Plays a minor role in starch synthesis in storage organs (tubers), but 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, soluble. High expression in leaves and very low in tubers. Constant expression in light and darkness. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.