Ito Shogo, Kawamura Hideaki, Niwa Yusuke, Nakamichi Norihito, Yamashino Takafumi, Mizuno Takeshi
Nagoya University, Chikusa-ku, Japan.
Plant Cell Physiol. 2009 Feb;50(2):290-303. doi: 10.1093/pcp/pcn198. Epub 2008 Dec 19.
In Arabidopsis thaliana, a consistent multiloop clock model has been widely adopted in many recent publications. This tentative model consists of three interactive feedback loops, namely the core CCA1/LHY-TOC1/X loop, the morning CCA1/LHY-PRR9/PRR7 loop and the evening Y-TOC1 loop, in which the undefined Y gene might be GI. The model in its current form provides us with a basis on which to address a number of fundamental issues for a better understanding of the molecular mechanism by which the central oscillator generates circadian rhythms. We have been conducting a series of genetic studies through the establishment of a set of combinatorial mutants. We have already characterized a prr9 prr7 double loss-of-function mutant that has lost the morning loop, and a cca1 lhy toc1 triple mutant that lacks the core loop. Extension of this line of study required characterization of a gi toc1 double loss-of-function mutant, which is expected to have no evening loop, and a prr9 prr7 toc1 triple mutant, lacking both the morning and evening loops. Genetic analysis of both these lines is reported here. From the results, we have clarified the genetic linkages between GI and TOC1 and those between PRR9/PRR7 and TOC1 with reference to the circadian clock-associated phenotypes, including: (i) length of hypocotyls during early photomor-phogenesis; (ii) photoperiodic control of flowering time; and (iii) expression profiles of CCA1 and LHY under free-running conditions. These results indicate that GI is not sufficient to fulfill the Y role, but plays more complicated clock-associated roles and, interestingly, that no epistatic interaction between PRR9/PRR7 and TOC1 was observed. Furthermore, these clock-defective mutants could still generate robust, free-running rhythms at the level of transcription. Therefore, we speculate that an as yet undefined oscillator (or loop) continues to generate rhythms within the plants lacking GI/TOC1 or PRR9/PRR7/TOC1.
在拟南芥中，一种一致的多环时钟模型已在许多近期出版物中被广泛采用。这个初步模型由三个相互作用的反馈环组成，即核心的CCA1/LHY-TOC1/X环、早晨的CCA1/LHY-PRR9/PRR7环和傍晚的Y-TOC1环，其中未定义的Y基因可能是GI。当前形式的模型为我们提供了一个基础，据此可以解决一些基本问题，以便更好地理解中央振荡器产生昼夜节律的分子机制。我们一直在通过建立一组组合突变体进行一系列遗传学研究。我们已经鉴定了一个失去早晨环的prr9 prr7双功能缺失突变体，以及一个缺乏核心环的cca1 lhy toc1三突变体。这项研究的扩展需要鉴定一个预计没有傍晚环的gi toc1双功能缺失突变体，以及一个同时缺乏早晨和傍晚环的prr9 prr7 toc1三突变体。本文报道了对这两个株系的遗传分析。从结果中，我们参照与昼夜节律相关的表型，阐明了GI与TOC1之间以及PRR9/PRR7与TOC1之间的遗传联系，这些表型包括：(i)早期光形态建成期间下胚轴的长度；(ii)开花时间的光周期控制；以及(iii)在自由运行条件下CCA1和LHY的表达谱。这些结果表明，GI不足以发挥Y的作用，但发挥着更复杂的与时钟相关的作用，有趣的是，未观察到PRR9/PRR7与TOC1之间的上位性相互作用。此外，这些时钟缺陷突变体在转录水平上仍能产生强劲的自由运行节律。因此，我们推测在缺乏GI/TOC1或PRR9/PRR7/TOC1的植物中，一个尚未明确的振荡器（或环）继续产生节律。
