Lee Sang-Kyu, Hwang Seon-Kap, Han Muho, Eom Joon-Seob, Kang Hong-Gyu, Han Yulyi, Choi Sang-Bong, Cho Man-Ho, Bhoo Seong Hee, An Gynheung, Hahn Tae-Ryong, Okita Thomas W, Jeon Jong-Seong
Graduate School of Biotechnology & Plant Metabolism Research Center, Kyung Hee University, Yongin 446-701, Korea.
Plant Mol Biol. 2007 Nov;65(4):531-46. doi: 10.1007/s11103-007-9153-z. Epub 2007 Apr 4.
ADP-glucose pyrophosphorylase (AGP) catalyzes the first committed step of starch biosynthesis in higher plants. 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 (SSU) and four genes, OsAGPL1, OsAGPL2, OsAGPL3 and OsAGPL4, encoding large subunits (LSU) of this enzyme heterotetrameric complex. Subcellular localization studies using green fluorescent protein (GFP) fusion constructs indicate that OsAGPS2a, the product of the leaf-preferential transcript of OsAGPS2, and OsAGPS1, OsAGPL1, OsAGPL3, and OsAGPL4 are plastid-targeted isoforms. In contrast, two isoforms, SSU OsAGPS2b which is a product of a seed-specific transcript of OsAGPS2, and LSU OsAGPL2, are 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. In leaves, however, only the osagps2 mutant appears to severely reduce the transitory starch content. Interestingly, the osagps2 mutant was indistinguishable from wild type during vegetative plant growth. Western blot analysis of the osagp mutants and wild type plants demonstrated that OsAGPS2a is an SSU isoform mainly present in leaves, and that OsAGPS2b and OsAGPL2 are the major SSU and LSU isoforms, respectively, in the endosperm. Finally, we propose a spatiotemporal complex model of OsAGP SSU and LSU isoforms in leaves and in developing endosperm of rice plants.
ADP - 葡萄糖焦磷酸化酶（AGP）催化高等植物淀粉生物合成的第一步关键反应。为了鉴定水稻植株库源组织中该生物合成过程所必需的AGP同工型，我们分析了水稻AGP基因家族，该家族由两个编码小亚基（SSU）的基因OsAGPS1和OsAGPS2，以及四个编码该酶异源四聚体复合物大亚基（LSU）的基因OsAGPL1、OsAGPL2、OsAGPL3和OsAGPL4组成。使用绿色荧光蛋白（GFP）融合构建体进行的亚细胞定位研究表明，OsAGPS2叶片优先转录本的产物OsAGPS2a，以及OsAGPS1、OsAGPL1、OsAGPL3和OsAGPL4是定位于质体的同工型。相比之下，两种同工型，即OsAGPS2种子特异性转录本的产物SSU OsAGPS2b和LSU OsAGPL2，定位于细胞质。对osagps2和osagpl2突变体的分析表明，两种细胞质同工型之一OsAGPL2和OsAGPS2b的损伤会导致淀粉合成显著减少，从而使胚乳萎缩。然而，在叶片中，只有osagps2突变体似乎严重降低了暂存淀粉含量。有趣的是，osagps2突变体在营养生长阶段与野生型没有区别。对osagp突变体和野生型植株的蛋白质免疫印迹分析表明，OsAGPS2a是主要存在于叶片中的SSU同工型，而OsAGPS2b和OsAGPL2分别是胚乳中主要的SSU和LSU同工型。最后，我们提出了水稻叶片和发育中的胚乳中OsAGP SSU和LSU同工型的时空复合模型。
