Chloroplast Starch Metabolism: A New Regulatory Junction for Redox and Protein Phosphorylation

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Scientific MeritStarch 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. Although the biosynthesis and degradation of glycogen in animals are regulated by protein phosphorylation via mechanisms that are textbook material, the mechanism by which diurnal cycles control the biosynthesis and degradation of transitory chloroplast starch has long remained a mystery. In a recent study, this group obtained 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. Two regulatory factors linked to light and dark i.e., pH andredox 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. The strategy used suggests that DSP4 acts as a bridge between light-induced redox changes and protein phosphorylation in the regulation of starch accumulation. This is also the first study that establishes a functional connection between protein phosphorylation and starch metabolism, providing a stepping stone for further understanding the mechanism underlying starch balance in plants. This project will: (1) identify the mechanism of DSP4 regulation by the light-mediated redox system, (2) characterize the potential target GSK3-like kinase and its function in starch metabolism, (3) explore the proteome on the starch granule and determine which proteins are modified by protein phosphorylation and regulated by DSP4, (4) determine whether other DSP4-related protein phosphatases function in starch metabolism. Information gained in these studies will be fundamental to understanding the molecular mechanism underlying diurnal regulation of starch metabolism in the chloroplast.Broader ImpactIn addition to contributing to the body of fundamental science, this research will impact society as well as education. Undergraduate students associated with the project will apply findings made in their research to food improvement via an ongoing multi-institutional project in the Co-PI's laboratory. This research is designed to improve the nutritional properties of sorghum a grain that serves as a major food staple for the world's poorest people. If the findings made with Arabidopsis can be extended to sorghum, the work of undergraduates affiliated with this NSF research could have significant impact on humanity and world agriculture. The research will have an additional effect on undergraduate education through major courses the PIs teach at Berkeley and through independent research programs in PIs' laboratories. The NSF project will also significantly enhance graduate education not only through direct lab training but also through the existing international collaboration in plant biology between the Unversity of California, Berkeley and the Chinese Academy of Sciences, which will positively influence the experience of graduate students at the international level. Finally, the project will assist ongoing efforts focused on local high schools to encourage minority students to obtain a higher education and enter biology.