Legume–Rhizobia Symbiosis and Interactions in Agroecosystems

In the present scenario, when the population of the world is expected to become 8–9 billion by 2040, the major concern is to maintain sustained food supply. Production of high-quality protein-rich food is extremely dependent on the availability of sufficient nitrogen. Nitrogen though abundant on Earth is unavailable to plants. Indiscriminate use of nitrogenous chemical fertilisers has significantly increased food production and quality but at the same time affected ecosystem sustainability. Hence, the process of biological nitrogen fixation (BNF) has gained considerable significance. BNF is both free-living as well as symbiotic. Symbiotic N2 fixation accounts for about 65 % of the total biologically fixed nitrogen. Frankia and rhizobia are two groups that fix atmospheric nitrogen symbiotically. Out of these, rhizobia–legume symbiosis accounts for about 45 % of nitrogen being used in agriculture. Rhizobia and legumes both are diverse. Currently 98 species of legume-nodulating bacteria have been identified within 13 bacterial genera, 11 in α-proteobacteria, whereas 2 in β-proteobacteria. Similarly, 13,000 species have been identified in 700 legume genera. Specificity of nodulation is an important attribute of legume–rhizobia symbiosis and is governed by both legume and rhizobial signals. For any successful legume–rhizobia symbiosis, interaction with other belowground microbes like AM fungi is also important. Here we give an account of rhizobial diversity and systematics, signals governing legume–rhizobia symbiosis, genes regulating nodulation and nitrogen fixation and legume–rhizobia–AM interactions.