Flowering time is a fundamental factor determining the global distribution and final yield of rice (Oryza sativa). Although diverse flowering time genes have been reported in this crop, the transcriptional regulation of its key flowering genes are poorly understood. Here, we report that a basic leucine zipper transcription factor, bZIP71, functions as a flowering repressor. The overexpression of bZIP71 delays flowering, while the bzip71 mutant flowers early in both long-day and short-day conditions. A genetic analysis showed that the regulation of flowering by bZIP71 might be independent of Heading date 2 (Hd2), Hd4, and Hd5. Importantly, bZIP71 directly associates with the Early heading date 1 (Ehd1) promoter and represses its transcription, and genetically the function of bZIP71 is impaired in the ehd1 mutant. Moreover, bZIP71 interacts with major components of polycomb repressive complex 2 (PRC2), SET domain group protein 711 (SDG711), and Fertilization independent endosperm 2 (FIE2), through which bZIP71 regulates the H3K27me3 level of Ehd1. Taken together, we present a transcriptional regulatory mechanism in which bZIP71 enhances the H3K27me3 level of Ehd1 and transcriptionally represses its expression, which not only offers a novel insight into a flowering pathway, but also provides a valuable putative target for the genetic engineering and breeding of elite rice cultivars.
水稻抽穗期（heading date，HD）是指水稻從播種到抽穗（開花）所經(jīng)歷的時間，是決定水稻種植區(qū)域和適應(yīng)性的一個重要農(nóng)藝性狀。調(diào)控水稻抽穗期基因關(guān)系網(wǎng)絡(luò)復(fù)雜，目前認為在水稻中存在兩條相對保守的調(diào)控途徑（Hd1-Hd3a/RFT1）和（Ghd7-Ehd1-Hd3a/RFT1）。Ehd1通過調(diào)控成花素基因Hd3a和RFT1的表達來控制水稻開花，Ehd1作為開花途徑的核心因子，受多種因子調(diào)控。盡管很多個水稻抽穗期基因已被鑒定，但水稻抽穗期關(guān)系網(wǎng)絡(luò)并不完善，依然需要不斷挖掘新的調(diào)控基因，同時進行功能解析，才能進一步應(yīng)用于水稻的分子設(shè)計育種。該研究發(fā)現(xiàn)一個堿性亮氨酸拉鏈轉(zhuǎn)錄因子bZIP71是水稻開花的抑制子，且遺傳分析表明，bZIP71發(fā)揮功能不依賴于Heading date 2（Hd2）、Hd4和Hd5等，而是通過與多梳抑制復(fù)合體2（PRC2）的兩個成員SDG711（SET domain group protein 711）和FIE2（Fertilization independent endosperm 2）的互作，調(diào)控Ehd1的H3K27me3水平。
研究發(fā)現(xiàn)bZIP71過量表達延遲水稻開花，bzip71敲除突變體提早開花。Ehd1、Hd3a和RFT1在bZIP71過量表達植株中低于野生型，在bzip71突變體中高于野生型，表明了bZIP71負調(diào)控水稻的抽穗期；bZIP71蛋白定位于細胞核中，具有轉(zhuǎn)錄激活活性及DNA結(jié)合能力；進一步實驗表明，bZIP71發(fā)揮功能不依賴于Ehd1上游的調(diào)控因子Hd4、Hd5、和Hd2等，而是直接結(jié)合到Ehd1的啟動子區(qū)域并抑制其表達。那么，bZIP71作為一個具有轉(zhuǎn)錄激活因子是如何發(fā)揮抑制靶基因Ehd1表達的呢？深入研究發(fā)現(xiàn)，bZIP71與SDG711和FIE2互作，進而招募這兩個PRC2成員到Ehd1啟動子區(qū)域，調(diào)控Ehd1的H3K27me3水平，導(dǎo)致bZIP71過量表達植株中 Ehd1的H3K27me3水平顯著升高，在bzip71突變體中降低；此外， bZIP71位于Ehd1的遺傳學(xué)上游，而且二者呈現(xiàn)相反的光周期節(jié)律性表達模式。