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Oh, Eunkyoo,Yamaguchi, Shinjiro,Kamiya, Yuji,Bae, Gabyong,Chung, Won-Il,Choi, Giltsu Blackwell Publishing Ltd 2006 The Plant journal Vol.47 No.1
<P>Summary</P><P>Angiosperm seeds integrate various environmental signals, such as water availability and light conditions, to make a proper decision to germinate. Once the optimal conditions are sensed, gibberellin (GA) is synthesized, triggering germination. Among environmental signals, light conditions are perceived by phytochromes. However, it is not well understood how phytochromes regulate GA biosynthesis. Here we investigated whether phytochromes regulate GA biosynthesis through PIL5, a phytochrome-interacting bHLH protein, in Arabidopsis. We found that <I>pil5</I> seed germination was inhibited by paclobutrazol, the <I>ga1</I> mutation was epistatic to the <I>pil5</I> mutation, and the inhibitory effect of <I>PIL5</I> overexpression on seed germination could be rescued by exogenous GA, collectively indicating that <I>PIL5</I> regulates seed germination negatively through GA. Expression analysis revealed that PIL5 repressed the expression of GA biosynthetic genes (<I>GA3ox1</I> and <I>GA3ox2</I>), and activated the expression of a GA catabolic gene (<I>GA2ox</I>) in both PHYA- and PHYB-dependent germination assays. Consistent with these gene-expression patterns, the amount of bioactive GA was higher in the <I>pil5</I> mutant and lower in the <I>PIL5</I> overexpression line. Lastly, we showed that red and far-red light signals trigger PIL5 protein degradation through the 26S proteasome, thus releasing the inhibition of bioactive GA biosynthesis by PIL5. Taken together, our data indicate that phytochromes promote seed germination by degrading PIL5, which leads to increased GA biosynthesis and decreased GA degradation.</P>
Oh, Eunkyoo,Yamaguchi, Shinjiro,Hu, Jianhong,Yusuke, Jikumaru,Jung, Byunghyuck,Paik, Inyup,Lee, Hee-Seung,Sun, Tai-ping,Kamiya, Yuji,Choi, Giltsu American Society of Plant Physiologists 2007 The Plant cell Vol.19 No.4
<P>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.</P>
Oh, Eunkyoo,Kang, Hyojin,Yamaguchi, Shinjiro,Park, Jeongmoo,Lee, Doheon,Kamiya, Yuji,Choi, Giltsu American Society of Plant Physiologists 2009 The Plant cell Vol.21 No.2
<P>PHYTOCHROME INTERACTING FACTOR 3-LIKE5 (PIL5) is a basic helix-loop-helix transcription factor that inhibits seed germination by regulating the expression of gibberellin (GA)- and abscisic acid (ABA)-related genes either directly or indirectly. It is not yet known, however, whether PIL5 regulates seed germination solely through GA and ABA. Here, we used Chromatin immunoprecipitation-chip (ChIP-chip) analysis to identify 748 novel PIL5 binding sites in the Arabidopsis thaliana genome. Consistent with the molecular function of PIL5 as a transcription regulator, most of the identified binding sites are located in gene promoter regions. Binding site analysis shows that PIL5 binds to its target sites mainly through the G-box motif in vivo. Microarray analysis reveals that phytochromes regulate a large number of genes mainly through PIL5 during seed germination. Comparison between the ChIP-chip and microarray data indicates that PIL5 regulates 166 genes by directly binding to their promoters. Many of the identified genes encode transcription regulators involved in hormone signaling, while some encode enzymes involved in cell wall modification. Interestingly, PIL5 directly regulates many transcription regulators of hormone signaling and indirectly regulates many genes involved in hormone metabolism. Taken together, our data indicate that PIL5 inhibits seed germination not just through GA and ABA, but also by coordinating hormone signals and modulating cell wall properties in imbibed seeds.</P>