Ammonia: An Emerging Gasotransmitter in Plant Growth and Response to Environmental Stress

Ammonia (NH₃), which is an intermediate of nitrogen metabolism, has been found to be a gasotransmitter in plants. It has a dual physiological effect in a concentration-dependent manner, namely as a signaling molecule at low concentrations and a cytotoxin at high concentrations. In plants, NH₃, as a gasotransmitter, can maintain homeostasis by multiple pathways, which are involving in biosynthesis, assimilation, and transport. Gasotransmitter NH₃ can regulate plant growth, development, and response to multiple environmental stresses by crosstalk with other signaling molecules. However, a few reviews have summarized NH₃ homeostasis and its signaling role in plant growth and stress response. Hence, in this review, based on the progress in NH₃, whose toxicity, metabolism, and membrane transport were summarized. Also, the signaling role of NH₃ in cell division, seed germination, and root system architecture was discussed. Furthermore, NH₃-induced stress resistance, including drought, heat, salt, iron deficiency, elevated CO₂, and pathogen infection tolerance, was summed up. This review is to further understanding the gasotransmitter role of NH₃, and lays the foundation for designing and developing climate-resilient crops for food safety and sustainable agriculture.