Cytosolic Glutamine Synthetase Isozymes Play Redundant Roles in Ammonium Assimilation Under Low-Ammonium Conditions in Roots of Arabidopsis thaliana

Abstract

Ammonium is a major nitrogen source for plants; it is assimilated into glutamine via a reaction catalyzed by glutamine synthetase (GLN). Arabidopsis expresses four cytosolic GLN genes, GLN1; 1, GLN1; 2, GLN1; 3 and GLN1; 4, in roots. However, the function and organization of these GLN1 isozymes in ammonium assimilation in roots remain unclear. In this study, we aimed to characterize the four GLN1 isozymes. The levels of growth of the wild type and gln1 single and multiple knockout lines were compared in a hydroponic culture at ammonium concentrations of 0.1 and 3 mM. Under the low-ammonium concentration, in single mutants for each GLN1 gene, there was little effect on growth, whereas the triple mutant for GLN1; 1, GLN1; 2 and GLN1; 3 grew slowly and accumulated ammonium. Under the high-ammonium concentration, the single mutant for GLN1; 2 showed 50% decreases in fresh weight and glutamine, whereas the other gln1 single mutants did not show notable changes in the phenotype. The double mutant for GLN1; 1 and GLN1; 2 showed less growth and a lower glutamine concentration than the single mutant for GLN1; 2. Promoter analysis indicated an overlapping expression of GLN1; 1 with GLN1; 2 in the surface layers of the roots. We thus concluded that: (i) at a low concentration, ammonium was assimilated by GLN1; 1, GLN1; 2 and GLN1; 3, and they were redundant; (ii) low-affinity GLN1; 2 could contribute to ammonium assimilation at concentrations ranging from 0.1 to 3 mM; and (iii) GLN1; 1 supported GLN1; 2 within the outer cell layers of the root.

Three cytosolic glutamine synthetase isoforms localized in different-order veins act together for N remobilization and seed filling in Arabidopsis

“…The reason why the total GS activity was unchanged in the shoots of the gln1;2 mutants under low-nitrate while the GS1 protein content showed a decrease can be explained by the fact that GLN1;2 has low affinity for ammonium ( Ishiyama et al , 2004 ). Nevertheless, the lower GS1 activity in roots under both low- and high-nitrate conditions could explain the smaller rosette size observed in the triple-mutant, ( Konishi et al , 2017 , 2018 ).…”

“… Guan et al (2015) showed that seed production and germination were impaired in the gln1;2 single and the gln1;2 - gln1;1 double knock-out mutants. As a result of grafting experiments, Guan and Schjoerring (2016) proposed that the prominent role of GLN1;2 was in the shoot; however, in their studies, Konishi et al (2017 , 2018 ) showed that both GLN1;2 and GLN1;3 are essential for ammonium assimilation in the roots.…”

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

“…Barley mutants lacking GS2 were unable to grow under normal atmospheric conditions as they were not able to re-assimilate photorespiratory ammonium and accumulated relatively high levels of ammonium in the leaves (Blackwell et al, 1987;Wallsgrove et al, 1987). GS1 plays a role in primary NH 4 + assimilation in the roots (Funayama et al, 2017;Guan et al, 2016;Konishi et al, 2018) and is, in addition, involved in the assimilation and recycling of NH 4 + released from protein degradation during senescence (Avila-Ospina et al, 2014;Bernard and Habash, 2009;Krapp, 2015). Individual GS1 isoforms may exert different functions reflecting their kinetic properties and expression patterns.…”

Protein Phosphatase (PP2C9) Induces Protein Expression Differentially to Mediate Nitrogen Utilization Efficiency in Rice under Nitrogen-Deficient Condition

“…However, the overexpression of nitrite reductase (NiR) and nitrate reductase (NR) in tobacco and Arabidopsis decreases nitrate levels in plant tissues but fail to improve the biomass or grain yield [ 12 , 20 ]. The ectopic expression of cytoplasmic glutamine synthetase1 (GS1) [ 21 ] and glutamine synthetase 2 (GS2) [ 22 , 23 ] have some positive effects on plant biomass and grain yield. Studies suggest that targeted manipulation of one gene or just one component of the N signaling pathway may not be enough to significantly improve overall NUE because it is a quantitative trait controlled by many genes and interacting regulatory pathways that are linked with the actual NUE of a crop plant [ 24 ].…”