Plays an important role during endosperm starch synthesis. Determines the type of amylopectin structure of starch grain. Synthesizes long B1 amylopectin chains by elongating short A and B1 chains, independently of the other soluble starch synthases. Barely active in japonica subspecies. [(1->4)-alpha-D-glucosyl](n) + ADP-alpha-D-glucose = [(1->4)-alpha-D-glucosyl](n+1) + ADP + H(+) Glycan biosynthesis; starch biosynthesis. Amyloplast or chloroplast, granule-bound and soluble. Expressed most exclusively in endosperm. Expressed in developing caryopsis at 5 to 15 days after flowering. Expressed exclusively in the endosperm at 5 to 10 days after flowering. Increased activity due to variations in subspecies indica is associated with the synthesis of the L-type amylopectin, which contains much lower proportion of short amylopectin chains than the S-type amylopectin found in most japonica varieties. The difference in amylopectin chain composition explains why starch from japonica subspecies has a lower gelatinisation temperature than starch from indica subspecies. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.