[(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. Widely expressed. Most highly expressed in early embryos. Levels decline in later stages of development. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.