May be responsible for the synthesis of amylose. [(1->4)-alpha-D-glucosyl](n) + an NDP-alpha-D-glucose = [(1->4)-alpha-D-glucosyl](n+1) + a ribonucleoside 5'-diphosphate + H(+) Glycan biosynthesis; starch biosynthesis. Amyloplast or chloroplast, granule-bound. Expressed in pods and leaves. No expression in flowers or stipules. Expressed at all stages of embryonic development with highest levels in later developmental stages. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.