Involved in the synthesis of short glycan chains within amylopectin in leaves. Is required to generate chains up to about a degree of polymerization of 10 (DP10). [(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, stems, buds and flowers. Circadian-regulation with the lowest levels at the end of the dark period. No visible phenotype under normal growth conditions, but mutant plants have reduced starch content in leaves. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.