The protective action of antioxidant and nitrogen fixation systems in soybean–Bradyrhizobium symbiosis under varying water conditions

Abstract

Abstract Background and Aims Legumes can be supplied with environmental nitrogen and protected against climate change by the formation of effective symbiotic relationships with nodule bacteria. To create effective stress-resistant symbiotic systems with optimal nitrogen fixation potential and high tolerance to water stress, it is important to understand the roles of key pro-oxidant and antioxidant systems in the response to different water supplies. Methods Various symbiotic systems of soybean with Bradyrhizobium strains, differing in activity and virulence, were subjected to microbiological, biochemical and physiological testing. Results While antioxidant enzyme activity generally increased with length of water stress, it varied according to the enzyme complex and symbiotic system. Soybeans inoculated with effective rhizobia strains demonstrated effective regeneration after stress, with enzyme levels quickly recovering to control levels. All systems exhibited lipid peroxidation activation as a universal reaction to water deficit; however, the intensity varied with the level of antioxidant protection in the symbiosis. The less-effective and non-effective systems had less antioxidant protection, and thus high hydrogen peroxide and lipid peroxidation levels. The effective symbiotic systems exhibited minimal differences in nodulation between control and stressed plants, and efficient nitrogen fixation after re-watering. Conclusions The soybean-rhizobial systems that effectively used key antioxidant enzymes to regulate oxidative processes were able to more effectively maintain their optimal prooxidant-antioxidant balance and better tolerate poor water supply. However, the coordinated participation of both the macrosymbiont and microsymbiont was needed to maximize nitrogen fixation and support their protective mechanisms in conditions of water stress.