Starch granule-associated phosphoglucan phosphatase involved in the control of starch accumulation. Acts as major regulator of the initial steps of starch degradation at the granule surface. Functions during the day by dephosphorylating the night-accumulated phospho-oligosaccharides. Can release phosphate from both the C6 and the C3 positions, but dephosphorylates preferentially the C6 position (PubMed:20018599, PubMed:26231210). Associated with starch granule. A number of isoforms are produced. According to EST sequences. Expressed with a circadian rhythm showing a peak at the end of the day and then decreasing to reach the lowest levels at the end of the night. Contains a C-terminal polysaccharide-binding domain which interacts with the phosphatase domain; this interaction is required for glucan phosphatase activity. Reduced plant size, slightly delayed flowering, leaves with large round starch granules and starch in excess. Starch binding efficiency is dependent on pH and redox conditions.