Stress-Responsive Regulatory Networks in Legume–Rhizobium Symbiosis: Implications for Climate-Smart Agriculture

Legume–rhizobium symbiosis is fundamental to sustainable agriculture because it supplies biologically fixed nitrogen, improves soil fertility, and reduces reliance on synthetic fertilizers. However, abiotic and chemical stresses, including drought, salinity, flooding, temperature extremes, heavy metals, and organic pollutants, disrupt nodulation and biological nitrogen fixation, limiting crop productivity and ecosystem sustainability. This review synthesizes current knowledge of the regulatory networks that enable legume–rhizobium symbiosis to adapt to environmental stress. We discuss how stress influences symbiotic signaling, infection, oxygen homeostasis, nitrogenase protection, phytohormonal regulation, antioxidant defenses, exopolysaccharide production, and plasmid-mediated adaptation. We further highlight the roles of root nodule-associated microorganisms and microbial interactions in maintaining symbiotic stability under adverse conditions. Finally, recent advances in multi-omics, genome editing, synthetic biology, and microbial consortia are evaluated for their potential to improve stress-resilient bioinoculants. Collectively, this review emphasizes that resilience of the legume–rhizobium symbiosis is an integrated property of both plant and microbes and identifies key regulatory mechanisms that can be exploited to develop climate-resilient and sustainable agricultural systems.