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

Plant Physiol. 2018 Dec;178(4):1473-1488. doi: 10.1104/pp.18.00410. Epub 2018 Oct 18.

Abstract

A high concentration of ammonium (NH4 +) 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 NH4 + 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 NH4NO3 and (NH4)2SO4 media to assess the function of NRT1.1 in NH4 + 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 (NH4)2SO4 medium. Grafting experiments between Col-0 and chl1-5 further confirmed that NH4 + toxicity is influenced by NRT1.1. In (NH4)2SO4 medium, NRT1.1 induced the expression of NH4 + transporters, increasing NH4 + 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 NH4 + accumulation. By contrast, the NRT1.1 mutants showed reduced NH4 + accumulation and enhanced NH4 + 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 (NH4)2SO4 suggests that ethylene is involved in NH4 + toxicity responses. This study showed that NH4 + toxicity is related to a nitrate-independent signaling function of NRT1.1 in Arabidopsis, characterized by enhanced NH4 + accumulation and altered NH4 + metabolism, which stimulates ethylene synthesis, leading to plant senescence.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Ammonium Compounds / pharmacokinetics*
  • Ammonium Compounds / toxicity*
  • Anion Transport Proteins / genetics
  • Anion Transport Proteins / metabolism*
  • Arabidopsis / drug effects*
  • Arabidopsis / genetics
  • Arabidopsis / metabolism
  • Carbon / metabolism
  • Enzymes / metabolism
  • Ethylenes / metabolism
  • Gene Expression Regulation, Plant / drug effects
  • Mutation
  • Plant Proteins / genetics
  • Plant Proteins / metabolism*
  • Plant Roots / drug effects
  • Plant Roots / genetics
  • Plant Roots / metabolism
  • Plant Shoots / drug effects
  • Plant Shoots / genetics
  • Plant Shoots / metabolism
  • Signal Transduction

Substances

  • Ammonium Compounds
  • Anion Transport Proteins
  • Enzymes
  • Ethylenes
  • NRT1.1 protein, Arabidopsis
  • Plant Proteins
  • Carbon
  • ethylene