May 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 leaves and weakly in endosperm and roots. Expressed in developing caryopsis at 1 to 5 days after flowering. Expressed exclusively in the pericarp at 5 days after flowering. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.