Involved in the synthesis of glycan chains within amylopectin in leaves. May play a regulatory role in the control of starch accumulation in plastids. [(1->4)-alpha-D-glucosyl](n) + ADP-alpha-D-glucose = [(1->4)-alpha-D-glucosyl](n+1) + ADP + H(+) Glycan biosynthesis; starch biosynthesis. A number of isoforms are produced. According to EST sequences. Expressed in leaves and flowers. No visible phenotype under normal growth conditions, but mutant plants accumulate increased levels of starch and have starch granules with alterated morphology. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily. Truncated N-terminus.