Mature wheat endosperm contains A-, B-, C-type starch granules, and each class has unique physiochemical

properties which determine the quality of starch. The dynamics of the starch granule size

distribution, activities of starch synthases and expression of starch synthase encoding genes were

studied in superior and inferior grains during grain filling. Compared with inferior grains, superior grains

showed higher grain weight, contents of starch, amylose and amylopectin. The formation of A-, B-, C-type

starch granules initiated at 4, 8, 20 DAF, respectively, and was well consistent with the temporally change

patterns of starch synthase activities and relative gene expression levels. For instance, activities of

soluble and granule-bound starch synthases (designated SSS and GBSS) peaked at 20 and 24 DAF. Genes

encoding isoforms of starch synthases expressed at different grain filling periods. In addition, SS I was

generally expressed over the grain filling stage; the SS II and SS III were expressed over the early and mid

grain filling stage, and the GBSS I was expressed during the mid to late grain filling stage. In addition, the

time-course changes in activities of starch synthases and expression of starch synthase encoding genes

explained well the dynamics of the starch granule size distribution.