Legume-Rhizobia Symbiosis Under Stress

Legume-rhizobia symbioses are of practical importance in provision of sustained food supply. Currently, it provides approximately 45 % of N used in agriculture. This quantity will have to be augmented as the world’s population increases and natural resources used in production of fertilizer-N diminish. The major constraints for food security are soil salinity, sodicity, nutrient deficiency, and toxicity which are driven by indiscriminate use of agrochemicals including chemical fertilizers and pesticides, utilization of excess water resource coupled with climate changes, and periodic droughts. In this scenario, legume-rhizobia symbioses are nonpolluting, cost-effective ways to enhance high N2-fixing potential under stress conditions. Several symbiotic systems that are tolerant in extreme conditions of salinity, alkalinity, acidity, drought, toxic doses of fertilizer, and metal toxicity have been identified including rhizobial symbiosis in arid region. Extreme environmental conditions adversely affect rhizobia-legume interactions wherein rhizobial partner utilizes endogenous or exogenous osmolytes and secretes specific proteins to alleviate the problem of aridity, salinity, and toxicity. Thus, change in rhizobial population can be an indicator of soil fertility. Hence, osmoadapted rhizobial strains can be used as biofertilizers for salt-sensitive crops in saline soils. In this context, interaction among rhizobia, plant growth-promoting rhizobacteria (PGPR), and mycorrhiza are also important. Here, we give an account of relevance of biological nitrogen fixation (BNF) in sustained food supply, effects of extreme conditions on legume-rhizobia symbiosis, as well as interaction of rhizobia with belowground microbial diversity including mycorrhiza and adaptive strategies of rhizobia under condition of stress. We also discuss about models wherein osmotolerant rhizobia can be used as biofertilizers and sustain green revolution to evergreen revolution.