Ability of nitrogen-fixing bacteria to alleviate drought stress in cowpea varies depending on the origin of the inoculated strain

Drought is one of the main causes of global crop decline. Plant growth-promoting rhizobacteria enhance plant tolerance to adverse environmental conditions. This study aimed to determine whether the rhizobacteria Microvirga vignae (BR 3296 and BR 3299) and Bradyrhizobium sp. (BR 3301) can maintain cowpea growth under drought stress. We analyzed biomass, nodulation, nitrogen accumulation, and physiological traits of the inoculated plants. Rhizobacterial strains were assessed for exopolysaccharide (EPS) and indole acetic acid (IAA) production, growth, and biofilm formation in a water-stress medium induced by polyethylene glycol (PEG)-6000. The expression of genes associated with abscisic acid (ABA) biosynthesis in root nodules was also investigated. All evaluated strains were grown in a culture medium supplemented with PEG. M. vignae strains exhibited increased biofilm formation and EPS production, while Bradyrhizobium showed high IAA production. Cowpea plants inoculated with Bradyrhizobium exhibit higher levels of nodulation, biomass, and nitrogen accumulation. Conversely, M. vignae strains were more efficient at alleviating drought stress and maintaining nodulation, biomass, nitrogen accumulation, and stomatal conductance similar to well-watered plants. Drought-inducible genes were more strongly upregulated in the nodules of plants inoculated with Bradyrhizobium than in those inoculated with M. vignae. Our results suggest that M. vignae strains, isolated from a semi-arid region, help plants withstand water-stress, whereas the strain of Bradyrhizobium sp. isolated from a wet region did not effectively alleviate drought stress. However, Bradyrhizobium sp. conferred growth and nitrogen accumulation to cowpea superior to M. vignae and like plants supplied with nitrogen fertilizer.