Mutation of NRT1.1 enhances ammonium/low pH-tolerance in Arabiopsis thaliana

NRT1.1 is a dual-affinity nitrate transporter and a nitrate sensor that plays a role in nitrate-dependent signaling pathway. Recently it has been revealed that NRT1.1 mutants show enhanced tolerance to ammonium and/or low pH conditions in the absence of nitrate, which indicates a nitrate-independent function of NRT1.1. Detailed underlying mechanisms for the ammonium/low pH-tolerance of the NRT1.1 mutants will be revealed by paying attention to auxin behavior, phosphorylated status of NRT1.1 and other components related to the primary nitrate response and nitrate transport

Excessive assimilation of ammonium by plastidic glutamine synthetase is a major cause of ammonium toxicity in Arabidopsis thaliana

Plants use nitrate and ammonium in the soil as their main nitrogen sources. Recently, ammonium has attracted attention due to evidence suggesting that, in C3 species, an elevated CO2 environment inhibits nitrate assimilation. However, high concentrations of ammonium as the sole nitrogen source for plants causes impaired growth, i.e. ammonium toxicity. Although ammonium toxicity has been studied for a long time, the primary cause remains to be elucidated. Here, we show that ammonium assimilation in plastids rather than ammonium accumulation is a primary cause for toxicity. Our genetic screen of ammonium-tolerant Arabidopsis lines with enhanced shoot growth identified plastidic GLUTAMINE SYNTHETASE 2 (GLN2) as the causal gene. Our reciprocal grafting of wild-type and GLN2 or GLN1;2-deficient lines suggested that shoot GLN2 activity results 34 in ammonium toxicity, whilst root GLN1;2 activity prevents it. With exposure to toxic levels of ammonium, the shoot GLN2 reaction produced an abundance of protons within cells, thereby elevating shoot acidity and stimulating expression of acidic stress responsive genes. Application of an alkaline ammonia solution to the toxic ammonium medium efficiently alleviated the ammonium toxicity with a concomitant reduction in shoot acidity. Consequently, we conclude that a primary cause of ammonium toxicity is acidic stress in the shoot. This fundamental insight provides a framework for enhanced understanding of ammonium toxicity in plants