NADH-dependent glutamate synthase plays a crucial role in assimilating ammonium in the Arabidopsis root

Abstract

Plant roots under nitrogen deficient conditions with access to both ammonium and nitrate ions, will take up ammonium first. This preference for ammonium rather than nitrate emphasizes the importance of ammonium assimilation machinery in roots. Glutamine synthetase (GS) and glutamate synthase (GOGAT) catalyze the conversion of ammonium and 2-oxoglutarate to glutamine and glutamate. Higher plants have two GOGAT species, ferredoxin-dependent glutamate synthase (Fd-GOGAT) and nicotinamide adenine dinucleotide (NADH)-GOGAT. While Fd-GOGAT participates in the assimilation of ammonium, which is derived from photorespiration in leaves, NADH-GOGAT is highly expressed in roots and its importance needs to be elucidated. While ammonium as a minor nitrogen form in most soils is directly taken up, nitrate as the major nitrogen source needs to be converted to ammonium prior to uptake. The aim of this study was to investigate and quantify the contribution of NADH-GOGAT to the ammonium assimilation in Arabidopsis (Arabidopsis thaliana Columbia) roots. Quantitative real-time polymerase chain reaction (PCR) and protein gel blot analysis showed an accumulation of NADH-GOGAT in response to ammonium supplied to the roots. In addition the localization of NADH-GOGAT and Fd-GOGAT did not fully overlap. Promoter-β-glucuronidase (GUS) fusion analysis and immunohistochemistry showed that NADH-GOGAT was highly accumulated in non-green tissue like vascular bundles, shoot apical meristem, pollen, stigma and roots. Reverse genetic approaches suggested a reduction in glutamate production and biomass accumulation in NADH-GOGAT transfer DNA (T-DNA) insertion lines under normal CO2 condition. The data emphasize the importance of NADH-GOGAT in the ammonium assimilation in Arabidopsis roots.

Effect of brassinosteroids on ammonium uptake via regulation of ammonium transporter and N-metabolism genes in Arabidopsis

“…Furthermore, NH 4 + assimilation-related enzyme expressions were analyzed. In Arabidopsis roots, GS/GOGAT genes have been reported to be induced following NH 4 + re-supply (Konishi et al 2014), and the six genes shown in Fig. 4 that encode GS or GOGAT are differentially expressed in bes1-D compared to En2 (Yu et al 2011).…”

“…Furthermore, the regulatory mechanism underlying this activity has been determined to involve NH 4 + assimilation products, rather than NH 4 + itself being responsible for AMT expression (Rawat et al 1999). In Arabidopsis roots, GS/GOGAT genes, encoding glutamine synthetase and glutamine oxoglutarate aminotransferase, also known as glutamate synthase, are upregulated following re-supply of NH 4 + (Konishi et al 2014). Phytohormones play important roles in NH 4 + -related plant growth and stress responses.…”

Lack of malate valve capacities lead to improved N-assimilation and growth in transgenicA. thalianaplants

“…These results indicate altered patterns in photorespiration in mutants lacking NADP-MDH 12 . Since Fd-GOGATs play an important role in photorespiratory ammonium assimilation, 15 the expression pattern of both Fd-GOGAT isoforms was analyzed in nadp-mdh knockout plants (Fig. 2).…”

“…1). While the NADH-GOGAT is crucial for ammonium assimilation in roots, 15 Fd-GOGATs contribute to the assimilation of ammonium derived from photorespiration which doesn’t seem to be influenced in line 159.…”

“…Due to the fact that plNAD-MDH as well as NADH-GOGAT are both mainly expressed in plastids of non-green tissues, 11 , 14 it was very interesting that recently a tissue-specific expression pattern during the life cycle of Arabidopsis for NADH-GOGAT was demonstrated 15 which is very similar to our findings concerning the plNAD-MDH expression during plant growth 11 . Both analyses were performed using gene promoter-GUS fusions and show a high β-glucuronidase activity in the tip of leaves and the root tip 11 , 15 .…”