Department of Biology, University of Washington, Seattle, WA 98195-1800, USA.
Proc Natl Acad Sci U S A. 2012 Feb 28;109(9):3582-7. doi: 10.1073/pnas.1118876109. Epub 2012 Feb 14.
Many plants monitor day-length changes throughout the year and use the information to precisely regulate the timing of seasonal flowering for maximum reproductive success. In Arabidopsis thaliana, transcriptional regulation of the CONSTANS (CO) gene and posttranslational regulation of CO protein are crucial mechanisms for proper day-length measurement in photoperiodic flowering. Currently, the CYCLING DOF FACTOR proteins are the only transcription factors known to directly regulate CO gene expression, and the mechanisms that directly activate CO transcription have remained unknown. Here we report the identification of four CO transcriptional activators, named FLOWERING BHLH 1 (FBH1), FBH2, FBH3, and FBH4. All FBH proteins are related basic helix-loop-helix-type transcription factors that preferentially bind to the E-box cis-elements in the CO promoter. Overexpression of all FBH genes drastically elevated CO levels and caused early flowering regardless of photoperiod, whereas CO levels were reduced in the fbh quadruple mutants. In addition, FBH1 is expressed in the vascular tissue and bound near the transcription start site of the CO promoter in vivo. Furthermore, FBH homologs in poplar and rice induced CO expression in Arabidopsis. These results indicate that FBH proteins positively regulate CO transcription for photoperiodic flowering and that this mechanism may be conserved in diverse plant species. Our results suggest that the diurnal CO expression pattern is generated by a concert of redundant functions of positive and negative transcriptional regulators.
许多植物会全年监测日照长度的变化，并利用这些信息精确地调节季节性开花的时间，以达到最大的生殖成功。在拟南芥中，CONSTANS（CO）基因的转录调控和 CO 蛋白的翻译后调控是光周期开花中正确测量日照长度的关键机制。目前，CYCLING DOF FACTOR 蛋白是已知可直接调节 CO 基因表达的唯一转录因子，而直接激活 CO 转录的机制仍不清楚。在这里，我们鉴定了四个 CO 转录激活因子，命名为 FLOWERING BHLH 1（FBH1）、FBH2、FBH3 和 FBH4。所有 FBH 蛋白都是相关的基本螺旋-环-螺旋型转录因子，它们优先结合 CO 启动子中的 E-box 顺式元件。所有 FBH 基因的过表达都极大地提高了 CO 的水平，并导致提前开花，而不管光周期如何，而 fbh 四重突变体中的 CO 水平降低。此外，FBH1 在维管束组织中表达，并在体内结合 CO 启动子的转录起始位点附近。此外，杨树和水稻中的 FBH 同源物在拟南芥中诱导 CO 表达。这些结果表明，FBH 蛋白正向调节 CO 转录以进行光周期开花，并且这种机制可能在不同的植物物种中保守。我们的研究结果表明，昼夜 CO 表达模式是由正、负转录调节因子的冗余功能共同产生的。
