Nitrogen-dependent regulation of cytosolic glutamine synthetase in Arabidopsis roots

Abstract

Glutamine synthetase (GS) catalyzes the synthesis of glutamine from ammonium and glutamate. Four distinct cytosolic GS (GS1, GLN1;1-4) are expressed in Arabidopsis roots. Characterization of the recombinant GS1 enzymes, and the analysis of nitrogen response and cell-type specific expression of GS1 isoforms indicated that GLN1;1 isoenzyme showed high affinities to ammonium and glutamate, and its expression was induced in the epidermis of the roots during nitrogen limitation. In addition, genes for ammonium transporters (AMT1;1 and AMT1;3) were also expressed in the epidermis under the same condition. By contrast, the low-affinity isoenzymes, GLN1;2 and GLN1;3, were localized at the vasculature. GLN1;2 mRNA was increased by application of ammonium, and the activity of GLN1;3 was significantly inhibited in the presence of high concentration of Glu.