State Key Laboratory of Rice Biology, China National Center for Rice Improvement, China National Rice Research Institute, Hangzhou 310006, China.
State Key Laboratory of Rice Biology, China National Center for Rice Improvement, China National Rice Research Institute, Hangzhou 310006, China; Institute of Agricultural Science in Jiangsu Coastal Areas, Yancheng 224002, China.
Plant Commun. 2022 Nov 14;3(6):100463. doi: 10.1016/j.xplc.2022.100463. Epub 2022 Oct 18.
Starch and storage proteins are the main components of rice (Oryza sativa L.) grains. Despite their importance, the molecular regulatory mechanisms of storage protein and starch biosynthesis remain largely elusive. Here, we identified a rice opaque endosperm mutant, opaque3 (o3), that overaccumulates 57-kDa proglutelins and has significantly lower protein and starch contents than the wild type. The o3 mutant also has abnormal protein body structures and compound starch grains in its endosperm cells. OPAQUE3 (O3) encodes a transmembrane basic leucine zipper (bZIP) transcription factor (OsbZIP60) and is localized in the endoplasmic reticulum (ER) and the nucleus, but it is localized mostly in the nucleus under ER stress. We demonstrated that O3 could activate the expression of several starch synthesis-related genes (GBSSI, AGPL2, SBEI, and ISA2) and storage protein synthesis-related genes (OsGluA2, Prol14, and Glb1). O3 also plays an important role in protein processing and export in the ER by directly binding to the promoters and activating the expression of OsBIP1 and PDIL1-1, two major chaperones that assist with folding of immature secretory proteins in the ER of rice endosperm cells. High-temperature conditions aggravate ER stress and result in more abnormal grain development in o3 mutants. We also revealed that OsbZIP50 can assist O3 in response to ER stress, especially under high-temperature conditions. We thus demonstrate that O3 plays a central role in rice grain development by participating simultaneously in the regulation of storage protein and starch biosynthesis and the maintenance of ER homeostasis in endosperm cells.
淀粉和贮藏蛋白是水稻（Oryza sativa L.）籽粒的主要成分。尽管它们很重要，但贮藏蛋白和淀粉生物合成的分子调控机制在很大程度上仍不清楚。在这里，我们鉴定了一个水稻 opaque 粉质突变体 opaque3（o3），其 57-kDa 类谷蛋白过度积累，且蛋白和淀粉含量明显低于野生型。o3 突变体的胚乳细胞中也存在异常的蛋白体结构和复合淀粉粒。OPAQUE3（O3）编码一个跨膜碱性亮氨酸拉链（bZIP）转录因子（OsbZIP60），定位于内质网（ER）和细胞核，但在 ER 应激下主要定位于细胞核。我们证明 O3 可以激活几个淀粉合成相关基因（GBSSI、AGPL2、SBEI 和 ISA2）和贮藏蛋白合成相关基因（OsGluA2、Prol14 和 Glb1）的表达。O3 还通过直接结合启动子并激活 OsBIP1 和 PDIL1-1（两种主要的伴侣蛋白）的表达，在 ER 中对蛋白质加工和输出发挥重要作用，这两种伴侣蛋白有助于水稻胚乳细胞 ER 中未成熟分泌蛋白的折叠。高温条件加剧了 ER 应激，导致 o3 突变体中更多异常的籽粒发育。我们还揭示了 OsbZIP50 可以协助 O3 应对 ER 应激，特别是在高温条件下。因此，我们证明 O3 通过参与贮藏蛋白和淀粉生物合成的调节以及胚乳细胞内质网的稳态维持，在水稻籽粒发育中发挥核心作用。
