The C‐terminal 17 amino acids of the photoreceptor UVR8 is involved in the fine‐tuning of UV‐B signaling

Plant UV-B responses are mediated by the photoreceptor UV RESISTANCE LOCUS 8(UVR8). In response to UV-B irradiation, UVR8 homodimers dissociate into monomers that bind to the E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC1(COP1). The interaction of the C27 domain in the C-terminal tail of UVR8 with the WD40 domain of COP1 is critical for UV-B signaling. However, the function of the last 17 amino acids(C17) of the C-terminus of UVR8, which are adjacent to C27, is unknown, although they are largely conserved in land plants. In this study, we established that Arabidopsis thaliana UVR8 C17 binds to full-length UVR8, but not to COP1, and reduces COP1 binding to the remaining portion of UVR8, including C27. We hypothesized that overexpression of C17 in a wild-type background would have a dominant negative effect on UVR8 activity;however, C17 overexpression caused strong silencing of endogenous UVR8, precluding a detailed analysis. We therefore generated YFP-UVR8~(N423) transgenic lines, in which C17 was deleted, to examine C17 function indirectly. YFP-UVR8~(N423) was more active than YFP-UVR8,suggesting that C17 inhibits UV-B signaling by attenuating binding between C27 and COP1. Our study reveals an inhibitory role for UVR8 C17 in fine-tuning UVR8–COP1 interactions during UV-B signaling.     

Dimer/monomer status and in vivo function of salt‐bridge mutants of the plant UV‐B photoreceptor UVR8

UV RESISTANCE LOCUS8 (UVR8) is a photoreceptor for ultraviolet‐B (UV‐B) light that initiates photomorphogenic responses in plants. UV‐B photoreception causes rapid dissociation of dimeric UVR8 into monomers that interact with CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) to initiate signal transduction. Experiments with purified UVR8 show that the dimer is maintained by salt‐bridge interactions between specific charged amino acids across the dimer interface. However, little is known about the importance of these charged amino acids in determining dimer/monomer status and UVR8 function in plants. Here we evaluate the use of different methods to examine dimer/monomer status of UVR8 and show that mutations of several salt‐bridge amino acids affect dimer/monomer status, interaction with COP1 and photoreceptor function of UVR8 in vivo. In particular, the salt‐bridges formed between arginine 286 and aspartates 96 and 107 are key to dimer formation. Mutation of arginine 286 to alanine impairs dimer formation, interaction with COP1 and function in vivo, whereas mutation to lysine gives a weakened dimer that is functional in vivo, indicating the importance of the positive charge of the arginine/lysine residue for dimer formation. Notably, a UVR8 mutant in which aspartates 96 and 107 are conservatively mutated to asparagine is strongly impaired in dimer formation but mediates UV‐B responses in vivo with a similar dose–response relationship to wild‐type. The UV‐B responsiveness of this mutant does not correlate with dimer formation and monomerisation, indicating that monomeric UVR8 has the potential for UV‐B photoreception, initiating signal transduction and responses in plants.     

Regulation of UVR8 photoreceptor dimer/monomer photo‐equilibrium in Arabidopsis plants grown under photoperiodic conditions

The UV RESISTANCE LOCUS 8 (UVR8) photoreceptor specifically mediates photomorphogenic responses to UV‐B. Photoreception induces dissociation of dimeric UVR8 into monomers to initiate responses. However, the regulation of dimer/monomer status in plants growing under photoperiodic conditions has not been examined. Here we show that UVR8 establishes a dimer/monomer photo‐equilibrium in plants growing in diurnal photoperiods in both controlled environments and natural daylight. The photo‐equilibrium is determined by the relative rates of photoreception and dark‐reversion to the dimer. Experiments with mutants in REPRESSOR OF UV‐B PHOTOMORPHOGENESIS 1 (RUP1) and RUP2 show that these proteins are crucial in regulating the photo‐equilibrium because they promote reversion to the dimer. In plants growing in daylight, the UVR8 photo‐equilibrium is most strongly correlated with low ambient fluence rates of UV‐B (up to 1.5 μmol m^?2 s^?1), rather than higher fluence rates or the amount of photosynthetically active radiation. In addition, the rate of reversion of monomer to dimer is reduced at lower temperatures, promoting an increase in the relative level of monomer at approximately 8–10 °C. Thus, UVR8 does not behave like a simple UV‐B switch under photoperiodic growth conditions but establishes a dimer/monomer photo‐equilibrium that is regulated by UV‐B and also influenced by temperature.     

Interaction of COP1 and UVR8 regulates UV‐B‐induced photomorphogenesis and stress acclimation in Arabidopsis

The ultraviolet‐B (UV‐B) portion of the solar radiation functions as an environmental signal for which plants have evolved specific and sensitive UV‐B perception systems. The UV‐B‐specific UV RESPONSE LOCUS 8 (UVR8) and the multifunctional E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) are key regulators of the UV‐B response. We show here that uvr8‐null mutants are deficient in UV‐B‐induced photomorphogenesis and hypersensitive to UV‐B stress, whereas overexpression of UVR8 results in enhanced UV‐B photomorphogenesis, acclimation and tolerance to UV‐B stress. By using sun simulators, we provide evidence at the physiological level that UV‐B acclimation mediated by the UV‐B‐specific photoregulatory pathway is indeed required for survival in sunlight. At the molecular level, we demonstrate that the wild type but not the mutant UVR8 and COP1 proteins directly interact in a UV‐B‐dependent, rapid manner in planta. These data collectively suggest that UV‐B‐specific interaction of COP1 and UVR8 in the nucleus is a very early step in signalling and responsible for the plant's coordinated response to UV‐B ensuring UV‐B acclimation and protection in the natural environment.     

UV‐B radiation delays flowering time through changes in the PRC2 complex activity and miR156 levels in Arabidopsis thaliana

UV‐B is a high‐energy component of the solar radiation perceived by the plant and induces a number of modifications in plant growth and development, including changes in flowering time. However, the molecular mechanisms underlying these changes are largely unknown. In the present work, we demonstrate that Arabidopsis plants grown under white light supplemented with UV‐B show a delay in flowering time, and this developmental reprogramming is mediated by the UVR8 photoreceptor. Using a combination of gene expression analyses and UV‐B irradiation of different flowering mutants, we gained insight into the pathways involved in the observed flowering time delay in UV‐B‐exposed Arabidopsis plants. We provide evidence that UV‐B light downregulates the expression of MSI1 and CLF, two of the components of the polycomb repressive complex 2, which in consequence drives a decrease in H3K27me3 histone methylation of MIR156 and FLC genes. Modification in the expression of several flowering time genes as a consequence of the decrease in the polycomb repressive complex 2 activity was also determined. UV‐B exposure of flowering mutants supports the involvement of this complex in the observed delay in flowering time, mostly through the age pathway.     

The UVB photoreceptor UVR8 mediates accumulation of UV‐absorbing pigments,but not changes in plant morphology,under outdoor conditions

UVB radiation is biologically active; in plants, it can induce a range of molecular, biochemical, morphological and developmental responses. Although much progress has been made in elucidating UVB perception and signalling pathways under controlled laboratory conditions, understanding of the adaptive, ecological role of UVB responses is still very limited. In this study, we looked at the functional role of UVR8 under outdoor light conditions, by studying growth, photosynthetic competence and accumulation of UV absorbing pigments in a mutant lacking functional UVR8 protein. It was found that the influence of UVB on morphology is restricted to summer and is independent of UVR8. In contrast, UVB had an effect on the content of UV‐absorbing pigments and the maximal efficiency of photosystem II of photosynthesis in the uvr8‐1 mutant throughout the year. It is concluded that the UVR8 photoreceptor plays a role throughout the year, in the temperate climate zone, even when UVB levels are relatively low.     

UV‐radiation and elevated temperatures induce formation of reactive oxygen species in gametophytes of cold‐temperate/Arctic kelps (Laminariales,Phaeophyceae)

Enhanced UV‐radiation (UVR) through stratospheric ozone depletion and global warming are crucial stressors to marine macroalgae. Damages may arise through formation of reactive oxygen species (ROS) in gametophytes of ecologically important kelps, brown algae of the order Laminariales, Such stress‐induced damages may have a negative impact on their fitness and further impact their following life stages. In our study, gametophytes of three kelp species Alaria esculenta (L.) Grev., Laminaria digitata (Huds.) Lamour., Saccharina latissima (L.) Lane, Mayes, Druehl, Saunders from the Arctic, and of L. hyperborea (Gunnerus) Foslie from the North Sea were exposed to photosynthetically active radiation, UV‐A, and UV‐B radiation and four temperatures (2–18°C). ROS are formed predominantly in the peripheral cytoplasm and in chloroplasts especially after exposure to UVR. Superoxide (O₂*^‐) is additionally formed in small, globular cytoplasmic structures, possibly mitochondria. In the surrounding medium O₂*^‐‐concentration increased markedly at elevated temperatures and under UV stress in some cases. Ultrastructural damage was negligible pointing to a high stress tolerance of this developmental stage. Our data indicate that stress tolerant gametophytes of three Arctic kelp species should sustain their crucial function as seed bank for kelp populations even under prospective rising environmental perturbations.     

Sporogenic and vegetative tissues of Saccharina latissima (Laminariales,Phaeophyceae) exhibit distinctive sensitivity to experimentally enhanced ultraviolet radiation : photosynthetically active radiation ratio

Fertile Saccharina latissima sporophytes, collected in the Kongsfjorden, Ny‐Ålesund, Spitsbergen, Norway (78°56.87′ N, 11°51.64′ E) were investigated in relation to its sensitivity to experimentally enhanced ultraviolet radiation : photosynthetically active radiation (UVR : PAR) ratios. Irradiance of UVR were 4.30 W m^?2 of UV‐A (320–400 nm) and 0.40 W m^?2 of UV‐B (280–320 nm), and PAR (400–700 nm) was ～4.30 W m^?2 (=20 µmol photons m^?2 s^?1). Excised soral (sporogenic) and non‐soral (vegetative) tissues were separately irradiated for 16 h at 7°C. Transmission electron microscopy showed abundant occurrence of physodes, electron dense particles (～300–600 nm) in the sorus. Paraphysis cells, with partly crystalline content, large mitochondria and abundant golgi bodies were towering over the sporangia. In soral tissue, cells were not visibly altered by the PAR + UVR irradiation. The chloroplasts, flagella and nucleus of unreleased meiospores inside the sporangial parent cells were visibly intact. Severe changes in the chloroplast structure of vegetative tissue occurred after PAR + UVR irradiation. These changes included wrinkling and dilatation of the thylakoid membranes, and appearance of electron translucent areas inside the chloroplasts. In vegetative cells exposed to PAR + UVR, the total amount of physodes, was slightly higher as in cells exposed to PAR only. Initial values of optimum quantum yield of photosystem II (F_v/F_m) were 0.743 ± 0.04 in non‐soral and 0.633 ± 0.04 in soral tissue. Vegetative tissue was observed to be more sensitive to radiant exposure of PAR and PAR + UVR compared to reproductive tissue. Under PAR, a 20% reduction in Fv/Fm was observed in non‐soral compared to no reduction in soral tissue, whereas under PAR + UVR, 60% and 33% reduction in Fv/Fm was observed in non‐soral and soral tissues, respectively. This can be attributed to the corresponding three times higher antiradical power (ARP) capacity in soral compared to non‐soral tissue.     

EFFECTS OF INCREASING UV‐B RADIATION DUE TO OZONE DEPLETION ON PHOTOSYNTHESIS OF ANTARCTIC CYANOBACTERIA

Ground level ultraviolet‐B (UV‐B; 290–320 nm) fluxes in Antarctica have been increasing due to stratospheric ozone depletion. Although mat‐forming cyanobacteria are major component of freshwater algal biomass in Antarctica, little is known about their response to increasing ultraviolet radiation (UVR). The present study evaluated the sensitivity to UVR of two strains of mat‐forming cyanobacteria with different cell size, Phormidium murrayi (6.0 x 3.2 μm) and Schizothrix calcicola (2.2 x 2.3 μm). Cyanobacterial photosynthesis was measured under different UV spectral quality and quantity achieved by polychromatic filters with different cutoff wavelengths and neutral density screens. The productivity and irradiance data were used to generate biological weighting functions (BWF) for the assessment of UV inhibition on photosynthesis. The kinetics of UV inhibition, as determined by PAM fluorometry, differed between the two species so that inhibition of P. murrayi and S. calcicola were modeled based on UV‐irradiance and cumulative exposure, respectively. After a one hour exposure, BWF's did not differ between the two isolates of cyanobacteria despite their differences in cell size. To evaluate the negative impact of increased UV‐B exposure due to ozone depletion on cyanobacteria, the BWF's were applied to two solar spectra obtained from McMurdo Station, one on a day when the ozone hole was prominent (O₃ = 170 Dobson units; DU = 10‐3 cm O₃), and the other on a day with high ozone concentration (O₃ = 328 DU). The decrease in ozone level would reduce productivity by 3–8%. Seasonal variation of UVR has a bigger impact on cyanobacterial productivity than ozone depletion.     

Solar UV‐B radiation and ethylene play a key role in modulating effective defenses against Anticarsia gemmatalis larvae in field‐grown soybean

Solar UV‐B radiation has been reported to enhance plant defenses against herbivore insects in many species. However, the mechanism and traits involved in the UV‐B mediated increment of plant resistance are unknown in crops species, such as soybean. Here, we studied defense‐related responses in undamaged and Anticarsia gemmatalis larvae‐damaged leaves of two soybean cultivars grown under attenuated or full solar UV‐B radiation. We determined changes in jasmonates, ethylene (ET), salicylic acid, trypsin protease inhibitor activity, flavonoids, and mRNA expression of genes related with defenses. ET emission induced by Anticarsia gemmatalis damage was synergistically increased in plants grown under solar UV‐B radiation and was positively correlated with malonyl genistin concentration, trypsin proteinase inhibitor activity and expression of IFS2, and the pathogenesis protein PR2, while was negatively correlated with leaf consumption. The precursor of ET, aminocyclopropane‐carboxylic acid, applied exogenously to soybean was sufficient to strongly induce leaf isoflavonoids. Our results showed that in field‐grown soybean isoflavonoids were regulated by both herbivory and solar UV‐B inducible ET, whereas flavonols were regulated by solar UV‐B radiation only and not by herbivory or ET. Our study suggests that, although ET can modulate UV‐B‐mediated priming of inducible plant defenses, some plant defenses, such as isoflavonoids, are regulated by ET alone.     

Exposure of eggs to solar UV‐B leads to reduced hatching rates in two sparid fishes,red sea bream Pagrus major and black sea bream Acanthopagrus schlegeli

Hatching success was examined under exposure to solar ultraviolet radiation (UVR) using filters to give three different light conditions [C1: UV‐B, UV‐A and photosynthetically active radiation (PAR), C2: UV‐A and PAR, C3: PAR] in red Pagrus major and black Acanthopagrus schlegeli sea bream. Hatching rate of both species was reduced by an exposure over a 2 day period to UVR and was not significantly different between two species under the three light conditions.     

UV‐B impairs growth and gas exchange in grapevines grown in high altitude

We previously demonstrated that solar ultraviolet‐B (UV‐B) radiation levels in high altitude vineyards improve berry quality in Vitis vinifera cv. Malbec, but also reduce berry size and yield, possibly as a consequence of increased oxidative damage and growth reductions (lower photosynthesis). The defense mechanisms toward UV‐B signal and/or evoked damage promote production of antioxidant secondary metabolites instead of primary metabolites. Purportedly, the UV‐B effects will depend on tissues developmental stage and interplay with other environmental conditions, especially stressful situations. In this work, grapevines were exposed to high solar UV‐B (+UV‐B) and reduced (by filtering) UV‐B (?UV‐B) treatments during three consecutive seasons, and the effects of UV‐B, developmental stages and seasons on the physiology were studied, i.e. growth, tissues morphology, photosynthesis, photoprotective pigments, proline content and antioxidant capacity of leaves. The +UV‐B reduced photosynthesis and stomatal conductance, mainly through limitation in gas exchange, reducing plant's leaf area, net carbon fixation and growth. The +UV‐B augmented leaf thickness, and also the amounts of photoprotective pigments and proline, thereby increasing the antioxidant capacity of leaves. The defense mechanisms triggered by + UV‐B reduced lipid peroxidation, but they were insufficient to protect the photosynthetic pigments per leaf dry weight basis. The +UV‐B effects depend on tissues developmental stage and interplay with other environmental conditions such as total radiation and air temperatures.     

Tissue‐specific and light‐dependent regulation of phytochrome gene expression in rice

Phytochromes are red‐ and far red light photoreceptors in higher plants. Rice (Oryza sativa L.) has three phytochromes (phyA, phyB and phyC), which play distinct as well as cooperative roles in light perception. To gain a better understanding of individual phytochrome functions in rice, expression patterns of three phytochrome genes were characterized using promoter‐GUS fusion constructs. The phytochrome genes PHYA and PHYB showed distinct patterns of tissue‐ and developmental stage‐specific expression in rice. The PHYA promoter‐GUS was expressed in all leaf tissues in etiolated seedlings, while its expression was restricted to vascular bundles in expanded leaves of light‐grown seedlings. These observations suggest that light represses the expression of the PHYA gene in all cells except vascular bundle cells in rice seedlings. Red light was effective, but far red light was ineffective in gene repression, and red light‐induced repression was not observed in phyB mutants. These results indicate that phyB is involved in light‐dependent and tissue‐specific repression of the PHYA gene in rice.