PIL5, a phytochrome-interacting bHLH protein, regulates gibberellin responsiveness by binding directly to the GAI and RGA promoters in Arabidopsis seeds.
Previous work showed that PHYTOCHROME-INTERACTING FACTOR3-LIKE5 (PIL5), a light-labile basic helix-loop-helix protein, inhibits seed germination by repressing GIBBERELLIN 3beta-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.
Oh
Eunkyoo
E
Department of Biological Sciences, KAIST, Daejeon 305-701, Korea.
Yamaguchi
Shinjiro
S
Hu
Jianhong
J
Yusuke
Jikumaru
J
Jung
Byunghyuck
B
Paik
Inyup
I
Lee
Hee-Seung
HS
Sun
Tai-ping
TP
Kamiya
Yuji
Y
Choi
Giltsu
G
D017360:Arabidopsis / Q000235:genetics / Q000254:growth & development / Q000502:physiology*; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000502:physiology*; D001483:Base Sequence; D051792:Basic Helix-Loop-Helix Proteins / Q000502:physiology*; D018506:Gene Expression Regulation, Plant; D018525:Germination; D005875:Gibberellins / Q000378:metabolism*; D008027:Light; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D012097:Repressor Proteins / Q000235:genetics* / Q000378:metabolism; D012639:Seeds / Q000502:physiology*
D029681:Arabidopsis Proteins; D051792:Basic Helix-Loop-Helix Proteins; D005875:Gibberellins; D012097:Repressor Proteins; C109555:GAI protein, Arabidopsis; C508468:RGA protein, Arabidopsis; C508068:At2g20180 protein, Arabidopsis