Plant Cell Advance Online Publication
Published on April 20, 2007; 10.1105/tpc.107.050153
Received January 8, 2007
Returned for revision February 20, 2007
Accepted April 5, 2007
PIL5, a Phytochrome-Interacting bHLH Protein, Regulates Gibberellin Responsiveness by Binding Directly to the GAI and RGA Promoters in Arabidopsis Seeds
Eunkyoo Oh 1, Shinjiro Yamaguchi 2, Jianhong Hu 3, Jikumaru Yusuke 2, Byunghyuck Jung 4, Inyup Paik 1, Hee-Seung Lee 4, Tai-ping Sun 3, Yuji Kamiya 2, and Giltsu Choi 1*
1 Department of Biological Sciences, KAIST, Daejeon 305-701, Korea
2 RIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, Japan
3 Department of Biology, Duke University, Durham, North Carolina 27708-1000
4 Department of Chemistry, KAIST, Daejeon 305-701, Korea
* To whom correspondence should be addressed. E-mail: gchoi{at}kaist.ac.kr.
Previous work showed that PHYTOCHROME-INTERACTING FACTOR3-LIKE5 (PIL5), a light-labile basic helix-loop-helix protein, inhibits seed germination by repressing GIBBERELLIN 3 -HYDROXYLASE1 (GA3ox1) and GA3ox2 and activating a gibberellic acid (GA) catabolic gene (GA2ox2). However, we show persistent light-dependent and PIL5-inhibited germination behavior in the absence of both de novo GA biosynthesis and deactivation by GA2ox2, suggesting that PIL5 regulates not only GA metabolism but also GA responsiveness. PIL5 increases the expression of two GA repressor (DELLA) genes, GA-INSENSITIVE (GAI) and REPRESSOR OF GA1-3 (RGA/RGA1), in darkness. The hypersensitivity of gai-t6 rga-28 to red light and the suppression of germination defects of a rga-28 PIL5 overexpression line show the significant role of this transcriptional regulation in seed germination. PIL5 also increases abscisic acid (ABA) levels by activating ABA biosynthetic genes and repressing an ABA catabolic gene. PIL5 binds directly to GAI and RGA promoters but not to GA and ABA metabolic gene promoters. Together, our results show that light signals perceived by phytochromes cause a reduction in the PIL5 protein level, which in turn regulates the transcription of two DELLA genes directly and that of GA and ABA metabolic genes indirectly.
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