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

Abstract 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.