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Plant and Cell Physiology Advance Access published online on December 19, 2008
Plant and Cell Physiology, doi:10.1093/pcp/pcn198
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© The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

A Genetic Study of the Arabidopsis Circadian Clock with Reference to the TIMING OF CAB EXPRESSION 1 (TOC1) Gene

Shogo Ito1, Hideaki Kawamura1, Yusuke Niwa1, Norihito Nakamichi2, Takafumi Yamashino1 and Takeshi Mizuno1

1 Laboratory of Molecular Microbiology, School of Agriculture, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan
2 Division of Biological Science, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan.

Corresponding author: Shogo Ito, E-mail: nqi33717{at}nifty.com Tel: +81-52-789-4090 Fax: +81-52-789-4091


  Abstract

In Arabidopsis thaliana, a consistent multiloop clock model has been adopted widely in many recent publications. This tentative model consists of three interactive feedback loops, namely, core CCA1/LHY-TOC1/X loop, morning CCA1/LHY-PRR9/PRR7 loop, and 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 may 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. Both genetic analysis of these lines 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 photomorphogenesis; (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.

Keywords: Arabidopsis thaliana - Circadian rhythm - Clock component - Pseudo - response regulators - photomorphogenesis

(Received October 22, 2008; Accepted December 15, 2008)
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