Pivotal role of nitric oxide in regulating beneficial plant-microbe interactions under both physiological and stress conditions.

Plant-microbe interactions involve a complex communication network, where microbes can serve as either beneficial partners or harmful pathogens. Nitric oxide (NO) is a crucial signaling molecule that regulates plant growth and development, while enhancing tolerance to both biotic and abiotic stresses. It does this by interacting directly with reactive oxygen species (ROS) and regulating antioxidant enzyme activity through post-translational modifications (PTMs), such as S-nitrosation and nitration. Additionally, NO affects transcription factors that regulate gene expression and modulates phytohormone signaling, which helps alleviate oxidative stress. NO plays a pivotal role in symbiotic relationships between plant roots and microbes. In legumes, it is essential for rhizobial recognition, root hair curling, infection thread formation, nodule initiation, development, and senescence. Similarly, NO supports positive interactions with mycorrhizal fungi and plant growth-promoting rhizobacteria (PGPR), which enhance nutrient availability, particularly nitrogen and phosphorus, and improve plant resilience under stress. At the molecular level, NO regulates symbiosis-related gene expression and modifies proteins through PTMs such as S-nitrosation of nitrogenase and tyrosine nitration of glutamine synthetase and leghemoglobin, while maintaining cellular redox balance. This review provides an updated and comprehensive overview of NO signaling in beneficial plant-microbe interactions under both physiological and stress conditions. It further analyzes the specific sources, regulatory mechanisms, and molecular targets of NO, which remain poorly understood. In addition, it highlights the application of omics-based approaches to unravel these processes, offering insights that could guide the development of new strategies to enhance crop productivity through the targeted exploitation of symbiotic associations.