The Arabidopsis Target of Rapamycin (TOR) kinase regulates ammonium assimilation and glutamine metabolism

In Eukaryotes, Target of Rapamycin (TOR) is a well conserved kinase that controls cell metabolism and growth in response to nutrients and environmental factors. Nitrogen (N) is an essential element for plants and TOR functions as a crucial N and amino acid sensor in animals and yeast. However, the knowledge on the connections between TOR and the overall N metabolism and assimilation in plants is still limited. In this study, we investigate the regulation of TOR in Arabidopsis by the N source as well as the impact of TOR deficiency on N metabolism. Inhibition of TOR globally decreases ammonium uptake while triggering a massive accumulation of amino acids such as Gln, but also of polyamines. Coherently, TOR complex mutants were found to be hypersensitive to Gln. We also show that the glutamine synthetase inhibitor glufosinate abolishes Gln accumulation resulting from TOR inhibition and improves the growth of TOR complex mutants. These results suggest that a high level of Gln contributes to the reduction in plant growth resulting from TOR inhibition. Glutamine synthetase activity was reduced by TOR inhibition while the enzyme amount increased. In conclusion our findings show that the TOR pathway is intimately connected to N metabolism and that a decrease in TOR activity results in a glutamine synthetase-dependent Gln and amino acids accumulation. One sentence summary The conserved Target of Rapamycin (TOR) kinase is an important sensor and regulator of the nitrogen metabolism and here we show that inhibiting this kinase affects ammonium uptake and results in Gln accumulation in a glutamine synthetase-dependent manner.