Implication of the glutamine synthetase/glutamate synthase pathway in conditioning the amino acid metabolism in bundle sheath and mesophyll cells of maize leaves

Abstract

In the C 4 plant maize, inorganic nitrate reduction to ammonium and subsequent ammonium assimilation into amino acids occur in two different photosynthetic cells: bundle sheath cells (BSCs) and mesophyll cells (MCs). Nitrate taken up by roots moves in part, via the vascular bundle, to leaves for reduction. We investigated the role of glutamine synthetases (cytosolic GS1 and chloroplast GS2) and glutamate synthases (ferredoxin-GOGAT and NADH-GOGAT) in the inorganic nitrogen assimilation and reassimilation into amino acids between bundle sheath cells and mesophyll cells for the remobilization of amino acids during the early phase of grain filling in Zea mays L. The plants responded to a light ⁄ dark cycle at the level of nitrate, ammonium and amino acids in the second leaf, upward from the primary ear, which acted as the source organ. The assimilation of ammonium issued from distinct pathways and amino acid synthesis were evaluated from the diurnal rhythms of the transcripts and the encoded enzyme activities of nitrate reductase, nitrite reductase, GS1, GS2, ferredoxin-GOGAT, NADH-GOGAT, NADH-glutamate dehydrogenase and asparagine synthetase. We discerned the specific role of the isoproteins of ferredoxin and ferredoxin:NADP + oxidoreductase in providing ferredoxin-GOGAT with photoreduced or enzymatically reduced ferredoxin as the electron donor. The spatial distribution of ferredoxin-GOGAT supported its role in the nitrogen (re)assimilation and reallocation in bundle sheath cells and mesophyll cells of the source leaf. The diurnal nitrogen recycling within the plants took place via the specific amino acids in the phloem and xylem exudates. Taken together, we conclude that the GS1 ⁄ ferredoxin-GOGAT cycle is the main pathway of inorganic nitrogen assimilation and recycling into glutamine and glutamate, and preconditions amino acid interconversion and remobilization.

Novel aspects of glutamine synthetase (GS) regulation revealed by a detailed expression analysis of the entire GS gene family of Medicago truncatulaunder different physiological conditions

“…However there are very few reports on the diurnal regulation of GS expression, and most of them have been devoted to roots [28-32]. Our results reveal that the circadian clock influences the expression of GS genes in M. truncatula at the transcriptional, translational and post-translational levels.…”

“…MtGS2a mRNA accumulates during the start of the light period and this is accompanied by a slight increase in polypeptide abundance, however total GS activity in the leaves decreases within the first 30 minutes of light. Similarly, it was reported that ZmGln2 mRNA peaks early in the light period in maize leaves [32]. The main function of the plastid-located GS in leaves of C 3 plants is the assimilation of ammonium resulting from photorespiration [13].…”

Interactions of C4 Subtype Metabolic Activities and Transport in Maize Are Revealed through the Characterization of DCT2 Mutants

“…As shown in Figure 6C, the linear incorporation of 14 C into polar metabolites including 2-OG was similar or slightly less in dct2-1 relative to the wild type and suggested that nitrogen exchange in BS cells was not significantly compromised by the mutation. Although the malate availability for counter-port activity could be compromised in dct2-1 plants, alternative reactions catalyzed by glutamine or asparagine synthetase, glutamate synthase, and glutamate dehydrogenase outside of the chloroplast could also be involved in ammonia recycling (Becker et al, 2000;Miflin and Habash, 2002;Valadier et al, 2008). Transcriptionally upregulated asparagine synthetase (Supplemental Data Set 1) and the accumulation of asparagine (Supplemental Table 1) were observed in dct2-1.…”

Assimilation of excess ammonium into amino acids and nitrogen translocation in Arabidopsis thaliana– roles of glutamate synthases and carbamoylphosphate synthetase in leaves

“…Inasmuch as arginine synthesis from CP relies on the regulation of glutamate conversion to ornithine [6], we studied the impact of CPSase on overall arginine synthesis in the carB mutant. In fact, amino acid synthesis is tightly correlated with amino acid transport under the fine control of the cellular and subcellular expression of the nitrogen assimilatory genes and of the encoded enzymes [12]. Despite their primary importance, the spatial location and expression patterns have not been investigated for Fd‐GOGAT isoenzymes, NADH‐GOGAT and CPSase in Arabidopsis .…”