The Plant-Rhizobial Symbiotic Interactions Provide Benefits to the Host beyond Nitrogen Fixation That Promote Plant Growth and Productivity

Rhizobial symbiotic interactions are known for nitrogen fixation, providing commercial crops and other plants with self-sufficiency in nitrogen requirements. An enormous contribution from nitrogen fixation is vital to the global nitrogen cycle. The symbiotic nitrogen reduces the carbon footprint of crop cultivation, which underlines its importance in agricultural sustainability. Extensive research efforts have been made to understand the symbiotic relationship at molecular, physiological, and ecological levels. This led to the isolation and modification of symbiotic strains for enhanced nitrogen efficiency. During the evaluation of strains for nitrogen fixation in exchange for supporting the bacterium in terms of space and resources, it has been observed that the accrued benefits to the host plants extend well beyond the nitrogen fixation. The symbiotic interaction has been advantageous to the host for better growth and development, tolerating a stressful environment, and even keeping the pathogenic microbial enemies at bay. Additionally, it enabled the availability of the mineral nutrients, which otherwise were inaccessible to the host. In this chapter, we bring together the information with a focus on the role of rhizobial symbiotic interactions that promote plant growth and productivity through phytohormone synthesis, by facilitating the availability of mineral nutrients, and by improving the plant tolerance to sub-optimal growth conditions.