An Arabidopsis mutant defective in UV-B light-mediated responses

Genome-Wide Analysis of the DC1 Domain Protein Gene Family in Tomatoes under Abiotic Stress

DC1 (Divergent C1) domain proteins are a new class of proteins that have been discovered in recent years, which play an important role in plant growth, development, and stress response. In order to better study the distribution and function of DC1 domain proteins in tomatoes, a genome-wide identification was conducted. It was found that there are twenty-one DC1 domain protein genes distributed on nine chromosomes of tomatoes, named SlCHP1-21. Phylogenetic analysis shows that twenty-one SlCHP genes are divided into six subfamilies. Most of the SlCHP genes in tomatoes have no or very short introns. All SlCHP proteins, with the exception of SlCHP8 and SlCHP17, contain variable amounts of C1 domain. Analysis of the SlCHP gene promoter sequence revealed multiple cis-elements responsive to plant stress. qRT-CR analysis showed that most members of SlCHP gene expressed in the roots. The SlCHP11, 13, 16, 17, and SlCHP20 genes showed specific responses to high temperature, low temperature, salt, and drought stress. In addition, the subcellular localization and interaction proteins of SlCHP were analyzed and predicted. Together, these results provides a theoretical basis for further exploration of the function and mechanism of the SlCHP gene in tomatoes.

The DC1 Domain Protein BINUCLEATE POLLEN is Required for POLLEN Development in Arabidopsis thaliana

The development of the male gametophyte is a tightly regulated process that requires the precise control of cell division and gene expression. A relevant aspect to understand the events underlying pollen development regulation constitutes the identification and characterization of the genes required for this process. In this work, we showed that the DC1 domain protein BINUCLEATE POLLEN (BNP) is essential for pollen development and germination. Pollen grains carrying a defective BNP alleles failed to complete mitosis II and exhibited impaired pollen germination. By yeast two-hybrid analysis and bimolecular fluorescence complementation assays, we identified a set of BNP-interacting proteins. Among confirmed interactors, we found the NAC family transcriptional regulators Vascular Plant One-Zinc Finger 1 (VOZ1) and VOZ2. VOZ1 localization changes during pollen development, moving to the vegetative nucleus at the tricellular stage. We observed that this relocalization requires BNP; in the absence of BNP in pollen from bnp/BNP plants, VOZ1 nuclear localization is impaired. As the voz1voz2 double mutants showed the same developmental defect observed in bnp pollen grains, we propose that BNP requirement to complete microgametogenesis could be linked to its interaction with VOZ1/2 proteins. BNP could have the role of a scaffold protein, recruiting VOZ1/2 to the endosomal system into assemblies that are required for their further translocation to the nucleus, where they act as transcriptional regulators.

The characterization of Mediator 12 and 13 as conditional positive gene regulators in Arabidopsis

Mediator 12 (MED12) and MED13 are components of the Mediator multi-protein complex, that facilitates the initial steps of gene transcription. Here, in an Arabidopsis mutant screen, we identify MED12 and MED13 as positive gene regulators, both of which contribute broadly to morc1 de-repressed gene expression. Both MED12 and MED13 are preferentially required for the expression of genes depleted in active chromatin marks, a chromatin signature shared with morc1 re-activated loci. We further discover that MED12 tends to interact with genes that are responsive to environmental stimuli, including light and radiation. We demonstrate that light-induced transient gene expression depends on MED12, and is accompanied by a concomitant increase in MED12 enrichment during induction. In contrast, the steady-state expression level of these genes show little dependence on MED12, suggesting that MED12 is primarily required to aid the expression of genes in transition from less-active to more active states.

Mediator is a multiprotein complex required to activate gene transcription by RNAPII. Here, the authors report that MED12 and MED13 are conditional positive regulators that facilitate the expression of genes depleted in active chromatin marks and the induction of gene expression in response to environmental stimuli in Arabidopsis.

UV-B Induces Distinct Transcriptional Re-programing in UVR8-Signal Transduction, Flavonoid, and Terpenoids Pathways in Camellia sinensis

Plants are known to respond to Ultraviolet-B radiation (UV-B: 280-320 nm) by generating phenolic metabolites which absorbs UV-B light. Phenolics are extraordinarily abundant in Camellia sinensis leaves and are considered, together with pleasant volatile terpenoids, as primary flavor determinants in tea beverages. In this study, we focused on the effects of UV-B exposure (at 35 μW cm^-2 for 0, 0.5, 2, and 8 h) on tea transcriptional and metabolic alterations, specifically related to tea flavor metabolite production. Out of 34,737 unigenes, a total of 18,081 differentially expressed genes (DEGs) due to UV-B treatments were identified. Additionally, the phenylpropanoid pathway was found as one of the most significantly UV-B affected top 20 KEGG pathways while flavonoid and monoterpenoid pathway-related genes were enhanced at 0.5 h. In the UVR8-signal transduction pathway, UVR8 was suppressed at both short and long exposure of UV-B with genes downstream differentially expressed. Divergent expression of MYB4 at different treatments could have differentially altered structural and regulatory genes upstream of flavonoid biosynthesis pathways. Suppression of MYB4-1&3 at 0.5 h could have led to the up-regulation of structural CCOAOMT-1&2, HST-1&2, DFR-4, ANR-2, and LAR-1&3 genes resulting in accumulation of specialized metabolites at a shorter duration of UV-B exposure. Specialized metabolite profiling revealed the correlated alterations in the abundances of catechins and some volatile terpenoids in all the treatments with significant accumulation of specialized metabolites at 0.5 h treatment. A significant increase in specialized metabolites at 0.5 h treatment and no significant alteration observed at longer UVB treatment suggested that shorter exposure to UV-B led to different display in gene expression and accumulation of specialized metabolites in tea shoots in response to UV-B stress. Taken together, our results indicated that the UV-B treatment applied in this study differentially altered the UVR8-signal transduction, flavonoid and terpenoid pathways at transcriptional and metabolic levels in tea plants. Our results show strong potential for UV-B application in flavor improvement in tea at the industrial level.

A perspective on ecologically relevant plant-UV research and its practical application

Plants perceive ultraviolet-B (UV-B) radiation through the UV-B photoreceptor UV RESISTANCE LOCUS 8 (UVR8), and initiate regulatory responses via associated signalling networks, gene expression and metabolic pathways. Various regulatory adaptations to UV-B radiation enable plants to harvest information about fluctuations in UV-B irradiance and spectral composition in natural environments, and to defend themselves against UV-B exposure. Given that UVR8 is present across plant organs and tissues, knowledge of the systemic signalling involved in its activation and function throughout the plant is important for understanding the context of specific responses. Fine-scale understanding of both UV-B irradiance and perception within tissues and cells requires improved application of knowledge about UV-attenuation in leaves and canopies, warranting greater consideration when designing experiments. In this context, reciprocal crosstalk among photoreceptor-induced pathways also needs to be considered, as this appears to produce particularly complex patterns of physiological and morphological response. Through crosstalk, plant responses to UV-B radiation go beyond simply UV-protection or amelioration of damage, but may give cross-protection over a suite of environmental stressors. Overall, there is emerging knowledge showing how information captured by UVR8 is used to regulate molecular and physiological processes, although understanding of upscaling to higher levels of organisation, i.e. organisms, canopies and communities remains poor. Achieving this will require further studies using model plant species beyond Arabidopsis, and that represent a broad range of functional types. More attention should also be given to plants in natural environments in all their complexity, as such studies are needed to acquire an improved understanding of the impact of climate change in the context of plant-UV responses. Furthermore, broadening the scope of experiments into the regulation of plant-UV responses will facilitate the application of UV radiation in commercial plant production. By considering the progress made in plant-UV research, this perspective highlights prescient topics in plant-UV photobiology where future research efforts can profitably be focussed. This perspective also emphasises burgeoning interdisciplinary links that will assist in understanding of UV-B effects across organisational scales and gaps in knowledge that need to be filled so as to achieve an integrated vision of plant responses to UV-radiation.

UV RESISTANCE LOCUS 8 From Chrysanthemum morifolium Ramat (CmUVR8) Plays Important Roles in UV-B Signal Transduction and UV-B-Induced Accumulation of Flavonoids

UV Resistance Locus 8 (UVR8), an ultraviolet-B (UV-B; 280-315 nm) photoreceptor, participates in the regulation of various plant growth and developmental processes. UV-B radiation is an important factor enhancing the production of active components in medicinal plants. To-date, however, studies on UV-B photoreceptors have largely focused on Arabidopsis, and the functions of UVR8 in medicinal plants are still largely unknown. In the present study, a homolog of Arabidopsis UVR8, CmUVR8, was isolated from Chrysanthemum morifolium Ramat, and its structure and function were analyzed in detail. Protein sequence analysis showed that CmUVR8 contained nine conserved regulators of chromosome condensation 1 repeats, seven conserved bladed propellers, one C27 region, three "GWRHT" motifs and several crucial amino acid residues (such as 14 Trps and 2 Args), similar to AtUVR8. 3-D structural analysis of CmUVR8 indicated that its structure was similar to AtUVR8. Heterologous expression of CmUVR8 could rescued the deficient phenotype of uvr8-6, a mutant of UVR8 in Arabidopsis, indicating the role of CmUVR8 in the regulation of hypocotyl elongation and HY5 gene expression under UV-B irradiation. Moreover, CmUVR8 regulates UV-B-induced expression of four flavonoids biosynthesis-related genes and the UV-B-induced accumulation of flavonoids. Furthermore, the interaction between CmUVR8 and CmCOP1 were confirmed using a yeast two-hybrid assay. These results indicated that CmUVR8 plays important roles in UV-B signal transduction and the UV-B-induced accumulation of flavonoids, as a counterpart of AtUVR8.

Photomorphogenic responses to ultraviolet-B light

Exposure to ultraviolet B (UV-B) light regulates numerous aspects of plant metabolism, morphology and physiology through the differential expression of hundreds of genes. Photomorphogenic responses to UV-B are mediated by the photoreceptor UV RESISTANCE LOCUS8 (UVR8). Considerable progress has been made in understanding UVR8 action: the structural basis of photoreceptor function, how interaction with CONSTITUTIVELY PHOTOMORPHOGENIC 1 initiates signaling and how REPRESSOR OF UV-B PHOTOMORPHOGENESIS proteins negatively regulate UVR8 action. In addition, recent research shows that UVR8 mediates several responses through interaction with other signaling pathways, in particular auxin signaling. Nevertheless, many aspects of UVR8 action remain poorly understood. Most research to date has been undertaken with Arabidopsis, and it is important to explore the functions and regulation of UVR8 in diverse plant species. Furthermore, it is essential to understand how UVR8, and UV-B signaling in general, regulates processes under natural growth conditions. Ultraviolet B regulates the expression of many genes through UVR8-independent pathways, but the activity and importance of these pathways in plants growing in sunlight are poorly understood.

Photoreceptor Mediated Plant Growth Responses: Implications for Photoreceptor Engineering toward Improved Performance in Crops

Rising temperatures during growing seasons coupled with altered precipitation rates presents a challenging task of improving crop productivity for overcoming such altered weather patterns and cater to a growing population. Light is a critical environmental factor that exerts a powerful influence on plant growth and development ranging from seed germination to flowering and fruiting. Higher plants utilize a suite of complex photoreceptor proteins to perceive surrounding red/far-red (phytochromes), blue/UV-A (cryptochromes, phototropins, ZTL/FKF1/LKP2), and UV-B light (UVR8). While genomic studies have also shown that light induces extensive reprogramming of gene expression patterns in plants, molecular genetic studies have shown that manipulation of one or more photoreceptors can result in modification of agronomically beneficial traits. Such information can assist researchers to engineer photoreceptors via genome editing technologies to alter expression or even sensitivity thresholds of native photoreceptors for targeting aspects of plant growth that can confer superior agronomic value to the engineered crops. Here we summarize the agronomically important plant growth processes influenced by photoreceptors in crop species, alongwith the functional interactions between different photoreceptors and phytohormones in regulating these responses. We also discuss the potential utility of synthetic biology approaches in photobiology for improving agronomically beneficial traits of crop plants by engineering designer photoreceptors.

The DC1-domain protein VACUOLELESS GAMETOPHYTES is essential for development of female and male gametophytes in Arabidopsis

In this work we identified VACUOLELESS GAMETOPHYTES (VLG) as a DC1 domain-containing protein present in the endomembrane system and essential for development of both female and male gametophytes. VLG was originally annotated as a gene coding for a protein of unknown function containing DC1 domains. DC1 domains are cysteine- and histidine-rich zinc finger domains found exclusively in the plant kingdom that have been named on the basis of similarity with the C1 domain present in protein kinase C (PKC). In Arabidopsis, both male and female gametophytes are characterized by the formation of a large vacuole early in development; this is absent in vlg mutant plants. As a consequence, development is arrested in embryo sacs and pollen grains at the first mitotic division. VLG is specifically located in multivesicular bodies or pre-vacuolar compartments, and our results suggest that vesicular fusion is affected in the mutants, disrupting vacuole formation. Supporting this idea, AtPVA12 - a member of the SNARE vesicle-associated protein family and previously related to a sterol-binding protein, was identified as a VLG interactor. A role for VLG is proposed mediating vesicular fusion in plants as part of the sterol trafficking machinery required for vacuole biogenesis in plants.

Transcriptomic Profiling of Soybean in Response to High-Intensity UV-B Irradiation Reveals Stress Defense Signaling

The depletion of the ozone layer in the stratosphere has led to a dramatic spike in ultraviolet B (UV-B) intensity and increased UV-B light levels. The direct absorption of high-intensity UV-B induces complex abiotic stresses in plants, including excessive light exposure, heat, and dehydration. However, UV-B stress signaling mechanisms in plants including soybean (Glycine max [L.]) remain poorly understood. Here, we surveyed the overall transcriptional responses of two soybean genotypes, UV-B-sensitive Cheongja 3 and UV-B-resistant Buseok, to continuous UV-B irradiation for 0 (control), 0.5, and 6 h using RNA-seq analysis. Homology analysis using UV-B-related genes from Arabidopsis thaliana revealed differentially expressed genes (DEGs) likely involved in UV-B stress responses. Functional classification of the DEGs showed that the categories of immune response, stress defense signaling, and reactive oxygen species (ROS) metabolism were over-represented. UV-B-resistant Buseok utilized phosphatidic acid-dependent signaling pathways (based on subsequent reactions of phospholipase C and diacylglycerol kinase) rather than phospholipase D in response to UV-B exposure at high fluence rates, and genes involved in its downstream pathways, such as ABA signaling, mitogen-activated protein kinase cascades, and ROS overproduction, were upregulated in this genotype. In addition, the DEGs for TIR-NBS-LRR and heat shock proteins are positively activated. These results suggest that defense mechanisms against UV-B stress at high fluence rates are separate from the photomorphogenic responses utilized by plants to adapt to low-level UV light. Our study provides valuable information for deep understanding of UV-B stress defense mechanisms and for the development of resistant soybean genotypes that survive under high-intensity UV-B stress.

Molecular cloning and functional analysis of a UV-B photoreceptor gene, MdUVR8 (UV Resistance Locus 8), from apple

UVR8 (UV Resistance Locus 8) is an ultraviolet-B (UV-B; 280-315nm) light receptor that is involved in regulating many aspects of plant growth and development. UV-B irradiation can increase the development of flower and fruit coloration in many fruit trees, such as grape, pear and apple. Previous investigations of the structure and functions of UVR8 in plants have largely focused on Arabidopsis. Here, we isolated the UVR8 gene from apple (Malus domestica) and analyzed its function in transgenic Arabidopsis. Genomic and protein sequence analysis showed that MdUVR8 shares high similarity with the AtUVR8 protein from Arabidopsis, including the conserved seven-bladed β-propeller, the C27 region, the 3 "GWRHT" motifs and crucial amino-acid residues (14 Trps, 2 Args). A point mutation prediction and three-dimensional structural analysis of MdUVR8 indicated that it has a similar structure to AtUVR8 and that the crucial residues are also important in MdUVR8. In terms of transcript levels, MdUVR8 expression was up-regulated by UV-B light, which suggests that its expression follows a 24-h circadian rhythm. Using heterologous expression of MdUVR8 in both uvr8-1 mutant and wild-type (WT) Arabidopsis, we found that MdUVR8 regulates hypocotyl elongation and gene expression under UV-B light. These data provide functional evidence for a role of MdUVR8 in controlling photomorphogenesis under UV-B light and indicate that the function of UVR8 is conserved between Arabidopsis and apple. Furthermore, we examined the interaction between MdUVR8 and MdCOP1 (constitutive photomorphogenic1) using a yeast two-hybrid assay and a co-immunoprecipitation assay. This interaction provides a direction for investigating the regulatory mechanisms of the UV-B-light pathway in apple.

Characterization of Brassica napus Flavonol Synthase Involved in Flavonol Biosynthesis in Brassica napus L

Recently, Brassica napus has become a very important crop for plant oil production. Flavonols, an uncolored flavonoid subclass, have a high antioxidative effect and are known to have antiproliferative, antiangiogenic, and neuropharmacological properties. In B. napus, some flavonoid structural genes have been identified, such as, BnF3H-1, BnCHS, and BnC4H-1. However, no studies on FLS genes in B. napus have been conducted. Thus, in this study, we cloned and characterized the function of BnFLS gene B. napus. By overexpression of the BnFLS gene, flavonol (kaempferol and quercetin) levels were recovered in the Arabidopsis atfls1-ko mutant. In addition, we found that the higher endogenous flavonol levels of BnFLS-ox in vitro shoots correlated with slightly higher ROS scavenging activities. Thus, our results indicate that the BnFLS gene encodes for a BnFLS enzyme that can be manipulated to specifically increase flavonol accumulation in oilseed plants and other species such as Arabidopsis.

Molecular Cloning and Functional Analysis of UV RESISTANCE LOCUS 8 (PeUVR8) from Populus euphratica

Ultraviolet-B (UV-B; 280–315 nm) light, which is an integral part of the solar radiation reaching the surface of the Earth, induces a broad range of physiological responses in plants. The UV RESISTANCE LOCUS 8 (UVR8) protein is the first and only light photoreceptor characterized to date that is specific for UV-B light and it regulates various aspects of plant growth and development in response to UV-B light. Despite its involvement in the control of important plant traits, most studies on UV-B photoreceptors have focused on Arabidopsis and no data on UVR8 function are available for forest trees. In this study, we isolated a homologue of the UV receptor UVR8 of Arabidopsis, PeUVR8, from Populus euphratica (Euphrates poplar) and analyzed its structure and function in detail. The deduced PeUVR8 amino acid sequence contained nine well-conserved regulator of chromosome condensation 1 (RCC1) repeats and the region 27 amino acids from the C terminus (C27) that interact with COP1 (CONSTITUTIVELY PHOTOMORPHOGENIC1). Secondary and tertiary structure analysis showed that PeUVR8 shares high similarity with the AtUVR8 protein from Arabidopsis thaliana. Using heterologous expression in Arabidopsis, we showed that PeUVR8 overexpression rescued the uvr8 mutant phenotype. In addition, PeUVR8 overexpression in wild-type background seedlings grown under UV-B light inhibited hypocotyl elongation and enhanced anthocyanin accumulation. Furthermore, we examined the interaction between PeUVR8 and AtCOP1 using a bimolecular fluorescence complementation (BiFC) assay. Our data provide evidence that PeUVR8 plays important roles in the control of photomorphogenesis in planta.

A hormone-responsive C1-domain-containing protein At5g17960 mediates stress response in Arabidopsis thaliana

Phytohormones play a critical role in mediating plant stress response. They employ a variety of proteins for coordinating such processes. In Arabidopsis thaliana, some members of a Cys-rich protein family known as C1-clan proteins were involved in stress response, but the actual function of the protein family is largely unknown. We studied At5g17960, a C1-clan protein member that possesses three unique C1 signature domains viz. C1_2, C1_3 and ZZ/PHD type. Additionally, we identified 72 other proteins in A. thaliana that contain all three unique signature domains. Subsequently, the 73 proteins were phylogenetically classified into IX subgroups. Promoter motif analysis of the 73 genes identified the presence of hormone-responsive and stress-responsive putative cis-regulatory elements. Furthermore, we observed that transcript levels of At5g17960 were induced in response to different hormones and stress treatments. At1g35610 and At3g13760, two other members of subgroup IV, also showed upregulation upon GA3, biotic and abiotic stress treatments. Moreover, seedlings of independent transgenic A. thaliana lines ectopically expressing or suppressing At5g17960 also showed differential regulation of several abiotic stress-responsive marker genes. Thus, our data suggest that C1-domain-containing proteins have a role to play in plant hormone-mediated stress responses, thereby assigning a putative function for the C1-clan protein family.

Hydrogen-rich water reestablishes ROS homeostasis but exerts differential effects on anthocyanin synthesis in two varieties of radish sprouts under UV-A irradiation

The aims of the study were to investigate whether hydrogen gas (H2) was involved in regulation of anthocyanin biosynthesis in two contrasting radish (Raphanus sativus L.) varieties (low [LA] and high [HA] level of anthocyanin) under UV irradiation. The results showed that hydrogen-rich water (HRW) significantly blocked the UV-A-induced increase of H2O2 and O2(•-) accumulation, and enhanced the UV-A-induced increase of superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities in LA and HA. Furthermore, UV-A-induced increase of anthocyanin and total phenols was further enhanced only in HA sprouts cotreated with HRW. LC-MS/MS analysis showed that five anthocyanidins existed in HA sprouts, but only two in LA sprouts. Meanwhile, the cyanidin was the most abundant anthocyanidin in HA, and the cyanidin was 2-fold higher cotreated with HRW than UV-A. Molecular analyses showed that the anthocyanin biosynthesis-related genes were upregulated significantly in both HA (in particular) and LA sprouts treated with HRW plus UV-A. These data imply that HRW reestablishes reactive oxygen species homeostasis in both LA and HA, but exerts different effects on anthocyanin accumulation between them under UV-A.

Role of Arabidopsis UV RESISTANCE LOCUS 8 in plant growth reduction under osmotic stress and low levels of UV-B

In high-light environments, plants are exposed to different types of stresses, such as an excess of UV-B, but also drought stress which triggers a common morphogenic adaptive response resulting in a general reduction of plant growth. Here, we report that the Arabidopsis thaliana UV RESISTANCE LOCUS 8 (UVR8) gene, a known regulator of the UV-B morphogenic response, was able to complement a Saccharomyces cerevisiae osmo-sensitive mutant and its expression was induced after osmotic or salt stress in Arabidopsis plants. Under low levels of UV-B, plants overexpressing UVR8 are dwarfed with a reduced root development and accumulate more flavonoids compared to control plants. The growth defects are mainly due to the inhibition of cell expansion. The growth inhibition triggered by UVR8 overexpression in plants under low levels of UV-B was exacerbated by mannitol-induced osmotic stress, but it was not significantly affected by ionic stress. In contrast, uvr8-6 mutant plants do not differ from wild-type plants under standard conditions, but they show an increased shoot growth under high-salt stress. Our data suggest that UVR8-mediated accumulation of flavonoid and possibly changes in auxin homeostasis are the underlying mechanism of the observed growth phenotypes and that UVR8 might have an important role for integrating plant growth and stress signals.

UVR8 mediated plant protective responses under low UV-B radiation leading to photosynthetic acclimation

The UV-B photoreceptor UVR8 regulates the expression of several genes leading to acclimation responses in plants. Direct role of UVR8 in maintaining the photosynthesis is not defined but it is known to increase the expression of some chloroplastic proteins like SIG5 and ELIP. It provides indirect protection to photosynthesis by regulating the synthesis of secondary metabolites and photomorphogenesis. Signaling cascades controlled by UVR8 mediate many protective responses thus promotes plant acclimation against stress and secures its survival.

The effects of a stimulating intron on the expression of heterologous genes in Arabidopsis thaliana

Introns are often added to transgenes to increase expression, although the mechanism through which introns stimulate gene expression in plants and other eukaryotes remains mysterious. While introns vary in their effect on expression, it is unknown whether different genes respond similarly to the same stimulatory intron. Furthermore, the degree to which gene regulation is preserved when expression is increased by an intron has not been thoroughly investigated. To test the effects of the same intron on the expression of a range of genes, GUS translational fusions were constructed using the promoters of eight Arabidopsis genes whose expression was reported to be constitutive (GAE1, CNGC2 and ROP10), tissue specific (ADL1A, YAB3 and AtAMT2) or regulated by light (ULI3 and MSBP1). For each gene, a fusion containing the first intron from the UBQ10 gene was compared to fusions containing the gene's endogenous first intron (if the gene has one) or no intron. In every case, the UBQ10 intron increased expression relative to the intronless control, although the magnitude of the change and the level of expression varied. The UBQ10 intron also changed the expression patterns of the CNGC2 and YAB3 fusions to include strong activity in roots, indicating that tissue specificity was disrupted by this intron. In contrast, the regulation of the ULI3 and MSBP1 genes by light was preserved when their expression was stimulated by the intron. These findings have important implications for biotechnology applications in which a high level of transgene expression in only certain tissues is desired.

From ozone depletion to agriculture: understanding the role of UV radiation in sustainable crop production

Largely because of concerns regarding global climate change, there is a burgeoning interest in the application of fundamental scientific knowledge in order to better exploit environmental cues in the achievement of desirable endpoints in crop production. Ultraviolet (UV) radiation is an energetic driver of a diverse range of plant responses and, despite historical concerns regarding the damaging consequences of UV-B radiation for global plant productivity as related to stratospheric ozone depletion, current developments representative of a range of organizational scales suggest that key plant responses to UV-B radiation may be exploitable in the context of a sustainable contribution towards the strengthening of global crop production, including alterations in secondary metabolism, enhanced photoprotection, up-regulation of the antioxidative response and modified resistance to pest and disease attack. Here, we discuss the prospect of this paradigm shift in photobiology, and consider the linkages between fundamental plant biology and crop-level outcomes that can be applied to the plant UV-B response, in addition to the consequences for related biota and many other facets of agro-ecosystem processes.

UV-Induced cell death in plants

Plants are photosynthetic organisms that depend on sunlight for energy. Plants respond to light through different photoreceptors and show photomorphogenic development. Apart from Photosynthetically Active Radiation (PAR; 400-700 nm), plants are exposed to UV light, which is comprised of UV-C (below 280 nm), UV-B (280-320 nm) and UV-A (320-390 nm). The atmospheric ozone layer protects UV-C radiation from reaching earth while the UVR8 protein acts as a receptor for UV-B radiation. Low levels of UV-B exposure initiate signaling through UVR8 and induce secondary metabolite genes involved in protection against UV while higher dosages are very detrimental to plants. It has also been reported that genes involved in MAPK cascade help the plant in providing tolerance against UV radiation. The important targets of UV radiation in plant cells are DNA, lipids and proteins and also vital processes such as photosynthesis. Recent studies showed that, in response to UV radiation, mitochondria and chloroplasts produce a reactive oxygen species (ROS). Arabidopsis metacaspase-8 (AtMC8) is induced in response to oxidative stress caused by ROS, which acts downstream of the radical induced cell death (AtRCD1) gene making plants vulnerable to cell death. The studies on salicylic and jasmonic acid signaling mutants revealed that SA and JA regulate the ROS level and antagonize ROS mediated cell death. Recently, molecular studies have revealed genes involved in response to UV exposure, with respect to programmed cell death (PCD).

Rapid reversion from monomer to dimer regenerates the ultraviolet-B photoreceptor UV RESISTANCE LOCUS8 in intact Arabidopsis plants

Arabidopsis (Arabidopsis thaliana) UV RESISTANCE LOCUS8 (UVR8) is a photoreceptor that specifically mediates photomorphogenic responses to ultraviolet (UV)-B in plants. UV-B photoreception induces the conversion of the UVR8 dimer into a monomer that interacts with the CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) protein to regulate gene expression. However, it is not known how the dimeric photoreceptor is regenerated in plants. Here, we show, by using inhibitors of protein synthesis and degradation via the proteasome, that the UVR8 dimer is not regenerated by rapid de novo synthesis following destruction of the monomer. Rather, regeneration occurs by reversion from the monomer to the dimer. However, regeneration of dimeric UVR8 in darkness following UV-B exposure occurs much more rapidly in vivo than in vitro with illuminated plant extracts or purified UVR8, indicating that rapid regeneration requires intact cells. Rapid dimer regeneration in vivo requires protein synthesis, the presence of a carboxyl-terminal 27-amino acid region of UVR8, and the presence of COP1, which is known to interact with the carboxyl-terminal region. However, none of these factors can account fully for the difference in regeneration kinetics in vivo and in vitro, indicating that additional proteins or processes are involved in UVR8 dimer regeneration in vivo.

root uv-b sensitive mutants are suppressed by specific mutations in ASPARTATE AMINOTRANSFERASE2 and by exogenous vitamin B6

Vitamin B6 (vitB6) serves as an essential cofactor for more than 140 enzymes. Pyridoxal 5'-phosphate (PLP), active cofactor form of vitB6, can be photolytically destroyed by trace amounts of ultraviolet-B (UV-B). How sun-exposed organisms cope with PLP photosensitivity and modulate vitB6 homeostasis is currently unknown. We previously reported on two Arabidopsis mutants, rus1 and rus2, that are hypersensitive to trace amounts of UV-B light. We performed mutagenesis screens for second-site suppressors of the rus mutant phenotype and identified mutations in the ASPARTATE AMINOTRANSFERASE2 (ASP2) gene. ASP2 encodes for cytosolic aspartate aminotransferase (AAT), a PLP-dependent enzyme that plays a key role in carbon and nitrogen metabolism. Genetic analyses have shown that specific amino acid substitutions in ASP2 override the phenotypes of rus1 and rus2 single mutants as well as rus1 rus2 double mutant. These substitutions, all shown to reside at specific positions in the PLP-binding pocket, resulted in no PLP binding. Additional asp2 mutants that abolish AAT enzymatic activity, but which alter amino acids outside of the PLP-binding pocket, fail to suppress the rus phenotype. Furthermore, exogenously adding vitB6 in growth media can rescue both rus1 and rus2. Our data suggest that AAT plays a role in vitB6 homeostasis in Arabidopsis.

RiceFOX: a database of Arabidopsis mutant lines overexpressing rice full-length cDNA that contains a wide range of trait information to facilitate analysis of gene function

Identification of gene function is important not only for basic research but also for applied science, especially with regard to improvements in crop production. For rapid and efficient elucidation of useful traits, we developed a system named FOX hunting (Full-length cDNA Over-eXpressor gene hunting) using full-length cDNAs (fl-cDNAs). A heterologous expression approach provides a solution for the high-throughput characterization of gene functions in agricultural plant species. Since fl-cDNAs contain all the information of functional mRNAs and proteins, we introduced rice fl-cDNAs into Arabidopsis plants for systematic gain-of-function mutation. We generated >30,000 independent Arabidopsis transgenic lines expressing rice fl-cDNAs (rice FOX Arabidopsis mutant lines). These rice FOX Arabidopsis lines were screened systematically for various criteria such as morphology, photosynthesis, UV resistance, element composition, plant hormone profile, metabolite profile/fingerprinting, bacterial resistance, and heat and salt tolerance. The information obtained from these screenings was compiled into a database named 'RiceFOX'. This database contains around 18,000 records of rice FOX Arabidopsis lines and allows users to search against all the observed results, ranging from morphological to invisible traits. The number of searchable items is approximately 100; moreover, the rice FOX Arabidopsis lines can be searched by rice and Arabidopsis gene/protein identifiers, sequence similarity to the introduced rice fl-cDNA and traits. The RiceFOX database is available at http://ricefox.psc.riken.jp/.

The tricks plants use to reach appropriate light

The perception of ambient light signals that produce a relevant response to ensure exposure to appropriate levels of light energy is vital for plants. In response to this, intricate molecular mechanisms to mediate light signaling have evolved in plants. Among the responses induced by light, seedling extension is a determining event for plant survival in darkness, especially in the initial stage of plant growth. Here we review previous studies and recent progress towards an understanding of light signaling that regulates seedling elongation. We focus on the three regions of the sunlight spectrum that primarily control seedling elongation, namely red/far-red light, blue/UV-A light and UV-B light, and summarize the four signaling pathways that correspond to the three effective spectra.

Chromosomal loci important for cotyledon opening under UV-B in Arabidopsis thaliana

BACKGROUND

Understanding of the genetic architecture of plant UV-B responses allows extensive targeted testing of candidate genes or regions, along with combinations of those genes, for placement in metabolic or signal transduction pathways.

RESULTS

Composite interval mapping and single-marker analysis methods were used to identify significant loci for cotyledon opening under UV-B in four sets of recombinant inbred lines. In addition, loci important for canalization (stability) of cotyledon opening were detected in two mapping populations. One candidate locus contained the gene HY5. Mutant analysis demonstrated that HY5 was required for UV-B-specific cotyledon opening.

CONCLUSIONS

Structured mapping populations provide key information on the degree of complexity in the genetic control of UV-B-induced cotyledon opening in Arabidopsis. The loci identified using quantitative trait analysis methods are useful for follow-up testing of candidate genes.

Photobiological properties of the inhibition of etiolated Arabidopsis seedling growth by ultraviolet-B irradiation

Alteration of 'normal' levels of ultraviolet-B light (UV-B, 280-320 nm) can affect plant chemical composition as well as growth; however, little is known about how plants perceive UV-B light. We have carried out fluence response curves, and demonstrated that the growth inhibition of etiolated Arabidopsis thaliana seedlings by low fluence UV light is specific to UV-B and not UV-A (320-390 nm). The response shows reciprocity between duration and intensity, at least over a limited range, and thus depends only on photon fluence and not on photon flux. The action spectrum for this response indicates a peak of maximum effectiveness at 290 nm, and response spectra at different fluences indicate that the most effective wavelength at 30,000 micromol m(-2) is 290 nm, whereas 300 nm light was the most effective at 100,000 micromol m(-2). This response occurs in mutant seedlings deficient in cryptochrome, phytochrome or phototropin, suggesting that none of the known photoreceptors is the major UV-B photoreceptor. Some null mutants in DNA repair enzymes show hypersensitivity to UV-B, suggesting that even at low fluence rates, direct damage to DNA may be one component of the response to UV-B.

Signal Transduction in Responses to UV-B Radiation

UV-B radiation is a key environmental signal that initiates diverse responses in plants that affect metabolism, development, and viability. Many effects of UV-B involve the differential regulation of gene expression. The response to UV-B depends on the nature of the UV-B treatment, the extent of adaptation and acclimation to UV-B, and interaction with other environmental factors. Responses to UV-B are mediated by both nonspecific signaling pathways, involving DNA damage, reactive oxygen species, and wound/defense signaling molecules, and UV-B-specific pathways that mediate photomorphogenic responses to low levels of UV-B. Importantly, photomorphogenic signaling stimulates the expression of genes involved in UV-protection and hence promotes plant survival in UV-B. Photomorphogenic UV-B signaling is mediated by the UV-B-specific component UV RESISTANCE LOCUS8 (UVR8). Both UVR8 and CONSTITUTIVE PHOTOMORPHOGENESIS1 (COP1) are required for UV-B-induced expression of the ELONGATED HYPOCOTYL5 (HY5) transcription factor, which plays a central role in the regulation of genes involved in photomorphogenic UV-B responses.

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.

Role of root UV-B sensing in Arabidopsis early seedling development

All sun-exposed organisms are affected by UV-B [(UVB) 280-320 nm], an integral part of sunlight. UVB can cause stresses or act as a developmental signal depending on its fluence levels. In plants, the mechanism by which high-fluence-rate UVB causes damages and activates DNA-repair systems has been extensively studied. However, little is known about how nondamaging low-fluence-rate UVB is perceived to regulate plant morphogenesis and development. Here, we report the identification of an Arabidopsis mutant, root UVB sensitive 1 (rus1), whose primary root is hypersensitive to very low-fluence-rate (VLF) UVB. Under standard growth-chamber fluorescent white light, rus1 displays stunted root growth and fails to form postembryonic leaves. Experiments with different monochromatic light sources showed that rus1 phenotypes can be rescued if the seedlings are allowed to grow in light conditions with minimum UVB. We determined that roots, not other organs, perceive the UVB signal. Genetic and molecular analyses confirmed that the root light-sensitive phenotypes are independent of all other known plant photoreceptors. Three rus1 alleles have been identified and characterized. A map-based approach was used to identify the RUS1 locus. RUS1 encodes a protein that contains DUF647 (domain of unknown function 647), a domain highly conserved in eukaryotes. Our results demonstrate a root VLF UVB-sensing mechanism that is involved in Arabidopsis early seedling morphogenesis and development.

An unidentified ultraviolet-B-specific photoreceptor mediates transcriptional activation of the cyclobutane pyrimidine dimer photolyase gene in plants

Cyclobutane pyrimidine dimers (CPDs) constitute a majority of DNA lesions caused by ultraviolet-B (UVB). CPD photolyase, which rapidly repairs CPDs, is essential for plant survival under sunlight containing UVB. Our earlier results that the transcription of the cucumber CPD photolyase gene (CsPHR) was activated by light have prompted us to propose that this light-driven transcriptional activation would allow plants to meet the need of the photolyase activity upon challenges of UVB from sunlight. However, molecular mechanisms underlying the light-dependent transcriptional activation of CsPHR were unknown. In order to understand spectroscopic aspects of the plant response, we investigated the wavelength-dependence (action spectra) of the light-dependent transcriptional activation of CsPHR. In both cucumber seedlings and transgenic Arabidopsis seedlings expressing reporter genes under the control of the CsPHR promoter, the action spectra exhibited the most predominant peak in the long-wavelength UVB waveband (around 310 nm). In addition, a 95-bp cis-acting region in the CsPHR promoter was identified to be essential for the UVB-driven transcriptional activation of CsPHR. Thus, we concluded that the photoperception of long-wavelength UVB by UVB photoreceptor(s) led to the induction of the CsPHR transcription via a conserved cis-acting element.

Flavonoid profiling and biosynthetic gene expression in flesh and peel of two tomato genotypes grown under UV-B-depleted conditions during ripening

The effect of shielding solar ultraviolet B radiation on the accumulation of some flavonoids and their precursor hydroxycinnamic acids in tomato (Solanum lycopersicum) was evaluated by liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). In particular, flesh and peel of two tomato hybrids, DRW 5981 and Esperanza, were separately analyzed. The hybrids have been chosen for their different responses to the light, since it was previously reported that they show different pigmentation and opposite behavior under UV-B in terms of carotenoids and ascorbic acid content at different ripening stages. To determine the effect of UV-B radiation during tomato ripening, we also measured the expression of some flavonoid biosynthetic genes by real-time reverse transcription-polymerase chain reaction (RT-PCR) analysis. The results allowed us to conclude that UV-B radiation deeply and differentially affects the content of the considered flavonoids and hydroxycinnamic acids as well as the expression of some of their biosynthetic genes in both flesh and peel during the ripening process. On the other hand, the collected data clearly showed that this influence varies between different genotypes. We conclude that the use of specific plastic covers able to eliminate UV-B radiation may be an environmentally friendly approach to modulate the expression of structural genes and, in turn, to enhance healthy antioxidant compounds in fruits of specific tomato cultivars.

Two separate UV-B radiation wavelength regions control expression of different molecular markers in Arabidopsis thaliana

Fluence-response curves were obtained at nine wavelengths in the interval 280-360 nm for mRNA transcripts of four molecular markers induced by ultraviolet-B (UV-B) radiation in Arabidopsis thaliana (L.) Heynh.: CHS (encoding chalcone synthase), PDX1.3 (encoding an enzyme involved in formation of pyridoxine), MEB5.2 (encoding a protein with unknown function but which is strongly upregulated by UV-B), and LHCB1*3 (encoding a chlorophyll a/b binding protein). Intact Arabidopsis plants were irradiated for 3 h using a high intensity deuterium radiation source and narrow bandwith filters without supplementary PAR. The results obtained suggest the existence of two distinct UV-B signal responses: one sensitive between 300 and 310 nm and the other sensitive around 280-290 nm. Among the investigated molecular markers, CHS and PDX1.3 were regulated through the chromophore absorbing around 300 nm, whereas MEB5.2 and LHCB1*3 were regulated through the chromophore absorbing at 280-290 nm. The results obtained show that at least two signal transduction pathways exist that regulate gene expression as a result of absorption of UV-B radiation in plants.

Identification of a novel cis-regulatory element for UV-B-induced transcription in Arabidopsis

Ultraviolet-B light (UV-B) regulates the expression of genes in a wavelength- and fluence rate-dependent fashion. A signaling pathway consisting of CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1) and UV RESISTANCE LOCUS 8 (UVR 8) mediates responsiveness to longer wavelength, low intensity UV-B light-activating, for example, HY5 gene expression. By contrast, transcription of another group of genes, including ANAC13, modulated by shorter wavelength, higher intensity UV-B is controlled by a yet unknown and largely COP1-independent signaling cascade. Here we provide evidence by promoter deletion analysis, and characterization of genetic mutants displaying aberrant expression patterns, that two cis-regulatory elements, designated MRE(ANAC13) and UVBox(ANAC13), are required for maximal UV-B induction of the ANAC13 gene in transgenic plants. These elements are located in the proximal 150-bp region of the ANAC13 promoter. They show no significant similarity to each other; the putative MRE(ANAC13) (-AACCTT-) is closely related to MRE(CHS) (-AACCTA-) found in the CHALCONE SYNTHASE (CHS) gene, whereas UVBox(ANAC13) (with core sequence CAAG) represents a novel cis-regulatory element. The novel UVBox(ANAC13) sequence is significantly enriched in the promoter region of a subset of UV-B-induced genes with similar activation properties as ANAC13. In addition, we demonstrate that expression of a chimeric gene containing only the dimerized 12-mer containing UVBox(ANAC13) fused to a minimal CaMV35S promoter/luciferase reporter is (i) efficiently induced by shorter wavelength, higher intensity UV-B, but (ii) does not respond either to longer wavelength UV-B and red light or (iii) to abscisic acid treatment and osmotic, salt, heat and cold stresses.

UV-B signaling pathways with different fluence-rate response profiles are distinguished in mature Arabidopsis leaf tissue by requirement for UVR8, HY5, and HYH

UV-B signaling is an important but poorly understood aspect of light responsiveness in plants. Arabidopsis (Arabidopsis thaliana) UV RESISTANCE LOCUS8 (UVR8) is a recently identified UV-B-specific signaling component that regulates UV-protective responses. Using the uvr8 mutant, we defined genetically distinct UVR8-dependent and UVR8-independent pathways that stimulate different sets of genes in mature Arabidopsis leaf tissue. Both pathways operate at 1 micromol m(-2) s(-1) UV-B and above, but the UVR8-dependent pathway is able to stimulate UV-protective genes even in response to 0.1 micromol m(-2) s(-1) UV-B. Both pathways function in mutants lacking phytochromes, cryptochromes, or phototropins. Genes encoding the ELONGATED HYPOCOTYL5 (HY5) and HY5 HOMOLOG (HYH) transcription factors are induced at low UV-B fluence rates (0.1 micromol m(-2) s(-1)). Experiments with hy5 and hyh mutants reveal that both these factors mediate responses of the UVR8-dependent pathway, acting with partial or complete redundancy to stimulate expression of particular genes. Furthermore, evidence is presented that all UVR8 pathway genes are likely to be regulated by HY5/HYH and that these transcription factors do not mediate UV-B responses independent of UVR8. Finally, we highlight the functions of HY5 and HYH in UV protection and show that HY5 plays the more critical role. This research provides evidence that, in UV-B signaling, UVR8, HY5, and HYH act together in a photoregulatory pathway and demonstrates a new role for HYH in UV-B responses.

Overexpression of rice WRKY89 enhances ultraviolet B tolerance and disease resistance in rice plants

WRKY proteins are a large family of transcriptional regulators involved in a variety of biological processes in plants. Here we report functional characterization of a rice WRKY gene, OsWRKY89. RNA gel blot analysis indicated that OsWRKY89 was strongly induced by treatments of methyl jasmonate and UV-B radiation. The transient expression analysis of the OsWRKY89-eGFP reporter in onion epidermal cells revealed that OsWRKY89 was targeted to nuclei. Transcriptional activity assays of OsWRKY89 and its mutants fused with a GAL4 DNA binding domain indicated that the 67 C-terminal amino acids were required for the transcriptional activation and that the leucine zipper region at the N-terminus enhanced its transcriptional activity. Overexpression of OsWRKY89 led to growth retardation at the early stage and reduction of internode length. Scanning electron microscopy revealed an increase in wax deposition on leaf surfaces of the OsWRKY89 overexpression lines and a decrease in wax loading in the RNAi-mediated OsWRKY89 suppression lines. Moreover, extractable and cell-wall-bound phenolic compounds were decreased in the overexpressor lines, but its SA levels were increased. Lignin staining showed an increase in lignification in culms of the overexpressor lines. Interestingly, overexpression of the OsWRKY89 gene enhanced resistance to the rice blast fungus and white-backed planthopper as well as tolerance to UV-B irradiation. These results suggest that OsWRKY89 plays an important role in response to biotic and abiotic stresses.

Genes up-regulated during red coloration in UV-B irradiated lettuce leaves

Molecular analysis of gene expression differences between green and red lettuce leaves was performed using the SSH method. BlastX comparisons of subtractive expressed sequence tags (ESTs) indicated that 7.6% of clones encoded enzymes involved in secondary metabolism. Such clones had a particularly high abundance of flavonoid-metabolism proteins (6.5%). Following SSH, 566 clones were rescreened for differential gene expression using dot-blot hybridization. Of these, 53 were found to overexpressed during red coloration. The up-regulated expression of six genes was confirmed by Northern blot analyses. The expression of chalcone synthase (CHS), flavanone 3-hydroxylase (F3H), and dihydroflavonol 4-reductase (DFR) genes showed a positive correlation with anthocyanin accumulation in UV-B-irradiated lettuce leaves; flavonoid 3',5'-hydroxylase (F3',5'H) and anthocyanidin synthase (ANS) were expressed continuously in both samples. These results indicated that the genes CHS, F3H, and DFR coincided with increases in anthocyanin accumulation during the red coloration of lettuce leaves. This study show a relationship between red coloration and the expression of up-regulated genes in lettuce. The subtractive cDNA library and EST database described in this study represent a valuable resource for further research for secondary metabolism in the vegetable crops.

Ultraviolet A-specific induction of anthocyanin biosynthesis in the swollen hypocotyls of turnip (Brassica rapa)

Ultraviolet A (UV-A)-mediated regulation of anthocyanin biosynthesis was investigated in swollen hypocotyls of the red turnip 'Tsuda'. The shaded swollen hypocotyls which contained negligible anthocyanin were exposed to artificial light sources including low fluence UV-B, UV-A, blue, red, far-red, red plus UV-A, far-red plus UV-A, and blue plus red. Among these lights, only UV-A induced anthocyanin biosynthesis and co-irradiation of red or far-red with UV-A did not affect the extent of UV-A-induced anthocyanin accumulation. The expression of phenylalanine ammonia lyase (PAL; EC 4.3.1.5), chalcone synthase (CHS; EC 2.3.1.74), flavanone 3-hydroxylase (F3H; EC 1.14.11.9), dihydroflavonol 4-reductase (DFR; EC 1.1.1.219), and anthocyanidin synthase (ANS; EC 1.14.11.19) genes was increased with time during a 24 h exposure to UV-A. In contrast, irradiation with red, blue, UV-B, and a combination of blue with red failed to induce CHS expression. Microarray analysis showed that only a few genes, including CHS and F3H, were induced significantly by UV-A, while a separate set of many genes was induced by low fluence UV-B. The UV-A-specific induction of anthocyanin biosynthesis and the unique gene expression profile upon UV-A irradiation as compared with blue and UV-B demonstrated that the observed induction of anthocyanin biosynthesis in red turnips was mediated by a distinct UV-A-specific photoreceptor, but not by phytochromes, UV-A/blue photoreceptors, or UV-B photoreceptors.

CONSTITUTIVELY PHOTOMORPHOGENIC1 is required for the UV-B response in Arabidopsis

CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) is a negative regulator of photomorphogenesis in Arabidopsis thaliana. COP1 functions as an E3 ubiquitin ligase, targeting select proteins for proteasomal degradation in plants as well as in mammals. Among its substrates is the basic domain/leucine zipper (bZIP) transcription factor ELONGATED HYPOCOTYL5 (HY5), one of the key regulators of photomorphogenesis under all light qualities, including UV-B responses required for tolerance to this environmental threat. Here, we report that, in contrast with the situation in visible light, COP1 is a critical positive regulator of responses to low levels of UV-B. We show that in the cop1-4 mutant, flavonoid accumulation and genome-wide expression changes in response to UV-B are blocked to a large extent. COP1 is required for HY5 gene activation, and both COP1 and HY5 proteins accumulate in the nucleus under supplementary UV-B. SUPPRESSOR OF PHYTOCHROME A-105 family proteins (SPA1 to SPA4) that are required for COP1 function in dark and visible light are not essential in the response to UV-B. We conclude that COP1 performs a specific and novel role in the plants' photomorphogenic response to UV-B, coordinating HY5-dependent and -independent pathways, which eventually results in UV-B tolerance.

Supplementary ultraviolet-B irradiation reveals differences in stress responses between Arabidopsis thaliana ecotypes

Irradiation of Arabidopsis thaliana ecotypes C24, Wassilewskija (Ws) and Columbia-0 (Col-0) with supplementary ultraviolet-A+B (UV-A+B) radiation revealed ecotype-specific differences in expression of the gene for the pathogenesis-related protein PR-5. C24 showed an increased expression level of PR-5 (5- and 20-fold higher compared with Col-0 and Ws, respectively). Expression of other molecular markers such as CHS (encoding chalcone synthase), MEB5.2 [encoding a gene strongly up-regulated by ultraviolet-B (UV-B)] and PYROA [encoding a pyridoxine (Vitamin B6) biosynthesis enzyme] only showed slight differences between ecotypes. Oxidative stress during UVA+B exposure was monitored by staining for H2O2. This analysis also revealed important ecotype-specific differences. 'H2O2 hot spots' were found in C24, whereas an even distribution of H2O2 was found in Ws and Col-0. Necrotic lesions also appeared on C24 leaves after prolonged UV-B exposure. There was a reverse correlation between the H2O2 steady-state concentration and the PR-5 gene expression; Ws showed the highest level of H2O2 accumulation but the lowest expression level of the PR-5 gene. Furthermore, application of paraquat on the rosettes led to similar PR-5 expression and H2O2 accumulation patterns as were found after UV-A+B irradiation. The observed ecotypic differences were also reflected in a statistically significant UV-B-dependent decrease in biomass, rosette size and leaf area for Ws, but not for C24 and Col-0. Our results show that a significant ecotype-specific genetic variability in general UV-B responses in Arabidopsis exists. Moreover, the signal transduction or gene regulation pathway for PR-5 differs from the other molecular markers used in this study.

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.

Signalling and gene regulation in response to ultraviolet light

In contrast to phytochrome-, cryptochrome- and phototropin-sensing systems, about which considerable knowledge has accumulated, the ultraviolet-B (UVB) photoreceptor is not yet known at the molecular level. Information about the downstream signalling events that underlie UVB-provoked physiological responses is limited. Recent whole-genome transcript profiling, isolation of mutants that are impaired in specific UVB-induced responses and detailed photobiological studies suggest that responses that are triggered by shorter wavelength UVB and longer wavelength UVB are mediated by two different sensory systems. The bZIP transcription factor HY5 was recently identified as an important player in the long-wavelength UVB-induced signal transduction cascade. Advances in the development of luciferase-reporter lines will make it feasible to perform high-throughput genetic screens to isolate novel mutants that are impaired in sensing or transducing signals downstream of the putative UVB photoreceptor(s).

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.

Light signal transduction in higher plants

Plants utilize several families of photoreceptors to fine-tune growth and development over a large range of environmental conditions. The UV-A/blue light sensing phototropins mediate several light responses enabling optimization of photosynthetic yields. The initial event occurring upon photon capture is a conformational change of the photoreceptor that activates its protein kinase activity. The UV-A/blue light sensing cryptochromes and the red/far-red sensing phytochromes coordinately control seedling establishment, entrainment of the circadian clock, and the transition from vegetative to reproductive growth. In addition, the phytochromes control seed germination and shade-avoidance responses. The molecular mechanisms involved include light-regulated subcellular localization of the photoreceptors, a large reorganization of the transcriptional program, and light-regulated proteolytic degradation of several photoreceptors and signaling components.