[(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, soluble. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.