A redox-regulated chloroplast protein phosphatase binds to starch diurnally and functions in its accumulation.

Starch is the ultimate storage molecule formed in the photosynthetic fixation of carbon dioxide by chloroplasts. Starch accumulates during the day and is degraded at night to intermediates that are exported to heterotrophic organs. The mechanism by which diurnal cycles control the transitory biosynthesis and degradation of chloroplast starch has long remained a mystery. We now report evidence that a dual-specificity protein phosphatase, DSP4, binds to starch granules during the day and dissociates at night. Disruption of the DSP4 gene resulted in a dramatic increase in the level of starch in mutant Arabidopsis plants. Moreover, although composition was apparently unchanged, the morphology of the starch granule was significantly altered compared to the wild type counterpart. Two regulatory factors linked to light (i.e., pH and redox status) changed both the activity and the starch-binding capacity of DSP4. The results further revealed that DSP4 represents a major fraction of granule-bound phosphatase activity during the day but not at night. Our study suggests that DSP4 acts as a bridge between light-induced redox changes and protein phosphorylation in the regulation of starch accumulation.