Abstract
Glutamine synthetase (GS) is central for ammonium assimilation and consists of cytosolic (GS1) and chloroplastic (GS2) isoenzymes. During plant ageing, GS2 protein decreases due to chloroplast degradation, and GS1 activity increases to support glutamine biosynthesis and N remobilization from senescing leaves. The role of the different Arabidopsis GS1 isoforms in nitrogen remobilization was examined using 15N tracing experiments. Only the gln1;1-gln1;2-gln1;3 triple-mutation affecting the three GLN1;1, GLN1;2, and GLN1;3 genes significantly reduced N remobilization, total seed yield, individual seed weight, harvest index, and vegetative biomass. The triple-mutant accumulated a large amount of ammonium that could not be assimilated by GS1. Alternative ammonium assimilation through asparagine biosynthesis was increased and was related to higher ASN2 asparagine synthetase transcript levels. The GS2 transcript, protein, and activity levels were also increased to compensate for the lack of GS1-related glutamine biosynthesis. Localization of the different GLN1 genes showed that they were all expressed in the phloem companion cells but in veins of different order. Our results demonstrate that glutamine biosynthesis for N-remobilization occurs in veins of all orders (major and minor) in leaves, it is mainly catalysed by the three major GS1 isoforms (GLN1;1, GLN1;2, and GLN1;3), and it is alternatively supported by AS2 in the veins and GS2 in the mesophyll cells.
Highlights
Because their structures are more plastic than those of ani- nutrients from their senescing organs and tissues to facilitate mals, plants regularly grow new organs to replace inefficient the growth of the new organs, including the production and and senescing ones
The data obtained were used to calculate the harvest index (HI; g of seeds g–1 of total dry matter), the nitrogen harvest index (NHI; g of N g–1 of total dry matter), which corresponds to the quantity of N in the seeds compared to the total N within the plant, and the 15N harvest index (15NHI; g of 15N g–1 of total dry matter), which corresponds to the quantity of 15N in the seeds compared to the total 15N within the plant
Studies of the GS1 gene family in barley (Hordeum vulgare), maize (Zea mays), wheat (Triticum aestivum), rice (Oriza sativa), oilseed rape (Brassica napus), and Arabidopsis have shown that the GS1 isoforms are differentially expressed depending on the organ, the developmental stage, and the nitrogen availability, and that they play different roles in determining
Summary
Because their structures are more plastic than those of ani- nutrients from their senescing organs and tissues to facilitate mals, plants regularly grow new organs to replace inefficient the growth of the new organs, including the production and and senescing ones. The cytosolic glutamine synthetases (GS1; EC 6.3.1.2) have been identified in many plant species (Guo et al, 2004; Martin et al, 2006; Diaz et al, 2008; Avila-Ospina et al, 2014)
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