Glutamine synthetase (GS), the key enzyme in plant nitrogen assimilation, is strictly regulated at multiple levels, but the most relevant reports focus on the mRNA level.

1. Introduction

Nitrogen (N) is a key limiting factor in the yield and quality of crops, and large quantities of nitrogen fertilizers are required to attain maximal growth and productivity [1,2]^[1][2]. To increase crop production, nitrogen fertilizers are often applied excessively, leading to severe nitrogen pollution on a global scale [3,4]^[3][4]. Therefore, there is a need to improve nitrogen use efficiency (NUE) to make agriculture more sustainable [4,5]^[4][5].

In order to improve crop NUE, glutamine synthetase (GS; EC 6.3.1.2) has been studied numerous times owing to its essential role in the assimilation of inorganic N [1,6,7,8,9,10]^{[1][6][7][8][9][10]}. GS catalyzes the ATP-dependent fixation of ammonium (NH4+) to glutamate (Glu) to form glutamine (Gln) [11]^[11]. Plant GS is classified into two groups according to its subcellular location: Cytosolic glutamine synthetase (GS1) and chloroplast glutamine synthetase (GS2) [12,13]^[12][13]. GS2 is encoded by a single gene, while GS1 is encoded by a multigene family [5]^[5].

2. GS isozymes are Involved in Gln Synthesis

Although all GS isozymes are involved in Gln synthesis, GS isozymes play different roles in nitrogen assimilation or transportation in plants. GS2 is involved in assimilating NH4+ derived from photorespiration and nitrate (NO3−) reduction [14,15]^[14][15]. GS1 has multiple isoforms with distinct affinities for NH4+ and glutamate [16^[16][17],17], and each GS1 isoform may have a different function in nitrogen assimilation or transportation. Wheat is an important crop for mankind. individual wheat GS (TaGS) isozymes are classified into four subfamilies: TaGS1, TaGSr, TaGSe, and TaGS2 [9]^[9]. Thomsen et al. clustered GS isozymes of cereals into four categories: GS1;1, GS1;2, GS1;3, and GS2 [5]^[5]. Based on the cluster of TaGS isoforms, researchers renamed TaGS1, TaGSr, and TaGSe genes as TaGS1;1, TaGS1;2, and TaGS1;3, respectively.

The physiological functions of GS isozymes have been studied according to the cellular localization and expression characteristics. In Arabidopsis, the green fluorescent protein (GFP) signal driven by the AtGln1;1 promoter is recorded in the epidermal cells of the root elongation zone and can affect primary root development in response to exogenous N provision [18]^[18]. The promoter of AtGln1;2 can drive reporter gene expression in the mesophyll and vasculature of developed leaves [19,20]^[19][20]; vascular cells, cortex, and epidermis of roots [18,21]^[18][21]; epidermal cells of sepals; and veins of petals and stamens [18]^[18]. The mRNA level of AtGln1;2 can be upregulated to relieve NH4+ toxicity under ample nitrate (NO3−) supply and high NH4+ supply conditions [19,20,21]^[19][20][21]. Promoter::GFP fusion has shown that AtGln1;3 expression is localized in the pericycle, suggesting a role in loading glutamine to the xylem [21]^[21]. A more recent study showed that β-glucuronidase (GUS) activity driven by Gln1;1−5 promoters was localized in phloem companion cells but in veins of different order, and AtGln1;1, AtGln1;2, and AtGln1;3 act together for N remobilization and seed filling [22]^[22].

In maize, ZmGln1−3 in the mesophyll cells has a role in the synthesis of Gln following NO3− reduction until plant maturity [7,23]^[7][23]. ZmGln1−4 in bundle sheath cells has a role in the reassimilation of NH4+ released during protein degradation in senescing leaves [7,24]^[7][24]. In rice, OsGS1;1, with its transcript located in vascular tissue of mature leaves, has a role in grain filling [25,26]^[25][26]. OsGS1;2, with its transcript located in surface cells of roots in an NH4+-dependent manner, is important in the primary assimilation of NH4+ taken up by rice roots [16,27]^[16][27]. OsGS1;3 transcript is mainly expressed in the spikelet, indicating a key role in grain ripening and/or germination [28]^[28]. In wheat, TaGS1 transcript is present in the perifascicular sheath cells, and TaGSr transcripts are confined to the vascular cells [9,29]^[9][29]. During leaf senescence, TaGS1 and TaGSr have high mRNA levels, suggesting major roles in assimilating ammonia during the critical phases of remobilization of nitrogen to the grain [9]^[9]. However, since GS genes are highly homologous and their gene products are indistinguishable at the protein level by any GS antibody, previous studies about the cellular localization and expression characteristics of individual GS isozymes were mainly focused on the mRNA level [9,12,29,30]^[30].

In cells, the inorganic nitrogen assimilation process that GS participates in consumes a substantial amount of energy; therefore, GS must be tightly regulated at the gene, transcript, and protein level [5,11,31,32]^{[5][11][31][32]}. The regulation of each step of this process may affect the localization and activity of GS. In transgenic alfalfa, constitutively overexpressed GS1 genes significantly increased the level of GS1 transcripts in the leaves, but it did not significantly change the level of GS1 polypeptides^[33] [33]. 

3. Conclusion

In plants, each GS isozyme plays a different role in nitrogen metabolism, and the expression of GS is strictly regulated at multiple levels [5,11,31,32]^{[5][11][31][32]}. GS proteins are responsible for the catalytic activity. However, previous studies about GS isoforms mainly focused on the mRNA level. In this study, using antibodies specific to individual TaGS isozymes, the expression differences of TaGS isoforms at the protein level were analyzed. Moreover, some new functions of TaGS isoforms were discovered by analyzing the effects of N supply on their expression and localization at the protein level, and their kinetic properties and nitrogen metabolism.

Reference

1. Kichey, T.; Heumez, E.; Pocholle, D.; Pageau, K.; Vanacker, H.; Dubois, F.; Le Gouis, J.; Hirel, B. Combined agronomic and physiological aspects of nitrogen management in wheat highlight a central role for glutamine synthetase. New Phytol. 2006, 169, 265–278. [Google Scholar] [CrossRef] [PubMed]
2. Kaur, G.; Asthir, B.; Bains, N.S.; Farooq, M. Nitrogen Nutrition, its Assimilation and Remobilization in Diverse Wheat Genotypes. Int. J. Agric. Biol. 2015, 17, 531–538. [Google Scholar] [CrossRef]
3. Robertson, G.P.; Vitousek, P.M. Nitrogen in Agriculture: Balancing the Cost of an Essential Resource. Annu. Rev. Environ. Res. 2009, 34, 97–125. [Google Scholar] [CrossRef]
4. Kant, S.; Bi, Y.; Rothstein, S.J. Understanding plant response to nitrogen limitation for the improvement of crop nitrogen use efficiency. J. Exp. Bot. 2010, 62, 1499–1509. [Google Scholar] [CrossRef]
5. Thomsen, H.C.; Eriksson, D.; Møller, I.S.; Schjoerring, J.K. Cytosolic glutamine synthetase: A target for improvement of crop nitrogen use efficiency? Trends Plant Sci. 2014, 19, 656–663. [Google Scholar] [CrossRef]
6. Fuentes, S.I.; Allen, D.J.; Ortiz-Lopez, A.; Hernández, G. Over-expression of cytosolic glutamine synthetase increases photosynthesis and growth at low nitrogen concentrations. J. Exp. Bot. 2001, 52, 1071–1081. [Google Scholar] [CrossRef]
7. Martin, A.; Lee, J.; Kichey, T.; Gerentes, D.; Zivy, M.; Tatout, C.; Dubois, F.; Balliau, T.; Valot, B.; Davanture, M.; et al. Two Cytosolic Glutamine Synthetase Isoforms of Maize Are Specifically Involved in the Control of Grain Production. Plant Cell 2006, 18, 3252–3274. [Google Scholar] [CrossRef]
8. Tobin, A.; Ridley, S.; Stewart, G. Changes in the activities of chloroplast and cytosolic isoenzymes of glutamine synthetase during normal leaf growth and plastid development in wheat. Planta 1985, 163, 544–548. [Google Scholar] [CrossRef]
9. Bernard, S.M.; Møller, A.L.B.; Dionisio, G.; Kichey, T.; Jahn, T.P.; Dubois, F.; Baudo, M.; Lopes, M.S.; Tercé-Laforgue, T.; Foyer, C.H.; et al. Gene expression, cellular localisation and function of glutamine synthetase isozymes in wheat (Triticum aestivum L.). Plant Mol. Biol. 2008, 67, 89–105. [Google Scholar] [CrossRef]
10. Nigro, D.; Fortunato, S.; Giove, S.L.; Paradiso, A.; Gu, Y.Q.; Blanco, A.; de Pinto, M.C.; Gadaleta, A. Glutamine synthetase in Durum Wheat: Genotypic Variation and Relationship with Grain Protein Content. Front. Plant Sci. 2016, 7, 971. [Google Scholar] [CrossRef]
11. Bernard, S.M.; Habash, D.Z. The importance of cytosolic glutamine synthetase in nitrogen assimilation and recycling. New Phytol. 2009, 182, 608–620. [Google Scholar] [CrossRef] [PubMed]
12. Goodall, A.J.; Kumar, P.; Tobin, A.K. Identification and expression analyses of cytosolic glutamine synthetase genes in barley (Hordeum vulgare L.). Plant Cell Physiol. 2013, 54, 492–505. [Google Scholar] [CrossRef] [PubMed]
13. Sun, F.F.; Wang, Z.; Mao, X.Y.; Zhang, C.W.; Wang, D.S.; Wang, X.; Hou, X.L. Overexpression of BcGS2 gene in non-heading Chinese cabbage (Brassica campestris) enhanced GS activity and total amino acid content in transgenic seedlings. Sci. Hortic. Amst. 2015, 186, 129–136. [Google Scholar] [CrossRef]
14. Wallsgrove, R.M.; Turner, J.C.; Hall, N.P.; Kendall, A.C.; Bright, S.W. Barley mutants lacking chloroplast glutamine synthetase—Biochemical and genetic analysis. Plant Physiol. 1987, 83, 155–158. [Google Scholar] [CrossRef] [PubMed]
15. Kozaki, A.; Takeba, G. Photorespiration protects C3 plants from photooxidation. Nature 1996, 384, 557–560. [Google Scholar] [CrossRef]
16. Ishiyama, K.; Inoue, E.; Tabuchi, M.; Yamaya, T.; Takahashi, H. Biochemical background and compartmentalized functions of cytosolic glutamine synthetase for active ammonium assimilation in rice roots. Plant Cell Physiol. 2004, 45, 1640–1647. [Google Scholar] [CrossRef]
17. Ishiyama, K.; Inoue, E.; Watanabe-Takahashi, A.; Obara, M.; Yamaya, T.; Takahashi, H. Kinetic properties and ammonium-dependent regulation of cytosolic isoenzymes of glutamine synthetase in Arabidopsis. J. Biol. Chem. 2004, 279, 16598–16605. [Google Scholar] [CrossRef]
18. Guan, M.; Møller, I.; Schjoerring, J. Two cytosolic glutamine synthetase isoforms play specific roles for seed germination and seed yield structure in Arabidopsis. J. Exp. Bot. 2015, 66, 203–212. [Google Scholar] [CrossRef]
19. Guan, M.; de Bang, T.C.; Pedersen, C.; Schjoerring, J.K. Cytosolic Glutamine Synthetase Gln1;2 Is the Main Isozyme Contributing to GS1 Activity and Can Be Up-Regulated to Relieve Ammonium Toxicity. Plant Physiol. 2016, 171, 1921–1933. [Google Scholar] [CrossRef]
20. Lothier, J.; Gaufichon, L.; Sormani, R.; Lemaître, T.; Azzopardi, M.; Morin, H.; Chardon, F.; Reisdorf-Cren, M.; Avice, J.-C.; Masclaux-Daubresse, C. The cytosolic glutamine synthetase GLN1; 2 plays a role in the control of plant growth and ammonium homeostasis in Arabidopsis rosettes when nitrate supply is not limiting. J. Exp. Bot. 2011, 62, 1375–1390. [Google Scholar] [CrossRef]
21. Konishi, N.; Ishiyama, K.; Beier, M.P.; Inoue, E.; Kanno, K.; Yamaya, T.; Takahashi, H.; Kojima, S. Contributions of two cytosolic glutamine synthetase isozymes to ammonium assimilation in Arabidopsis roots. J. Exp. Bot. 2017, 68, 613–625. [Google Scholar] [CrossRef] [PubMed]
22. Moison, M.; Marmagne, A.; Dinant, S.; Soulay, F.; Azzopardi, M.; Lothier, J.; Citerne, S.; Morin, H.; Legay, N.; Chardon, F.; et al. Three cytosolic glutamine synthetase isoforms localized in different-order veins act together for N remobilization and seed filling in Arabidopsis. J. Exp. Bot. 2018, 69, 4379–4393. [Google Scholar] [CrossRef] [PubMed]
23. Hirel, B.; Martin, A.; Tercé-Laforgue, T.; Gonzalez-Moro, M.B.; Estavillo, J.M. Physiology of maize I: A comprehensive and integrated view of nitrogen metabolism in a C4 plant. Physiol. Plant. 2005, 124, 167–177. [Google Scholar] [CrossRef]
24. Martin, A.; Belastegui-Macadam, X.; Quillere, I.; Floriot, M.; Valadier, M.H.; Pommel, B.; Andrieu, B.; Donnison, I.; Hirel, B. Nitrogen management and senescence in two maize hybrids differing in the persistence of leaf greenness: Agronomic, physiological and molecular aspects. New Phytol. 2005, 167, 483–492. [Google Scholar] [CrossRef] [PubMed]
25. Tabuchi, M.; Sugiyama, K.; Ishiyama, K.; Inoue, E.; Sato, T.; Takahashi, H.; Yamaya, T. Severe reduction in growth rate and grain filling of rice mutants lacking OsGS1;1, a cytosolic glutamine synthetase1;1. Plant J. 2005, 42, 641–651. [Google Scholar] [CrossRef]
26. Kusano, M.; Tabuchi, M.; Fukushima, A.; Funayama, K.; Diaz, C.; Kobayashi, M.; Hayashi, N.; Tsuchiya, Y.N.; Takahashi, H.; Kamata, A.; et al. Metabolomics data reveal a crucial role of cytosolic glutamine synthetase 1;1 in coordinating metabolic balance in rice. Plant J. 2011, 66, 456–466. [Google Scholar] [CrossRef]
27. Funayama, K.; Kojima, S.; Tabuchi-Kobayashi, M.; Sawa, Y.; Nakayama, Y.; Hayakawa, T.; Yamaya, T. Cytosolic Glutamine Synthetase1;2 is Responsible for the Primary Assimilation of Ammonium in Rice Roots. Plant Cell Physiol. 2013, 54, 934–943. [Google Scholar] [CrossRef]
28. Yamaya, T.; Kusano, M. Evidence supporting distinct functions of three cytosolic glutamine synthetases and two NADH-glutamate synthases in rice. J. Exp. Bot. 2014, 65, 5519–5525. [Google Scholar] [CrossRef]
29. Caputo, C.; Criado, M.V.; Roberts, I.N.; Gelso, M.A.; Barneix, A.J. Regulation of glutamine synthetase 1 and amino acids transport in the phloem of young wheat plants. Plant Physiol. Biochem. 2009, 47, 335–342. [Google Scholar] [CrossRef]
30. Zhang, Z.; Xiong, S.; Wei, Y.; Meng, X.; Wang, X.; Ma, X. The role of glutamine synthetase isozymes in enhancing nitrogen use efficiency of N-efficient winter wheat. Sci. Rep. 2017, 7, 1000. [Google Scholar] [CrossRef]
31. Harper, C.J.; Hayward, D.; Kidd, M.; Wiid, I.; van Helden, P. Glutamate dehydrogenase and glutamine synthetase are regulated in response to nitrogen availability in Myocbacterium smegmatis. BMC Microbiol. 2010, 10, 1–12. [Google Scholar] [CrossRef]
32. Wei, Y.; Shi, A.; Jia, X.; Zhang, Z.; Ma, X.; Gu, M.; Meng, X.; Wang, X. Nitrogen Supply and Leaf Age Affect the Expression of TaGS1 or TaGS2 Driven by a Constitutive Promoter in Transgenic Tobacco. Genes 2018, 9, 406. [Google Scholar] [CrossRef] [PubMed]
33. Ortega, J.L.; Temple, S.J.; Sengupta-Gopalan, C. Constitutive Overexpression of Cytosolic Glutamine Synthetase (GS1) Gene in Transgenic Alfalfa Demonstrates That GS1 May Be Regulated at the Level of RNA Stability and Protein Turnover. Plant Physiol. 2001, 126, 109–121.