B-box family proteins BBX18 and BBX23 control thermoresponsive hypocotyl growth in Arabidopsis

Plants coordinate their growth and developmental programs with various endogenous signals and environmental challenges such as seasonal and diurnal temperature fluctuations. The bHLH transcription factor PIF4 plays critical roles in thermoresponsive hypocotyl growth in Arabidopsis, and the evening complex component ELF3 negatively regulates PIF4's activity for downstream gene expression and hypocotyl elongation at elevated temperature. However, how warm temperature signal is transmitted to ELF3 is not known. Here, we report the identification of two B-Box protein BBX18/BBX23 as new regulators of thermomorphogenesis in Arabidopsis. Mutations of BBX18/BBX23 confer reduced thermoresponsive hypocotyl elongation. Overexpression of BBX18 enhances the sensitivity of hypocotyl growth to elevated temperature, which is dependent on the function of PIF4 and RING E3 ligase COP1, respectively. Both BBX18 and BBX23 interact with ELF3 or COP1, relegating the protein abundance of ELF3 at warm temperature. Further, the expression of multiple thermoresponsive genes is impaired in both the PIF4 single mutant and BBX18/BBX23 double mutant. In addition, both the transcription and protein levels of BBX18/BBX23 are up-regulated by elevated ambient temperature. Thus, our findings reveal the important roles of B-Box proteins in plant thermomorphogenesis, and build a new connection from warm temperature information to ELF3 and its downstream signaling components. Totally 18 samples with three genotypes and two treatments,both of which are in triplicates.

植物会将自身生长与发育程序与各类内源信号以及季节、昼夜温度波动等环境胁迫相协调。碱性螺旋-环-螺旋（bHLH）转录因子PIF4在拟南芥热响应下胚轴生长过程中发挥关键作用，傍晚复合体组分ELF3可在高温条件下负调控PIF4的活性，进而影响下游基因表达与下胚轴伸长。然而，目前尚不清楚温暖温度信号是如何传递至ELF3的。本研究鉴定得到两个B-Box蛋白BBX18与BBX23，二者为拟南芥温度形态建成的新型调控因子。BBX18与BBX23的突变会导致热响应下胚轴伸长能力减弱。BBX18的过表达可增强下胚轴生长对高温的敏感性，这一过程分别依赖于PIF4与环指型E3泛素连接酶COP1的功能。BBX18与BBX23均可与ELF3或COP1发生相互作用，进而在温暖温度条件下降低ELF3的蛋白丰度。此外，在PIF4单突变体与BBX18/BBX23双突变体中，多个热响应基因的表达均受到损伤。除此之外，BBX18与BBX23的转录水平与蛋白水平均可在升高的环境温度下被上调。综上，本研究揭示了B-Box蛋白在植物温度形态建成中的重要作用，并搭建了一条从温暖温度信号传递至ELF3及其下游信号组分的全新调控通路。本实验共设置18个样本，涵盖3种基因型与2种处理，每组均设置3次生物学重复。