Enhanced alkali tolerance of rhizobia-inoculated alfalfa correlates with altered proteins and metabolic processes as well as decreased oxidative damage

Abstract Aims Alkaline salt is one of the most devastating environmental factors limiting alfalfa productivity, however, the mechanisms underlying adaptation of alfalfa to alkaline remain unclear. Our aim is to investigate proteomic and metabolomic differences in growth and root of alfalfa under alkaline salt in Rhizobium-alfalfa symbiotic relationships. Methods Rhizobium-inoculated and non-inoculated alfalfa plants were treated with 200 mmol/L NaHCO3 to investigate physiological, metabolic, and proteomic responses of root-nodule symbiosis under alkaline-induced stress, using an integrated approach combining metabolome and proteome analysis with measurements of physiological parameters. Results The improved tolerance to alkalinity was observed in RI-plants compared with NI-plants. RI-plants accumulated more proline and MDH, and had higher antioxidant activity and relatively high RWC but low MDA content and low Na+/K+ ratio. The stress-related genes (P5CS, GST13, H+-Ppase, NADP-Me, SDH, and CS) were actively upregulated in RI plants under alkaline stress. In RI-plants, damage caused by alkaline stress was mainly alleviated by decreasing oxidative damage, enhancing the organic acid and amino acid metabolic processes, and scavenging harmful ROS by activating the phenylpropanoid biosynthetic pathway. Conclusions We revealed distinct proteins and metabolites related to alkali tolerance in RI-plants compared to NI-plants. Alkali tolerance of rhizobia-inoculated alfalfa was enhanced by altered proteins and metabolic processes as well as decreased oxidative damage.