The counteracting role of nitrate during ammonium toxicity in plants

Ammonium toxicity in plants remains poorly understood despite extensive research. While nitrate is known to benefit plant growth, the synergistic effects of nitrate in mitigating ammonium toxicity, even at low concentrations, are not fully elucidated. This review delves into the physiological and molecular nature of this phenomenon. To date, nitrate-dependent alleviation of ammonium toxicity is the result of cumulative consequences of the role of nitrate as a nutrient and signal in plant performance. The ability to counteract the ammonium-induced acidification through nitrate uptake and metabolism, the enhancement of potassium uptake as an essential nitrate counterion, and the nitrate-dependent signaling of key factors involved in ammonium assimilation, ROS scavenging, and growth hormone biosynthesis, are the most relevant hallmarks. In addition, evidence suggests that the availability of nitrate and ammonium has driven ecological selection in plants, determining current N preferences, and may have led to the selection of nitrate-dependent and ammonium-sensitive domesticated crops and the inefficient use of N fertilizers in agriculture. As ammonium toxicity limits N fertilization options and reduces agricultural yields, when it could be a more sustainable and cheaper alternative to nitrate, this review provides a better understanding of how plants use nitrate to counteract the problematic aspects of ammonium nutrition.