Molecular cloning and functional analysis of a UV-B photoreceptor gene, BpUVR8 (UV Resistance Locus 8), from birch and its role in ABA response

A birch ELONGATED HYPOCOTYL 5 gene enhances UV-B and drought tolerance

UV-B radiation and drought majorly restrict plant growth, particularly in summer. ELONGATED HYPOCOTYL 5 (HY5), a bZIP transcription factor (TF), has a beneficial impact on photomorphogenesis. However, the sequence of HY5 from Betula platyphylla (BpHY5) has not been identified and the gene functions remain unclarified. We cloned the sequence of BpHY5, which was targeted to the nucleus. The hypocotyl phenotypes of heterologous expression in Arabidopsis thaliana and reverse mutation showed that BpHY5 is homologous to AtHY5. The expression of BpHY5 was increased in response to UV-B radiation, drought conditions, and the presence of abscisic acid (ABA). The overexpression of BpHY5 resulted in increased tolerance to UV-B radiation and drought and decreased ABA sensitivity with higher germination and greening rate, more developmental root system, stronger reactive oxygen species scavenging ability, and lower damage degree. The lignin content under UV-B condition of BpHY5/Col was higher than that of Col. Furthermore, overexpressing BpHY5 up-regulated the expression of genes related to tolerance (NCED3/9, ABI5, DREB2A, RD20, ERF4, NDB2, and APX2). In brief, the study suggests that BpHY5 from birch serves as a beneficial modulator of plant responses to UV-B radiation and drought stress.

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.

Selection and optimization of reference genes for RT-qPCR normalization: A case study in Solanum lycopersicum exposed to UV-B

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.

Light and abscisic acid interplay in early seedling development

Abscisic acid (ABA) plays a crucial role in plant development, regulating germination, seedling development and stomatal movements, especially under adverse conditions. Light interacts with the ABA signalling pathway to fine tune these processes. Here, we provide an overview of the recent investigations on ABA-light interplay during early plant development after germination. We discuss the multilayered and reciprocal interactions between ABA signalling components and several light signalling modulators, including photoreceptors, transcription factors and posttranslational modifiers. ABSCISIC ACID INSENSITIVE5 acts as a central convergence point for these interactions during postgermination seedling development. ABA also regulates the adaptation of seedlings to challenging light environments. Furthermore, we enlist the role of ABA-light cross-talk in regulating seedling establishment in crops and highlight open questions for future investigations.

Physiological and Molecular Mechanism Involved in Cold Stress Tolerance in Plants

Previous studies have reported that low temperature (LT) constrains plant growth and restricts productivity in temperate regions. However, the underlying mechanisms are complex and not well understood. Over the past ten years, research on the process of adaptation and tolerance of plants during cold stress has been carried out. In molecular terms, researchers prioritize research into the field of the ICE-CBF-COR signaling pathway which is believed to be the important key to the cold acclimation process. Inducer of CBF Expression (ICE) is a pioneer of cold acclimation and plays a central role in C-repeat binding (CBF) cold induction. CBFs activate the expression of COR genes via binding to cis-elements in the promoter of COR genes. An ICE-CBF-COR signaling pathway activates the appropriate expression of downstream genes, which encodes osmoregulation substances. In this review, we summarize the recent progress of cold stress tolerance in plants from molecular and physiological perspectives and other factors, such as hormones, light, and circadian clock. Understanding the process of cold stress tolerance and the genes involved in the signaling network for cold stress is essential for improving plants, especially crops.

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

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

The influence of spectral composition on spring and autumn phenology in trees

Several recent reviews highlight the molecular mechanisms that underpin phenological responses to temperature and photoperiod; however, these have mostly overlooked the influence of solar radiation and its spectral composition on these processes. For instance, solar radiation in the blue and ultraviolet (UV) regions of the spectrum, as well as the red/far-red (R:FR) ratio, can influence spring and autumn phenology. Solar radiation reaching the Earth changes diurnally and seasonally; however, rising global temperatures, latitudinal range shifts and light pollution are likely to produce novel combinations of phenological cues for tree species. Here, we review the literature on phenological responses to spectral composition. Our objective was to explore the natural variation in spectral composition using radiative transfer models and to reveal any species-specific or ecotype-specific responses relating to latitudinal origin. These responses are likely to be most pronounced at high latitudes where spectral composition varies most throughout the year. For instance, trees from high latitudes tend to be more sensitive to changes in R:FR than those from low latitudes. The effects of blue light and UV radiation on phenology have not been studied as much as those of R:FR, but the limited results available suggest both could be candidate cues affecting autumn leaf colouration and senescence. Failure of more-southern species and ecotypes to adapt and use spectral cues during northwards range shifts could result in mistimed phenology, potentially resulting in frost damage, reduced fitness and limited range expansion. Future areas for research should look to establish how consistently different functional types of tree respond to spectral cues and identify photoreceptor-mediated mechanisms that allow plants to combine information from multiple light cues to coordinate the timing of phenological events. It should then be feasible to consider the synchronous or sequential action of light cues within a hierarchy of environmental factors regulating phenology.

Beyond Arabidopsis: Differential UV-B Response Mediated by UVR8 in Diverse Species

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.