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 (By similarity). In dormant or cold-stressed trees, localizes to both the amyloplast stroma and nucleus. In conditions of vegetative growth, detected in the nucleus only. Expressed in phloem parenchyma of 16-24 week old seedlings and 2 year old trees (at protein level). Expressed in leaves of 16-24 week old seedlings and 2 year old trees. Shows seasonal variation in expression levels with maximal levels from November-March (at protein level). Shows diurnal fluctuations in expression level, with peak levels at dusk, under conditions of natural light but not under conditions of continuous light: may not have true circadian rhythmicity. By cold stress for at least 9 days at 4 degrees Celsius. Contains a C-terminal polysaccharide-binding domain which interacts with the phosphatase domain; this interaction is required for glucan phosphatase activity.