Analysis of Sucrose to Starch Metabolism Pathway in Developing Grains of Common Wheat

Starch comprises majority of grain dry weight in wheat, making it the major storage reserve in this crop. During the reproductive growth of wheat, most of the sucrose synthesized in the source tissues is transported into the developing grains, where it is used as a substrate for the synthesis of starch. To better understand the coordination of sucrose-starch metabolic pathways, this study investigates the dynamics of sucrose catabolism and starch synthesis in wheat seeds at both molecular and metabolite levels. Our data show that a gene encoding sucrose synthase (TaSuSy2) is highly expressed during the rapid phase of starch accumulation; reflecting that sucrose hydrolysis in developing wheat seeds is catalyzed mainly by the TaSuSy2 isoform. ADP-glucose pyrophosphorylase (AGPase) has been shown previously to be a regulatory starch biosynthetic enzyme in plants. Thus, the high expression of AGPL1 and AGPS1a, encoding the large and small subunits of AGPase, respectively, found in the seeds during the same period  of development, suggest that these two genes contribute the majority of AGPase activity required for converting the hexose sugars to starch. Furthermore, the upregulation of TaSuSy2, AGPL1 and AGPS1a was coordinated with increased expression of specific members of genes encoding granular bound and soluble starch synthases. Cellular localization of some of these genes was also studied to complement our data