May account for most of the soluble starch synthase activity in the tubers. Contributes only a tiny percentage of the granule-bound activity, but may also 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. Tuber, sink and source leaves. By sucrose under illumination. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.