Abstract
Four distinct isoenzymes of glutamine synthetase (GS1) are known to share their functions in maintaining ammonium assimilation in Arabidopsis root. Among them, GLN1;2 showed unique characteristics favorable for an ammonium-rich environment in root: (i) transcripts of GLN1;2 were highly accumulated by ammonium supplementation; (ii) GLN1;2 showed low substrate affinity for ammonium (Km = 2.5 mM); (iii) GLN1;2 promoter activity was localized in root epidermis and root hairs in response to ammonium supplementation, thus, it is likely that GLN1;2 is one of the most essential components in primary ammonium assimilation in roots. We further investigated the GLN1;2 promoter region responsive to ammonium supplementation using GFP as a reporter. The ammonium-responsive region was narrowed down to a 21-bp sequence which was located 3.6-kb upstream of GLN1;2 coding region and was essentially required for driving the expression of GFP in root epidermis and root hairs under ammonium-rich conditions.
