earticle

영어

The process for the synthesis of storage polysaccharides, glycogen and starch, respectively, in bacteria and plants, takes place with ADP-glucose(ADP-Glu) as the glucosyl donor for the elongation of the α-1,4-glucosidic chain. In these organisms, ADP-Glc and inorganic pyrophosphate (PPi) are synthesized from ATP and Glc-1-P in a reaction catalyzed by ADP-glucose pyrophosphorylase(AGP). To identify AGP isoforms essential for this biosynthetic process in sink and source tissues of rice plants, we analyzed the rice AGP gene family which consists of two genes, OsAGPS1 and OsAGPS2, encoding small subunits and four genes, OsAGPL1, OsAGPL2, OsAGPL3 and OsAGPL4, encoding large subunits of this enzyme heterotetrameric complex. Subcellular localization, expression profile and functional studies of AGP subunits revealed that OsAGPS2b, an alternative spliced form of OsAGPS2 and OsAGPL2 are endosperm-specifically expressed, localized in the cytosol. Analysis of osagps2 and osagpl2 mutants revealed that a lesion of one of the two cytosolic isoforms, OsAGPL2 and OsAGPS2b, causes a shrunken endosperm due to a remarkable reduction in starch synthesis. Interestingly, they exhibited reduced not only the level of starch but also that of the major seed storage proteins and protein modifying proteins such as protein disulfide isomerase and the dnaK-type endoplasmic reticulum chaperone BiP. They also had much lower levels of malate dehydrogenase and alanine aminotransferase, which likely function in maintaining an adequate carbohydrate supply into storage proteins of seed endosperms under low oxygen condition. Additionally, PPi dependent isoforms, PPi-dependent fructose-6-phosphate 1-phosphotransferase and PPi-dependent pyruvate-orthophosphate dikinase (PPDK) are preferentially expressed in seed endosperm. Surprisingly, the knockout mutants of all these genes show opaque endosperms. These results strongly support the idea that the PPi produced by AGP in the cytosol of seed endosperm, not in the plastid, is necessary not only for the starch synthesis but for the supply of PPi, an alternative energy currency of ATP to produce storage proteins.