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.