Required for the synthesis of amylose (PubMed:25710501). Destroyed as it is released from the starch granules during the night (PubMed:15347792). The circadian expression is controlled by CCA1 and LHY transcription factors (PubMed:12777053). [(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. Interacts with PTST. This interaction is critical for the localization to starch granules. Exclusively bound to starch granules and found in the soluble fraction only if PTST is absent. Expressed in roots, inflorescences, flowers, fruits and at much higher levels in leaves. Circadian-regulation. Strong up-regulation at the end of the night and the beginning of the light period. Production of amylose-free starch. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.