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Plant and Cell Physiology Advance Access published online on May 31, 2007
Plant and Cell Physiology, doi:10.1093/pcp/pcm067
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© The Author 2007. 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

Genetic Linkages of the Circadian Clock-Associated Genes, TOC1, CCA1, and LHY, in the Photoperiodic Control of Flowering Time in Arabidopsis thaliana

Yusuke Niwa1,5, Shogo Ito1,4,5, Norihito Nakamichi2, Tsuyoshi Mizoguchi3, Kanae Niinuma3, 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.
3 Institute of Biological Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan

4Corresponding author: Shogo Ito. FAX: +81-52-789-4091, E-mail: i052001d{at}mbox.nagoya-u.ac.jp


  Abstract

In Arabidopsis thaliana, the flowering time is regulated through the circadian clock that measures day-length and modulates the photoperiodic CO-FT output pathway in accordance to the external coincidence model. Nevertheless, the genetic linkages between the major clock-associated TOC1, CCA1, and LHY genes and the canonical CO-FT flowering pathway are less clear. By employing a set of mutants including an extremely early flowering toc1 cca1 lhy triple mutant, here we showed that CCA1 and LHY act redundantly as negative regulators for the photoperiodic flowering pathway. The partly redundant CCA1/LHY functions are largely, but not absolutely, dependent on the upstream TOC1 gene that serves as an activator. The results of examination with reference to the expression profiles of CO and FT in the mutants indicated that this clock circuitry is indeed linked to the CO-FT output pathway, if not exclusively. For this linkage, the phase control of certain flowering-associated genes, GI, CDF1, and FKF1 appears to be crucial. Furthermore, the genetic linkage between TOC1 and CCA1/LHY is well compatible with the negative and positive feedback loop, which is currently believed to be a core of the circadian clock. The results of this study suggested that the circadian clock might open an exit for a photoperiodic output pathway during daytime. In the context of current clock model, these results will be discussed in connection to the previous finding that the same clock might open an exit for the early photomorphogenic output pathway during nighttime.

5These two authors contributed equally to this study.


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