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Plant and Cell Physiology Advance Access originally published online on May 31, 2007
Plant and Cell Physiology 2007 48(7):925-937; 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 email: journals.permissions@oxfordjournals.org

Rapid Paper

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

Yusuke Niwa1,4, Shogo Ito1,4,*, Norihito Nakamichi2, Tsuyoshi Mizoguchi3, Kanae Niinuma3, Takafumi Yamashino1 and Takeshi Mizuno1

1Laboratory of Molecular Microbiology, School of Agriculture, Nagoya University, Chikusa-ku, Nagoya, 464-8601 Japan
2Division of Biological Science, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya, 464-8602 Japan
3Institute of Biological Sciences, University of Tsukuba, Tsukuba, Ibaraki, 305-8572 Japan

*Corresponding author: E-mail, i052001d{at}mbox.nagoya-u.ac.jp; Fax, +81-52-789-4091.


  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 with 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 of 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 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 the daytime. In the context of the current clock model, these results will be discussed in connection with the previous finding that the same clock might open an exit for the early photomorphogenic output pathway during the night-time.

Keywords: Arabidopsis thaliana - Circadian rhythm - Clock - CO-FT - Photoperiodic flowering

Abbreviations: CCA1, CIRCADIAN CLOCK-ASSOCIATED 1; CO, CONSTANS; FT, flowering locus T; LD, long day; LHY, LATE ELONGATED HYPOCOTYL; PRR, PSEUDO-RESPONSE REGULATOR; SD, short day; TOC1, TIMING OF CAB EXPRESSION 1.

4These authors contributed equally to this work.

(Received May 9, 2007; Accepted May 27, 2007)
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