Glutamine synthetase in Medicago truncatula, unveiling new secrets of a very old enzyme

Abstract

Glutamine synthetase (GS) catalyzes the first step at which nitrogen is brought into cellular metabolism and is also involved in the reassimilation of ammonium released by a number of metabolic pathways. Due to its unique position in plant nitrogen metabolism, GS plays essential roles in all aspects of plant development, from germination to senescence, and is a key component of nitrogen use efficiency (NUE) and plant yield. Understanding the mechanisms regulating GS activity is therefore of utmost importance and a great effort has been dedicated to understand how GS is regulated in different plant species. The present review summarizes exciting recent developments concerning the structure and regulation of GS isoenzymes, using the model legume Medicago truncatula. These include the understanding of the structural determinants of both the cytosolic and plastid located isoenzymes, the existence of a seed-specific GS gene unique to M. truncatula and closely related species and the discovery that GS isoenzymes are regulated by nitric oxide at the post-translational level. The data is discussed and integrated with the potential roles of the distinct GS isoenzymes within the whole plant context.

Cisgenic overexpression of cytosolic glutamine synthetase improves nitrogen utilization efficiency in barley and prevents grain protein decline under elevated CO2

“…A similar positive interaction between GS1 and GS2 activities in young leaves was observed in transgenic barley plants overexpressing the HvGS1‐1 gene driven by a constitutive promoter (Gao et al ., ). The increase in GS2 activity may reflect different levels of regulation affecting the assembly of the GS1 and GS2 holoenzymes (Wang et al ., ) or the association of GS2 or GS1 polypeptides with other proteins (Seabra and Carvalho, ; Seabra et al ., ). Overall, the interaction between GS1 and GS2 may serve to balance N and C metabolism (Bao et al ., ; Németh et al ., ).…”

Multiple mutualist effects on genomewide expression in the tripartite association between Medicago truncatula, nitrogen‐fixing bacteria and mycorrhizal fungi

“…) were affected by these symbionts, and one gene (Medtr3g065250) was affected nonadditively with expression being upregulated synergistically in the presence of multiple partners. Because glutamine synthetase is the first enzyme in the main pathway of ammonium assimilation in higher plants, it a key target for improving nitrogen use efficiency in crop species (Miflin & Habash ; Seabra & Carvalho ) and illustrates the potential importance of understanding interactive effects of partner species on the molecular phenotype. While beyond the scope of our global analyses of expression patterns, follow‐up work with methods like RT–PCR could be important for validation of promising genes like these and is a critical next step in understanding the link between phenotypic and molecular consequences of multiple mutualists and applied goals.…”

Cytosolic Glutamine Synthetase Gln1;2 Is the Main Isozyme Contributing to GS1 Activity and Can Be Up-Regulated to Relieve Ammonium Toxicity

“…Regulation of GS may occur at levels of transcription, translation, subcellular localization, subunit assembly, and post-translational modification of the protein and its turnover (Thomsen et al, 2014;He et al, 2015;Seabra and Carvalho, 2015;Wang et al, 2015). While the regulation of GS gene expression has been studied in many cases, much less is known about responses at protein level.…”