NRT1.1-Related NH4+ Toxicity Is Associated with a Disturbed Balance between NH4+ Uptake and Assimilation

A high concentration of ammonium (NH 4 + ) as the sole source of nitrogen in the growth medium often is toxic to plants. The nitrate transporter NRT1.1 is involved in mediating the effects of NH 4 + toxicity; however, the mechanism remains undefined. In this study, wild-type Arabidopsis ( Arabidopsis thaliana Columbia-0 [Col-0]) and NRT1.1 mutants ( chl1-1 and chl1-5 ) were grown hydroponically in NH 4 NO 3 and (NH 4 ) 2 SO 4 media to assess the function of NRT1.1 in NH 4 + stress responses. All the plants grew normally in medium containing mixed nitrogen sources, but Col-0 displayed more chlorosis and lower biomass and photosynthesis than the NRT1.1 mutants in (NH 4 ) 2 SO 4 medium. Grafting experiments between Col-0 and chl1-5 further confirmed that NH 4 + toxicity is influenced by NRT1.1. In (NH 4 ) 2 SO 4 medium, NRT1.1 induced the expression of NH 4 + transporters, increasing NH 4 + uptake. Additionally, the activities of glutamine synthetase and glutamate synthetase in roots of Col-0 plants decreased and soluble sugar accumulated significantly, whereas pyruvate kinase-mediated glycolysis was not affected, all of which contributed to NH 4 + accumulation. By contrast, the NRT1.1 mutants showed reduced NH 4 + accumulation and enhanced NH 4 + assimilation through glutamine synthetase, glutamate synthetase, and glutamate dehydrogenase. Moreover, the up-regulation of genes involved in ethylene synthesis and senescence in Col-0 plants treated with (NH 4 ) 2 SO 4 suggests that ethylene is involved in NH 4 + toxicity responses. This study showed that NH 4 + toxicity is related to a nitrate-independent signaling function of NRT1.1 in Arabidopsis, characterized by enhanced NH 4 + accumulation and altered NH 4 + metabolism, which stimulates ethylene synthesis, leading to plant senescence.