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Light is a critical environmental signal that is perceived by various photoreceptors and is of great significance in the photosynthetic growth of algae and plants. The phytochrome‐lacking model green alga Chlamydomonas reinhardtii possesses heme oxygenase (HMOX1) and phycocyanobilin ferredoxin oxidoreductase (PCYA1) to synthesize the linear tetrapyrrole bilin from heme in the chloroplast. The hmox1 mutant has photosynthetic growth deficiency and accumulation of photosystem I proteins such as LHCA1 is severely inhibited, and these defects could be rescued by exogenous bilin feeding in a blue light‐dependent manner. To investigate the contribution of the typical blue/ultraviolet light photoreceptors PHOT, aCRY, pCRY, and UVR8 in the process of bilin and blue light‐dependent recovery of LHCA1 protein in hmox1, we generated double mutants of these photoreceptors in hmox1, as well as a triple mutant of phot uvr8 hmox1, to analyze the LHCA1 protein abundance in these mutants. Results clearly showed that PHOT, CRYs, and UVR8 do not participate in this process. In addition, transcriptome profiling analysis of the hmox1 and its genetically complemented strain ho1C2 during dark‐to‐blue light transition revealed a total of 269 blue light‐responsive genes independent of bilin (|fold change| ≥ 2). RNA‐seq also identified a set of 249 differentially expressed genes that are dependent on both blue light and bilin. These findings provide valuable insights for elucidating the role of bilin in mediating blue light signaling pathways in Chlamydomonas. [ABSTRACT FROM AUTHOR]

UV-B radiation can substantially impact plant growth. To study UV-B effects, broadband UV-B tubes are commonly used. Apart from UV-B, such tubes also emit UV-A wavelengths. This study aimed to distinguish effects of different UV-B intensities on Arabidopsis thaliana wildtype and UVR8 mutant rosette morphology, from those by accompanying UV-A. UV-A promotes leaf-blade expansion along the proximal–distal, but not the medio-lateral, axis. Consequent increases in blade length: width ratio are associated with increased light capture. However, petiole length is not affected by UV-A exposure. This scenario is distinct from the shade avoidance driven by low red to far-red ratios, whereby leaf blade elongation is impeded but petiole elongation is promoted. Thus, the UV-A mediated elongation response is phenotypically distinct from classical shade avoidance. UV-B exerts inhibitory effects on petiole length, blade length and leaf area, and these effects are mediated by UVR8. Thus, UV-B antagonises aspects of both UV-A mediated elongation and classical shade avoidance. Indeed, this study shows that accompanying UV-A wavelengths can mask effects of UV-B. This may lead to potential underestimates of the magnitude of the UV-B induced morphological response using broadband UV-B tubes. [ABSTRACT FROM AUTHOR]

The zygnematophytes are the closest relatives of land plants and comprise several lineages that adapted to a life on land. Species of the genus Serritaenia form colorful, mucilaginous capsules, which surround the cells and block harmful solar radiation, one of the major terrestrial stressors. In eukaryotic algae, this 'sunscreen mucilage' represents a unique photoprotective strategy, whose induction and chemical background are unknown. We generated a de novo transcriptome of Serritaenia testaceovaginata and studied its gene regulation under moderate UV radiation (UVR) that triggers sunscreen mucilage under experimental conditions. UVR induced the repair of DNA and the photosynthetic apparatus as well as the synthesis of aromatic specialized metabolites. Specifically, we observed pronounced expressional changes in the production of aromatic amino acids, phenylpropanoid biosynthesis genes, potential cross-membrane transporters of phenolics, and extracellular, oxidative enzymes. Interestingly, the most up-regulated enzyme was a secreted class III peroxidase, whose embryophyte homologs are involved in apoplastic lignin formation. Overall, our findings reveal a conserved, plant-like UVR perception system (UVR8 and downstream factors) in zygnematophyte algae and point to a polyphenolic origin of the sunscreen pigment of Serritaenia , whose synthesis might be extracellular and oxidative, resembling that of plant lignins. [ABSTRACT FROM AUTHOR]

The phytohormone jasmonate (JA) coordinates stress and growth responses to increase plant survival in unfavorable environments. Although JA can enhance plant UV‐B stress tolerance, the mechanisms underlying the interaction of UV‐B and JA in this response remain unknown. In this study, we demonstrate that the UV RESISTANCE LOCUS 8 ‐ TEOSINTE BRANCHED1, Cycloidea and PCF 4 ‐ LIPOXYGENASE2 (UVR8‐TCP4‐LOX2) module regulates UV‐B tolerance dependent on JA signaling pathway in Arabidopsis thaliana. We show that the nucleus‐localized UVR8 physically interacts with TCP4 to increase the DNA‐binding activity of TCP4 and upregulate the JA biosynthesis gene LOX2. Furthermore, UVR8 activates the expression of LOX2 in a TCP4‐dependent manner. Our genetic analysis also provides evidence that TCP4 acts downstream of UVR8 and upstream of LOX2 to mediate plant responses to UV‐B stress. Our results illustrate that the UV‐B‐dependent interaction of UVR8 and TCP4 serves as an important UVR8‐TCP4‐LOX2 module, which integrates UV‐B radiation and JA signaling and represents a new UVR8 signaling mechanism in plants. [ABSTRACT FROM AUTHOR]

This article presents the findings of a study on the response of the green alga Serritaenia testaceovaginata to moderate UV radiation. The study used comparative transcriptomics to analyze gene expression in the alga and discovered that it possesses a plant-like system for UV perception and signal transduction, as well as components of major plant photoreceptor systems. This suggests that adaptations to light conditions on land may rely on the adaptation of existing systems rather than the development of new components. The study emphasizes the importance of understanding the genetic and physiological changes that enabled the transition of plants from water to land. Additionally, the study found that genes involved in the production of aromatic amino acids and phenylpropanoid biosynthesis were up-regulated under UV irradiance, indicating a polyphenolic origin of the sunscreen pigment in Serritaenia. The study also identified components of various photoreceptor systems in Serritaenia, including UVR8, which is involved in UV radiation signaling. The authors call for further research on light signaling in non-seed plant species. [Extracted from the article]

Abstract Background Artemisia annua is the major source for artemisinin production. The artemisinin content in A. annua is affected by different types of light especially the UV light. UVR8, a member of RCC1 gene family was found to be the UV-B receptor in plants. The gene structures, evolutionary history and expression profile of UVR8 or RCC1 genes remain undiscovered in A. annua. Results Twenty-two RCC1 genes (AaRCC1) were identified in each haplotype genome of two diploid strains of A. annua, LQ-9 and HAN1. Varied gene structures and sequences among paralogs were observed. The divergence of most RCC1 genes occurred at 46.7 – 51 MYA which overlapped with species divergence of core Asteraceae during the Eocene, while no recent novel RCC1 members were found in A. annua genome. The number of RCC1 genes remained stable among eudicots and RCC1 genes underwent purifying selection. The expression profile of AaRCC1 is analogous to that of Arabidopsis thaliana (AtRCC1) when responding to environmental stress. Conclusions This study provided a comprehensive characterization of the AaRCC1 gene family and suggested that RCC1 genes were conserved in gene number, structures, constitution of amino acids and expression profiles among eudicots.

Summary: Leaves are the main photosynthetic organs in plants, and their anatomy is optimized for light interception and gas exchange. Although each species has a characteristic leaf anatomy, which depends on the genotype, leaves also show a large degree of developmental plasticity. Light and temperature regulate leaf development from primordia differentiation to late stages of blade expansion. While the molecular mechanisms of light and temperature signaling have been mostly studied in seedlings, in the latest years, research has focused on leaf development. Here, I will describe the latest work carried out in the environmental regulation of Arabidopsis leaf development, comparing signaling mechanisms between leaves and seedlings, highlighting the new discoveries, and pointing out the most exciting open questions. [ABSTRACT FROM AUTHOR]

Background: Artemisia annua is the major source for artemisinin production. The artemisinin content in A. annua is affected by different types of light especially the UV light. UVR8, a member of RCC1 gene family was found to be the UV-B receptor in plants. The gene structures, evolutionary history and expression profile of UVR8 or RCC1 genes remain undiscovered in A. annua. Results: Twenty-two RCC1 genes (AaRCC1) were identified in each haplotype genome of two diploid strains of A. annua, LQ-9 and HAN1. Varied gene structures and sequences among paralogs were observed. The divergence of most RCC1 genes occurred at 46.7 – 51 MYA which overlapped with species divergence of core Asteraceae during the Eocene, while no recent novel RCC1 members were found in A. annua genome. The number of RCC1 genes remained stable among eudicots and RCC1 genes underwent purifying selection. The expression profile of AaRCC1 is analogous to that of Arabidopsis thaliana (AtRCC1) when responding to environmental stress. Conclusions: This study provided a comprehensive characterization of the AaRCC1 gene family and suggested that RCC1 genes were conserved in gene number, structures, constitution of amino acids and expression profiles among eudicots. [ABSTRACT FROM AUTHOR]

Oriented movement (phototaxis) is an efficient way to optimize light‐driven processes and to avoid photodamage for motile algae. In Chlamydomonas the receptors for phototaxis are the channelrhodopsins ChR1 and ChR2. Both are directly light‐gated, plasma membrane‐localized cation channels. To optimally adjust its overall light‐dependent responses, Chlamydomonas must tightly control the ChRs cellular abundance and integrate their activities into its general photoprotective network. How this is achieved is largely unknown. Here we show that the ChR1 protein level decreases upon illumination in a light‐intensity and quality‐dependent manner, whereas it is stable in prolonged darkness. Analysis of knockout strains of six major photoreceptors absorbing in the blue–violet range, which is most effective in evoking ChR1 degradation, revealed that only phototropin (PHOT) is involved. Notably, ChR2 degradation was normal in a ΔPHOT strain. Further, our results indicate that a COP1‐SPA1 E3 ubiquitin ligase, the transcription factor Hy5 as well as changes in the cellular redox poise and cyclic nucleotide levels are additional components involved in this light acclimation response of Chlamydomonas. Our data highlight the presence of an adaptive framework connecting phototaxis with general photoprotective mechanisms via the use of overlapping signaling components already at the level of the primary photoreceptor. Summary Statement: How Chlamydomonas controls the level of ChR1, the main photoreceptor for its movement responses, is largely unknown. Here we show that regulatory elements involved in photoprotective gene expression are parts of this cascade(s), supporting an integrating link between phototaxis and photoprotection. [ABSTRACT FROM AUTHOR]

SUMMARY: Reductions in red to far‐red ratio (R:FR) provide plants with an unambiguous signal of vegetational shade and are monitored by phytochrome photoreceptors. Plants integrate this information with other environmental cues to determine the proximity and density of encroaching vegetation. Shade‐sensitive species respond to reductions in R:FR by initiating a suite of developmental adaptations termed shade avoidance. These include the elongation of stems to facilitate light foraging. Hypocotyl elongation is driven by increased auxin biosynthesis promoted by PHYTOCHROME INTERACTING FACTORs (PIF) 4, 5 and 7. UV‐B perceived by the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor rapidly inhibits shade avoidance, in part by suppressing PIF4/5 transcript accumulation and destabilising PIF4/5 protein. Here, we show that longer‐term inhibition of shade avoidance is sustained by ELONGATED HYPOCOTYL 5 (HY5) and HY5 HOMOLOGUE (HYH), which regulate transcriptional reprogramming of genes involved in hormone signalling and cell wall modification. HY5 and HYH are elevated in UV‐B and suppress the expression of XYLOGLUCAN ENDOTRANSGLUCOSYLASE/HYDROLASE (XTH) genes involved in cell wall loosening. They additionally increase expression GA2‐OXIDASE1 (GA2ox1) and GA2ox2, encoding gibberellin catabolism enzymes that act redundantly to stabilise the PIF‐inhibiting DELLA proteins. UVR8 therefore regulates temporally distinct signalling pathways to first rapidly inhibit and subsequently maintain suppression of shade avoidance following UV‐B exposure. Significance Statement: UV‐B perceived by the photoreceptor UVR8 provides a strong brake on plant shade avoidance responses. Following the rapid depletion of PHYTOCHROME INTERACTING FACTORS (PIFs), UV‐B‐mediated suppression of shade avoidance is maintained by the transcription factors HY5 and HYH, which regulate wide‐scale transcriptional reprogramming of genes involved in hormone signalling and cell wall loosening, stabilising PIF‐inhibiting DELLA proteins. [ABSTRACT FROM AUTHOR]

Ultraviolet-B (UV-B) radiation is a major abiotic stress that dragon trees are exposed to during their growth and development; however, it is also an environmental signal perceived by plants that affects the flavonoid pathway. Previous studies have demonstrated that amounts of flavonoids are contained in dragon tree resin, otherwise known as dragon's blood. However, the traits and mechanism involved in the UV-B-mediated increase in flavonoids in dragon trees are still unknown. Here, we studied the response of Dracaena cambodiana under full solar UV-B radiation. The results showed that the contents of total flavonoids in D. cambodiana significantly increased after UV-B radiation exposure. Then, the transcriptome was used for determining the interactive mechanism of flavonoid accumulation and UV-B stress. Differential expression analyses identified 34 differentially expressed genes (DEGs) involved in flavonoid synthesis; specifically, 24 of the identified DEGs were significantly up-regulated after UV-B radiation exposure. In addition, 57 DEGs involved in Ca2+/kinase sensors, 58 DEGs involved in ROS scavenging and the plant hormone pathway, and 116 DEGs transcription factors in 5 families were further identified and analyzed. Finally, we deduced the potential mechanism of UV-B-promoting flavonoid formation to neutralize ROS damage derived from UV-B radiation in D. cambodiana based on the gene co-expression network and previous studies from other plants. Considering that wild dragon tree populations are currently highly threatened by anthropogenic and natural stressors, the interactive studies between D. cambodiana plants and UV-B radiation provide valuable information toward understanding the mechanism of dragon's blood formation and help us reveal the evolution of D. cambodiana, with the eventual goal of aiding in the global conservation of this precious biological resource. [ABSTRACT FROM AUTHOR]

Abstract Ultraviolet-B (UV-B) light is an intrinsic part of sunlight that reaches the earth’s surface, and affects plant survival and adaptation. How plants respond to UV-B light is regulated by the wavelength, intensity and duration of UV-B radiation, and is also regulated by photosynthetically active radiation perceived by phytochrome and cryptochrome photoreceptors. Non-damaging UV-B light promotes plant photomorphogenesis and UV-B acclimation which enhances plant tolerance against UV-B stress. However, high-level UV-B radiation induces DNA damage, generates reactive oxygen species (ROS) and impairs photosynthesis. Plants have evolved efficient mechanisms to utilize informational UV-B signal, and protect themselves from UV-B stress. UV RESISTANCE LOCUS8 (UVR8) is a conserved plant-specific UV-B photoreceptor. It interacts with CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) to initiate UV-B-specific light signaling and regulate UV-B responsive gene expression. A set of transcription factors such as ELONGATED HYPOCOTYL5 (HY5) function downstream of the UVR8-COP1 module to promote seedling de-etiolation for photomorphogenic development and biosynthesis of sunscreen flavonoids for UV-B stress tolerance. In addition to UVR8 signaling pathways, plants subjected to damaging UV-B radiation initiate stress protection and repair mechanisms through UVR8-independent pathways. In this review, we summarize the emerging mechanisms underlying UV-B stress acclimation and protection in plants, primarily revealed in the model plant Arabidopsis thaliana.

SUMMARY: COP1 is a critical repressor of plant photomorphogenesis in darkness. However, COP1 plays distinct roles in the photoreceptor UVR8 pathway in Arabidopsis thaliana. COP1 interacts with ultraviolet B (UV‐B)‐activated UVR8 monomers and promotes their retention and accumulation in the nucleus. Moreover, COP1 has a function in UV‐B signaling, which involves the binding of its WD40 domain to UVR8 and HY5 via conserved Val‐Pro (VP) motifs of these proteins. UV‐B‐activated UVR8 interacts with COP1 via both the core domain and the VP motif, leading to the displacement of HY5 from COP1 and HY5 stabilization. However, it remains unclear whether the function of COP1 in UV‐B signaling is solely dependent on its VP motif binding capacity and whether UV‐B regulates the subcellular localization of COP1. Based on published structures of the COP1 WD40 domain, we generated a COP1 variant with a single amino acid substitution, COP1C509S, which cannot bind to VP motifs but retains the ability to interact with the UVR8 core domain. UV‐B only marginally increased nuclear YFP‐COP1 levels and significantly promoted YFP‐COP1 accumulation in the cytosol, but did not exert the same effects on YFP‐COP1C509S. Thus, the full UVR8–COP1 interaction is important for COP1 accumulation in the cytosol. Notably, UV‐B signaling including activation of HY5 transcription was obviously inhibited in the Arabidopsis lines expressing YFP‐COP1C509S, which cannot bind VP motifs. We conclude that the full binding of UVR8 to COP1 leads to the predominant accumulation of COP1 in the cytosol and that COP1 has an additional function in UV‐B signaling besides VP binding‐mediated protein destabilization. Significance Statement: We demonstrate that COP1 can promote UV‐B signaling in both VP motif binding‐dependent and ‐independent manners by using the COP1 variant COP1C509S, which is defective in binding to the VP motif of UVR8. Moreover, with this COP1 variant, we show that the full binding of UVR8 to COP1 is required for UV‐B‐induced COP1 accumulation in the cytosol. [ABSTRACT FROM AUTHOR]

Ultraviolet-B radiation (UV-B, 280–315 nm), as an environmental signal and potential abiotic stressor, has a wide range of effects on plant growth and development. Plants sense UV-B through the UV-B photoreceptor UV RESISTANCE LOCUS 8, which regulates downstream gene expression. On the one hand, UV-B regulates a series of photomorphogenesis events, such as inhibition of hypocotyl and stem elongation, which are largely related to the endogenous levels of phytohormones. Plants down-regulate auxin, gibberellin, and brassinosteroid signals in response to UV-B photomorphogenesis. On the other hand, UV-B causes DNA damage, accumulation of reactive oxygen species and damage to the photosynthetic system. Plants prevent UV-B damage by accumulating a variety of 'sunscreens', in particular flavonoids, which are typical UV-absorbers. It is noteworthy that phytohormones may also be involved in UV-B acclimation. UV-B up-regulates the levels of endogenous phytohormones, including abscisic acid, melatonin, jasmonate, and salicylic acid. Here, we review the molecular pathways of UV-B photomorphogenesis and acclimation that have been reported in flowering plants. We also highlight the important role of phytohormones in response to UV-B to understand the mechanism of UV-B acclimation from different perspectives. • Plants perceive UV-B using the UV-B photoreceptor UV RESISTANCE LOCUS 8. • Phytohormones play important roles in UV-B-regulated plant photomorphogenesis. • Phytohormones may also be involved in UV-B acclimation [ABSTRACT FROM AUTHOR]

The plant UV-B photoreceptor UV RESISTANCE LOCUS 8 (UVR8) exists as a homodimer in its inactive ground state. Upon UV-B exposure, UVR8 monomerizes and interacts with a downstream key regulator, the CONSTITUTIVE PHOTOMORPHOGENIC 1/SUPPRESSOR OF PHYA (COP1/SPA) E3 ubiquitin ligase complex, to initiate UV-B signaling. Two WD40 proteins, REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1 (RUP1) and RUP2 directly interact with monomeric UVR8 and facilitate UVR8 ground state reversion, completing the UVR8 photocycle. Here, we reconstituted the RUP-mediated UVR8 redimerization process in vitro and reported the structure of the RUP2-UVR8W285A complex (2.0 Å). RUP2 and UVR8W285A formed a heterodimer via two distinct interfaces, designated Interface 1 and 2. The previously characterized Interface 1 is found between the RUP2 WD40 domain and the UVR8 C27 subregion. The newly identified Interface 2 is formed through interactions between the RUP2 WD40 domain and the UVR8 core domain. Disruption of Interface 2 impaired UV-B induced photomorphogenic development in Arabidopsis thaliana. Further biochemical analysis indicated that both interfaces are important for RUP2-UVR8 interactions and RUP2-mediated facilitation of UVR8 redimerization. Our findings suggest that the two-interface-interaction mode is adopted by both RUP2 and COP1 when they interact with UVR8, marking a step forward in understanding the molecular basis that underpins the interactions between UVR8 and its photocycle regulators.

Photosensory proteins known as photoreceptors (PHRs) are crucial for delineating light environments in synchronization with other environmental cues and regulating their physiological variables in plants. However, this has not been well studied in the Brassica genus, which includes several important agricultural and horticultural crops. Herein, we identified five major PHR gene families—phytochrome (PHY), cryptochrome (CRY), phototropin (PHOT), F-box containing flavin binding proteins (ZTL/FKF1/LKP2), and UV RESISTANCE LOCUS 8 (UVR8)—genomic scales and classified them into subfamilies based on their phylogenetic clustering with Arabidopsis homologues. The molecular evolution characteristics of Brassica PHR members indicated indirect expansion and lost one to six gene copies at subfamily levels. The segmental duplication was possibly the driving force of the evolution and amplification of Brassica PHRs. Gene replication retention and gene loss events of CRY, PHY, and PHOT members found in diploid progenitors were highly conserved in their tetraploid hybrids. However, hybridization events were attributed to quantitative changes in UVR8 and ZTL/FKF1/LKP2 members. All PHR members underwent purifying selection. In addition, the transcript expression profiles of PHR genes in different tissue and in response to exogenous ABA, and abiotic stress conditions suggested their multiple biological significance. This study is helpful in understanding the molecular evolution characteristics of Brassica PHRs and lays the foundation for their functional characterization. [ABSTRACT FROM AUTHOR]

A regulator of chromosome condensation 1 (RCC1) family protein has been functionally characterized to be involved in various cellular processes. In this study, one RCC1 gene named SaRCC1 was cloned from the full-length cDNA library of Spartinaalterniflora. The open reading frame (ORF) of SaRCC1 was 1440 bp, and it encoded 479 amino acids with a calculated molecular mass of 51.65 kDa. Multiple amino acid sequence alignments showed that SaRCC1 had high identity with other plant RCC1s, and the phylogenetic analysis indicated that SaRCC1 had a closer affinity to Zea mays RCC1 family protein (ZmRCC1). SaRCC1 gene was induced under salt stress conditions, and its encoded protein was located in peroxisome. In order to further investigate the function of SaRCC1, transgenic Arabidopsis plants ectopically both sense-overexpressing and antisense-overexpressing SaRCC1 were generated. SaRCC1-overexpressing lines exhibited an increased salt and ABA hypersensitivity and reduced resistance to salinity stress. On the other hand, the transcripts of some stress-responsive genes in the SaRCC1 transgenic plants were affected in response to salinity stress. Our results provide evidence for the involvement of SaRCC1, negatively regulating salt stress responses by affecting stress-related gene expression in Arabidopsis. [ABSTRACT FROM AUTHOR]

In the field, plants face constantly changing light conditions caused by both atmospheric effects and neighbouring vegetation. This interplay creates a complex, fluctuating light environment within plant canopies. Shade-intolerant species rely on light cues from competitors to trigger shade avoidance responses, ensuring access to light for photosynthesis. While research often uses controlled growth chambers with steady light to study shade avoidance responses, the influence of light fluctuations in real-world settings remains unclear. This review examines the dynamic light environments found in woodlands, grasslands, and crops. We explore how plants respond to some fluctuations but not others, analyse the potential reasons for these differences, and discuss the possible molecular mechanisms regulating this sensitivity. We propose that studying shade avoidance responses under fluctuating light conditions offers a valuable tool to explore the intricate regulatory network behind them., (© The Author(s) 2024. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

SUMMARY: The UV RESISTANCE LOCUS 8 (UVR8) photoreceptor mediates plant responses to Ultraviolet‐B (UV‐B) wavelengths. The UVR8 dimer dissociates into monomers following UV‐B photoreception, a process accompanied by conformational changes that facilitate interaction of UVR8 with proteins that initiate responses. However, the importance of particular amino acids in maintaining UVR8 conformation and modulating protein interactions is poorly understood. Here we examine the roles of cysteine amino acids C231 and C335 in UVR8 structure and function. UVR8C231S,C335S mutant protein forms dimers and monomerizes similarly to wild‐type UVR8. UVR8C231S,C335S interacts with CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) in plants to initiate photomorphogenic responses to UV‐B, although the interaction is weaker when examined in yeast two‐hybrid assays. Similarly, the interaction of UVR8C231S,C335S with REPRESSOR OF UV‐B PHOTOMORPHOGENESIS (RUP) proteins is weaker in both plants and yeast compared with wild‐type UVR8. Re‐dimerization of UVR8 in plants, which is mediated by RUP proteins, occurs with reduced efficiency in UVR8C231S,C335S. Fluorescence resonance energy transfer analysis indicates that UVR8C231S,C335S has an altered conformation in plants, in that the N‐ and C‐termini appear closer together, which may explain the altered protein interactions. Significance Statement: Photomorphogenic responses to ultraviolet‐B (UV‐B) wavelengths are initiated by the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor, but the importance of particular amino acids in maintaining UVR8 conformation and modulating protein interactions is poorly understood. Photobiological analyses of UVR8 mutants indicate that specific cysteine amino acids have a role in determining UVR8 conformation. [ABSTRACT FROM AUTHOR]

SUMMARY: Plants undergo photomorphogenic development in the presence of light. Photomorphogenesis is repressed by the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1), which binds to substrates through their valine–proline (VP) motifs. The UV RESISTANCE LOCUS 8 (UVR8) photoreceptor senses UV‐B and inhibits COP1 through the cooperative binding of its own VP motif and photosensing core to COP1, thereby preventing COP1 binding to substrates, including the basic leucine zipper (bZIP) transcriptional regulator ELONGATED HYPOCOTYL 5 (HY5). As a key promoter of visible light and UV‐B photomorphogenesis, HY5 requires coregulators for its function. The B‐box family transcription factors BBX20–BBX22 were recently described as HY5 rate‐limiting coactivators under red light, but their role in UVR8 signaling was unknown. Here we describe a hypermorphic bbx21‐3D mutant with enhanced photomorphogenesis, carrying a proline‐to‐leucine mutation at position 314 in the VP motif that impairs the interaction with and regulation by COP1. We show that BBX21 and BBX22 are UVR8‐dependently stabilized after UV‐B exposure, which is counteracted by a repressor induced by HY5/BBX activity. bbx20 bbx21 bbx22 mutants under UV‐B are impaired in hypocotyl growth inhibition, photoprotective pigment accumulation and the expression of several HY5‐dependent genes under continuous UV‐B, but the immediate induction of marker genes after exposure to UV‐B remains surprisingly rather unaffected. We conclude that BBX20–BBX22 contribute to HY5 activity in a subset of UV‐B responses, but that additional, presently unknown, coactivators for HY5 are functional in early UVR8 signaling. Significance Statement: A subfamily of B‐box transcription factors promotes plant responses to visible light. In this work, we describe a hypermorphic bbx21 mutant with enhanced photomorphogenesis, and we show that BBX proteins are involved in HY5‐dependent, UVR8 photoreceptor‐mediated responses to UV‐B. BBX21 and BBX22 are stabilized under UV‐B, their stability is fine‐tuned through a HY5‐dependent feedback loop and they promote the expression of phenylpropanoid biosynthesis genes under continuous UV‐B, but apparently not the acute induction of those genes. [ABSTRACT FROM AUTHOR]

The tremendously growing global human population and increasing level of UV radiation necessitates the need for developing a cost-effective, convenient and environment-friendly method for improving the quality and quantity of food crops. UV-B radiation has particular regulatory functions in plant growth and development, which can lead to increased plant hardiness, increased plant resistance to herbivores and diseases and improved agricultural product quality, all of which have implications for food security. UV-B priming is one of the most efficient techniques helpful in increasing yield and improving quality of the crop. The review paper scrutinizes the previous research studies on effect of UV priming on plants, mainly focusing UV-B priming. The overall significance of this technique and the mode of action through which the photomorphogenic responses are brought about in plants after UV exposure are discussed in detail. We found certain gaps in research that are needed to be addressed. Exposing plants to low doses of UV-B increases the production of antioxidants, phenolics, pigments and enzymes, overall strengthening defense system and increasing the efficiency of photosystem of the plant. Experiments on the effects of UV-B light needs precise monitoring of UV radiation. The development of tools and procedures that permit for the estimation of pragmatic UV radiation exposures of discrete plants, natural ecosystems and agricultural contexts need further attention. Ecologists are to forecast the expected effects of natural ecosystems; investigations of the combined effects of environmental variables and UV-B radiation will be required for the development of stress-tolerant crop varieties. A better understanding of the mechanism of UV-B priming mechanisms will aid in the development of seed production technologies. The role of UV-B photo receptor, UVR8, in nucleoplasmic gene transcription and the link between UVR8 activation and repression of hormone production or signaling are all questions that needs to be explored further. [ABSTRACT FROM AUTHOR]

Inflorescence movements in response to natural gradients of sunlight are frequently observed in the plant kingdom and are suggested to contribute to reproductive success. Although the physiological and molecular bases of light‐mediated tropisms in vegetative organs have been thoroughly investigated, the mechanisms that control inflorescence orientation in response to light gradients under natural conditions are not well understood. In this work, we have used a combination of laboratory and field experiments to investigate light‐mediated re‐orientation of Arabidopsis thaliana inflorescences. We show that inflorescence phototropism is promoted by photons in the UV and blue spectral range (≤500 nm) and depends on multiple photoreceptor families. Experiments under controlled conditions show that UVR8 is the main photoreceptor mediating the phototropic response to narrowband UV‐B radiation, and phototropins and cryptochromes control the response to narrowband blue light. Interestingly, whereas phototropins mediate bending in response to low irradiances of blue, cryptochromes are the principal photoreceptors acting at high irradiances. Moreover, phototropins negatively regulate the action of cryptochromes at high irradiances of blue light. Experiments under natural field conditions demonstrate that cryptochromes are the principal photoreceptors acting in the promotion of the heliotropic response of inflorescences under full sunlight. We investigated the phototropic response of A. thaliana inflorescences in the laboratory and in the field. Under natural conditions, plants displayed heliotropic inflorescences. The heliotropic response under full sunlight was activated by UV‐A/blue light photons, acting through the cryptochromes. [ABSTRACT FROM AUTHOR]

A regulator of chromosome condensation 1 (RCC1) family protein has been functionally characterized to be involved in various cellular processes. In this study, one RCC1 gene named SaRCC1 was cloned from the full-length cDNA library of Spartinaalterniflora. The open reading frame (ORF) of SaRCC1 was 1440 bp, and it encoded 479 amino acids with a calculated molecular mass of 51.65 kDa. Multiple amino acid sequence alignments showed that SaRCC1 had high identity with other plant RCC1s, and the phylogenetic analysis indicated that SaRCC1 had a closer affinity to Zea mays RCC1 family protein (ZmRCC1). SaRCC1 gene was induced under salt stress conditions, and its encoded protein was located in peroxisome. In order to further investigate the function of SaRCC1, transgenic Arabidopsis plants ectopically both sense-overexpressing and antisense-overexpressing SaRCC1 were generated. SaRCC1-overexpressing lines exhibited an increased salt and ABA hypersensitivity and reduced resistance to salinity stress. On the other hand, the transcripts of some stress-responsive genes in the SaRCC1 transgenic plants were affected in response to salinity stress. Our results provide evidence for the involvement of SaRCC1, negatively regulating salt stress responses by affecting stress-related gene expression in Arabidopsis.

Quantitative RT- PCR is one of the most common methods to study gene expression in response to stress. Therefore, it is crucial to have suitable reference genes (RGs) for result normalization. Although several reports describe UV-B-modulated gene expression in Solanum lycopersicum , there are no suitable RGs identified until now. The aim of this work was to evaluate the suitability of seven traditional genes: actin (ACT), tubulin (TUB), ubiquitin (UBI), glyceraldehyde- 3 phosphate dehydrogenase (GAPDH), elongation factor 1α (EF1α), phosphatase 2A catalytic subunit (PP2A) and GAGA binding transcriptional activator (GAGA); and two non-traditional genes: thioredoxin h1 (TRX h1) and UV-B RESISTANCE LOCUS 8 (UVR8), as candidate RGs for their potential use as reliable internal controls in leaves, stems and roots of tomato seedlings exposed to acute and chronic UV-B. The stability of these genes expression was evaluated using five statistical algorithms: geNorm, NormFinder, BestKeeper, Delta Ct and ANOVA. Considering the comprehensive stability ranking, we recommend ACT + TUB as the best pair of RGs for leaves, PP2A + GAPDH + TRX h1 for stems and TUB + UVR8 for roots. The reliability of the selected RGs for each tissue was verified amplifying tomato chalcone synthase 1 (CHS1) and cyclobutane pyrimidine dimer (CPD) photolyase (PHR1-LIKE). Under UV-B treatment, CHS1 was upregulated in leaves, stems and roots whereas PHR1-LIKE was only upregulated in leaves and stems. This interpretation differs when the most and least stable RGs are chosen. This is the first report regarding suitable RGs selection for accurate normalization of gene expression in tomato seedlings exposed to UV-B irradiation. • Nine candidate reference genes (RGs) were validated in tomato seedlings exposed to UV-B • Four conditions and three tissues were analysed with five statistical algorithms. • ACT +(TUB/PP2A/TRX h1) were the suggested RG in leaves, PP2A + GAPDH + TRX h1 in stems and TUB + UVR8 in roots. • UV-B induced CHS1 and PHR1-LIKE mRNA in leaves and stems whereas CHS1 in roots. • This work will facilitate RT-qPCR studies of UV-B exposed tomato seedlings. [ABSTRACT FROM AUTHOR]

The photoreceptor UVR8 mediates many plant responses to UV-B and short wavelength UV-A light. UVR8 functions through interactions with other proteins which lead to extensive changes in gene expression. Interactions with particular proteins determine the nature of the response to UV-B. It is therefore important to understand the molecular basis of these interactions: how are different proteins able to bind to UVR8 and how is differential binding regulated? This concise review highlights recent developments in addressing these questions. Key advances are discussed with regard to: identification of proteins that interact with UVR8; the mechanism of UVR8 accumulation in the nucleus; the photoactivation of UVR8 monomer; the structural basis of interaction between UVR8 and CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and REPRESSOR OF UV-B PHOTOMORPHOGENESIS (RUP) proteins; the role of UVR8 phosphorylation in modulating interactions and responses to UV-B. Nevertheless, much remains to be understood and the need to extend future research to the growing list of interactors is emphasised., (© The Author(s) 2024. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

Light acts catalytically to initiate a cascade of events to eventually regulate different aspects of plant development. The cascade of light signal transduction involves several components that can be broadly grouped as photoreceptors, early signaling factors, central integrators and downstream effectors. ELONGATED HYPOCOTYL 5 (HY5), a bZIP transcription factor, is the most well characterized downstream effector in Arabidopsis, which acts as a positive regulator of photomorphogenesis. HY5 acts as a master regulator and binds to promoters of close to 4000 genes, thereby, regulating wide diversity of photomorphogenic responses. It physically interacts with CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a negative regulator of photomorphogenesis. HY5-COP1 interaction mediates cross-talk among multiple pathways, thereby, enhancing the phenotypic plasticity of plants. This interaction regulates not only the upstream components of light signaling pathway like photoreceptors but also regulates a number of downstream effectors and early signaling factors, which help to regulate a number of developmental processes in plants. This review highlights the key interaction between HY5 and COP1 and how it regulates seedling photomorphogenesis, shade avoidance, circadian clock, root architecture, flowering, thermomorphogenesis, hormone signaling and stress responses, in conjunction with other signaling components and transcription factors. [ABSTRACT FROM AUTHOR]

Abstract Background Tea is the most popular nonalcoholic beverage worldwide for its pleasant characteristics and healthful properties. Catechins, theanine and caffeine are the major natural products in tea buds and leaves that determine tea qualities such as infusion colors, tastes and fragrances, as well as their health benefits. Shading is a traditional and effective practice to modify natural product accumulation and to enhance the tea quality in tea plantation. However, the mechanism underlying the shading effects is not fully understood. This study aims to explore the regulation of flavonoid biosynthesis in Camellia sinensis under shading by using both metabolomic and transcriptional analyses. Results While shading enhanced chlorophyll accumulation, major catechins, including C, EC, GC and EGC, decreased significantly in tea buds throughout the whole shading period. The reduction of catechins and flavonols were consistent with the simultaneous down-regulation of biosynthetic genes and TFs associated with flavonoid biosynthesis. Of 16 genes involved in the flavonoid biosynthetic pathway, F3’H and FLS significantly decreased throughout shading while the others (PAL, CHSs, DFR, ANS, ANR and LAR, etc.) temporally decreased in early or late shading stages. Gene co-expression cluster analysis suggested that a number of photoreceptors and potential genes involved in UV-B signal transductions (UVR8_L, HY5, COP1 and RUP1/2) showed decreasing expression patterns consistent with structural genes (F3’H, FLS, ANS, ANR, LAR, DFR and CHSs) and potential TFs (MYB4, MYB12, MYB14 and MYB111) involved in flavonoid biosynthesis, when compared with genes in the UV-A/blue and red/far-red light signal transductions. The KEGG enrichment and matrix correlation analyses also attributed the regulation of catechin biosynthesis to the UVR8-mediated signal transduction pathway. Further UV-B treatment in the controlled environment confirmed UV-B induction on flavonols and EGCG accumulation in tea leaves. Conclusions We proposed that catechin biosynthesis in C. sinensis leaves is predominantly regulated by UV through the UVR8-mediated signal transduction pathway to MYB12/MYB4 downstream effectors, to modulate flavonoid accumulation. Our study provides new insights into our understanding of regulatory mechanisms for shading-enhanced tea quality.

Summary: The photoreceptor UVR8 mediates numerous photomorphogenic responses of plants to UV‐B wavelengths by regulating transcription. Studies with purified UVR8 and seedlings not previously exposed to UV‐B have generated a model for UVR8 action in which dimeric UVR8 rapidly monomerises in response to UV‐B exposure to initiate signalling. However, the mechanism of UVR8 action in UV‐B‐acclimated plants growing under photoperiodic conditions, where UVR8 exists in a dimer/monomer photo‐equilibrium, is poorly understood.We examined UVR8 dimer/monomer status, gene expression responses, amounts of key UVR8 signalling proteins and their interactions with UVR8 in UV‐B‐acclimated Arabidopsis.We show that in UV‐B‐acclimated plants UVR8 can mediate a response to a 15‐fold increase in UV‐B without any increase in abundance of UVR8 monomer. Following transfer to elevated UV‐B, monomers show increased interaction with both COP1, to initiate signalling and RUP2, to maintain the photo‐equilibrium when the dimer/monomer cycling rate increases. Native RUP1 is present in low abundance compared with RUP2.We present a model for UVR8 action in UV‐B‐acclimated plants growing in photoperiodic conditions that incorporates dimer and monomer photoreception, dimer/monomer cycling, abundance of native COP1 and RUP proteins, and interactions of the monomer population with COP1, RUP2 and potentially other proteins. [ABSTRACT FROM AUTHOR]

The photoreceptors UV RESISTANCE LOCUS 8 (UVR8) and CRYPTOCHROMES 1 and 2 (CRYs) play major roles in the perception of UV‐B (280–315 nm) and UV‐A/blue radiation (315–500 nm), respectively. However, it is poorly understood how they function in sunlight. The roles of UVR8 and CRYs were assessed in a factorial experiment with Arabidopsis thaliana wild‐type and photoreceptor mutants exposed to sunlight for 6 or 12 hr under five types of filters with cut‐offs in UV and blue‐light regions. Transcriptome‐wide responses triggered by UV‐B and UV‐A wavelengths shorter than 350 nm (UV‐Asw) required UVR8 whereas those induced by blue and UV‐A wavelengths longer than 350 nm (UV‐Alw) required CRYs. UVR8 modulated gene expression in response to blue light while lack of CRYs drastically enhanced gene expression in response to UV‐B and UV‐Asw. These results agree with our estimates of photons absorbed by these photoreceptors in sunlight and with in vitro monomerization of UVR8 by wavelengths up to 335 nm. Motif enrichment analysis predicted complex signaling downstream of UVR8 and CRYs. Our results highlight that it is important to use UV waveband definitions specific to plants' photomorphogenesis as is routinely done in the visible region. How UVR8 and cryptochrome photoreceptors function in the natural environment remains poorly understood. We show that transcriptome responses to solar UV‐B and UV‐A below 350 nm (UVsw) are mediated by UVR8 whereas those induced by blue and UV‐A longer than 350 nm require cryptochromes. UVR8 modulates blue light induced changes in gene expression while cryptochromes antagonize solar UVsw responses mediated by UVR8. [ABSTRACT FROM AUTHOR]

Non-damaging ultraviolet B (UV-B) light promotes photomorphogenic development and stress acclimation through UV-B-specific signal transduction in Arabidopsis. UV-B irradiation induces monomerization and nuclear translocation of the UV-B photoreceptor UV RESISTANCE LOCUS 8 (UVR8). However, it is not clear how the nuclear localization of UVR8 leads to changes in global gene expression. Here, we reveal that nuclear UVR8 governs UV-B-responsive transcriptional networks in concert with several previously known transcription factors, including ELONGATED HYPOCOTYL 5 (HY5) and PHYTOCHROME INTERACTING FACTOR 4 (PIF4). Based on the transcriptomic analysis, we identify MYB13 as a novel positive regulator in UV-B-induced cotyledon expansion and stress acclimation. MYB13 is UV-B inducible and is predominantly expressed in the cotyledons. Our results demonstrate that MYB13 protein functions as a transcription factor to regulate the expression of genes involved in auxin response and flavonoid biosynthesis through direct binding with their promoters. In addition, photoactivated UVR8 interacts with MYB13 in a UV-B-dependent manner and differentially modulates the affinity of MYB13 with its targets. Taken together, our results elucidate the cooperative function of the UV-B photoreceptor UVR8 with various transcription factors in the nucleus to orchestrate the expression of specific sets of downstream genes and, ultimately, mediate plant responses to UV-B light. The nucleus-localized UVR8 photoreceptor regulates global UV-B-responsive gene expression via multiple transcription factors. MYB13 was identified as a novel UV-B-inducible transcription factor involved in the regulation of UV-B-induced cotyledon expansion and stress acclimation. UVR8 interacts with MYB13 and differentially modulates MYB13 binding with its targets. [ABSTRACT FROM AUTHOR]

Summary: Inhibition of hypocotyl growth is a well‐established UV‐B‐induced photomorphogenic response that is mediated by the UV‐B photoreceptor UV RESISTANCE LOCUS 8 (UVR8). However, the molecular mechanism by which UVR8 signaling triggers inhibition of hypocotyl growth is poorly understood. The bZIP protein ELONGATED HYPOCOTYL 5 (HY5) functions as the main positive regulatory transcription factor in the UVR8 signaling pathway, with HY5‐HOMOLOG (HYH) playing a minor role. However, here we demonstrate that hy5 hyh double mutants maintain significant UVR8‐dependent hypocotyl growth inhibition. We identify UVR8‐dependent inhibition of the activities of bHLH transcription factors PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5 as part of the UVR8 signaling pathway, which results in inhibition of hypocotyl growth. The UVR8‐mediated repression of several hypocotyl elongation‐related genes is independent of HY5 and HYH but largely associated with UVR8‐dependent degradation of PIF4 and PIF5, a process that consequently diminishes PIF4/5 target promoter occupancy. Taken together, our data indicate that UVR8‐mediated inhibition of hypocotyl growth involves degradation of PIF4 and PIF5. These findings contribute to our mechanistic understanding of UVR8‐induced photomorphogenesis and further support the function of PIFs as integrators of different photoreceptor signaling pathways. Significance Statement: This work describes a role for degradation of PIF4 and PIF5 in the core UVR8 signaling pathway associated with UV‐B‐induced gene repression and inhibition of hypocotyl growth. [ABSTRACT FROM AUTHOR]

Ultraviolet- B (UV-B) radiation exerts a dual effect on plants. While high doses induce stress, lower doses act as a signal. The sensing of UV-B triggers a photomorphogenic response, causing changes like the hypocotyl shortening. This response is strongly regulated by the UV-B photoreceptor UV RESISTANCE LOCUS 8 (UVR8). Although nitric oxide (NO) was proven to inhibit hypocotyl elongation in Arabidopsis thaliana , its role in UV-B-induced photomorphogenesis remains unclear. By using the NO probe DAF-FM DA, we found that UV-B radiation induced the accumulation of NO in the hypocotyl. This event is dependent on the presence of UVR8. Moreover, the NO donor S-nitrosoglutathione (GSNO), restored the WT phenotype when the Arabidopsis line lacking UVR8 was grown in the presence of UV-B. Also, we found that DELLA proteins were necessary for GSNO sensing in UV-B-induced photomorphogenesis. We showed that, through DELLAs, GSNO negatively controlled the levels of gibberellin (GA) bioprocessing-related genes. Therefore, a downregulation of GA signaling by NO is proposed. On the other hand, GSNO supplementation before UV-B treatment inhibited the induction of genes of the UVR8 signaling pathway: HY5 , CHS , and PHR1. This resulted in augmented DNA damage in short exposure to UV-B scenarios. Altogether, these results point to a complex regulation by NO of the UV-B-induced photomorphogenesis. • UV-B-induced photomorphogenesis requires nitric oxide (NO) accumulation. • NO is generated in hypocotyls by the UVR8 signaling pathway. • NO promotes inhibition of hypocotyl elongation by DELLA proteins. • A negative feedback of HY5 regulated UV-B response occurs at high levels of NO. [ABSTRACT FROM AUTHOR]

We examined the impact of UV-B irradiation on chloroplast movements in Arabidopsis leaves. Directional chloroplast movements induced by blue light have been described in multiple plant species. In weak light, chloroplasts accumulate at periclinal cell walls to increase light capture. In strong light, chloroplasts exhibit the avoidance response, as they move towards anticlinal walls to protect the photosynthetic apparatus from light-induced damage. In Arabidopsis, chloroplast movements are triggered by phototropins, phot1 and phot2, which are known as blue/UV-A photoreceptors. We found that irradiation with UV-B of 3.3 µmol·m−2·s−1 induced chloroplast accumulation in wild-type plants. UV-B-triggered accumulation was dependent on the presence of phototropins, especially phot1, but not on UVR8 (the canonical UV-B photoreceptor). Irradiation with strong UV-B of 20 µmol·m−2·s−1 did not induce substantial chloroplast relocations in wild-type leaves. However, in the jac1 mutant, which is defective in chloroplast accumulation, strong UV-B elicited chloroplast avoidance. This indicated that UV-B can also activate signaling to the avoidance response. To assess the possibility of indirect effects of UV-B on chloroplast movements, we examined the impact of UV-B on the actin cytoskeleton, which serves as the motile system for chloroplast movements. While irradiation with UV-B of 3.3 µmol·m−2·s−1 did not affect the actin cytoskeleton, strong UV-B disrupted its structure as shown using an Arabidopsis line expressing Lifeact-green fluorescent protein (GFP). In wild-type plants, pretreatment with strong UV-B attenuated chloroplast responses triggered by subsequent blue light irradiation, further indicating that this UV-B intensity also indirectly affects chloroplast movements. Taken together, our results suggest that the effect of UV-B on chloroplast movement is twofold: it directly induces phototropin-mediated movements; however, at higher intensities, it attenuates the movements in a nonspecific manner.

Ultraviolet-B radiation (UV-B, 280–315 nm) is an important environmental signal that regulates growth and development in plants. Two dose-dependent UV-B response pathways were described in plants: a specific one, mediated by UVR8 (the specific UV-B receptor) and an unspecific one, activated by the oxidative damage produced by radiation. The constitutively expressed receptor appears inactive as a dimer, with the two monomers dissociating upon UV-B irradiation. The monomer then interacts with COP1, an ubiquitin ligase, hindering its ability to poly-ubiquitinate transcriptional factor HY5, thus averting its degradation and activating the photomorphogenic response. HY5 induces the synthesis of proteins RUP1 and RUP2, which interact with UVR8, releasing COP1, and inducing the re-dimerization of UVR8. This mechanism has been thoroughly characterized in Arabidopsis, where studies have demonstrated that the UVR8 receptor is key in UV-B response. Although Arabidopsis importance as a model plant many mechanisms described in this specie differ in other plants. In this paper, we review the latest information regarding UV-B response mediated by UVR8 in different species, focusing on the differences reported compared to Arabidopsis. For instance, UVR8 is not only induced by UV-B but also by other agents that are expressed differentially in diverse tissues. Also, in some of the species analyzed, proteins with low homology to RUP1 and RUP2 were detected. We also discuss how UVR8 is involved in other developmental and stress processes unrelated to UV-B. We conclude that the receptor is highly versatile, showing differences among species.

The ability to manipulate expression of exogenous genes in particular regions of living organisms has profoundly transformed the way we study biomolecular processes involved in both normal development and disease. Unfortunately, most of the classical inducible systems lack fine spatial and temporal accuracy, thereby limiting the study of molecular events that strongly depend on time, duration of activation, or cellular localization. By exploiting genetically engineered photo sensing proteins that respond to specific wavelengths, we can now provide acute control of numerous molecular activities with unprecedented precision. In this review, we present a comprehensive breakdown of all of the current optogenetic systems adapted to regulate gene expression in both unicellular and multicellular organisms. We focus on the advantages and disadvantages of these different tools and discuss current and future challenges in the successful translation to more complex organisms.

We examined the impact of UV-B irradiation on chloroplast movements in Arabidopsis leaves. Directional chloroplast movements induced by blue light have been described in multiple plant species. In weak light, chloroplasts accumulate at periclinal cell walls to increase light capture. In strong light, chloroplasts exhibit the avoidance response, as they move towards anticlinal walls to protect the photosynthetic apparatus from light-induced damage. In Arabidopsis , chloroplast movements are triggered by phototropins, phot1 and phot2, which are known as blue/UV-A photoreceptors. We found that irradiation with UV-B of 3.3 µmol·m−2·s−1 induced chloroplast accumulation in wild-type plants. UV-B-triggered accumulation was dependent on the presence of phototropins, especially phot1, but not on UVR8 (the canonical UV-B photoreceptor). Irradiation with strong UV-B of 20 µmol·m−2·s−1 did not induce substantial chloroplast relocations in wild-type leaves. However, in the jac1 mutant, which is defective in chloroplast accumulation, strong UV-B elicited chloroplast avoidance. This indicated that UV-B can also activate signaling to the avoidance response. To assess the possibility of indirect effects of UV-B on chloroplast movements, we examined the impact of UV-B on the actin cytoskeleton, which serves as the motile system for chloroplast movements. While irradiation with UV-B of 3.3 µmol·m−2·s−1 did not affect the actin cytoskeleton, strong UV-B disrupted its structure as shown using an Arabidopsis line expressing Lifeact-green fluorescent protein (GFP). In wild-type plants, pretreatment with strong UV-B attenuated chloroplast responses triggered by subsequent blue light irradiation, further indicating that this UV-B intensity also indirectly affects chloroplast movements. Taken together, our results suggest that the effect of UV-B on chloroplast movement is twofold: it directly induces phototropin-mediated movements; however, at higher intensities, it attenuates the movements in a nonspecific manner. [ABSTRACT FROM AUTHOR]

Cryptochromes (CRYs) and UV RESISTANCE LOCUS 8 (UVR8) photoreceptors perceive UV-A/blue (315–500 nm) and UV-B (280–315 nm) radiation in plants, respectively. While the roles of CRYs and UVR8 have been studied in separate controlled-environment experiments, little is known about the interaction between these photoreceptors. Here, Arabidopsis wild-type L er , CRYs and UVR8 photoreceptor mutants (uvr8-2 , cry1cry2 and cry1cry2uvr8-2), and a flavonoid biosynthesis-defective mutant (tt4) were grown in a sun simulator. Plants were exposed to filtered radiation for 17 d or for 6 h, to study the effects of blue, UV-A, and UV-B radiation. Both CRYs and UVR8 independently enabled growth and survival of plants under solar levels of UV, while their joint absence was lethal under UV-B. CRYs mediated gene expression under blue light. UVR8 mediated gene expression under UV-B radiation, and in the absence of CRYs, also under UV-A. This negative regulation of UVR8-mediated gene expression by CRYs was also observed for UV-B. The accumulation of flavonoids was also consistent with this interaction between CRYs and UVR8. In conclusion, we provide evidence for an antagonistic interaction between CRYs and UVR8 and a role of UVR8 in UV-A perception. [ABSTRACT FROM AUTHOR]

Ultraviolet-B radiation (UV-B, 280–315 nm) is an important environmental signal that regulates growth and development in plants. Two dose-dependent UV-B response pathways were described in plants: a specific one, mediated by UVR8 (the specific UV-B receptor) and an unspecific one, activated by the oxidative damage produced by radiation. The constitutively expressed receptor appears inactive as a dimer, with the two monomers dissociating upon UV-B irradiation. The monomer then interacts with COP1, an ubiquitin ligase, hindering its ability to poly-ubiquitinate transcriptional factor HY5, thus averting its degradation and activating the photomorphogenic response. HY5 induces the synthesis of proteins RUP1 and RUP2, which interact with UVR8, releasing COP1, and inducing the re-dimerization of UVR8. This mechanism has been thoroughly characterized in Arabidopsis, where studies have demonstrated that the UVR8 receptor is key in UV-B response. Although Arabidopsis importance as a model plant many mechanisms described in this specie differ in other plants. In this paper, we review the latest information regarding UV-B response mediated by UVR8 in different species, focusing on the differences reported compared to Arabidopsis. For instance, UVR8 is not only induced by UV-B but also by other agents that are expressed differentially in diverse tissues. Also, in some of the species analyzed, proteins with low homology to RUP1 and RUP2 were detected. We also discuss how UVR8 is involved in other developmental and stress processes unrelated to UV-B. We conclude that the receptor is highly versatile, showing differences among species. [ABSTRACT FROM AUTHOR]

The growth–defense trade-off in plant biology has gained enormous traction in the last two decades, highlighting the importance of understanding how plants deal with two of the greatest challenges for their survival and reproduction. It has been well established that in response to competition signals perceived by informational photoreceptors, shade-intolerant plants typically activate the shade-avoidance syndrome (SAS). In turn, in response to signals of biotic attack, plants activate a suite of defense responses, many of which are directed to minimize the loss of plant tissue to the attacking agent (broadly defined, the defense syndrome, DS). We argue that components of the SAS, including increased elongation, apical dominance, reduced leaf mass per area (LMA), and allocation to roots, are in direct conflict with configurational changes that plants require to maximize defense. We hypothesize that these configurational trade-offs provide a functional explanation for the suppression of components of the DS in response to competition cues. Based on this premise, we discuss recent advances in the understanding of the mechanisms by which informational photoreceptors, by interacting with jasmonic acid (JA) signaling, help the plant to make intelligent allocation and developmental decisions that optimize its configuration in complex biotic contexts. [ABSTRACT FROM AUTHOR]

ContentsSummary1247I.Introduction1247II.The UVR8–COP1 pathway1248III.The UVR8–WRKY36 pathway1248IV.The UVR8–BES1/BIM1 pathway1249V.Other pathways1250VI.Conclusion and perspectives1250Acknowledgements1251References1251 Summary: Ultraviolet‐B (UV‐B) light is an intrinsic part of sunlight that has significant effects on plant development and acclimation responses. UVR8 (UV Resistance Locus 8) is the long sought‐after UV‐B photoreceptor that mediates UV‐B light perception and signal transduction. UV‐B irradiation induces the monomerization and nuclear accumulation of UVR8 in plant cells to activate the UV‐B signaling pathway. The photoactivated UVR8 could transduce UV‐B signal via multiple mechanisms to regulate transcription and plant growth. Here, we summarize current understanding of UVR8‐mediated UV‐B signal transduction pathways, including UVR8–COP1 (CONSTITUTIVELY PHOTOMORPHOGENIC 1) and UVR8–WRKY36 (WRKY DNA‐BINDING PROTEIN 36), UVR8–BES1 (BRI1‐EMS‐SUPPRESSOR1) and BIM1 (BES1‐INTERACTING MYC‐LIKE 1). [ABSTRACT FROM AUTHOR]

Abstract Ultraviolet-B (UV B) radiation impacts the plant behaviour in many ways, including modifying their secondary metabolism. Although several studies have quantified the UV-B effects on phenolic composition, most of them focused on leaves or investigated a limited amount of phenolics. The present work aimed to investigate the phenolic changes after two postharvest UV-B treatments, 10 and 60 min (1.39 kJ m−2 and 8.33 kJ m−2, respectively), on peach (Prunus persica cv Fairtime) fruit with a non-targeted, whole profiling approach, and targeted gene expression analysis on skin. After both UV-B exposures, peach fruit were harvested at 24 and 36 h for "phenol-omics" analysis, while additional 6 h and 12 h recovery times were used for gene expression analysis. Our results revealed that both UV-B exposures resulted in a decrease of several phenolic compounds, such as anthocyanins, after 24 h from the exposure. In contrast, the expression of the UV-B signalling components, the phenylpropanoid biosynthesis genes and their transcriptional regulators increased 6 h after the treatment, mostly with a UV-B-dose dependent behaviour, preceding an accumulation of most phenolics in both the UV-B treatments at 36 h compared to 24 h. Orthogonal projections to latent structures discriminant analysis (OPLS-DA) revealed that flavonoids, particularly anthocyanins, were the main phenolic subclasses accumulated after UV-B exposure. Highlights • UV-B exposure increased expression of flavonoid and UVR8 genes. • Accumulation of most phenolics occurred 36 h after UV-B irradiation. • 60 min-UV B-exposure determined a greater phenolic accumulation than 10 min-UV B. • Flavonoids, particularly anthocyanins, were the most UV-B-affected phenolics. • UV-B treatment determined a higher complexity level of anthocyanins after 36 h. [ABSTRACT FROM AUTHOR]

UVR8 is a plant photoreceptor protein that regulates photomorphogenic and protective responses to UV light. The inactive, homodimeric state absorbs UV-B light, resulting in dissociation into monomers, which are considered to be the active state and comprise a β-propeller core domain and intrinsically disordered N- and C-terminal tails. The C terminus is required for functional binding to signaling partner COP1. To date, however, structural studies have only been conducted with the core domain where the terminal tails have been truncated. Here, we report structural investigations of full-length UVR8 using native ion mobility mass spectrometry adapted for photoactivation. We show that, while truncated UVR8 photoconverts from a single conformation of dimers to a single monomer conformation, the full-length protein exists in numerous conformational families. The full-length dimer adopts both a compact state and an extended state where the C terminus is primed for activation. In the monomer the extended C terminus destabilizes the core domain to produce highly extended yet stable conformations, which we propose are the fully active states that bind COP1. Our results reveal the conformational diversity of full-length UVR8. We also demonstrate the potential power of native mass spectrometry to probe functionally important structural dynamics of photoreceptor proteins throughout nature. [ABSTRACT FROM AUTHOR]

Summary: Damaging UVB radiation is a major abiotic stress facing land plants. In angiosperms the UV RESISTANCE LOCUS8 (UVR8) photoreceptor coordinates UVB responses, including inducing biosynthesis of protective flavonoids. We characterised the UVB responses of Marchantia polymorpha (marchantia), the model species for the liverwort group of basal plants. Physiological, chemical and transcriptomic analyses were conducted on wild‐type marchantia exposed to three different UVB regimes. CRISPR/Cas9 was used to obtain plant lines with mutations for components of the UVB signal pathway or the flavonoid biosynthetic pathway, and transgenics overexpressing the marchantia UVR8 sequence were generated. The mutant and transgenic lines were analysed for changes in flavonoid content, their response to UVB exposure, and transcript abundance of a set of 48 genes that included components of the UVB response pathway characterised for angiosperms. The marchantia UVB response included many components in common with Arabidopsis, including production of UVB‐absorbing flavonoids, the central activator role of ELONGATED HYPOCOTYL5 (HY5), and negative feedback regulation by REPRESSOR OF UV‐B PHOTOMORPHOGENESIS1 (RUP1). Notable differences included the greater importance of CHALCONE ISOMERASE‐LIKE (CHIL). Mutants disrupted in the response pathway (hy5) or flavonoid production (chalcone isomerase, chil) were more easily damaged by UVB. Mutants (rup1) or transgenics (35S:MpMYB14) with increased flavonoid content had increased UVB tolerance. The results suggest that UVR8‐mediated flavonoid induction is a UVB tolerance character conserved across land plants and may have been an early adaptation to life on land. Significance statement: Inducible flavonoid biosynthesis regulated by the photoreceptor UVR8 is key for tolerance to UVB radiation in flowering plants, but whether this is the case for other plant groups is unclear. In this study, CRISPR/Cas9 mutagenesis is used in the model species Marchantia polymorpha to show that both the UVR8 signal transduction pathway and the importance of flavonoids for UVB tolerance are conserved in one of the most distantly related groups of plants to flowering plants, the liverworts. [ABSTRACT FROM AUTHOR]

The ability to manipulate expression of exogenous genes in particular regions of living organisms has profoundly transformed the way we study biomolecular processes involved in both normal development and disease. Unfortunately, most of the classical inducible systems lack fine spatial and temporal accuracy, thereby limiting the study of molecular events that strongly depend on time, duration of activation, or cellular localization. By exploiting genetically engineered photo sensing proteins that respond to specific wavelengths, we can now provide acute control of numerous molecular activities with unprecedented precision. In this review, we present a comprehensive breakdown of all of the current optogenetic systems adapted to regulate gene expression in both unicellular and multicellular organisms. We focus on the advantages and disadvantages of these different tools and discuss current and future challenges in the successful translation to more complex organisms. [ABSTRACT FROM AUTHOR]