bradyrhizobium and plant growth-promoting bacteria and exposed to salt stress displayed significant alterations in the total, reduced and oxidized forms of ascorbate and glutathione. Inoculation with Bradyrhizobium and plant growth-promoting bacteria induced increased superoxide dismutase, catalase and phenol peroxidase activities in the nodules of cowpea plants exposed to salt stress. The catalase activity in plants co-inoculated with Bradyrhizobium and Streptomyces was 55% greater than in plants inoculated with Bradyrhizobium alone, and this value was remarkably greater than that in the other treatments. These results reinforce the beneficial effects of plant growth-promoting bacteria on the antioxidant system that detoxifies reactive oxygen species. We concluded that the combination of Bradyrhizobium and plant growth-promoting bacteria induces positive responses for coping with salt-induced oxidative stress in cowpea nodules, mainly in plants co-inoculated with Bradyrhizobium and P. graminis or co-inoculated with Bradyrhizobium and Bacillus.