UVR8 interacts with WRKY36 to regulate HY5 transcription and hypocotyl elongation in Arabidopsis

The transcription factor EjNAC5 regulates loquat fruit chilling lignification

Changes in both lignin biosynthesis and DNA methylation have been reported to be associated with chilling stress in plants. When stored at low temperatures, red-fleshed loquat is prone to lignification, with increased lignin content and fruit firmness, which has deleterious effects on taste and eating quality. Here, we found that 5 °C storage mitigated the increasing firmness and lignin content of red-fleshed 'Dahongpao' ('DHP') loquat fruit that occurred during 0 °C storage. EjNAC5 was identified by integrating RNA sequencing with whole-genome bisulfite sequencing analysis of 'DHP' loquat fruit. The transcript levels of EjNAC5 were positively correlated with changes in firmness and negatively correlated with changes in DNA methylation level of a differentially methylated region in the EjNAC5 promoter. In white-fleshed 'Baisha' ('BS') loquat fruit, which do not undergo chilling-induced lignification at 0 °C, the transcripts of EjNAC5 remained low and the methylation level of the differentially methylated region in the EjNAC5 promoter was higher, compared with 'DHP' loquat fruit. Transient overexpression of EjNAC5 in loquat fruit and stable overexpression in Arabidopsis and liverwort led to an increase in lignin content. Furthermore, EjNAC5 interacts with EjERF39 and EjHB1 and activates the transcription of Ej4CL1 and EjPRX12 genes involved in lignin biosynthesis. This regulatory network involves different transcription factors from those involved in the lignification pathway. Our study indicates that EjNAC5 promoter methylation modulates EjNAC5 transcript levels and identifies novel EjNAC5-EjERF39-Ej4CL1 and EjNAC5-EjHB1-EjPRX12 regulatory modules involved in chilling induced-lignification.

Comparative Insights into Photosynthetic, Biochemical, and Ultrastructural Mechanisms in Hibiscus and Pelargonium Plants

Understanding photosynthetic mechanisms in different plant species is crucial for advancing agricultural productivity and ecological restoration. This study presents a detailed physiological and ultrastructural comparison of photosynthetic mechanisms between Hibiscus (Hibiscus rosa-sinensis L.) and Pelargonium (Pelargonium zonale (L.) L'Hér. Ex Aiton) plants. The data collection encompassed daily photosynthetic profiles, responses to light and CO2, leaf optical properties, fluorescence data (OJIP transients), biochemical analyses, and anatomical observations. The findings reveal distinct morphological, optical, and biochemical adaptations between the two species. These adaptations were associated with differences in photochemical (AMAX, E, Ci, iWUE, and α) and carboxylative parameters (VCMAX, ΓCO2, gs, gm, Cc, and AJMAX), along with variations in fluorescence and concentrations of chlorophylls and carotenoids. Such factors modulate the efficiency of photosynthesis. Energy dissipation mechanisms, including thermal and fluorescence pathways (ΦPSII, ETR, NPQ), and JIP test-derived metrics highlighted differences in electron transport, particularly between PSII and PSI. At the ultrastructural level, Hibiscus exhibited optimised cellular and chloroplast architecture, characterised by increased chloroplast density and robust grana structures. In contrast, Pelargonium displayed suboptimal photosynthetic parameters, possibly due to reduced thylakoid counts and a higher proportion of mitochondria. In conclusion, while Hibiscus appears primed for efficient photosynthesis and energy storage, Pelargonium may prioritise alternative cellular functions, engaging in a metabolic trade-off.

Recent advances in response to environmental signals during Arabidopsis root development

Plants grow by anchoring their roots in the soil, acquiring essential water and nutrients for growth, and interacting with other signaling factors in the soil. Root systems are crucial for both the basic growth and development of plants and their response to external environmental stimuli. Under different environmental conditions, the configuration of root systems in plants can undergo significant changes, with their strength determining the plant's ability to adapt to the environment. Therefore, understanding the mechanisms by which environmental factors regulate root development is essential for crop root architecture improvement and breeding for stress resistance. This paper summarizes the research progress in genetic regulation of root development of the model plant Arabidopsis thaliana (L.) Heynh. amidst diverse environmental stimuli over the past five years. Specifically, it focuses on the regulatory networks of environmental signals, encompassing light, energy, temperature, water, nutrients, and reactive oxygen species, on root development. Furthermore, it provides prospects for the application of root architecture improvement in crop breeding for stress resistance and nutrient efficiency.

Ubiquitin-specific protease UBP14 stabilizes HY5 by deubiquitination to promote photomorphogenesis in Arabidopsis thaliana

Transcription factor ELONGATED HYPOCOTYL5 (HY5) is the central hub for seedling photomorphogenesis. E3 ubiquitin (Ub) ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) inhibits HY5 protein accumulation through ubiquitination. However, the process of HY5 deubiquitination, which antagonizes E3 ligase-mediated ubiquitination to maintain HY5 homeostasis has never been studied. Here, we identified that Arabidopsis thaliana deubiquitinating enzyme, Ub-SPECIFIC PROTEASE 14 (UBP14) physically interacts with HY5 and enhances its protein stability by deubiquitination. The da3-1 mutant lacking UBP14 function exhibited a long hypocotyl phenotype, and UBP14 deficiency led to the failure of rapid accumulation of HY5 during dark to light. In addition, UBP14 preferred to stabilize nonphosphorylated form of HY5 which is more readily bound to downstream target genes. HY5 promoted the expression and protein accumulation of UBP14 for positive feedback to facilitate photomorphogenesis. Our findings thus established a mechanism by which UBP14 stabilizes HY5 protein by deubiquitination to promote photomorphogenesis in A. thaliana.

Circadian Rhythm and Nitrogen Metabolism Participate in the Response of Boron Deficiency in the Root of Brassica napus

Boron (B) deficiency has been shown to inhibit root cell growth and division. However, the precise mechanism underlying B deficiency-mediated root tip growth inhibition remains unclear. In this study, we investigated the role of BnaA3.NIP5;1, a gene encoding a boric acid channel, in Brassica napus (B. napus). BnaA3.NIP5;1 is expressed in the lateral root cap and contributes to B acquisition in the root tip. Downregulation of BnaA3.NIP5;1 enhances B sensitivity in B. napus, resulting in reduced shoot biomass and impaired root tip development. Transcriptome analysis was conducted on root tips from wild-type B. napus (QY10) and BnaA3.NIP5;1 RNAi lines to assess the significance of B dynamics in meristematic cells during seedling growth. Differentially expressed genes (DEGs) were significantly enriched in plant circadian rhythm and nitrogen (N) metabolism pathways. Notably, the circadian-rhythm-related gene HY5 exhibited a similar B regulation pattern in Arabidopsis to that observed in B. napus. Furthermore, Arabidopsis mutants with disrupted circadian rhythm (hy5/cor27/toc1) displayed heightened sensitivity to low B compared to the wild type (Col-0). Consistent with expectations, B deficiency significantly disrupted N metabolism in B. napus roots, affecting nitrogen concentration, nitrate reductase enzyme activity, and glutamine synthesis. Interestingly, this disruption was exacerbated in BnaA3NIP5;1 RNAi lines. Overall, our findings highlight the critical role of B dynamics in root tip cells, impacting circadian rhythm and N metabolism, ultimately leading to retarded growth. This study provides novel insights into B regulation in root tip development and overall root growth in B. napus.

Genome-wide analysis of the bZIP gene family in Cinnamomum camphora ('Gantong 1') reveals the putative function in anthocyanin biosynthesis

Basic leucine zipper (bZIP) transcription factors (TFs) regulate plant development, growth, and secondary metabolism. The formation of red bark of new ornamental cultivar 'Gantong 1' is regulated mainly by anthocyanin anabolism. However, it is unclear whether and which bZIP TFs are involved in this process. We identified 89 genes encoding CcbZIP TFs in Cinnamomum camphora genome that could be divided into 14 subfamilies with similar gene structures and conserved motifs. CcbZIP38 and CcbZIP57 were highly conserved compared to HY5 in Arabidopsis thaliana and they were highly expressed in the bark and leaves of 'Gantong 1' at different stages. The target gene enrichment analysis showed that indicating indirect involvement of CcbZIP38 and CcbZIP57 in the regulation of anthocyanin synthesis. Our study contributes to understanding the molecular mechanism of anthocyanin synthesis regulation by CcbZIP TFs and provides a theoretical basis for genetic improvement of ornamental traits in C. camphora.

Environmental Control of Hypocotyl Elongation

The hypocotyl is the embryonic stem connecting the primary root to the cotyledons. Hypocotyl length varies tremendously depending on the conditions. This developmental plasticity and the simplicity of the organ explain its success as a model for growth regulation. Light and temperature are prominent growth-controlling cues, using shared signaling elements. Mechanisms controlling hypocotyl elongation in etiolated seedlings reaching the light differ from those in photoautotrophic seedlings. However, many common growth regulators intervene in both situations. Multiple photoreceptors including phytochromes, which also respond to temperature, control the activity of several transcription factors, thereby eliciting rapid transcriptional reprogramming. Hypocotyl growth often depends on sensing in green tissues and interorgan communication comprising auxin. Hypocotyl auxin, in conjunction with other hormones, determines epidermal cell elongation. Plants facing cues with opposite effects on growth control hypocotyl elongation through intricate mechanisms. We discuss the status of the field and end by highlighting open questions.

OsUVR8b, rather than OsUVR8a, plays a predominant role in rice UVR8-mediated UV-B response

UV RESISTANCE LOCUS 8 (UVR8) has been identified in Arabidopsis thaliana as the receptor mediating responses to UV-B radiation. However, UVR8-mediated UV-B signaling pathways in rice, which possesses two proteins (UVR8a and UVR8b) with high identities to AtUVR8, remain largely unknown. Here, UVR8a/b were found to be predominantly expressed in rice leaves and leaf sheaths, while the levels of UVR8b transcript and UVR8b protein were both higher than those of UVR8a. Compared to wild-type (WT) plants, uvr8b and uvr8a uvr8b rice mutants exposed to UV-B showed reduced UV-B-induced growth inhibition and upregulation of CHS and HY5 transcripts alongside UV-B acclimation. However, uvr8a mutant was similar to WT plants and exhibited changes comparable with WT. Overexpressing OsUVR8a/b enhanced UV-B-induced growth inhibition and acclimation to UV-B. UV-B was able to enhance the interaction between E3 ubiquitin ligase OsCOP1 and OsUVR8a/b, whereas the interaction of the homologous protein of Arabidopsis REPRESSOR OF UV-B PHOTOMORPHOGENESIS2 (AtRUP2) in rice with OsUVR8a/b was independent of UV-B. Additionally, OsUVR8a/b proteins were also found in the nucleus and cytoplasm even in the absence of UV-B. The abundance of OsUVR8 monomer showed an invisible change in the leaves of rice seedlings transferred from white light to that supplemented with UV-B, even though UV-B was able to weaken the interactions between OsUVR8a and OsUVR8b homo and heterodimers. Therefore, both OsUVR8a and OsUVR8b, which have different localization and response patterns compared with AtUVR8, function in the response of rice to UV-B radiation, whereas OsUVR8b plays a predominant role in this process.

Bridging the Gap: From Photoperception to the Transcription Control of Genes Related to the Production of Phenolic Compounds

Phenolic compounds are a group of secondary metabolites responsible for several processes in plants-these compounds are involved in plant-environment interactions (attraction of pollinators, repelling of herbivores, or chemotaxis of microbiota in soil), but also have antioxidative properties and are capable of binding heavy metals or screening ultraviolet radiation. Therefore, the accumulation of these compounds has to be precisely driven, which is ensured on several levels, but the most important aspect seems to be the control of the gene expression. Such transcriptional control requires the presence and activity of transcription factors (TFs) that are driven based on the current requirements of the plant. Two environmental factors mainly affect the accumulation of phenolic compounds-light and temperature. Because it is known that light perception occurs via the specialized sensors (photoreceptors) we decided to combine the biophysical knowledge about light perception in plants with the molecular biology-based knowledge about the transcription control of specific genes to bridge the gap between them. Our review offers insights into the regulation of genes related to phenolic compound production, strengthens understanding of plant responses to environmental cues, and opens avenues for manipulation of the total content and profile of phenolic compounds with potential applications in horticulture and food production.

Arabidopsis ERD15 regulated by BBX24 plays a positive role in UV-B signaling

Ultraviolet-B (UV-B, 280-315 nm) is a minor component of solar radiation, but it has a major regulatory impact on plant growth and development. Solar UV-B regulates numerous aspects of plant metabolism, morphology and physiology through altering the expression of hundreds of genes. EARLY RESPONSIVE TO DEHYDRATION 15 (ERD15) is a drought-induced rapid response gene, formerly known as a negative regulator of the abscisic acid (ABA) signaling pathway. It is unclear whether ERD15 is involved in UV-B-induced photomorphogenesis. Previously, we reported that the BBX24 transcriptional factor negatively regulated UV-B signaling. In the present study, we identified that ERD15 is involved in UV-B photomorphogenesis as a positive regulator at phenotypic, physiological and molecular levels. Our results indicated that ERD15 expression is suppressed by UV-B, inhibited the elongation of Arabidopsis hypocotyls in a UV-B-dependent manner, promoted the expression of related UV-B signaling genes and increased the total antioxidant capacity of Arabidopsis under UV-B. Genetic hybridization results show that ERD15 acts downstream of BBX24, and BBX24 protein mediated the expression of ERD15 by binding to its promoter. Thus, ERD15 is a novel positive regulator of the UV-B signaling pathway, which is downstream of BBX24 and regulated by BBX24 protein to participate in UV-B photomorphogenesis.

UVR8-TCP4-LOX2 module regulates UV-B tolerance in Arabidopsis

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.

RACK1A promotes hypocotyl elongation by scaffolding light signaling components in Arabidopsis

Plants deploy versatile scaffold proteins to intricately modulate complex cell signaling. Among these, RACK1A (Receptors for Activated C Kinase 1A) stands out as a multifaceted scaffold protein functioning as a central integrative hub for diverse signaling pathways. However, the precise mechanisms by which RACK1A orchestrates signal transduction to optimize seedling development remain largely unclear. Here, we demonstrate that RACK1A facilitates hypocotyl elongation by functioning as a flexible platform that connects multiple key components of light signaling pathways. RACK1A interacts with PHYTOCHROME INTERACTING FACTOR (PIF)3, enhances PIF3 binding to the promoter of BBX11 and down-regulates its transcription. Furthermore, RACK1A associates with ELONGATED HYPOCOTYL 5 (HY5) to repress HY5 biochemical activity toward target genes, ultimately contributing to hypocotyl elongation. In darkness, RACK1A is targeted by CONSTITUTIVELY PHOTOMORPHOGENIC (COP)1 upon phosphorylation and subjected to COP1-mediated degradation via the 26 S proteasome system. Our findings provide new insights into how plants utilize scaffold proteins to regulate hypocotyl elongation, ensuring proper skoto- and photo-morphogenic development.

A Regulatory Mechanism on Pathways: Modulating Roles of MYC2 and BBX21 in the Flavonoid Network

Genes of metabolic pathways are individually or collectively regulated, often via unclear mechanisms. The anthocyanin pathway, well known for its regulation by the MYB/bHLH/WDR (MBW) complex but less well understood in its connections to MYC2, BBX21, SPL9, PIF3, and HY5, is investigated here for its direct links to the regulators. We show that MYC2 can activate the structural genes of the anthocyanin pathway but also suppress them (except F3'H) in both Arabidopsis and Oryza when a local MBW complex is present. BBX21 or SPL9 can activate all or part of the structural genes, respectively, but the effects can be largely overwritten by the local MBW complex. HY5 primarily influences expressions of the early genes (CHS, CHI, and F3H). TF-TF relationships can be complex here: PIF3, BBX21, or SPL9 can mildly activate MYC2; MYC2 physically interacts with the bHLH (GL3) of the MBW complex and/or competes with strong actions of BBX21 to lessen a stimulus to the anthocyanin pathway. The dual role of MYC2 in regulating the anthocyanin pathway and a similar role of BBX21 in regulating BAN reveal a network-level mechanism, in which pathways are modulated locally and competing interactions between modulators may tone down strong environmental signals before they reach the network.

Phosphorylation of Arabidopsis UVR8 photoreceptor modulates protein interactions and responses to UV-B radiation

Exposure of plants to ultraviolet-B (UV-B) radiation initiates transcriptional responses that modify metabolism, physiology and development to enhance viability in sunlight. Many of these regulatory responses to UV-B radiation are mediated by the photoreceptor UV RESISTANCE LOCUS 8 (UVR8). Following photoreception, UVR8 interacts directly with multiple proteins to regulate gene expression, but the mechanisms that control differential protein binding to initiate distinct responses are unknown. Here we show that UVR8 is phosphorylated at several sites and that UV-B stimulates phosphorylation at Serine 402. Site-directed mutagenesis to mimic Serine 402 phosphorylation promotes binding of UVR8 to REPRESSOR OF UV-B PHOTOMORPHOGENESIS (RUP) proteins, which negatively regulate UVR8 action. Complementation of the uvr8 mutant with phosphonull or phosphomimetic variants suggests that phosphorylation of Serine 402 modifies UVR8 activity and promotes flavonoid biosynthesis, a key UV-B-stimulated response that enhances plant protection and crop nutritional quality. This research provides a basis to understand how UVR8 interacts differentially with effector proteins to regulate plant responses to UV-B radiation.

Root-specific photoreception directs early root development by HY5-regulated ROS balance

Root development is tightly controlled by light, and the response is thought to depend on signal transmission from the shoot. Here, we show that the root apical meristem perceives light independently from aboveground organs to activate the light-regulated transcription factor ELONGATED HYPOCOTYL5 (HY5). The ROS balance between H2O2 and superoxide anion in the root is disturbed under darkness with increased H2O2. We demonstrate that root-derived HY5 directly activates PER6 expression to eliminate H2O2. Moreover, HY5 directly represses UPBEAT1, a known inhibitor of peroxidases, to release the expression of PERs, partially contributing to the light control of ROS balance in the root. Our results reveal an unexpected ability in roots with specific photoreception and provide a mechanistic framework for the HY5-mediated interaction between light and ROS signaling in early root development.

Genome-Wide Identification of bZIP Transcription Factors in Cymbidium ensifolium and Analysis of Their Expression under Low-Temperature Stress

The basic leucine zipper (bZIP) transcription factors constitute the most widely distributed and conserved eukaryotic family. They play crucial roles in plant growth, development, and responses to both biotic and abiotic stresses, exerting strong regulatory control over the expression of downstream genes. In this study, a genome-wide characterization of the CebZIP transcription factor family was conducted using bioinformatic analysis. Various aspects, including physicochemical properties, phylogenetics, conserved structural domains, gene structures, chromosomal distribution, gene covariance relationships, promoter cis-acting elements, and gene expression patterns, were thoroughly analyzed. A total of 70 CebZIP genes were identified from the C. ensifolium genome, and they were randomly distributed across 18 chromosomes. The phylogenetic tree clustered them into 11 subfamilies, each exhibiting complex gene structures and conserved motifs arranged in a specific order. Nineteen pairs of duplicated genes were identified among the 70 CebZIP genes, with sixteen pairs affected by purifying selection. Cis-acting elements analysis revealed a plethora of regulatory elements associated with stress response, plant hormones, and plant growth and development. Transcriptome and qRT-PCR results demonstrated that the expression of CebZIP genes was universally up-regulated under low temperature conditions. However, the expression patterns varied among different members. This study provides theoretical references for identifying key bZIP genes in C. ensifolium that confer resistance to low-temperature stress, and lays the groundwork for further research into their broader biological functions.

Cooperative transcriptional regulation by ATAF1 and HY5 promotes light-induced cotyledon opening in Arabidopsis thaliana

The opening of the embryonic leaves (cotyledons) as seedlings emerge from the dark soil into the light is crucial to ensure the survival of the plant. Seedlings that sprout in the dark elongate rapidly to reach light but keep their cotyledons closed. During de-etiolation, the transition from dark to light growth, elongation slows and the cotyledons open. Here, we report that the transcription factor ACTIVATING FACTOR1 (ATAF1) participates in de-etiolation and facilitates light-induced cotyledon opening. The transition from dark to light rapidly induced ATAF1 expression and ATAF1 accumulation in cotyledons. Seedlings lacking or overexpressing ATAF1 exhibited reduced or enhanced cotyledon opening, respectively, and transcriptomic analysis indicated that ATAF1 repressed the expression of genes associated with growth and cotyledon closure. The activation of the photoreceptor phytochrome A (phyA) by far-red light induced its association with the ATAF1 promoter and stimulation of ATAF1 expression. The transcription factor ELONGATED HYPOCOTYL5 (HY5), which is also activated in response far-red light, cooperated with phyA to induce ATAF1 expression. ATAF1 and HY5 interacted with one another and cooperatively repressed the expression of growth-promoting and cotyledon closure genes. Together, our study reveals a mechanism through which far-red light promotes cotyledon opening.

Shoot-to-root communication via GmUVR8-GmSTF3 photosignaling and flavonoid biosynthesis fine-tunes soybean nodulation under UV-B light

Legume nodulation requires light perception by plant shoots and precise long-distance communication between shoot and root. Recent studies have revealed that TGACG-motif binding factors (GmSTFs) integrate light signals to promote root nodulation; however, the regulatory mechanisms underlying nodule formation in changing light conditions remain elusive. Here, we applied genetic engineering, metabolite measurement, and transcriptional analysis to study soybean (Glycine max) nodules. We clarify a fine-tuning mechanism in response to ultraviolet B (UV-B) irradiation and rhizobia infection, involving GmUVR8-dependent UV-B perception and GmSTF3/4-GmMYB12-GmCHS-mediated (iso)flavonoid biosynthesis for soybean nodule formation. GmUVR8 receptor-perceived UV-B signal triggered R2R3-MYB transcription factors GmMYB12-dependent flavonoid biosynthesis separately in shoot and root. In shoot, UV-B-triggered flavonoid biosynthesis relied on GmUVR8a, b, c receptor-dependent activation of GmMYB12L-GmCHS8 (chalcone synthase) module. In root, UV-B signaling distinctly promotes the accumulation of the isoflavones, daidzein, and its derivative coumestrol, via GmMYB12B2-GmCHS9 module, resulting in hypernodulation. The mobile transcription factors, GmSTF3/4, bind to cis-regulatory elements in the GmMYB12L, GmMYB12B2, and GmCHS9 promoters, to coordinate UV-B light perception in shoot and (iso)flavonoid biosynthesis in root. Our findings establish a novel shoot-to-root communication module involved in soybean nodulation and reveal an adaptive strategy employed by soybean roots in response to UV-B light.

WRKY transcription factors in passion fruit analysis reveals key PeWRKYs involved in abiotic stress and flavonoid biosynthesis

WRKY transcription factors (TFs) are a superfamily of regulators involved in plant responses to pathogens and abiotic stress. Passion fruit is famous for its unique flavor and nutrient-rich juice, but its growth is limited by environmental factors and pathogens. In this study, 55 WRKY genes were identified from the Passiflora edulis genome. The structure and evolutionary characteristics of PeWRKYs were analyzed using a bioinformatics approach. PeWRKYs were classified into seven subgroups (I, IIa, IIb, IIc, IId, IIe, III) according to their homologs in Arabidopsis thaliana. Group IIa PeWRKY48 gene was highly up-regulated under cold stress by RNA expression analysis, and transgenic PeWRKY48 in yeast and Arabidopsis showed resistance exposure to cold, salt, and drought stress. Metabolome and transcriptome co-expression analysis of two different disease resistance genotypes of P. edulis identified PeWRKY30 as a key TF co-expressed with flavonoid accumulation in yellow fruit P. edulis, which may contribute to biotic or abiotic resistance. The qRT-PCR verified the expression of key genes in different tissues of P. edulis and in different species of Passiflora. This study provides a set of WRKY candidate genes that will facilitate the genetic improvement of disease and abiotic tolerance in passion fruit.

Genome-wide characterization of regulator of chromosome condensation 1 (RCC1) gene family in Artemisia annua L. revealed a conservation evolutionary pattern

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.

UV-B responses in the spotlight: Dynamic photoreceptor interplay and cell-type specificity

Plants are constantly exposed to a multitude of external signals, including light. The information contained within the full spectrum of light is perceived by a battery of photoreceptors, each with specific and shared signalling outputs. Recently, it has become clear that UV-B radiation is a vital component of the electromagnetic spectrum, guiding growth and being crucial for plant fitness. However, given the large overlap between UV-B specific signalling pathways and other photoreceptors, understanding how plants can distinguish UV-B specific signals from other light components deserves more scrutiny. With recent evidence, we propose that UV-B signalling and other light signalling pathways occur within distinct tissues and cell-types and that the contribution of each pathway depends on the type of response and the developmental stage of the plant. Elucidating the precise site(s) of action of each molecular player within these signalling pathways is key to fully understand how plants are able to orchestrate coordinated responses to light within the whole plant body. Focusing our efforts on the molecular study of light signal interactions to understand plant growth in natural environments in a cell-type specific manner will be a next step in the field of photobiology.

Genome-Wide Identification and Expression Analysis of bZIP Family Genes in Stevia rebaudiana

The basic (region) leucine zippers (bZIPs) are evolutionarily conserved transcription factors widely distributed in eukaryotic organisms. In plants, they are not only involved in growth and development, defense and stress responses and regulation of physiological processes but also play a pivotal role in regulating secondary metabolism. To explore the function related to the bZIP gene family in Stevia rebaudiana Bertoni, we identified 105 SrbZIP genes at the genome-wide level and classified them into 12 subfamilies using bioinformation methods. Three main classes of cis-acting elements were found in the SrbZIP promoter regions, including development-related elements, defense and stress-responsive elements and phytohormone-responsive elements. Through protein-protein interaction network of 105 SrbZIP proteins, SrbZIP proteins were mainly classified into four major categories: ABF2/ABF4/ABI5 (SrbZIP51/SrbZIP38/SrbZIP7), involved in phytohormone signaling, GBF1/GBF3/GBF4 (SrbZIP29/SrbZIP63/SrbZIP60) involved in environmental signaling, AREB3 (SrbZIP88), PAN (SrbZIP12), TGA1 (SrbZIP69), TGA4 (SrbZIP82), TGA7 (SrbZIP31), TGA9 (SrbZIP95), TGA10 (SrbZIP79) and HY5 (SrbZIP96) involved in cryptochrome signaling, and FD (SrbZIP72) promoted flowering. The transcriptomic data showed that SrbZIP genes were differentially expressed in six S. rebaudiana cultivars ('023', '110', 'B1188', '11-14', 'GP' and 'GX'). Moreover, the expression levels of selected 15 SrbZIP genes in response to light, abiotic stress (low temperature, salt and drought), phytohormones (methyl jasmonate, gibberellic acid and salicylic acid) treatment and in different tissues were analyzed utilizing qRT-PCR. Some SrbZIP genes were further identified to be highly induced by factors affecting glycoside synthesis. Among them, three SrbZIP genes (SrbZIP54, SrbZIP63 and SrbZIP32) were predicted to be related to stress-responsive terpenoid synthesis in S. rebaudiana. The protein-protein interaction network expanded the potential functions of SrbZIP genes. This study firstly provided the comprehensive genome-wide report of the SrbZIP gene family, laying a foundation for further research on the evolution, function and regulatory role of the bZIP gene family in terpenoid synthesis in S. rebaudiana.

Light signaling as cellular integrator of multiple environmental cues in plants

Plants being sessile need to rapidly adapt to the constantly changing environment through modifications in their internal clock, metabolism, and gene expression. They have evolved an intricate system to perceive and transfer the signals from the primary environmental factors namely light, temperature and water to regulate their growth development and survival. Over past few decades rigorous research using molecular genetics approaches, especially in model plant Arabidopsis, has resulted in substantial progress in discovering various photoreceptor systems and light signaling components. In parallel several molecular pathways operating in response to other environmental cues have also been elucidated. Interestingly, the studies have shown that expression profiles of genes involved in photomorphogenesis can undergo modulation in response to other cues from the environment. Recently, the photoreceptor, PHYB, has been shown to function as a thermosensor. Downstream components of light signaling pathway like COP1 and PIF have also emerged as integrating hubs for various kinds of signals. All these findings indicate that light signaling components may act as central integrator of various environmental cues to regulate plant growth and development processes. In this review, we present a perspective on cross talk of signaling mechanisms induced in response to myriad array of signals and their integration with the light signaling components. By putting light signals on the central stage, we propose the possibilities of enhancing plant resilience to the changing environment by fine-tuning the genetic manipulation of its signaling components in the future.

Photobiotechnology for abiotic stress resilient crops: Recent advances and prospects

Massive crop failures worldwide are caused by abiotic stress. In plants, adverse environmental conditions cause extensive damage to the overall physiology and agronomic yield at various levels. Phytochromes are photosensory phosphoproteins that absorb red (R)/far red (FR) light and play critical roles in different physiological and biochemical responses to light. Considering the role of phytochrome in essential plant developmental processes, genetically manipulating its expression offers a promising approach to crop improvement. Through modulated phytochrome-mediated signalling pathways, plants can become more resistant to environmental stresses by increasing photosynthetic efficiency, antioxidant activity, and expression of genes associated with stress resistance. Plant growth and development in adverse environments can be improved by understanding the roles of phytochromes in stress tolerance characteristics. A comprehensive overview of recent findings regarding the role of phytochromes in modulating abiotic stress by discussing biochemical and molecular aspects of these mechanisms of photoreceptors is offered in this review.

Functions of Plant Phytochrome Signaling Pathways in Adaptation to Diverse Stresses

Phytochromes are receptors for red light (R)/far-red light (FR), which are not only involved in regulating the growth and development of plants but also in mediated resistance to various stresses. Studies have revealed that phytochrome signaling pathways play a crucial role in enabling plants to cope with abiotic stresses such as high/low temperatures, drought, high-intensity light, and salinity. Phytochromes and their components in light signaling pathways can also respond to biotic stresses caused by insect pests and microbial pathogens, thereby inducing plant resistance against them. Given that, this paper reviews recent advances in understanding the mechanisms of action of phytochromes in plant resistance to adversity and discusses the importance of modulating the genes involved in phytochrome signaling pathways to coordinate plant growth, development, and stress responses.

Functions of WRKYs in plant growth and development

As sessile organisms, plants must overcome various stresses. Accordingly, they have evolved several plant-specific growth and developmental processes. These plant processes may be related to the evolution of plant-specific protein families. The WRKY transcription factors originated in eukaryotes and expanded in plants, but are not present in animals. Over the past two decades, there have been many studies on WRKYs in plants, with much of the research concentrated on their roles in stress responses. Nevertheless, recent findings have revealed that WRKYs are also required for seed dormancy and germination, postembryonic morphogenesis, flowering, gametophyte development, and seed production. Thus, WRKYs may be important for plant adaptations to a sessile lifestyle because they simultaneously regulate stress resistance and plant-specific growth and development.

Inhibition of SIZ1-mediated SUMOylation of HOOKLESS1 promotes light-induced apical hook opening in Arabidopsis

The apical hook protects cotyledons and the shoot apical meristem from mechanical injuries during seedling emergence from the soil. HOOKLESS1 (HLS1) is a central regulator of apical hook development, as a terminal signal onto which several pathways converge. However, how plants regulate the rapid opening of the apical hook in response to light by modulating HLS1 function remains unclear. In this study, we demonstrate that the small ubiquitin-like modifier (SUMO) E3 ligase SAP AND MIZ1 DOMAIN-CONTAINING LIGASE1 (SIZ1) interacts with HLS1 and mediates its SUMOylation in Arabidopsis thaliana. Mutating SUMO attachment sites of HLS1 results in impaired function of HLS1, indicating that HLS1 SUMOylation is essential for its function. SUMOylated HLS1 was more likely to assemble into oligomers, which are the active form of HLS1. During the dark-to-light transition, light induces rapid apical hook opening, concomitantly with a drop in SIZ1 transcript levels, resulting in lower HLS1 SUMOylation. Furthermore, ELONGATED HYPOCOTYL5 (HY5) directly binds to the SIZ1 promoter and suppresses its transcription. HY5-initiated rapid apical hook opening partially depended on HY5 inhibition of SIZ1 expression. Taken together, our study identifies a function for SIZ1 in apical hook development, providing a dynamic regulatory mechanism linking the post-translational modification of HLS1 during apical hook formation and light-induced apical hook opening.

Genome-Wide Identification and Characterization of WRKY Transcription Factors and Their Expression Profile in Loropetalum chinense var. rubrum

The WRKY gene family plays important roles in plant growth and development, as well as in the responses to biotic and abiotic stresses. Loropetalum chinense var. rubrum has high ornamental and medicinal value. However, few WRKY genes have been reported in this plant, and their functions remain unknown. To explore the roles that the WRKY genes play in L. chinense var. rubrum, we identified and characterized 79 LcWRKYs through BLAST homology analysis and renamed them (as LcWRKY1-79) based on their distribution on the chromosomes of L. chinense var. rubrum. In this way, according to their structural characteristics and phylogenetic analysis, they were divided into three groups containing 16 (Group I), 52 (Group II), and 11 (Group III) WRKYs, respectively. LcWRKYs in the same group have similar motifs and gene structures; for instance, Motifs 1, 2, 3, 4, and 10 constitute the WRKY domain and zinc-finger structure. The LcWRKY promoter region contains light response elements (ACE, G-box), stress response elements (TC-rich repeats), hormone response elements (TATC-box, TCA-element), and MYB binding sites (MBS, MBSI). Synteny analysis of LcWRKYs allowed us to establish orthologous relationships among the WRKY gene families of Arabidopsis thaliana, Oryza sativa, Solanum lycopersicum L., Vitis vinifera L., Oryza sativa L., and Zea mays L.; furthermore, analysis of the transcriptomes of mature leaves and flowers from different cultivars demonstrated the cultivar-specific LcWRKY gene expression. The expression levels of certain LcWRKY genes also presented responsive changes from young to mature leaves, based on an analysis of the transcriptome in leaves at different developmental stages. White light treatment led to a significant decrease in the expression of LcWRKY6, 18, 24, 34, 36, 44, 48, 61, 62, and 77 and a significant increase in the expression of LcWRKY41, blue light treatment led to a significant decrease in the expression of LcWRKY18, 34, 50, and 77 and a significant increase in the expression of LcWRKY36 and 48. These results enable a better understanding of LcWRKYs, facilitating the further exploration of their genetic functions and the molecular breeding of L. chinense var. rubrum.

Maize WRKY28 interacts with the DELLA protein D8 to affect skotomorphogenesis and participates in the regulation of shade avoidance and plant architecture

Competition for light from neighboring vegetation can trigger the shade-avoidance response (SAR) in plants, which is detrimental to their yield. The molecular mechanisms regulating SAR are well established in Arabidopsis, and some regulators of skotomorphogenesis have been found to be involved in the regulation of the SAR and plant architecture. However, the role of WRKY transcription factors in this process has rarely been reported, especially in maize (Zea mays). Here, we report that maize Zmwrky28 mutants exhibit shorter mesocotyls in etiolated seedlings. Molecular and biochemical analyses demonstrate that ZmWRKY28 directly binds to the promoter regions of the Small Auxin Up RNA (SAUR) gene ZmSAUR54 and the Phytochrome-Interacting Factor (PIF) gene ZmPIF4.1 to activate their expression. In addition, the maize DELLA protein Dwarf Plant8 (D8) interacts with ZmWRKY28 in the nucleus to inhibit its transcriptional activation activity. We also show that ZmWRKY28 participates in the regulation of the SAR, plant height, and leaf rolling and erectness in maize. Taken together, our results reveal that ZmWRKY28 is involved in GA-mediated skotomorphogenic development and can be used as a potential target to regulate SAR for breeding of high-density-tolerant cultivars.

PIF8-WRKY42-mediated salicylic acid synthesis modulates red light induced powdery mildew resistance in oriental melon

Light signals and plant hormones are involved in regulating the growth, development and stress resistance of plants; however, it remains unclear whether light affects hormones and thus pathogen resistance in oriental melon. Here, we found that red light promoted salicylic acid (SA) accumulation and powdery mildew resistance by activating the transcription of CmICS, the key gene for SA biosynthesis, and silencing CmICS seriously weakened the induction effect of red light on powdery mildew resistance in oriental melon leaves. Further studies showed that red light induced the expression of CmWRKY42 under powdery mildew stress, and CmWRKY42 directly bound to the CmICS promoter to activate its expression and promote the accumulation of SA under red light. Furthermore, we found that PHYTOCHROME INTERACTING FACTOR 8 (PIF8), as a negative regulator of SA biosynthesis, inhibits CmWRKY42 transcriptional activation by binding to the CmWRKY42 promoter, and thus inhibits transcriptional activation of CmICS by CmWRKY42. Also, CmPIF8 binds to the CmICS promoter and directly inhibits its transcription. In conclusion, our study revealed a new molecular mechanism of the relationship between red light-SA-powdery mildew resistance and provided a theoretical basis for resistance breeding of oriental melon.

Green means go: Green light promotes hypocotyl elongation via brassinosteroid signaling

Although many studies have elucidated the mechanisms by which different wavelengths of light (blue, red, far-red, or ultraviolet-B [UV-B]) regulate plant development, whether and how green light regulates plant development remains largely unknown. Previous studies reported that green light participates in regulating growth and development in land plants, but these studies have reported conflicting results, likely due to technical problems. For example, commercial green light-emitting diode light sources emit a little blue or red light. Here, using a pure green light source, we determined that unlike blue, red, far-red, or UV-B light, which inhibits hypocotyl elongation, green light promotes hypocotyl elongation in Arabidopsis thaliana and several other plants during the first 2-3 d after planting. Phytochromes, cryptochromes, and other known photoreceptors do not mediate green-light-promoted hypocotyl elongation, but the brassinosteroid (BR) signaling pathway is involved in this process. Green light promotes the DNA binding activity of BRI1-EMS-SUPPRESSOR 1 (BES1), a master transcription factor of the BR pathway, thus regulating gene transcription to promote hypocotyl elongation. Our results indicate that pure green light promotes elongation via BR signaling and acts as a shade signal to enable plants to adapt their development to a green-light-dominant environment under a canopy.

A transcription factor WRKY36 interacts with AFP2 to break primary seed dormancy by progressively silencing DOG1 in Arabidopsis

The phytohormones abscisic acid (ABA) and gibberellic acid (GA) antagonistically control the shift between seed dormancy and its alleviation. DELAY OF GERMINATION1 (DOG1) is a critical regulator that determines the intensity of primary seed dormancy, but its underlying regulatory mechanism is unclear. In this study, we combined physiological, biochemical, and genetic approaches to reveal that a bHLH transcriptional factor WRKY36 progressively silenced DOG1 expression to break seed dormancy through ABI5-BINDING PROTEIN 2 (AFP2) as the negative regulator of ABA signal. AFP2 interacted with WRKY36, which recognizes the W-BOX in the DOG1 promoter to suppress its expression; Overexpressing WRKY36 broke primary seed dormancy, whereas wrky36 mutants showed strong primary seed dormancy. In addition, AFP2 recruited the transcriptional corepressor TOPLESS-RELATED PROTEIN2 (TPR2) to reduce histone acetylation at the DOG1 locus, ultimately mediating WRKY36-dependent inhibition of DOG1 expression to break primary seed dormancy. Our result proposes that the WRKY36-AFP2-TPR2 module progressively silences DOG1 expression epigenetically, thereby fine-tuning primary seed dormancy.

WRKY transcription factors: evolution, regulation, and functional diversity in plants

The recent advancements in sequencing technologies and informatic tools promoted a paradigm shift to decipher the hidden biological mysteries and transformed the biological issues into digital data to express both qualitative and quantitative forms. The transcriptomic approach, in particular, has added new dimensions to the versatile essence of plant genomics through the large and deep transcripts generated in the process. This has enabled the mining of super families from the sequenced plants, both model and non-model, understanding their ancestry, diversity, and evolution. The elucidation of the crystal structure of the WRKY proteins and recent advancement in computational prediction through homology modeling and molecular dynamic simulation has provided an insight into the DNA-protein complex formation, stability, and interaction, thereby giving a new dimension in understanding the WRKY regulation. The present review summarizes the functional aspects of the high volume of sequence data of WRKY transcription factors studied from different species, till date. The review focuses on the dynamics of structural classification and lineage in light of the recent information. Additionally, a comparative analysis approach was incorporated to understand the functions of the identified WRKY transcription factors subjected to abiotic (heat, cold, salinity, senescence, dark, wounding, UV, and carbon starvation) stresses as revealed through various sets of studies on different plant species. The review will be instrumental in understanding the events of evolution and the importance of WRKY TFs under the threat of climate change, considering the new scientific evidences to propose a fresh perspective.

Insight into the bZIP gene family in Lagenaria siceraria: Genome and transcriptome analysis to understand gene diversification in Cucurbitaceae and the roles of LsbZIP gene expression and function under cold stress

The basic leucine zipper (bZIP) as a well-known transcription factor family, figures prominently in diverse biological and developmental processes and response to abiotic/biotic stresses. However, no knowledge of the bZIP family is available for the important edible Cucurbitaceae crop bottle gourd. Herein, we identified 65 putative LsbZIP genes and characterized their gene structure, phylogenetic and orthologous relationships, gene expression profiles in different tissues and cultivars, and responsive genes under cold stress. The phylogenetic tree of 16 released Cucurbitaceae plant genomes revealed the evolutionary convergence and divergence of bZIP family. Based on the specific domains, LsbZIP family were classified into 12 clades (A-K, S) with similar motifs and exon-intron distribution. 65 LsbZIP genes have undergone 19 segmental and two tandem duplication events with purifying selection. The expression profiling of LsbZIP genes showed tissue-specific but no cultivar-specific pattern. The cold stress-responsive candidate LsbZIP genes were analyzed and validated by RNA-Seq and RT-PCR, providing new insights of transcriptional regulation of bZIP family genes in bottle gourd and their potential functions in cold-tolerant variety breeding.

Dissecting the functions of COP1 in the UVR8 pathway with a COP1 variant in Arabidopsis

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, COP1^C509S , 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-COP1^C509S . 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-COP1^C509S , 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.

RNAseq-Based Working Model for Transcriptional Regulation of Crosstalk between Simultaneous Abiotic UV-B and Biotic Stresses in Plants

Plants adjust their secondary metabolism by altering the expression of corresponding genes to cope with both abiotic and biotic stresses. In the case of UV-B radiation, plants produce protective flavonoids; however, this reaction is impeded during pattern-triggered immunity (PTI) induced by pathogens. Pathogen attack can be mimicked by the application of microbial associated molecular patterns (e.g., flg22) to study crosstalk between PTI and UV-B-induced signaling pathways. Switching from Arabidopsis cell cultures to in planta studies, we analyzed whole transcriptome changes to gain a deeper insight into crosstalk regulation. We performed a comparative transcriptomic analysis by RNAseq with four distinct mRNA libraries and identified 10778, 13620, and 11294 genes, which were differentially expressed after flg22, UV-B, and stress co-treatment, respectively. Focusing on genes being either co-regulated with the UV-B inducible marker gene chalcone synthase CHS or the flg22 inducible marker gene FRK1 identified a large set of transcription factors from diverse families, such as MYB, WRKY, or NAC. These data provide a global view of transcriptomic reprogramming during this crosstalk and constitute a valuable dataset for further deciphering the underlying regulatory mechanism(s), which appear to be much more complex than previously anticipated. The possible involvement of MBW complexes in this context is discussed.

RUP2 facilitates UVR8 redimerization via two interfaces

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-UVR8^W285A complex (2.0 Å). RUP2 and UVR8^W285A 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.

Plant responses to UV-B radiation: signaling, acclimation and stress tolerance

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.

Activation of anthocyanin biosynthesis in high light - what is the initial signal?

Due to their sessile nature, plants cannot escape adverse environmental conditions and evolved mechanisms to cope with sudden environmental changes. The reaction to variations in abiotic factors, also summarized as acclimation response, affects all layers of cellular functions and involves rapid modification of enzymatic activities, the metabolome, proteome and transcriptome on different timescales. One trait of plants acclimating to high light (HL) is the rapid transcriptional activation of the flavonoid biosynthesis (FB) pathway resulting in the accumulation of photoprotective and antioxidative flavonoids, such as flavonols and anthocyanins, in the leaf tissue. Although enormous progress has been made in identifying enzymes and transcriptional regulators of FB by forward and reverse genetic approaches in the past, the signals and signalling pathways permitting the conditional activation of FB in HL are still debated. With this Tansley Insight, we summarize the current knowledge on the proposed signals and downstream factors involved in regulating FB and will discuss their contribution to, particularly, HL-induced accumulation of anthocyanins.

Mechanisms of UV-B light-induced photoreceptor UVR8 nuclear localization dynamics

Light regulates the subcellular localization of plant photoreceptors, a key step in light signaling. Ultraviolet-B radiation (UV-B) induces the plant photoreceptor UV RESISTANCE LOCUS 8 (UVR8) nuclear accumulation, where it regulates photomorphogenesis. However, the molecular mechanism for the UV-B-regulated UVR8 nuclear localization dynamics is unknown. With fluorescence recovery after photobleaching (FRAP), cell fractionation followed by immunoblotting and co-immunoprecipitation (Co-IP) assays we tested the function of UVR8-interacting proteins including CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1), REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1 (RUP1) and RUP2 in the regulation of UVR8 nuclear dynamics in Arabidopsis thaliana. We showed that UV-B-induced rapid UVR8 nuclear translocation is independent of COP1, which previously was shown to be required for UV-B-induced UVR8 nuclear accumulation. Instead, we provide evidence that the UV-B-induced UVR8 homodimer-to-monomer photo-switch and the concurrent size reduction of UVR8 enables its monomer nuclear translocation, most likely via free diffusion. Nuclear COP1 interacts with UV-B-activated UVR8 monomer, thereby promoting UVR8 nuclear retention. Conversely, RUP1and RUP2, whose expressions are induced by UV-B, inhibit UVR8 nuclear retention via attenuating the UVR8-COP1 interaction, allowing UVR8 to exit the nucleus. Collectively, our data suggest that UV-B-induced monomerization of UVR8 promotes its nuclear translocation via free diffusion. In the nucleus, COP1 binding promotes UVR8 monomer nuclear retention, which is counterbalanced by the major negative regulators RUP1 and RUP2.

Transcription factor ZmWRKY20 interacts with ZmWRKY115 to repress expression of ZmbZIP111 for salt tolerance in maize

Maize (Zea mays) is an important cereal crop worldwide. However, its yield and quality are adversely affected by salt stress resulting from soil hypersalinity. Exploring the regulatory mechanisms of stress responses is of vital importance to increase maize seed production. In the present study, we screened ethyl methanesulfonate-induced maize mutants and identified a salt-tolerant mutant. A single base was mutated in ZmWRKY20, leading to the formation of a truncated protein variant. A detailed phenotypic analysis revealed that this mutant had significantly higher resistance to wilting and lower reactive oxygen species levels than the inbred line B73. ZmWRKY20 showed transcriptional activity in yeast and specifically bound W-boxes according to the results of our yeast one-hybrid, electrophoretic mobility shift, and dual-luciferase assays. Overexpression of ZmWRKY20 decreased salt tolerance in maize. Transcriptome profiling revealed that ZmWRKY20 overexpression extensively reprogrammed genes involved in regulating defense and oxidation-reduction responses. The results substantiate that ZmWRKY20 is directly targeted to the basic leucine zipper (bZIP) motif in the transcription factor ZmbZIP111. It was also verified that ZmWRKY20 interacts with ZmWRKY115 and both proteins act jointly to enhance ZmbZIP111 repression. The results indicate that the ZmWRKY20 and ZmWRKY115 transcription factors interact in the nucleus, leading to repression of ZmbZIP111 expression by directly binding its promoter, and increase the sensitivity of maize seedlings to salt stress. The current study improves our understanding of the complicated responses of maize to salt stress.

Effect of enhanced UV-B radiation on growth and photosynthetic physiology of Iris tectorum maxim

Iris tectorum Maxim. is an important plant that plays a very crucial role in the ecological welfare of wetlands. In this study, the effects of different intensities of UV-B radiation on the growth, photosynthetic pigment content, chlorophyll fluorescence characteristics, chloroplast ultrastructure, and gas exchange parameters of Iris tectorum Maxim. were studied. The results showed that enhanced UV-B radiation had a significant influence on the above-mentioned parameters of iris. Compared with the control, enhanced UV-B radiation caused certain damage to the leaf appearance. With the increasing intensity of radiation, the apparent damage degree became more serious. Enhanced UV-B radiation significantly decreased leaf chlorophyll contents, and the effect accumulated with the exposure time. Enhanced UV-B radiation increased Fo, significantly increased the non-photochemical quenching coefficient NPQ, reduced PSII and Qp, and significantly decreased the Fm, Fv/Fm, and Fv/Fo in leaves. The effect of UV-B radiation on PSII destruction of Iris tectorum Maxim. increased as the radiation intensity increased and the exposure time prolonged. The chloroplast structure was damaged under the enhanced UV-B radiation. More specifically, thylakoid lamellae were distorted, swelling and even blurred, and a large number of starch granules appeared. The effect of the high intensity of radiation on chloroplast ultrastructure was greater than that of lower intensity. Enhanced UV-B radiation reduced significantly the net photosynthetic rate, stomatal conductance, and transpiration rate, and the degree of degradation increased with the increasing irradiation intensity. However, the intercellular CO₂ content increased, which suggests that the main reason for the decrease of photosynthetic rate was the non-stomatal factors.

A seed-specific transcription factor, HSFA9, anticipates UV-B light responses by mimicking the activation of the UV-B receptor in tobacco

Sunflower heat shock factor A9 (HSFA9, hereafter A9) is a transcription factor involved in seed desiccation tolerance and longevity. A9 also links the regulation of seed maturation with that of seedling photomorphogenesis through visible light receptors. Analyses in transgenic Nicotiana tabacum (tobacco) indicated that A9 also affects responses mediated by NtUVR8, the receptor of ultraviolet light B (UV-B). We compared the effects of A9 and UV-B illumination on the nuclear localization of GFP-NtUVR8 in Nicotiana benthamiana leaves. We also used co-immunoprecipitation and limited proteolysis for analyzing the interaction between A9 and NtUVR8. We found that A9, by binding to NtUVR8, induced structural changes that resulted in enhancing the nuclear localization of NtUVR8 by hindering its nuclear export. The localization of UVR8 is crucial for receptor activation and function in Arabidopsis, where UV-B-activated nuclear UVR8 binds the E3 ubiquitin ligase COP1, leading to enhanced UV-B responses and photoprotection. A9 similarly activated NtUVR8 by enhancing COP1 binding without UV-B light. Seedlings and dark-germinated seeds that overexpress A9 showed primed UV-B light stress protection. Our results unveil a UV-B-independent activation mechanism and a role for UVR8 in plant seeds that might contribute to early stress protection, facilitating seedling establishment.

Integrative analysis of transcriptome and metabolome reveals flavonoid biosynthesis regulation in Rhododendron pulchrum petals

BACKGROUND

Color is the major ornamental feature of the Rhododendron genus, and it is related to the contents of flavonoid in petals. However, the regulatory mechanism of flavonoid biosynthesis in Rhododendron pulchrum remains unknown. The transcriptome and metabolome analysis of Rhododendron pulchrum with white, pink and purple color in this study aimed to reveal the mechanism of flavonoid biosynthesis and to provide insight for improving the petal color.

RESULTS

Flavonoids and flavonols are the major components of flavonoid metabolites in R.pulchrum, such as laricitrin, apigenin, tricin, luteolin, isoorientin, isoscutellarein, diosmetin and their glycosides derivatives. With transcriptome and metabolome analysis, we found CHS, FLS, F3'H, F3'5'H, DFR, ANS, GT, FNS, IFR and FAOMT genes showed significantly differential expression in cultivar 'Zihe'. FNS and IFR were discovered to be associated with coloration in R.pulchrum for the first time. The FNS gene existed in the form of FNSI. The IFR gene and its related metabolites of medicarpin derivatives were highly expressed in purple petal. In cultivar 'Fenhe', up-regulation of F3'H and F3'5'H and down-regulation of 4CL, DFR, ANS, and GT were associated with pink coloration. With the transcription factor analysis, a subfamily of DREBs was found to be specifically enriched in pink petals. This suggested that the DREB family play an important role in pink coloration. In cultivars 'Baihe', flavonoid biosynthesis was inhibited by low expression of CHS, while pigment accumulation was inhibited by low expression of F3'5'H, DFR, and GT, which led to a white coloration.

CONCLUSIONS

By analyzing the transcriptome and metabolome of R.pulchrum, principal differential expression genes and metabolites of flavonoid biosynthesis pathway were identified. Many novel metabolites, genes, and transcription factors associated with coloration have been discovered. To reveal the mechanism of the coloration of different petals, a model of the flavonoid biosynthesis pathway of R.pulchrum was constructed. These results provide in depth information regarding the coloration of the petals and the flavonoid metabolism of R.pulcherum. The study of transcriptome and metabolome profiling gains insight for further genetic improvement in Rhododendron.

Transcription factors BBX11 and HY5 interdependently regulate the molecular and metabolic responses to UV-B

UV-B radiation acts as a developmental cue and a stress factor for plants, depending on dose. Activation of the transcription factor ELONGATED HYPOCOTYL 5 (HY5) in a UV RESISTANCE LOCUS 8 (UVR8)-dependent manner leads to the induction of a broad set of genes under UV-B. However, the underlying molecular mechanisms regulating this process are less understood. Here, we use molecular, biochemical, genetic, and metabolomic tools to identify the B-BOX transcription factor B-BOX PROTEIN 11 (BBX11) as a component of the molecular response to UV-B in Arabidopsis (Arabidopsis thaliana). BBX11 expression is induced by UV-B in a dose-dependent manner. Under low UV-B, BBX11 regulates hypocotyl growth suppression, whereas it protects plants exposed to high UV-B radiation by promoting the accumulation of photo-protective phenolics and antioxidants, and inducing DNA repair genes. Our genetic studies indicate that BBX11 regulates hypocotyl elongation under UV-B partially dependent on HY5. Overexpression of BBX11 can partially rescue the high UV-B sensitivity of hy5, suggesting that HY5-mediated UV-B stress tolerance is partially dependent on BBX11. HY5 regulates the UV-B-mediated induction of BBX11 by directly binding to its promoter. BBX11 reciprocally regulates the mRNA and protein levels of HY5. We report here the role of a BBX11-HY5 feedback loop in regulating photomorphogenesis and stress tolerance under UV-B.

Cysteines have a role in conformation of the UVR8 photoreceptor

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. UVR8^C231S,C335S mutant protein forms dimers and monomerizes similarly to wild-type UVR8. UVR8^C231S,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 UVR8^C231S,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 UVR8^C231S,C335S . Fluorescence resonance energy transfer analysis indicates that UVR8^C231S,C335S has an altered conformation in plants, in that the N- and C-termini appear closer together, which may explain the altered protein interactions.

Arabidopsis B-box transcription factors BBX20-22 promote UVR8 photoreceptor-mediated UV-B responses

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.

A missense mutation in WRKY32 converts its function from a positive regulator to a repressor of photomorphogenesis

CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) mediates various cellular and physiological processes in plants by targeting a large number of substrates for ubiquitination and degradation. In this study, we reveal that a substitution of Pro for Leu at amino acid position 409 in WRKY32 largely suppresses the short hypocotyls and expanded cotyledon phenotypes of cop1-6. WRKY32^P409L promotes hypocotyl growth and inhibits the opening of cotyledons in Arabidopsis. Loss of WRKY32 function mutant seedlings display elongated hypocotyls, whereas overexpression of WRKY32 leads to shortened hypocotyls. WRKY32 directly associates with the promoter regions of HY5 to activate its transcription. COP1 interacts with and targets WRKY32 for ubiquitination and degradation in darkness. WRKY32^P409L exhibits enhanced DNA binding ability and affects the expression of more genes compared with WRKY32 in Arabidopsis. Our results not only reveal the basic role for WRKY32 in promoting photomorphogenesis, but also provide insights into manipulating plant growth by engineering key components of light signaling.

Parental UV-B radiation regulates the habitat selection of clonal Duchesnea indica in heterogeneous light environments

Habitat selection behaviour is an effective strategy adopted by clonal plants in heterogeneous understorey light environments, and it is likely regulated by the parental environment's ultraviolet-B radiation levels (UV-B) due to the photomorphogenesis of UV-B and maternal effects. Here, parental ramets of Duchesnea indica were treated with two UV-B radiation levels [high (UV5 group) and low (UV10 group)], newborn offspring were grown under a heterogeneous light environment (ambient light vs shade habitat), and the growth and DNA methylation variations of parents and offspring were analysed. The results showed that parental UV-B affected not only the growth of the parent but also the offspring. The offspring of different UV-B-radiated parents showed different performances. Although these offspring all displayed a tendency to escape from light environments, such as entering shade habitats earlier, and allocating more biomass under shade (33.06% of control, 42.28% of UV5 and 72.73% of UV10), these were particularly obvious in offspring of the high UV-B parent. Improvements in epigenetic diversity (4.77 of control vs 4.83 of UV10) and total DNA methylation levels (25.94% of control vs 27.15% of UV10) and the inhibition of shade avoidance syndrome (denser growth with shorter stolons and internodes) were only observed in offspring of high UV-B parents. This difference was related to the eustress and stress effects of low and high UV-B, respectively. Overall, the behaviour of D. indica under heterogeneous light conditions was regulated by the parental UV-B exposure. Moreover, certain performance improvements helped offspring pre-regulate growth to cope with future environments and were probably associated with the effects of maternal DNA methylation variations in UV-B-radiated parents.