Coregulation of glutamine synthetase1;2 (GLN1;2) and NADH-dependent glutamate synthase (GLT1) gene expression in Arabidopsis roots in response to ammonium supply

Abstract

Ammonium absorbed by roots is assimilated into amino acids. The glutamine synthetase/glutamate synthase (glutamine 2-oxoglutarate aminotransferase) (GS/GOGAT) cycle is essential to this biological process. In Arabidopsis thaliana, GLN1;2 and GLT1 are the GS and GOGAT isoenzymes induced in response to ammonium supply and playing key roles in ammonium utilization. Although recent studies suggest gene regulatory networks involved in transcriptional regulation of ammonium-responsive genes, direct regulatory mechanisms for ammonium-induced expression of GS/GOGAT remain unclear. In this study, we revealed that the expression of GLN1;2 and GLT1 in Arabidopsis is not directly induced by ammonium but is regulated by glutamine or post-glutamine metabolites produced by ammonium assimilation. Previously, we identified a promoter region required for ammonium-responsive expression of GLN1;2. In this study, we further dissected the ammonium-responsive region of the GLN1;2 promoter and also performed a deletion analysis of the GLT1 promoter, which led to the identification of a conserved ammonium-responsive region. Yeast one-hybrid screening using the ammonium-responsive region of the GLN1;2 promoter as a decoy sequence revealed a trihelix family transcription factor DF1 that binds to this region. A putative DF1 binding site was also found in the ammonium-responsive region of the GLT1 promoter.

Nitrogen Journey in Plants: From Uptake to Metabolism, Stress Response, and Microbe Interaction

“…These findings suggest that the NH 4 + -responsive forms of GS/GOGAT have a central role in the primary assimilation of NH 4 + within the roots [188]. As the expression of these isoenzyme-encoding genes increases with NH 4 + availability, it is plausible to infer that plants possess a regulatory transcriptional network that modulates their gene expression in response to NH 4 + availability [185].…”

“…The coordinated action of GS and GOGAT is responsible for these interconnected processes [181][182][183]. Experimental studies involving reverse genetic analysis have demonstrated that the absence of genes encoding NH 4 +responsive GS/GOGAT isoenzymes reduces NH 4 + assimilation, particularly in the roots, resulting in impaired growth [184][185][186][187]. These findings suggest that the NH 4 + -responsive forms of GS/GOGAT have a central role in the primary assimilation of NH 4 + within the roots [188].…”

“…In addition, the advancement of genome sequencing has unveiled various isoenzymes of GS/GOGAT present in plants. Among these, specific isoenzymes such as GS1,2 and NADH-GOGAT1 from rice [198] as well as GLN1,2 and NADH-GOGAT (GLT1) from Arabidopsis [185,199] are expressed in plant roots in response to NH 4 + supply. These isoenzymes' expression at transcript and protein levels highlights their specificity in NH 4 + assimilation.…”