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.redox regulation ͉ starch metabolism ͉ starch-binding phosphatase A s the ultimate storage form of photosynthetically fixed carbon, starch plays a central role in plants. During the day, chloroplasts convert carbon dioxide into soluble sugars that, with the collective action of starch synthases and branching enzymes, are converted to a mixture of linear and branched glucans (amylose and amylopectin) that determine the properties of the starch (1). At night, the starch granules are degraded by a combination of enzymes (2). The degradation products are transported out of the chloroplast and further exported to heterotrophic tissues in the form of sucrose (in most plants), thereby providing a carbon source for growth and energy. In this way, starch built up during the day and degraded at night meets the energy needs of the plant. The extent of starch synthesis and degradation in the chloroplast is tightly regulated by the lightdark cycle and by a number of environmental factors. However, little is known about the molecular mechanisms underlying the regulation of starch accumulation.Protein phosphorylation and dephosphorylation represents a ubiquitous mechanism for regulating cellular processes (3), including the enzymes of glycogen breakdown and synthesis. Furthermore, the protein kinases and phosphatases constituting the signaling pathways regulating these latter processes are now textbook material. By contrast, information on the regulation of starch metabolism by protein phosphorylation is scant.In an earlier effort to identify protein phosphatases in Arabidopsis that could dephosphorylate phosphotyrosine, we isolated both tyrosine-specific (4) and dual-specificity protein phosphatases (DSPs) (ref. 5 and L.N.S. and S.L., unpublished data) that may participate in a number of physiological and developmental processes (3). Several...
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