This book covers aspects of biological nitrogen fixation along with the unique signaling and interaction between the diazotrophic bacteria and plants, especially the non-legumes. Nitrogen is the most important growth-limiting nutrient in the ecosystems and biological nitrogen fixation involving microbial symbionts, mainly rhizobia and legumes holds enormous interest across the globe. However, free-living rhizobacteria of non-legumes especially cereals, also establish themselves within the root system, fixing nitrogen and contributing to plant productivity, soil fertility, and agricultural...