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Plant and Cell Physiology Advance Access originally published online on May 22, 2007
Plant and Cell Physiology 2007 48(7):971-983; doi:10.1093/pcp/pcm063
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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

Genetic Linkages Between Circadian Clock-Associated Components and Phytochrome-Dependent Red Light Signal Transduction in Arabidopsis thaliana

Shogo Ito1,3,*, Norihito Nakamichi1,3,4, Yuko Nakamura1, Yusuke Niwa1, Takahiko Kato1, Masaya Murakami1, Masanori Kita1, Tsuyoshi Mizoguchi2, Kanae Niinuma2, Takafumi Yamashino1 and Takeshi Mizuno1

1Laboratory of Molecular Microbiology, School of Agriculture, Nagoya University, Chikusa-ku, Nagoya, 464-8601 Japan
2Institute 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

The current best candidates for Arabidopsis thaliana clock components are CCA1 (CIRCADIAN CLOCK-ASSOCIATED 1) and its homolog LHY (LATE ELONGATED HYPOCOTYL). In addition, five members of a small family, PSEUDO-RESPONSE REGULATORS (including PRR1, PRR3, PRR5, PRR7 and PRR9), are believed to be another type of clock component. The originally described member of PRRs is TOC1 (or PRR1) (TIMING OF CAB EXPRESSION 1). Interestingly, seedlings of A. thaliana carrying a certain lesion (i.e. loss-of-function or misexpression) of a given clock-associated gene commonly display a characteristic phenotype of light response during early photomorphogenesis. For instance, cca1 lhy double mutant seedlings show a shorter hypocotyl length than the wild type under a given fluence rate of red light (i.e. hypersensitivity to red light). In contrast, both toc1 single and prr7 prr5 double mutant seedlings with longer hypocotyls are hyposensitive under the same conditions. These phenotypes are indicative of linkage between the circadian clock and red light signal transduction mechanisms. Here this issue was addressed by conducting combinatorial genetic and epistasis analyses with a large number of mutants and transgenic lines carrying lesions in clock-associated genes, including a cca1 lhy toc1 triple mutant and a cca1 lhy prr7 prr5 quadruple mutant. Taking these results together, we propose a genetic model for clock-associated red light signaling, in which CCA1 and LHY function upstream of TOC1 (PRR1) in a negative manner, in turn, TOC1 (PRR1) serves as a positive regulator. PRR7 and PRR5 also act as positive regulators, but independently from TOC1 (PRR1). It is further suggested that these signaling pathways are coordinately integrated into the phytochrome-mediated red light signal transduction pathway, in which PIF3 (PHYTOCHROME-INTERACTING FACTOR 3) functions as a negative regulator immediately downstream of phyB.

Keywords: Arabidopsis thaliana - Light signaling - Circadian clock - Hypocotyl elongation - Photomorphogenesis

Abbreviations: bHLH, basic Helix–Loop–Helix; CCA1, CIRCADIAN CLOCK-ASSOCIATED 1; LHY, LATE ELONGATED HYPOCOTYL; PIF/PIL, PHYTOCHROME-INTERACTING FACTOR/LIKE: PRR, PSEUDO-RESPONSE REGULATOR; TOC1, TIMING OF CAB EXPRESSION 1.

3These authors contributed equally to this work.

4Present address: Division of Biological Science, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya, 464-8602 Japan.

(Received April 20, 2007; Accepted May 21, 2007)
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