Ammonium aggravates salt stress in plants by entrapping them in a chloride over-accumulation state in an NRT1.1-dependent manner

Global climate change has exacerbated flooding in coastal areas affected by soil salinization. Ammonium (NH₄⁺) is the predominant form of nitrogen in flooded soils, but the role played by NH₄⁺ in the plant response to salt stress has not been fully clarified. We investigated the responses of Arabidopsis thaliana, Oryza sativa, and Nicotiana benthamiana plants fed with NH₄⁺. All species were hypersensitive to NaCl stress and accumulated more Cl^- and less Na⁺ than those fed with NO₃^-. Further investigation of A. thaliana indicated that salt hypersensitivity induced by the presence of NH₄⁺ was abolished by removing the Cl^- but was not affected by the removal of Na⁺, suggesting that excess accumulation of Cl^- rather than Na⁺ is involved in NH₄⁺-conferred salt hypersensitivity. The expression of nitrate transporter NRT1.1 protein was also up-regulated by NH₄⁺ treatment, which increased root Cl^- uptake due to the Cl^- uptake activity of NRT1.1 and the absence of uptake competition from NO₃^-. Knockout of NRT1.1 in plants decreased their root Cl^- uptake and retracted the NH₄⁺-conferred salt hypersensitivity. Our findings revealed that NH₄⁺-aggravated salt stress in plants is associated with Cl^- over-accumulation through the up-regulation of NRT1.1-mediated Cl^- uptake. These findings suggest the significant impact of Cl^- toxicity in flooded coastal areas, an issue of ecological significance.