REPRESSOR OF ULTRAVIOLET-B PHOTOMORPHOGENESIS function allows efficient phototropin mediated ultraviolet-B phototropism in etiolated seedlings

Mutation in shoot-to-root mobile transcription factor, ELONGATED HYPOCOTYL 5, leads to low nicotine levels in tobacco

Tobacco remains one of the most commercially important crops due to the parasympathomimetic alkaloid nicotine used in cigarettes. Most genes involved in nicotine biosynthesis are expressed in root tissues; however, their light-dependent regulation has not been studied. Here, we identified the ELONGATED HYPOCOTYL 5 homolog, NtHY5, from Nicotiana tabacum and demonstrated that NtHY5 could complement the Arabidopsis thaliana hy5 mutant at molecular, morphological and biochemical levels. We report the development of CRISPR/Cas9-based knockout mutant plants of tobacco, NtHY5CR, and show down-regulation of the nicotine and phenylpropanoid pathway genes leading to a significant reduction in nicotine and flavonol content, whereas NtHY5 overexpression (NtHY5OX) plants show the opposite effect. Grafting experiments using wild-type, NtHY5CR, and NtHY5OX indicated that NtHY5 moves from shoot-to-root to regulate nicotine biosynthesis in the root tissue. Shoot HY5, directly or through enhancing expression of the root HY5, promotes nicotine biosynthesis by binding to light-responsive G-boxes present in the NtPMT, NtQPT and NtODC promoters. We conclude that the mobility of HY5 from shoot-to-root regulates light-dependent nicotine biosynthesis. The CRISPR/Cas9-based mutants developed, in this study; with low nicotine accumulation in leaves could help people to overcome their nicotine addiction and the risk of death.

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.

UV-B and UV-C radiation trigger both common and distinctive signal perceptions and transmissions in Pinus tabuliformis Carr

In plants, ultraviolet (UV)-light is an important driver for growth and natural distribution, and is also a valuable tool for manipulating productivity as well as biotic interactions. Understanding of plant responses to different UV radiation is sparse, especially from a systems biology perspective and particularly for conifers. Here, we evaluated the physiological and transcriptomic responses to the short-term application of high-irradiance UV-B and UV-C waves on Pinus tabuliformis Carr., a major conifer in Northern China. By undertaking time-ordered gene coexpression network analyses and network comparisons incorporating physiological traits and gene expression variation, we uncovered communalities but also differences in P. tabuliformis responses to UV-B and UV-C. Both types of spectral bands caused a significant inhibition of photosynthesis, and conversely, the improvement of antioxidant capacity, flavonoid production and signaling pathways related to stress resistance, indicating a clear switch from predominantly primary metabolism to enhanced defensive metabolism in pine. We isolated distinct subnetworks for photoreceptor-mediated signal transduction, maximum quantum efficiency of photosystem II (Fv/Fm) regulation and flavonoid biosynthesis in response to UV-B and UV-C radiation. From these subnetworks, we further identified phototropins as potentially important elements in both UV-B and UV-C signaling and, for the first time, suggesting peptide hormones to be involved in promoting flavonoid biosynthesis against UV-B, while these hormones seem not to be implicated in the defense against UV-C exposure. The present study employed an effective strategy for disentangling the complex physiological and genetic regulatory mechanisms in a nonmodel plant species, and thus, provides a suitable reference for future functional evaluations and artificial UV-light mediated growing strategies in plant production.

Low Fluence Ultraviolet-B Promotes Ultraviolet Resistance 8-Modulated Flowering in Arabidopsis

Ultraviolet-B (UV-B) irradiation (280-320 nm) is an integral part of sunlight and a pivotal environmental cue that triggers various plant responses, from photoprotection to photomorphogenesis and metabolic processes. UV-B is perceived by ULTRAVIOLET RESISTANCE 8 (UVR8), which orchestrates UV-B signal transduction and transcriptional control of UV-B-responsive genes. However, there is limited information on the molecular mechanism underlying the UV-B- and UVR8-dependent regulation of flowering time in plants. Here, we investigate the role of UV-B and UVR8 in photoperiodic flowering in Arabidopsis thaliana. Our findings suggest that UV-B controls photoperiodic flowering in an ecotype-specific manner and that UVR8 acts as a negative regulator of UV-B-induced flowering. Overall, our research shows that UV-B modulates flowering initiation through the action of UVR8 at the transcriptional level.

Cryptochromes are the dominant photoreceptors mediating heliotropic responses of Arabidopsis inflorescences

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.

UV-B light and its application potential to reduce disease and pest incidence in crops

Ultraviolet-B radiation (280-315 nm), perceived by the plant photoreceptor UVR8, is a key environmental signal that influences plant growth and development and can reduce disease and pest incidence. The positive effect of UV-B on disease resistance and incidence in various plant species supports the implementation of supplemental UV-B radiation in sustainable crop production. However, despite many studies focusing on UV-B light, there is no consensus on the best mode of application. This review aims to analyze, evaluate, and organize the different application strategies of UV-B radiation in crop production with a focus on disease resistance. We summarize the physiological effects of UV-B light on plants and discuss how plants perceive and transduce UV-B light by the UVR8 photoreceptor as well as how this perception alters plant specialized metabolite production. Next, we bring together conclusions of various studies with respect to different UV-B application methods to improve plant resistance. In general, supplemental UV-B light has a positive effect on disease resistance in many plant-pathogen combinations, mainly through the induction of the production of specialized metabolites. However, many variables (UV-B light source, plant species, dose and intensity, timing during the day, duration, background light, etc.) make it difficult to compare and draw general conclusions. We compiled the information of recent studies on UV-B light applications, including e.g., details on the UV-B light source, experimental set-up, calculated UV-B light dose, intensity, and duration. This review provides practical insights and facilitates future research on UV-B radiation as a promising tool to reduce disease and pest incidence.

Protein Dynamics and Time Resolved Protein Crystallography at Synchrotron Radiation Sources: Past, Present and Future

The ultrabright and ultrashort pulses produced at X-ray free electron lasers (XFELs) has enabled studies of crystallized molecular machines at work under ‘native’ conditions at room temperature by the so-called time-resolved serial femtosecond crystallography (TR-SFX) technique. Since early TR-SFX experiments were conducted at XFELs, it has been largely reported in the literature that time-resolved X-ray experiments at synchrotrons are no longer feasible or are impractical due to the severe technical limitations of these radiation sources. The transfer of the serial crystallography approach to newest synchrotrons upgraded for higher flux density and with beamlines using sophisticated focusing optics, submicron beam diameters and fast low-noise photon-counting detectors offers a way to overcome these difficulties opening new and exciting possibilities. In fact, there is an increasing amount of publications reporting new findings in structural dynamics of protein macromolecules by using time resolved crystallography from microcrystals at synchrotron sources. This review gathers information to provide an overview of the recent work and the advances made in this filed in the past years, as well as outlines future perspectives at the next generation of synchrotron sources and the upcoming compact pulsed X-ray sources.

UV-B signalling in rice: Response identification, gene expression profiling and mutant isolation

Responses of rice seedlings to UV-B radiation (UV-B) were investigated, aiming to establish rice as a model plant for UV-B signalling studies. The growth of japonica rice coleoptiles, grown under red light, was inhibited by brief irradiation with UV-B, but not with blue light. The effective UV-B fluences (10^-1 -10³ μmol m^-2 ) were much lower than those reported in Arabidopsis. The response was much less in indica rice cultivars and its extent varied among Oryza species. We next identified UV-B-specific anthocyanin accumulation in the first leaf of purple rice and used this visible phenotype to isolate mutants. Some isolated mutants were further characterized, and one was found to have a defect in the growth response. Using microarrays, we identified a number of genes that are regulated by low-fluence-rate UV-B in japonica coleoptiles. Some up-regulated genes were analysed by real-time PCR for UV-B specificity and the difference between japonica and indica. More than 70% of UV-B-regulated rice genes had no homologs in UV-B-regulated Arabidopsis genes. Many UV-B-regulated rice genes are related to plant hormones and especially to jasmonate biosynthetic and responsive genes in apparent agreement with the growth response. Possible involvement of two rice homologs of UVR8, a UV-B photoreceptor, is discussed.

Phototropin 1 and 2 Influence Photosynthesis, UV-C Induced Photooxidative Stress Responses, and Cell Death

Phototropins are plasma membrane-associated photoreceptors of blue light and UV-A/B radiation. The Arabidopsis thaliana genome encodes two phototropins, PHOT1 and PHOT2, that mediate phototropism, chloroplast positioning, and stomatal opening. They are well characterized in terms of photomorphogenetic processes, but so far, little was known about their involvement in photosynthesis, oxidative stress responses, and cell death. By analyzing phot1, phot2 single, and phot1phot2 double mutants, we demonstrated that both phototropins influence the photochemical and non-photochemical reactions, photosynthetic pigments composition, stomata conductance, and water-use efficiency. After oxidative stress caused by UV-C treatment, phot1 and phot2 single and double mutants showed a significantly reduced accumulation of H₂O₂ and more efficient photosynthetic electron transport compared to the wild type. However, all phot mutants exhibited higher levels of cell death four days after UV-C treatment, as well as deregulated gene expression. Taken together, our results reveal that on the one hand, both phot1 and phot2 contribute to the inhibition of UV-C-induced foliar cell death, but on the other hand, they also contribute to the maintenance of foliar H₂O₂ levels and optimal intensity of photochemical reactions and non-photochemical quenching after an exposure to UV-C stress. Our data indicate a novel role for phototropins in the condition-dependent optimization of photosynthesis, growth, and water-use efficiency as well as oxidative stress and cell death response after UV-C exposure.

Ultraviolet Radiation From a Plant Perspective: The Plant-Microorganism Context

Ultraviolet (UV) radiation directly affects plants and microorganisms, but also alters the species-specific interactions between them. The distinct bands of UV radiation, UV-A, UV-B, and UV-C have different effects on plants and their associated microorganisms. While UV-A and UV-B mainly affect morphogenesis and phototropism, UV-B and UV-C strongly trigger secondary metabolite production. Short wave (<350 nm) UV radiation negatively affects plant pathogens in direct and indirect ways. Direct effects can be ascribed to DNA damage, protein polymerization, enzyme inactivation and increased cell membrane permeability. UV-C is the most energetic radiation and is thus more effective at lower doses to kill microorganisms, but by consequence also often causes plant damage. Indirect effects can be ascribed to UV-B specific pathways such as the UVR8-dependent upregulated defense responses in plants, UV-B and UV-C upregulated ROS accumulation, and secondary metabolite production such as phenolic compounds. In this review, we summarize the physiological and molecular effects of UV radiation on plants, microorganisms and their interactions. Considerations for the use of UV radiation to control microorganisms, pathogenic as well as non-pathogenic, are listed. Effects can be indirect by increasing specialized metabolites with plant pre-treatment, or by directly affecting microorganisms.

Stem phototropism toward blue and ultraviolet light

Positive phototropism is the process through which plants orient their organs toward a directional light source. While the blue light receptors phototropins (phot) play a major role in phototropism toward blue (B) and ultraviolet (UV) radiation, recent research showed that the UVB light receptor UVR8 also triggers phototropism toward UVB. In addition, new details of the molecular mechanisms underlying the activity of these receptors and interaction with other environmental signals have emerged in the past years. In this review, we summarize the current knowledge about hypocotyledoneous and inflorescence stem growth reorientation toward B and UVB, with a focus on the molecular mechanisms.

Unique and contrasting effects of light and temperature cues on plant transcriptional programs

Plants have adapted to tolerate and survive constantly changing environmental conditions by reprogramming gene expression in response to stress or to drive developmental transitions. Among the many signals that plants perceive, light and temperature are of particular interest due to their intensely fluctuating nature which is combined with a long-term seasonal trend. Whereas specific receptors are key in the light-sensing mechanism, the identity of plant thermosensors for high and low temperatures remains far from fully addressed. This review aims at discussing common as well as divergent characteristics of gene expression regulation in plants, controlled by light and temperature. Light and temperature signaling control the abundance of specific transcription factors, as well as the dynamics of co-transcriptional processes such as RNA polymerase elongation rate and alternative splicing patterns. Additionally, sensing both types of cues modulates gene expression by altering the chromatin landscape and through the induction of long non-coding RNAs (lncRNAs). However, while light sensing is channeled through dedicated receptors, temperature can broadly affect chemical reactions inside plant cells. Thus, direct thermal modifications of the transcriptional machinery add another level of complexity to plant transcriptional regulation. Besides the rapid transcriptome changes that follow perception of environmental signals, plant developmental transitions and acquisition of stress tolerance depend on long-term maintenance of transcriptional states (active or silenced genes). Thus, the rapid transcriptional response to the signal (Phase I) can be distinguished from the long-term memory of the acquired transcriptional state (Phase II - remembering the signal). In this review we discuss recent advances in light and temperature signal perception, integration and memory in Arabidopsis thaliana, focusing on transcriptional regulation and highlighting the contrasting and unique features of each type of cue in the process.

Association between blooming time and climatic adaptation in Prunus mume

Prunus mume Sieb. et Zucc. is an important fruit crop of the subtropical region, originating in China. It blooms earlier than other deciduous fruit trees, but different regions have different blooming periods. The time of anthesis is related to the dormancy period, and a certain amount of chilling promotes bud break and blooming. To identify the relationship between blooming time and the climatic adaptation of P. mume cultivars in China, the nuclear and chloroplast genomes of 19 cultivars from the main cultivation areas of P. mume in China were resequenced. The average depth of coverage was 34X-76X, and a total of 388,134 single nucleotide polymorphisms were located within the coding regions of the gene (CDs). Additionally, the 19 cultivar accessions were divided into three groups based on their blooming time: early, mid, and late. Associated with the blooming time groups, 21 selective sweep regions were identified, which could provide evidence supporting the possible model of P. mume domestication originating due to natural selection. Furthermore, we identified a flowering gene, FRIGIDA-LIKE 3 (FRL3), seems to affect the blooming time and the climatic adaptation of P. mume cultivars. This study is a major step toward understanding the climatic adaptation of P. mume cultivars in China.

UV-B Induces Chloroplast Movements in a Phototropin-Dependent Manner

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.

Determination of Phototropism by UV-B Radiation

UV-B phototropism in etiolated Arabidopsis seedlings has only been shown recently and needs further exploration. Here we elaborate on how to generate a customized setup with a unilateral UV-B light source, the required plant materials, different growth substrates, and a framework for data analysis.

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.

Developmental reprogramming by UV-B radiation in plants

Plants are extremely plastic organisms with the ability to adapt and respond to the changing environmental conditions surrounding them. Sunlight is one of the main resources for plants, both as a primary energy source for photosynthesis and as a stimulus that regulates different aspects of their growth and development. UV-B comprises wavelengths that correspond to a high energy region of the solar spectrum capable of reaching the biosphere, influencing plant growth. It is currently believed that plants are able to acclimate when growing under the influence of this radiation and perceive it as a signal, without stress signs. Nonetheless, many UV-B induced changes are elicited after DNA damage occurs as a consequence of exposure. In this review we focus on the influence of UV-B on leaf, flower and root development and emphasize the limited understanding of the molecular mechanisms for most of this developmental processes affected by UV-B documented over the years of research in this area.

Photomorphogenic responses to ultraviolet-B light

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