Abstract
Glutamine synthetase (GS) plays a major role in plant nitrogen metabolism, but the roles of individual GS isoforms in grains are unknown. Here, the localization and expression of individual TaGS isozymes in wheat grain were probed with TaGS isoenzyme-specific antibodies, and the nitrogen metabolism of grain during the grain filling stage were investigated. Immunofluorescence revealed that TaGS1;1, TaGS1;3, and TaGS2 were expressed in different regions of the embryo. In grain transporting tissues, TaGS1;2 was localized in vascular bundle; TaGS1;2 and TaGS1;1 were in chalaza and placentochalaza; TaGS1;1 and TaGS1;3 were in endosperm transfer cells; and TaGS1;3 and TaGS2 were in aleurone layer. GS exhibited maximum activity and expression at 8 days after flowering (DAF) with peak glutamine content in grains; from then, increased largely from reduction, glutamate dehydrogenase (GDH) aminating activity increased continuously, and the activities of GS and glutamate synthase (GOGAT) decreased, while only TaGS1;3 kept a stable expression in different TaGS isozymes. Hence, GS-GOGAT cycle and GDH play different roles in assimilation of grain in different stages of grain development; TaGS1;3, located in aleurone layer and endosperm transfer cells, plays a key role in Gln into endosperm for gluten synthesis. At 30 DAF, grain amino acids are mainly transported from maternal phloem.
Highlights
Nitrogen (N) is a vital macronutrient for crops, which is essential for crop growth and a primary driver of grain yield (Hirel et al, 2007; Xu et al, 2012)
The results showed that anti-TaGS1;1, anti-TaGS1;2, anti-TaGS1;3, and anti-TaGS2 antibodies were monospecific to TaGS1;1, TaGS1;2, TaGS1;3, and TaGS2 polypeptides, respectively, with an antibody dilution ratio of 1:30,000, 1:30,000, 1:10,000, and 1:10,000, respectively (Figure 1B)
The aminating activity of glutamate dehydrogenase (GDH) increased continuously while the activity of Glutamine synthetase (GS) and NADH-GOGAT decreased continuously and synchronously (Figures 5D,G,J). These results suggested that the GS-GOGAT cycle plays the main role of NH+4 assimilation in the earlier grain filling stage, while the GDH and GS-GOGAT pathways may involve in NH+4 assimilation in the later stage of grain filling
Summary
Nitrogen (N) is a vital macronutrient for crops, which is essential for crop growth and a primary driver of grain yield (Hirel et al, 2007; Xu et al, 2012). One of the optimization methods is to improve the crop N use efficiency (NUE), which is defined as the grain yield per unit of applied N fertilizer (McAllister et al, 2012; Xu et al, 2012; Thomsen et al, 2014). To improve crop NUE, glutamine synthetase (GS; EC 6.3.1.2) has been identified as one of the main objects of crop research, owing to its essential role in the assimilation of ammonium (NH+4 ).
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