Journal Article

A Link between Circadian-Controlled bHLH Factors and the APRR1/TOC1 Quintet in <i>Arabidopsis thaliana</i>

Takafumi Yamashino, Akinori Matsushika, Toru Fujimori, Shusei Sato, Tomohiko Kato, Satoshi Tabata and Takeshi Mizuno

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 44, issue 6, pages 619-629
Published in print June 2003 | ISSN: 0032-0781
Published online June 2003 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pcg078
A Link between Circadian-Controlled bHLH Factors and the APRR1/TOC1 Quintet in Arabidopsis thaliana

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  • Biochemistry
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  • Plant Sciences and Forestry

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APRR1 (ARABIDPSIS PSUEDO-RESPONSE REGULATOR 1) (or TOC1, TIMING OF CAB EXPRESSION 1) is believed to be a crucial component of biological clocks of Arabidopsis thaliana. Nevertheless, its molecular function remains to be fully elucidated. Based on the results of yeast two-hybrid and in vitro binding assays, we previously showed that APRR1/TOC1 interacts with certain bHLH factors (i.e. PIF3 and PIL1, which are PHYTOCHROME INTERACTING FACTOR 3 and its homolog (PIF3-LIKE 1), respectively). To critically examine the relevance of PIL1 with reference to the function of APRR1/TOC1, T-DNA insertion mutants were isolated for PIL1. No phenotype was observed for such homozygous pil1 mutants, in terms of circadian-associated events in plants. We then examined more extensively a certain set of bHLH factors, which are considerably similar to PIL1 in their structural designs. The results of extensive analyses of such bHLH factors (namely, HFR1, PIL2, PIF4, PIL5 and PIL6) in wild-type and APRR1-overexressing (APRR1-ox) transgenic lines provided us with several new insights into a link between APRR1/TOC1 and these bHLH factors. In yeast two-hybrid assays, APRR1/TOC1 showed the ability to interact with these proteins (except for HFR1), as well as PIL1 and PIF3. Among them, it was found that the expressions of PIF4 and PIL6 were regulated in a circadian-dependent manner, exhibiting free-running robust rhythms. The expressions of PIF4 and PIL6 were regulated also by light in a manner that their transcripts were rapidly accumulated upon exposure of etiolated seedlings to light. The light-induced expressions of PIF4 and PIL6 were severely impaired in APRR1-ox transgenic lines. Taken together, here we propose the novel view that these bHLH factors (PIF4 and PIL6) might play roles, in concert with APRR1/TOC1, in the integration of light-signals to control both circadian and photomorphogenic processes.

Keywords: Keywords: Arabidopsis thaliana — Circadian rhythm — bHLH factors — Light signalling.; Abbreviations: APRR, ARABIDOPSIS PSEUDO-RESPONSE REGULATORS; bHLH, basic helix-loop-helix; CCA1, CIRCADIAN CLOCK-ASSOCIATED 1; HFR1, LONG HYPOCOTYL IN FAR-RED 1; LHY, LATE ELONGATED HYPOCOTYL; PIF3, PHYTOCHROME INTERACTING FACTOR 3; PIL, PHYTOCHROME INTERACTING FACTOR-LIKE; PIL1, PIF3-LIKE 1; TOC1, TIMING OF CAB EXPRESSION 1.

Journal Article.  7525 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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