Factors Impacting Rhizobium-legume Symbiotic Nitrogen Fixation with the Physiological and Genetic Responses to Overcome the Adverse Conditions: A Review

Abstract

Symbiotic N2 fixation is essential for the plant’s growth because it can fix reactive nitrogen compounds in soil. However, all steps of this process can be hampered by several biotic and abiotic environmental factors. This study mainly focused on discussing the impacts of 12 major factors on this process by reviewing the significant numbers of research works. According to the information from these works, we found some significant physiological and genetic impacts caused by these factors like plasmid deletion, genomic mismanagement, abnormal molecular signals, toxicity, deficiency of minerals, deformation of rhizobial cells, protein denaturation, nucleic acid damage, acetylene reduction and nod factors limitation. Furthermore, expression of heat or acid shock proteins, internal buffering, genes spanning, extracellular immobilization, periplasmic allocation, change of lipopolysaccharides composition, intracellular accumulation of inorganic and organic solutes (Osmolyte) and activation of hydrogenase expression are shown by both micro and macro symbionts as a natural response to adapt to these stress conditions. Though the stress-tolerant strains like HR-3, HR-6, HR-10, HR-12, acta, actP, exoR, lpiA, actR, actS and phrR can be used to sense the external environment and make signals to change gene transcription during the adverse condition, the application of genetic engineering should be expanded more to promote the commercial inoculation by the production of novel stress-tolerant strains or modified genes of rhizobia and legumes.