Nitrogen-Fixing Bradyrhizobium spp. as Plant Growth-Promoting Bacteria to Improve Soil Quality and Plant Tolerance to Biotic and Abiotic Stresses

Nitrogen-fixing Bradyrhizobium species are Gram-negative soil bacteria belonging to the Bradyrhizobiaceae family. These bacteria provide a wide range of services and benefits to many legume and nonlegume plants, and in turn, they receive reduced carbon and other metabolites provided by the host plant. The most discussed ability is nitrogen fixation under symbiotic conditions with legumes. However, the ability of Bradyrhizobium species goes beyond supplying the host plant with fixed nitrogen. Whether under symbiotic or saprophytic lifestyle conditions, Bradyrhizobium species contribute to improving soil structure; increasing nutrient uptake; modulation of extracellular molecules; and secretion of molecules such as antibiotics, hormones, and various signal compounds, leading to improved plant growth. In this chapter, we discussed the mechanisms developed by Bradyrhizobium species to enhance plant growth. First, we described the mechanisms involved in providing nutrients, such as nitrogen, phosphate, and iron. Next, we discussed the compounds they release to enhance plant responses to biotic and abiotic stresses.