Poster: Clocks

Abs # 774: The circadian-associated APRR1/TOC1 quintet of Arabidopsis thaliana, (I): Interaction between APRR1 and a small family of PIF3-related bHLH factors

Presenter:	Yamashino, Takafumi , yamasino@agr.nagoya-u.ac.jp
Authors	Yamashino, Takafumi  (A)   Matsushika, Akinori  (A)   Fujimori, Toru  (A)   Sato, Shusei  (B)   Kato, Tomohiko  (B)   Tabata, Satoshi  (B)   Mizuno, Takeshi  (A)
Affiliations:	(A): Graduate School of Bioagricultural Sciences, Nagoya University (B): Kazusa DNA Research Institute

APRR1 (ARABIDOPSIS PSEUDO-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. We previously showed that APRR1/TOC1 interacts with certain bHLH factors (i.e., PIF3 and PIL1), which are a PHYTOCHROME INTERACTING FACTOR and its homolog (PIF3-LIKE 1), respectively. To examine the relevance of PIL1 with reference to the function of APRR1/TOC1, T-DNA insertion mutants were isolated for PIL1. No phenotype was demonstrated for such homozygous pil1 mutants, in terms of circadian-regulated events in plants. We then decided to examine more extensively a certain set of bHLH factors, which are highly similar to PIL1 in their structural designs. The results of analyses of these bHLH factors (named HFR1, PIL2, PIF4, PIL5 and PIL6) in wild-type and APRR1-overexressing transgenic lines provided us with several new insight into a link between APRR1/TOC1 and these bHLH factors. In yeast two-hybrid assays, APRR1/TOC1 showed the abilities to interact with these proteins, as well as PIL1 and PIF3. Among them, it was found that the expressions of PIF4 and PIF6 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 that a certain set of bHLH factors appears to play crucial roles, in concert with APRR1/TOC1, in the integration of light signals to control both circadian and morphogenic processes.