Environmental stress conditions affecting the N2 fixing Rhizobium-legume symbiosis and adaptation mechanisms

Rhizobia are bacteria which fix atmospheric nitrogen in association within the root or the stem nodules of legume plants and transform atmospheric nitrogen to ammonia. Biological nitrogen fixation is an important process for sustainable land management, because nitrogen is the principal crop production’s limiting factor. However, several environmental conditions such as salinity, temperature, acidity/alkalinity, drought, heavy metals, etc., are critical factors which can have detrimental effects on the steps involved in Rhizobium-legume symbiosis as infection process, nodule’s development and function, resulting in low nitrogen fixation and crop yield. The presence of Rhizobium- legume symbioses able to fix appreciable N2 amounts under unfavorable conditions is very interesting, because these symbioses represent the best source of nitrogen especially in arid and semi-arid regions, where they contribute to land stabilization and fertilization. Hence, the better understanding of rhizobial physiological responses to different intrinsic and extrinsic stresses factors is very important to improve crop production by harnessing biological nitrogen fixation process.