Involved in the synthesis of glycan chains within amylopectin in leaves. Is required to produce chains with a degree of polymerization of 12 to 25 (DP12-DP25). [(1->4)-alpha-D-glucosyl](n) + ADP-alpha-D-glucose = [(1->4)-alpha-D-glucosyl](n+1) + ADP + H(+) Glycan biosynthesis; starch biosynthesis. Expressed in roots, leaves and flowers. Reduced plant growth under short day photopheriod conditions and starch granules with alterated morphology. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.