May be involved in starch synthesis in endosperm amyloplasts and contribute to the deposition of transient starch in chloroplasts of leaves. [(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 in endosperm, leaves, and weakly in roots. Expressed in developing caryopsis at 1 to 20 days after flowering. Expressed in the pericarp and endosperm at 5 and 5 to 10 days after flowering respectively. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.