To optimize their growth and survival, plants perceive and respond to ultraviolet-B (UV-B) radiation. However, neither the molecular identity of the UV-B photoreceptor nor the photoperception mechanism is known. Here we show that dimers of the UVR8 protein perceive UV-B, probably by a tryptophan-based mechanism. Absorption of UV-B induces instant monomerization of the photoreceptor and interaction with COP1, the central regulator of light signaling. Thereby this signaling cascade controlled by UVR8 mediates UV-B photomorphogenic responses securing plant acclimation and thus promotes survival in sunlight.
The recently identified plant photoreceptor UVR8 triggers regulatory changes in gene expression in response to ultraviolet-B (UV-B) light via an unknown mechanism. Here, crystallographic and solution structures of the UVR8 homodimer, together with mutagenesis and far-UV circular dichroism spectroscopy, reveal its mechanisms for UV-B perception and signal transduction. β-propeller subunits form a remarkable, tryptophan-dominated, dimer interface stitched together by a complex salt-bridge network. Salt-bridging arginines flank the excitonically coupled cross-dimer tryptophan “pyramid” responsible for UV-B sensing. Photoreception reversibly disrupts salt bridges, triggering dimer dissociation and signal initiation. Mutation of a single tryptophan to phenylalanine re-tunes the photoreceptor to detect UV-C wavelengths. Our analyses establish how UVR8 functions as a photoreceptor without a prosthetic chromophore to promote plant development and survival in sunlight.
UV-B light initiates photomorphogenic responses in plants. Arabidopsis UV RESISTANCE LOCUS8 (UVR8) specifically mediates these responses by functioning as a UV-B photoreceptor. UV-B exposure converts UVR8 from a dimer to a monomer, stimulates the rapid accumulation of UVR8 in the nucleus, where it binds to chromatin, and induces interaction of UVR8 with CONSTITUTIVELY PHOTO-MORPHOGENIC1 (COP1), which functions with UVR8 to control photomorphogenic UV-B responses. Although the crystal structure of UVR8 reveals the basis of photoreception, it does not show how UVR8 initiates signaling through interaction with COP1. Here we report that a region of 27 amino acids from the C terminus of UVR8 (C27) mediates the interaction with COP1. The C27 region is necessary for UVR8 function in the regulation of gene expression and hypocotyl growth suppression in Arabidopsis. However, UVR8 lacking C27 still undergoes UV-B-induced monomerization in both yeast and plant protein extracts, accumulates in the nucleus in response to UV-B, and interacts with chromatin at the UVR8-regulated ELONGATED HYPOCOTYL5 (HY5) gene. The UV-B-dependent interaction of UVR8 and COP1 is reproduced in yeast cells and we show that C27 is both necessary and sufficient for the interaction of UVR8 with the WD40 domain of COP1. Furthermore, we show that C27 interacts in yeast with the REPRESSOR OF UV-B PHOTOMORPHO-GENESIS proteins, RUP1 and RUP2, which are negative regulators of UVR8 function. Hence the C27 region has a key role in UVR8 function.U V-B wavelengths (280-315 nm) are a minor component of sunlight but have a major impact on living organisms. The damaging effects of UV-B are well documented, but plants rarely show signs of UV-damage despite constant exposure to sunlight. This is because plants have evolved effective means of protection against UV-B, including the deposition of UV-absorbing phenolic compounds in the outer tissues and the production of efficient antioxidant and DNA repair systems (1-4). These UV-protective mechanisms are stimulated by low doses of UV-B through differential gene expression. Moreover, low levels of UV-B regulate other responses in plants, including the suppression of hypocotyl extension (5). Thus, in plants, UV-B acts as a key regulatory signal that initiates photomorphogenic responses and promotes survival.The low dose, photomorphogenic responses to UV-B are mediated by the photoreceptor UVR8 (3-7). UVR8 acts specifically in UV-B to regulate over 100 genes, many of which are involved in UV protection (5, 6). Arabidopsis uvr8 mutant plants are highly sensitive to UV-B because they fail to express UVprotective genes (6,8). Among the genes regulated by UVR8 is that encoding the ELONGATED HYPOCOTYL 5 (HY5) transcription factor, which mediates most, if not all, gene expression responses initiated by UVR8 (6, 7). UVR8 interacts with chromatin via histones, in particular H2B (9) at the HY5 gene (6, 9) and a number of other UVR8-regulated genes (9), which raises the possibility that UVR8 promotes recruitment or activation of...
Arabidopsis thaliana UV RESISTANCE LOCUS8 (UVR8) is a photoreceptor specifically for UV-B light that initiates photomorphogenic responses in plants. UV-B exposure causes rapid conversion of UVR8 from dimer to monomer, accumulation in the nucleus, and interaction with CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1), which functions with UVR8 in UV-B responses. Studies in yeast and with purified UVR8 implicate several tryptophan amino acids in UV-B photoreception. However, their roles in UV-B responses in plants, and the functional significance of all 14 UVR8 tryptophans, are not known. Here we report the functions of the UVR8 tryptophans in vivo. Three tryptophans in the b-propeller core are important in maintaining structural stability and function of UVR8. However, mutation of three other core tryptophans and four at the dimeric interface has no apparent effect on function in vivo. Mutation of three tryptophans implicated in UV-B photoreception, W233, W285, and W337, impairs photomorphogenic responses to different extents. W285 is essential for UVR8 function in plants, whereas W233 is important but not essential for function, and W337 has a lesser role. Ala mutants of these tryptophans appear monomeric and constitutively bind COP1 in plants, but their responses indicate that monomer formation and COP1 binding are not sufficient for UVR8 function.
In the present study, we combine theoretical and experimental approaches in order to gain insight into the electronic properties of both the high-temperature, rutile (metallic) and low-temperature, body-centered tetragonal (insulating) phase of niobium dioxide (NbO2) as well as the optical properties of the low-temperature phase. Theoretical calculations performed at the level of the local density approximation, Hubbard U correction, and hybrid functional are complemented with the spectroscopic ellipsometry (SE) of epitaxial films grown by molecular beam epitaxy. For the rutile phase, the local density approximation (LDA) gives the best description and predicts Fermi surface nesting consistent with wave vectors that lead to niobium-niobium dimerization during the phase transition. For the insulating phase, LDA provides a good quantitative description of the lattice, but only a qualitative description for the band gap. Including a Hubbard U correction opens the band gap at the expense of correctly describing the valence band and lattice of both phases. The hybrid functional slightly overestimates the band gap. Ellipsometric measurement is consistent with insulating behavior with a 1.0 eV band gap. Comparison with the theoretical dielectric functions, obtained utilizing a scissors operator to adjust the LDA band gap to reproduce the ellipsometry data, allows for identification of the SE peak features.
The following Supporting Information is available for this article: Figure S1. Solar spectrum at different times of the day when plants were moved outdoors. Figure S2. Photon irradiance for different wavebands in solar radiation.Figure S3. Multidimensional scaling of RNA-seq data.Figure S4. Comparison between RNA-seq and qRT-PCR data.Figure S5. Venn diagrams showing the number differentially expressed genes in RNA-seq data.Figure S6. Enrichment of KEGG pathways in RNA-seq data.Figure S7. In vitro absorption spectra of Arabidopsis UVR8 protein.Figure S8. Position weight matrices of the enriched DNA-binding motifs.Figure S9. Transcript abundance of seven genes measured using qRT-PCR.Table S1. Information of primers used and genes assessed in qRT-PCR. Table S2. Summary of the ANOVA of the qRT-PCR data.Methods S1. Description of the filters and the waveband contrasts.Dataset S1. Outcome of differential gene expression analysis for the three genotypes and multiple waveband contrasts combination. The dataset is included as a separate file in .Rda format and can be read using R.
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