Potential of Rhizosphere Bacteria for Improving Rhizobium-Legume Symbiosis

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About 60 % of the earth’s available nitrogen is fixed via biological nitrogen fixation (BNF). Being a major contributor to BNF, Rhizobium-legume symbiosis can provide well over half of the biological source of fixed nitrogen. Actually, Rhizobium-legume symbiosis results in the formation of nodules on legume roots where rhizobia fix nitrogen from the atmosphere. But nodulation and nitrogen fixation is a complex process and is dependent on the compatibility and potential of both partners of Rhizobium-legume symbiosis under variable soil and environmental conditions. Although, some selected efficient and effective traits of rhizobia and legumes have shown encouraging results, there is a need of consistent positive influence on nodulation and nitrogen fixation to maximize the growth and yield of legumes under variable conditions. Hence, the use of means capable of improving both the legume growth and the growth and function of symbiotic rhizobia is essential. Co-inoculation of Rhizobium species with favorably interacting traits of plant growth-promoting rhizobacteria (PGPR) is considered an applied, cost-effective, efficient, and environment-friendly approach to further improve legume growth and productivity under variable conditions because they can provide broad spectrum mechanisms of actions and improve reliability of inocula without genetic engineering. In addition, these PGPR when used in combination with rhizobia have also shown the strategies for dealing with stressful conditions like salinity, pH, temperature, drought, heavy metal, and pathogens which could further impose limitations on the capacity of Rhizobium-legume symbiosis. This chapter highlights various PGPR traits compatible with specific legume rhizobia and their phytostimulatory mechanisms contributing to augmentation in rhizobial growth and function for growth and yield enhancement of legumes under variable conditions.