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

Abstract

Cytosolic GS1 (Gln synthetase) is central for ammonium assimilation in plants. High ammonium treatment enhanced the expression of the GS1 isogene Gln-1;2 encoding a low-affinity high-capacity GS1 protein in Arabidopsis (Arabidopsis thaliana) shoots. Under the same conditions, the expression of the high-affinity low-capacity isoform Gln-1;1 was reduced. The expression of Gln-1;3 did not respond to ammonium treatment while Gln-1;4 and Gln-1;5 isogenes in all cases were expressed at a very low level. Gln-2 was highly expressed in shoots but only at a very low level in roots. To investigate the specific functions of the two isogenes Gln-1;1 and Gln-1;2 in shoots for ammonium detoxification, single and double knock-out mutants were grown under standard N supply or with high ammonium provision. Phenotypes of the single mutant gln1;1 were similar to the wild type, while growth of the gln1;2 single mutant and the gln1;1:gln1;2 double mutant was significantly impaired irrespective of N regime. GS1 activity was significantly reduced in both gln1;2 and gln1;1:gln1;2. Along with this, the ammonium content increased while that of Gln decreased, showing that Gln-1;2 was essential for ammonium assimilation and amino acid synthesis. We conclude that Gln-1;2 is the main isozyme contributing to shoot GS1 activity in vegetative growth stages and can be up-regulated to relieve ammonium toxicity. This reveals, to our knowledge, a novel shoot function of Gln-1;2 in Arabidopsis shoots.

Peering into the separate roles of root and shoot cytosolic glutamine synthetase 1;2 by use of grafting experiments in Arabidopsis

“…5 The important function of Gln1;2 in promoting shoot growth is not limited to generative growth stages as shown by the fact that the knockout mutant gln1;2 produced significantly lower mass of rosette leaves and other shoot components than Wt plants during vegetative growth. 3 The role of Gln1;2 may also be affected by the form of nitrogen supplied to plants because Gln1;2 expression is enhanced by ammonium supply relative to nitrate as the sole N source. 3,4 functions of Gln1;2 are primarily associated with nitrogen remobilization or with N signaling.…”

“…3 The role of Gln1;2 may also be affected by the form of nitrogen supplied to plants because Gln1;2 expression is enhanced by ammonium supply relative to nitrate as the sole N source. 3,4 functions of Gln1;2 are primarily associated with nitrogen remobilization or with N signaling. A high flux of N to juvenile inflorescences is essential for ensuring that a high number of the initiated primordia develop into fertile siliques with a high number of seeds.…”

“…10,12 The surface of the sand was covered by black vermiculate in order to avoid algae. Pots were watered 3 times per week over 24 h by immersion of the base of the pot in nutrient solutions containing 2 mM NH 4 NO 3 together with all other essential plant nutrients as specified in Guan et al (2016). 3 Results are means §SE.…”

“…Pots were watered 3 times per week over 24 h by immersion of the base of the pot in nutrient solutions containing 2 mM NH 4 NO 3 together with all other essential plant nutrients as specified in Guan et al (2016). 3 Results are means §SE. Asterisks indicate statistically significant differences between Wt shoot /Wt root and grafted plants, determined using Student's t-test: Ã , P < 0.05; ÃÃ , P < 0.01.…”

“…9 GS1 is also expressed in shoots and may during vegetative growth stages be up-regulated to accommodate requirements for ammonium assimilation during periods with high ammonium supply. 3 The most dramatic up-regulation of GS1 in shoots takes place during reproductive growth stages and is considered to play an important role for nitrogen remobilization from senescing tissues to new sinks such as seeds. 1 Arabidopsis has 5 isogenes for GS1 (Gln1;1-5) and among these Gln1;2 seems to play particular roles for ammonium assimilation in roots as well as for nitrogen remobilization during seed germination and seed production.…”