Exogenous Silicon Application Promotes Tolerance of Legumes and Their N2 Fixing Symbiosis to Salt Stress

Legumes, the second-most-important crop family, are a key source of biological nitrogen in agriculture and potentially contribute to sustainable cropping systems. Nevertheless, most legumes are salt sensitive, especially during biological nitrogen fixation (BNF). Therefore, improving legume growth and symbiosis efficiency under this constraint constitutes a great challenge to meet the increasing food demands and to protect soils from negative impacts of chemical fertilizers. In this perspective, silicon (Si) has been found to mitigate salt stress effect and improve legume development at the overall developmental stages. Whether direct or indirectly, Si counteracts salt stress effects on seed germination, plant growth and nodulation. The improvement of water uptake and nutrient homeostasis, the modification of gas exchange, the regulation of phytohormone and compatible solute biosynthesis and the regulation of the antioxidant metabolism under salinity are the key mechanisms evoked by plants upon Si treatment. Furthermore, during rhizobial symbiosis, Si has been shown to induce nodule formation and act on nodule functionality by increasing bacteroids and symbiosomes number, nitrogenase activity and leghemoglobin content under salinity. Here, we reviewed recent progress related to the role of exogenous Si in improving legume salt tolerance and highlighted the mechanisms through which Si could mediate salt tolerance. The needs of future research for better understanding how Si can promote salt tolerance in legumes are also addressed.