Hypocotyl elongation relies on directional cell expansion, a process under light and circadian clock control. Under short photoperiods (SD), hypocotyl elongation in Arabidopsis thaliana follows a rhythmic pattern, a process in which circadian morning-to-midnight waves of the transcriptional repressors PSEUDO-RESPONSE REGULATORS (PRRs) jointly gate PHYTOCHROME-INTERACTING FACTOR (PIF) activity to dawn. Previously, we described CYCLING DOF FACTOR 5 (CDF5) as a target of this antagonistic PRR/PIF dynamic interplay. Under SD, PIFs induce CDF5 accumulation specifically at dawn, when it promotes the expression of positive cell elongation regulators such as YUCCA8 to induce growth. In contrast to SD, hypocotyl elongation under long days (LD) is largely reduced. Here, we examine whether CDF5 is an actor in this photoperiod specific regulation. We report that transcription of CDF5 is robustly induced in SD compared to LD, in accordance with PIFs accumulating to higher levels in SD, and in contrast to other members of the CDF family, whose expression is mainly clock regulated and have similar waveforms in SD and LD. Notably, when CDF5 was constitutively expressed under LD, CDF5 protein accumulated to levels comparable to SD but was inactive in promoting cell elongation. Similar results were observed for CDF1. Our findings indicate that both CDFs can promote cell elongation specifically in shorter photoperiods, however their activity in LD is inhibited at the post-translational level. These data not only expand our understanding of the biological role of CDF transcription factors, but also identify a previously unrecognized regulatory layer in the photoperiodic response of hypocotyl elongation. This article is protected by copyright. All rights reserved.

中文翻译：

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下胚轴伸长的光周期反应涉及 CDF1 和 CDF5 活性的调节

下胚轴伸长依赖于定向细胞扩张，这是一个受光和生物钟控制的过程。在短光周期 (SD) 下，拟南芥中的下胚轴伸长遵循有节奏的模式，在这个过程中，转录抑制因子的昼夜节律性早到午夜波假性响应调节器 (PRRs) 联合门控光致色素相互作用因子 (PIF) 活动黎明。以前，我们将循环自由度因子 5 (CDF5) 描述为这种对抗性 PRR/PIF 动态相互作用的目标。在 SD 下，PIFs 特别在黎明诱导 CDF5 积累，当它促进阳性细胞伸长调节因子（如 YUCCA8）的表达以诱导生长时。与 SD 相比，长日照 (LD) 下的下胚轴伸长率大大降低。在这里，我们检查 CDF5 是否是这个光周期特定调控的参与者。我们报告说，与 LD 相比，CDF5 的转录在 SD 中被强烈诱导，这与在 SD 中积累到更高水平的 PIF 一致，并且与 CDF 家族的其他成员形成对比，后者的表达主要受时钟调节并在 SD 和LD。值得注意的是，当 CDF5 在 LD 下组成型表达时，CDF5 蛋白积累到与 SD 相当的水平，但在促进细胞伸长方面没有活性。对 CDF1 观察到类似的结果。我们的研究结果表明，这两种 CDF 都可以在较短的光周期内特异性地促进细胞伸长，但是它们在 LD 中的活性在翻译后水平受到抑制。这些数据不仅扩展了我们对 CDF 转录因子生物学作用的理解，但也确定了下胚轴伸长的光周期反应中以前未被识别的调节层。本文受版权保护。版权所有。