Accounts for only 10 to 15% of the total soluble starch synthase activity in tubers. [(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. Constant expression in light and darkness. May be the main starch synthase responsible for the incorporation of phosphorylated precursors into growing alpha-1,4-glucans. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.