A UV-B-specific signaling component orchestrates plant UV protection

UV-B radiation in sunlight has diverse effects on humans, animals, plants, and microorganisms. UV-B can cause damage to molecules and cells, and consequently organisms need to protect against and repair UV damage to survive in sunlight. In plants, low nondamaging levels of UV-B stimulate transcription of genes involved in UV-protective responses. However, remarkably little is known about the underlying mechanisms of UV-B perception and signal transduction. Here we report that Arabidopsis UV RESISTANCE LOCUS 8 (UVR8) is a UV-B-specific signaling component that orchestrates expression of a range of genes with vital UV-protective functions. Moreover, we show that UVR8 regulates expression of the transcription factor HY5 specifically when the plant is exposed to UV-B. We demonstrate that HY5 is a key effector of the UVR8 pathway, and that it is required for survival under UV-B radiation. UVR8 has sequence similarity to the eukaryotic guanine nucleotide exchange factor RCC1, but we found that it has little exchange activity. However, UVR8, like RCC1, is located principally in the nucleus and associates with chromatin via histones. Chromatin immunoprecipitation showed that UVR8 associates with chromatin in the HY5 promoter region, providing a mechanistic basis for its involvement in regulating transcription. We conclude that UVR8 defines a UV-B-specific signaling pathway in plants that orchestrates the protective gene expression responses to UV-B required for plant survival in sunlight

Plant Photoreceptors: Phylogenetic Overview

Plants possess photoreceptors to perceive light which controls most aspects of their lives. Three photoreceptor families are well characterized: cryptochromes (crys), phototropins (phots), and phytochromes (phys). Two putative families have been identified more recently: Zeitlupes (ZTLs) and UV-B photoreceptors (ULI). Using Arabidopsis thaliana and Oryza sativa photoreceptor sequences as references, we have searched for photoreceptor encoding genes in the major phyla of plant kingdom. For each photoreceptor family, using a phylogenetic tree based on the alignment of conserved amino acid sequences, we have tried to trace back the evolution and the emergence of the diverse photoreceptor ancestral sequences. The green alga Chlamydomonas contains one cry and one phot sequence, probably close to the corresponding ancestral sequences, and no phy-related sequence. The putative UV-B photoreceptors seem to be restricted to the Brassicacae. Except for mosses and ferns, which contain divergent photoreceptor numbers, the composition of the diverse photoreceptor families is conserved between species. A high conservation of the residues within domains is observed in each photoreceptor family. The complete phylogenic analysis of the photoreceptor families in plants has confirmed the existence of crucial evolutionary nodes between the major phyla. For each photoreceptor class, a major duplication occurred before the separation between Mono- and Eudicotyledons. This allowed postulating on the putative ancestral function of the photoreceptors

UVR8 in Arabidopsis thaliana regulates multiple aspects of cellular differentiation during leaf development in response to ultraviolet B radiation

Responses specific to ultraviolet B (UV-B) wavelengths are still poorly understood, both in terms of initial signalling and effects on morphogenesis. Arabidopsis thaliana UV RESISTANCE LOCUS8 (UVR8) is the only known UV-B specific signalling component, but the role of UVR8 in leaf morphogenesis is unknown. The regulatory effects of UVR8 on leaf morphogenesis at a range of supplementary UV-B doses were characterized, revealing both UVR8-dependent and independent responses to UV irradiation. Inhibition of epidermal cell division in response to UV-B is largely independent of UVR8. However, overall leaf growth under UV-B irradiation in wild-type plants is enhanced compared with a uvr8 mutant because of a UVR8-dependent compensatory increase of cell area in wild-type plants. UVR8 was also required for the regulation of endopolyploidy in response to UV-B, and the uvr8 mutant also has a lower density of stomata than the wild type in the presence of UV-B, indicating that UVR8 has a regulatory role in other developmental events. Our findings show that, in addition to regulating UV-protective gene expression responses, UVR8 is involved in controlling aspects of leaf growth and morphogenesis. This work extends our understanding of how UV-B response is orchestrated at the whole-plant level. New Phytologist (2009) 183: 315-326doi: 10.1111/j.1469-8137.2009.02855.x