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A UV-B-Specific Signaling Component Orchestrates Plant UV Protection

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A UV-B-Specific Signaling Component Orchestrates Plant UV Protection A UV-B-specific signaling component orchestrates plant UV protection Bobby A. Brown*†, Catherine Cloix*†, Guang Huai Jiang*‡, Eirini Kaiserli*, Pawel Herzyk§, Daniel J. Kliebenstein¶, and Gareth I. Jenkins*ʈ *Plant Science Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, and §Sir Henry Wellcome Functional Genomics Facility and Division of Biochemistry and Molecular Biology, University of Glasgow, Glasgow G12 8QQ, United Kingdom; and ¶Department of Plant Sciences, University of California, Davis, CA 95616 Edited by Alexander N. Glazer, University of California System, Oakland, CA, and approved October 20, 2005 (received for review August 22, 2005) UV-B radiation in sunlight has diverse effects on humans, animals, genes are involved, for instance in ameliorating oxidative stress and plants, and microorganisms. UV-B can cause damage to molecules repairing UV damage. Mutants lacking UV-protective compo- and cells, and consequently organisms need to protect against and nents, such as the flavonoids and sinapic acid esters, are highly repair UV damage to survive in sunlight. In plants, low nondam- sensitive to ambient levels of UV-B (13, 14). aging levels of UV-B stimulate transcription of genes involved in Although plant responses to low ambient fluence rates of UV-B UV-protective responses. However, remarkably little is known are key to survival, the underlying mechanisms of UV-B perception about the underlying mechanisms of UV-B perception and signal and signal transduction are very poorly understood, despite decades transduction. Here we report that Arabidopsis UV RESISTANCE of research. These responses are not mediated by the known plant LOCUS 8 (UVR8) is a UV-B-specific signaling component that or- photoreceptors and do not involve DNA damage signaling path- chestrates expression of a range of genes with vital UV-protective ways (5, 6). It has often been speculated that UV-B may be functions. Moreover, we show that UVR8 regulates expression of perceived by a novel class of photoreceptor, but no such molecule the transcription factor HY5 specifically when the plant is exposed has ever been identified. Moreover, although pharmacological, cell to UV-B. We demonstrate that HY5 is a key effector of the UVR8 physiological, and genetic approaches have provided some insights pathway, and that it is required for survival under UV-B radiation. into UV-B signal transduction processes (15–17), no UV-B-specific UVR8 has sequence similarity to the eukaryotic guanine nucleotide signaling pathway has been defined. Thus, understanding of the exchange factor RCC1, but we found that it has little exchange mechanisms of plant UV-B responses lags well behind knowledge activity. However, UVR8, like RCC1, is located principally in the of light responses mediated by the phytochrome, cryptochrome, nucleus and associates with chromatin via histones. Chromatin and phototropin photoreceptors. immunoprecipitation showed that UVR8 associates with chromatin Here we report that the Arabidopsis protein UV RESISTANCE in the HY5 promoter region, providing a mechanistic basis for its LOCUS 8 (UVR8) is a UV-B-specific signaling component that involvement in regulating transcription. We conclude that UVR8 regulates expression of a range of genes essential for UV-B defines a UV-B-specific signaling pathway in plants that orches- protection. In addition, we show that UVR8 controls expression of trates the protective gene expression responses to UV-B required the transcription factor HY5, and that HY5 is a key effector of the for plant survival in sunlight. UVR8 signaling pathway. Further, we show that the association of UVR8 with chromatin provides a basis for its action in regulating photomorphogenesis ͉ ultraviolet-B radiation ͉ UV RESISTANCE LOCUS 8 transcription. Materials and Methods V-B radiation (280–320 nm) is an integral component of Usunlight. UV-B can cause damage to macromolecules, includ- Plant Material and Treatments. For transcript measurements and ing DNA, and generate reactive oxygen species. Because UV-B transcriptome analyses, wild-type Arabidopsis L er, uvr8-1 (18), uvr8-2 (this study; backcrossed twice to wild type), and hy5-1 (19) affects the growth, development, reproduction, and survival of ␮ Ϫ2⅐ Ϫ1 many organisms, there is concern that any further increases in plants were grown in compost for 3 weeks in 25 mol m s ambient levels of UV-B, resulting from stratospheric ozone deple- continuous white light (warm white fluorescent tubes) at 21°C and exposed to 3 ␮mol mϪ2⅐sϪ1 UV-Bfor4hor100␮mol mϪ2⅐sϪ1 tion may have a significant impact on natural and agricultural UV-A for 6 h using light sources described previously (20). For ecosystems (1–4). Hence, it is important to understand how plants low-temperature treatment, plants were transferred to 7°C for 24 h and other organisms protect themselves against the potentially (20). Seedlings were grown in darkness for 4 days on 0.8% agar damaging effects of UV-B. ϫ plates containing 1 Murashige and Skoog salts, MS vitamins, 2% PLANT BIOLOGY Exposure to UV-B is obligatory for higher plants because of the sucrose, and 0.05% Mes (pH 5.7) and illuminated with 70 ␮mol need to maximize light capture for photosynthesis. The effects of mϪ2⅐sϪ1 far-red light (20) for 6 h. The effect of sucrose was UV-B on diverse species of plants have been reported in the examined in seedlings grown in darkness with or without 2% literature, and it is evident that different responses are observed at different UV-B fluence rates (5, 6). Exposure to high amounts of UV-B causes tissue necrosis and induces the expression of stress- Conflict of interest statement: No conflicts declared. associated genes in part through activation of pathogen-defense and This paper was submitted directly (Track II) to the PNAS office. wound-signaling pathways (5–7). At ambient UV-B levels, crosstalk Abbreviations: FDR, false discovery rate; CHS, chalcone synthase; GEF, guanine nucleotide between wound and UV-B signaling pathways modifies plant– exchange factor. insect interactions (8). Importantly, exposure to low nondamaging Data deposition: The Affymetrix microarray expression data reported in this paper have levels of UV-B has numerous regulatory effects on plant morphol- been deposited in the National Center for Biotechnology Information’s Gene Expression ogy, development, physiology, and biochemical composition (1, 5, Omnibus (GEO) database (accession no. GSE3533). 6, 9). Low fluence rates of UV-B promote the expression of a range †B.A.B. and C.C. contributed equally to this work. of genes involved in UV-B protection (5, 6, 10, 11). These include ‡Present address: National Key Laboratory of Plant Genomics, Institute of Genetics and genes concerned with the production of flavonoids and other Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China. phenolic compounds that accumulate in the epidermal layers and ʈTo whom correspondence should be addressed. E-mail: [email protected]. provide a UV-absorbing sun screen (12, 13). Other UV-B-induced © 2005 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0507187102 PNAS ͉ December 13, 2005 ͉ vol. 102 ͉ no. 50 ͉ 18225–18230 Downloaded by guest on September 28, 2021 sucrose, as described (20). For GFP-UVR8 localization and chro- matin association experiments, plants were grown in 25 ␮mol mϪ2⅐sϪ1 or 100 ␮mol mϪ2⅐sϪ1 continuous white light before UV-B illumination. For UV-B sensitivity assays, plants were grown in 120 ␮mol mϪ2⅐sϪ1 white light for 12 days, then exposed to 5 ␮mol mϪ2⅐sϪ1 UV-B with supplementary 40 ␮mol mϪ2⅐sϪ1 white light for 24 h. Plants were returned to 120 ␮mol mϪ2⅐sϪ1 white light for 5 days to determine survival. Transcript Measurements. RNA was isolated from leaf tissue by using the Purescript kit (Flowgen, Nottingham, U.K.). Chalcone synthase (CHS), HY5, and ACTIN2 transcripts were assayed by RT-PCR with gene-specific primers by using cycle numbers within the linear range of amplification (CHS-L 5Ј-ATCTTTGAGATG- GTGTCTGC-3Ј, CHS-R 5Ј-CGTCTAGTATGAAGAGAACG- 3Ј; HY5-L 5Ј-GCTGCAAGCTCTTTACCATC-3Ј, HY5-R 5Ј- AGCATCTGGTTCTCGTTCTG-3Ј; ACTIN2 primers as in ref. 21). Fig. 1. UVR8 acts specifically in the UV-B regulation of CHS expression. Shown are RT-PCR measurements of CHS and control ACTIN2 transcripts Transcriptome Analysis. Three independent RNA samples were (lower and upper bands, respectively, A–E). (A) Wild-type, uvr8-1, uvr8-2, and analyzed for each treatment to facilitate statistical analysis (22). one of the F1 progeny of uvr8-1 and uvr8-2 grown for 3 weeks in low fluence rate (25 ␮mol mϪ2⅐sϪ1) white light (L) and illuminated with ambient (3 ␮mol RNA quality was checked by using an Agilent (Austin, TX) RNA Ϫ Ϫ m 2⅐s 1) UV-B for 4 h (UB). (B) Plants grown as in A illuminated with UV-A (UA) BioAnalyzer 2100. RNA was reverse-transcribed, double-stranded for6h.(C) Four-day-old dark-grown (D) seedlings illuminated with far-red (FR) cDNA in vitro transcribed, and biotinylated cRNA hybridized to light for 6 h. (D) Plants grown as in A transferred to 7°C for 24 h. (E) Seedlings Affymetrix Arabidopsis ATH1 GeneChips as recommended by grown in darkness for 4 days with (ϩS) or without (ϪS) 2% sucrose. Affymetrix. Chips were washed and stained by using Affymetrix protocols on the Fluidics Station 400 and scanned on the Gene Array Scanner 2500. Data were analyzed by using FUNALYSE, mouse IgG conjugated to horseradish peroxidase, Promega), bands Version 2.0, an automated pipeline in Sir Henry Wellcome Func- were visualized by using ECLϩ reagent (Amersham Pharmacia). tional Genomics Facility, University of Glasgow. This analysis consisted of the Robust Multichip Average (23) normalization GFP-UVR8 Localization and Chromatin Association. UVR8 cDNA was followed by identification of differentially expressed genes by using inserted into the binary vector pEZR(K)-LC, a derivative of the Rank Products method (22).
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