Recent Advances in Biological Nitrogen Fixation in Agricultural Systems

Abstract

The environmental concerns due to the increasing amount of the reactive forms of nitrogen in atmosphere, originating from the manufacture and use of chemical fertilizers have resulted in a re-focus on the importance of biological nitrogen fixation (BNF), particularly by legumes. The worldwide contribution of BNF is estimated at 105 Tg yr. Nitrogen fixation is catalysed by the enzyme nitrogenase. Advances have included discovery of alternative nitrogenases, elucidation of more details on the mechanism of action, discovery of novel nitrogenases, existence of nitrogenase siblings in eukaryotes etc. There are fresh efforts to engineer cereal plants to nodulate and fix nitrogen. Discovery of common signalling mechanisms between mycorrhization and nodulation, discovery of cereal endophytes and advances in plastid physiology have given a fresh impetus to such approaches. However, the contribution of agronomically significant quantities of N through endophytes still requires more rigorous evidences. Advances in molecular biology have included a greater understanding of the native diversity of rhizobia and together with advances in plant breeding for selection of high nodulating genotypes, presents an attractive opportunity to maximize BNF. Recent advances include the discovery of novel rhizobial species, emphasis on study of climate change and adaptation to various abiotic stresses like high temperature, drought and salinity. This review focuses on major developments in BNF in last two decades with emphasis on rhizobia. The beneficial effects of applying organics and of rhizobial inoculation practices are discussed to bring out the strategies to improve the contribution of BNF in agriculture.