The signaling state of Arabidopsis cryptochrome 2 contains flavin semiquinone

Light reception of Phycomyces revisited: several white collar proteins confer blue- and red-light sensitivity and control dynamic range and adaptation

The giant-fruiting body, sporangiophore, of the fungus Phycomyces blakesleeanus grows toward near-UV/blue-light (phototropism). The blue-light photoreceptor, MadA, should contain FAD bound to the LOV domain, and forms a complex with MadB. Both proteins are homologs of white collar proteins WC-1 and WC-2 from the fungus Neurospora crassa and should be localized in nuclei, where they function as a light-sensitive transcription factor complex. The photoreceptor properties of two further Wc proteins, WcoA and WcoB, remain unclear because of lack of mutants. We propose that WcoA and/or WcoB play essential roles in photoreception by enlarging the dynamic range that help explain complex stimulus-response relationships. Even though red light does not elicit photo-movement or -differentiation in Phycomyces, it affects the effectiveness of blue light which indicates an underlying photochromic receptor. Protein sequence searches show that other fungal red-light receptors are absent in Phycomyces. The solution to the red-light riddle is thus sought in the ability of Wc complexes to generate after blue-light irradiation a neutral flavosemiquinone radical that absorbs red light and functions as primary photochemical signal. Phototropism requires Ras-GAP (MadC) as part of the signal transduction cascade and, we propose, to allocate photoreceptors in the plasmalemma of the growing zone, which allows for receptor dichroism, range adjustment and contrast recognition for spatial orientation. Phototropic signal chains must entail transduction networks between Wc receptors and small G-proteins and their associated Ras-GAP and Ras-GEF proteins. The interactions among these proteins should occur in trans-Golgi vesicles and the plasmalemma of the growing zone.

'Seeing' the electromagnetic spectrum: spotlight on the cryptochrome photocycle

Cryptochromes are widely dispersed flavoprotein photoreceptors that regulate numerous developmental responses to light in plants, as well as to stress and entrainment of the circadian clock in animals and humans. All cryptochromes are closely related to an ancient family of light-absorbing flavoenzymes known as photolyases, which use light as an energy source for DNA repair but themselves have no light sensing role. Here we review the means by which plant cryptochromes acquired a light sensing function. This transition involved subtle changes within the flavin binding pocket which gave rise to a visual photocycle consisting of light-inducible and dark-reversible flavin redox state transitions. In this photocycle, light first triggers flavin reduction from an initial dark-adapted resting state (FADox). The reduced state is the biologically active or 'lit' state, correlating with biological activity. Subsequently, the photoreduced flavin reoxidises back to the dark adapted or 'resting' state. Because the rate of reoxidation determines the lifetime of the signaling state, it significantly modulates biological activity. As a consequence of this redox photocycle Crys respond to both the wavelength and the intensity of light, but are in addition regulated by factors such as temperature, oxygen concentration, and cellular metabolites that alter rates of flavin reoxidation even independently of light. Mechanistically, flavin reduction is correlated with conformational change in the protein, which is thought to mediate biological activity through interaction with biological signaling partners. In addition, a second, entirely independent signaling mechanism arises from the cryptochrome photocycle in the form of reactive oxygen species (ROS). These are synthesized during flavin reoxidation, are known mediators of biotic and abiotic stress responses, and have been linked to Cry biological activity in plants and animals. Additional special properties arising from the cryptochrome photocycle include responsivity to electromagnetic fields and their applications in optogenetics. Finally, innovations in methodology such as the use of Nitrogen Vacancy (NV) diamond centers to follow cryptochrome magnetic field sensitivity in vivo are discussed, as well as the potential for a whole new technology of 'magneto-genetics' for future applications in synthetic biology and medicine.

Exploring the potential of Eu3+ and Mn4+ activated LaAlO3 phosphors as red and far-red emitters for horticulture lighting

The development of efficient red and far-red emitters, for efficient plant absorption in the Photosynthetically Active Radiation (PAR) region, holds significance in contemporary plant growth control. This study focuses on the synthesis and characterization of LaAlO3 as a host material, doped with Eu3+ and Mn4+ ions, using a solid-state reaction method. The investigation encompasses the creation and analysis of both single-doped and co-doped samples, employing techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. XRD analysis consistently confirmed the perovskite-like structure of all samples, devoid of detectable impurities or major structural changes due to doping. SEM images revealed a uniform distribution of regularly shaped particles for the co-doped sample. The PL spectroscopy showed that the doping led to strong photoluminescence, with the co-doped sample exhibiting the intensity of each of the ions independently neither exhibiting quenching nor energy transfer mechanisms. The excitation spectrum of Eu3+ exhibited a broad charge transfer band at approximately 328 nm, coupled with characteristic f-f excitation bands. On the other hand, the Mn4+ ion's excitation spectrum featured transitions from ground state (4A2g) electrons excited to higher excited states (4T1g, 2T2g, and 4T2g) centered at 350 nm and within the region 250-550 nm. The co-doped sample was excited at a common excitation wavelength of 460 nm and underwent an in-depth examination of its photoluminescent properties, including decay curves analysis and time dependence also. The results from this study suggest that the synthesized phosphor materials exhibit substantial potential for diverse applications, including but not limited to solid-state lighting for efficient plant growth.

A deazariboflavin chromophore kinetically stabilizes reduced FAD state in a bifunctional cryptochrome

An animal-like cryptochrome derived from Chlamydomonas reinhardtii (CraCRY) is a bifunctional flavoenzyme harboring flavin adenine dinucleotide (FAD) as a photoreceptive/catalytic center and functions both in the regulation of gene transcription and the repair of UV-induced DNA lesions in a light-dependent manner, using different FAD redox states. To address how CraCRY stabilizes the physiologically relevant redox state of FAD, we investigated the thermodynamic and kinetic stability of the two-electron reduced anionic FAD state (FADH-) in CraCRY and related (6-4) photolyases. The thermodynamic stability of FADH- remained almost the same compared to that of all tested proteins. However, the kinetic stability of FADH- varied remarkably depending on the local structure of the secondary pocket, where an auxiliary chromophore, 8-hydroxy-7,8-didemethyl-5-deazariboflavin (8-HDF), can be accommodated. The observed effect of 8-HDF uptake on the enhancement of the kinetic stability of FADH- suggests an essential role of 8-HDF in the bifunctionality of CraCRY.

Impact of varying light spectral compositions on photosynthesis, morphology, chloroplast ultrastructure, and expression of light-responsive genes in Marchantia polymorpha

Marchantia polymorpha is a convenient model for studying light of different spectral compositions on various physiological and biochemical processes because its photoreceptor system is vastly simplified. The influence of red light (RL, 660 nm), far-red light (FRL, 730 nm), blue light (BL, 450 nm), and green light (GL, 525 nm) compared to white light (high-pressure sodium light (HPSL), white LEDs (WL 450 + 580 nm) and white fluorescent light (WFL) on photosynthetic and transpiration rates, photosystem II (PSII) activity, photomorphogenesis, and the expression of light and hormonal signaling genes was studied. The ultrastructure of the chloroplasts in different tissues of the gametophyte M. polymorpha was examined. FRL led to the formation of agranal chloroplasts (in the epidermis and the chlorenchyma) with a high starch content (in the parenchyma), which led to a reduced intensity of photosynthesis. BL increased the transcription of genes for the biosynthesis of secondary metabolites - chalcone synthase (CHS), cellulose synthase (CELL), and L-ascorbate peroxidase (APOX3), which is consistent with the increased activity of low-molecular weight antioxidants. FRL increased the expression of phytochrome apoprotein (PHY) and cytokinin oxidase (CYTox) genes, but the expression of the phytochrome interacting factor (PIF) gene decreased, which was accompanied by a significant change in gametophyte morphology. Analysis of crosstalk gene expression, and changes in morphology and photosynthetic activity was carried out.

Cryptochromes in mammals: a magnetoreception misconception?

Cryptochromes are flavoproteins related to photolyases that are widespread throughout the plant and animal kingdom. They govern blue light-dependent growth in plants, control circadian rhythms in a light-dependent manner in invertebrates, and play a central part in the circadian clock in vertebrates. In addition, cryptochromes might function as receptors that allow animals to sense the Earth's magnetic field. As cryptochromes are also present in mammals including humans, the possibility of a magnetosensitive protein is exciting. Here we attempt to provide a concise overview of cryptochromes in mammals. We briefly review their canonical role in the circadian rhythm from the molecular level to physiology, behaviour and diseases. We then discuss their disputed light sensitivity and proposed role in the magnetic sense in mammals, providing three mechanistic hypotheses. Specifically, mammalian cryptochromes could form light-induced radical pairs in particular cellular milieus, act as magnetoreceptors in darkness, or as secondary players in a magnetoreception signalling cascade. Future research can test these hypotheses to investigate if the role of mammalian cryptochromes extends beyond the circadian clock.

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.

Cryptochromes and UBP12/13 deubiquitinases antagonistically regulate DNA damage response in Arabidopsis

Cryptochromes (CRYs) are evolutionarily conserved blue-light receptors that evolved from bacterial photolyases that repair damaged DNA. Today, CRYs have lost their ability to repair damaged DNA; however, prior reports suggest that human CRYs can respond to DNA damage. Currently, the role of CRYs in the DNA damage response (DDR) is lacking, especially in plants. Therefore, we evaluated the role of plant CRYs in DDR along with UBP12/13 deubiquitinases, which interact with and regulate the CRY2 protein. We found that cry1cry2 was hypersensitive, while ubp12ubp13 was hyposensitive to UVC-induced DNA damage. Elevated UV-induced cyclobutane pyrimidine dimers (CPDs) and the lack of DNA repair protein RAD51 accumulation in cry1cry2 plants indicate that CRYs are required for DNA repair. On the contrary, CPD levels diminished and RAD51 protein levels elevated in plants lacking UBP12 and UBP13, indicating their role in DDR repression. Temporal transcriptomic analysis revealed that DDR-induced transcriptional responses were subdued in cry1cry2, but elevated in ubp12ubp13 compared to WT. Through transcriptional modeling of the time-course transcriptome, we found that genes quickly induced by UVC (15 min) are targets of CAMTA 1-3 transcription factors, which we found are required for DDR. This transcriptional regulation seems, however, diminished in the cry1cry2 mutant, indicating that CAMTAs are required for CRY2-mediated DDR. Furthermore, we observed enhanced CRY2-UBP13 interaction and formation of CRY2 nuclear speckles under UVC, suggesting that UVC activates CRY2 similarly to blue light. Together, our data reveal the temporal dynamics of the transcriptional events underlying UVC-induced genotoxicity and expand our knowledge of the role of CRY and UBP12/13 in DDR.

An Optogenetic Toolkit for the Control of Phase Separation in Living Cells

Phase separation is a key mechanism for intracellular organization, driving the segregation of biomolecules into distinct condensates. Intracellular condensates play diverse functional roles including gene expression, stress response, and cell signaling. Technologies that enable the control of intracellular phase separation can be highly useful not only for a better understanding of the biophysical principles of phase separation processes but also for engineering novel condensates. Here, we describe an optogenetic approach for spatiotemporal control of phase separation in living cells.

Optogenetics at the presynapse

Optogenetic actuators enable highly precise spatiotemporal interrogation of biological processes at levels ranging from the subcellular to cells, circuits and behaving organisms. Although their application in neuroscience has traditionally focused on the control of spiking activity at the somatodendritic level, the scope of optogenetic modulators for direct manipulation of presynaptic functions is growing. Presynaptically localized opsins combined with light stimulation at the terminals allow light-mediated neurotransmitter release, presynaptic inhibition, induction of synaptic plasticity and specific manipulation of individual components of the presynaptic machinery. Here, we describe presynaptic applications of optogenetic tools in the context of the unique cell biology of axonal terminals, discuss their potential shortcomings and outline future directions for this rapidly developing research area.

Temperature Sensing in Plants: On the Dawn of Molecular Thermosensor Research

Although many studies on plant growth and development focus on the effects of light, a growing number of studies dissect plant responses to temperature and the underlying signaling pathways. The identity of plant thermosensing molecules (thermosensors) acting upstream of the signaling cascades in temperature responses was elusive until recently. During the past six years, a set of plant thermosensors has been discovered, representing a major turning point in the research on plant temperature responses and signaling. Here, we review these newly discovered plant thermosensors, which can be classified as sensors of warmth or cold. We compare between plant thermosensors and those from other organisms and attempt to define the subcellular thermosensing compartments in plants. In addition, we discuss the notion that photoreceptive thermosensors represent a novel class of thermosensors, the roles of which have yet to be described in non-plant systems.

Low Concentration of Anti-Auxin and Anti-Fungal Agent Accelerates the PLB Regeneration of Dendrobium okinawense under Green LED

Dendrobium okinawense is an endangered epiphytic orchid, and there has been no scientific report so far on its propagation. Protocorm is a mass of cells, and protocorm-like bodies (PLBs) are lookalike protocorms produced by vegetative explants in vitro. Regeneration of PLBs is a widely used technique for orchid micropropagation. We used different light-emitting diodes (LEDs) for the PLB regeneration of D. okinawense. The number of PLBs and fresh weight were increased by 81.1% and 80.8%, respectively, under green LED over the white fluorescent (WF) light. We added different concentrations of PCIB (p-Chlorophenoxyisobutyric acid, an anti-auxin) and HMI (3-Hydroxy-5-methyl isoxazole, an anti-fungal agent) in culture media. The number of PLBs was increased in media having 0.01 mg/L of PCIB (35.9%) compared to control (no PCIB), whereas 19.3% increased in media having 0.01 mL/L of HMI compared to control (no HMI). Green LED in combination with 0.01 mg/L of PCIB significantly increased the number of PLBs (69.0%) compared to the WF–without PCIB combination. In LEDs-PCIB and LED-HMI combinations, HMI did not show better PLBs regeneration compared with PCIB. The results suggested that a combination of low concentrations of PCIB and green LED have the potential to accelerate PLB regeneration of D. okinawense.

Optogenetic Control of PIP2 Interactions Shaping ENaC Activity

The activity of the epithelial Na⁺ Channel (ENaC) is strongly dependent on the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2). PIP2 binds two distinct cationic clusters within the N termini of β- and γ-ENaC subunits (βN1 and γN2). The affinities of these sites were previously determined using short synthetic peptides, yet their role in sensitizing ENaC to changes in PIP2 levels in the cellular system is not well established. We addressed this question by comparing the effects of PIP2 depletion and recovery on ENaC channel activity and intracellular Na⁺ levels [Na⁺]_i. We tested effects on ENaC activity with mutations to the PIP2 binding sites using the optogenetic system CIBN/CRY2-OCRL to selectively deplete PIP2. We monitored changes of [Na⁺]_i by measuring the fluorescent Na⁺ indicator, CoroNa Green AM, and changes in channel activity by performing patch clamp electrophysiology. Whole cell patch clamp measurements showed a complete lack of response to PIP2 depletion and recovery in ENaC with mutations to βN1 or γN2 or both sites, compared to wild type ENaC. Whereas mutant βN1 also had no change in CoroNa Green fluorescence in response to PIP2 depletion, γN2 did have reduced [Na⁺]_i, which was explained by having shorter CoroNa Green uptake and half-life. These results suggest that CoroNa Green measurements should be interpreted with caution. Importantly, the electrophysiology results show that the βN1 and γN2 sites on ENaC are each necessary to permit maximal ENaC activity in the presence of PIP2.

Layering contrasting photoselective filters improves the simulation of foliar shade

BACKGROUND

Neutral density shade cloth is commonly used for simulating foliar shade, in which it reduces light intensity without altering spectral quality. However, foliar shade also alters spectral quality, reducing the ratio of red to far-red (R:FR) light, altering the ratio of blue to green (B:G) light, and reducing ultraviolet light. Unlike shade cloth, photoselective filters can alter spectral quality, but the filters used in previous literature have not simulated foliar shade well. We examined the spectral quality of sunlight under color temperature blue (CTB), plus green (PG), and neutral density (ND) filters from LEE Filters, Rosco e-colour + and Cinegel brands either alone or layered, hypothesizing that the contrasting filter qualities would improve simulations. As a proof-of-concept, we collected spectral data under foliar shade to compare to data collected under photoselective filters.

RESULTS

Under foliar shade reductions in the R:FR ratio ranged from 0.11 to 0.54 (~ 1.18 in full sun), while reductions in the B:G ratio were as low as 0.53 in deep shade, or were as high as 1.11 in moderate shade (~ 0.87 in full sun). Neutral density filters led to near-neutral reductions in photosynthetically active radiation and reduced the R:FR ratio similar to foliar shade. Color temperature blue filters simulated the increased B:G ratio observed under moderate foliar shade, but did not reduce the R:FR ratio low enough. On their own, PG filters did not simulate any type of foliar shade. Different brands of the same filter type also had disparate effects on spectral quality. Layered CTB and ND filters improved the accuracy of moderate foliar shade simulations, and layering CTB, PG, and ND filters led to accurate simulations of deep foliar shade.

CONCLUSIONS

Layering photoselective filters with contrasting effects on the spectral quality of sunlight results in more accurate simulations of foliar shade compared to when these filters are used separately. Layered filters can re-create the spectral motifs of moderate and deep foliar shade; they could be used to simulate shade scenarios found in different cropping systems. Photoselective filters offer numerous advantages over neutral density shade cloth and could be a direct replacement for researchers currently using neutral density shade cloth.

Phytophthora infestans Ago1-associated miRNA promotes potato late blight disease

Phytophthora spp. cause serious damage to plants by exploiting a large number of effector proteins and small RNAs (sRNAs). Several reports have described modulation of host RNA biogenesis and defence gene expression. Here, we analysed Phytophthora infestans Argonaute (Ago) 1 associated small RNAs during potato leaf infection. Small RNAs were co-immunoprecipitated, deep sequenced and analysed against the P. infestans and potato genomes, followed by transcript analyses and transgenic assays on a predicted target. Extensive targeting of potato and pathogen-derived sRNAs to a range of mRNAs was observed, including 638 sequences coding for resistance (R) proteins in the host genome. The single miRNA encoded by P. infestans (miR8788) was found to target a potato alpha/beta hydrolase-type encoding gene (StABH1), a protein localized to the plasma membrane. Analyses of stable transgenic potato lines harbouring overexpressed StABH1 or artificial miRNA gene constructs demonstrated the importance of StABH1 during infection by P. infestans. miR8788 knock-down strains showed reduced growth on potato, and elevated StABH1 expression levels were observed when plants were inoculated with the two knock-down strains compared to the wild-type strain 88069. The findings of our study suggest that sRNA encoded by P. infestans can affect potato mRNA, thereby expanding our knowledge of the multifaceted strategies this species uses to facilitate infection.

Plant Cryptochromes Illuminated: A Spectroscopic Perspective on the Mechanism

Plant cryptochromes are central blue light receptors for the control of land plant and algal development including the circadian clock and the cell cycle. Cryptochromes share a photolyase homology region with about 500 amino acids and bind the chromophore flavin adenine dinucleotide. Characteristic for plant cryptochromes is a conserved aspartic acid close to flavin and an exceptionally long C-terminal extension. The mechanism of activation by excitation and reduction of the chromophore flavin adenine dinucleotide has been controversially discussed for many years. Various spectroscopic techniques have contributed to our understanding of plant cryptochromes by providing high time resolution, ambient conditions and even in-cell approaches. As a result, unifying and differing aspects of photoreaction and signal propagation have been revealed in comparison to members from other cryptochrome subfamilies. Here, we review the insight from spectroscopy on the flavin photoreaction in plant cryptochromes and present the current models on the signal propagation from flavin reduction to dissociation of the C-terminal extension.

Green light reduces elongation when partially replacing sole blue light independently from cryptochrome 1a

Although green light is sometimes neglected, it can have several effects on plant growth and development. Green light is probably sensed by cryptochromes (crys), one of the blue light photoreceptor families. The aim of this study is to investigate the possible interaction between green and blue light and the involvement of crys in the green light response of plant photomorphogenesis. We hypothesize that green light effects on morphology only occur when crys are activated by the presence of blue light. Wild-type Moneymaker (MM), cry1a mutant (cry1a), and two CRY2 overexpressing transgenic lines (CRY2-OX3 and CRY2-OX8) of tomato (Solanum lycopersicum) were grown in a climate chamber without or with green light (30 μmol m^-2  s^-1 ) on backgrounds of sole red, sole blue and red/blue mixture, with all treatments having the same photosynthetic photon flux density of 150 μmol m^-2  s^-1 . Green light showed no significant effects on biomass accumulation, nor on leaf characteristics such as leaf area, specific leaf area, and chlorophyll content. However, in all genotypes, green light significantly decreased stem length on a sole blue background, whereas green light hardly affected stem length on sole red and red/blue mixture background. MM, cry1a, and CRY2-OX3/8 plants all exhibited similar responses of stem elongation to green light, indicating that cry1a, and probably cry2, is not involved in this green light effect. We conclude that partially replacing blue light by green light reduces elongation and that this is independent of cry1a.

Perception of solar UV radiation by plants: photoreceptors and mechanisms

About 95% of the ultraviolet (UV) photons reaching the Earth's surface are UV-A (315-400 nm) photons. Plant responses to UV-A radiation have been less frequently studied than those to UV-B (280-315 nm) radiation. Most previous studies on UV-A radiation have used an unrealistic balance between UV-A, UV-B, and photosynthetically active radiation (PAR). Consequently, results from these studies are difficult to interpret from an ecological perspective, leaving an important gap in our understanding of the perception of solar UV radiation by plants. Previously, it was assumed UV-A/blue photoreceptors, cryptochromes and phototropins mediated photomorphogenic responses to UV-A radiation and "UV-B photoreceptor" UV RESISTANCE LOCUS 8 (UVR8) to UV-B radiation. However, our understanding of how UV-A radiation is perceived by plants has recently improved. Experiments using a realistic balance between UV-B, UV-A, and PAR have demonstrated that UVR8 can play a major role in the perception of both UV-B and short-wavelength UV-A (UV-Asw, 315 to ∼350 nm) radiation. These experiments also showed that UVR8 and cryptochromes jointly regulate gene expression through interactions that alter the relative sensitivity to UV-B, UV-A, and blue wavelengths. Negative feedback loops on the action of these photoreceptors can arise from gene expression, signaling crosstalk, and absorption of UV photons by phenolic metabolites. These interactions explain why exposure to blue light modulates photomorphogenic responses to UV-B and UV-Asw radiation. Future studies will need to distinguish between short and long wavelengths of UV-A radiation and to consider UVR8's role as a UV-B/UV-Asw photoreceptor in sunlight.

Wavelength-dependent effects of artificial light at night on phytoplankton growth and community structure

Artificial light at night (ALAN) is a disruptive form of pollution, impacting physiological and behavioural processes that may scale up to population and community levels. Evidence from terrestrial habitats show that the severity and type of impact depend on the wavelength and intensity of ALAN; however, research on marine organisms is still limited. Here, we experimentally investigated the effect of different ALAN colours on marine primary producers. We tested the effect of green (525 nm), red (624 nm) and broad-spectrum white LED ALAN, compared to a dark control, on the green microalgae Tetraselmis suesica and a diatom assemblage. We show that green ALAN boosted chlorophyll production and abundance in T. suesica. All ALAN wavelengths affected assemblage biomass and diversity, with red and green ALAN having the strongest effects, leading to higher overall abundance and selective dominance of specific diatom species, some known to cause harmful algal blooms. Our findings show that green and red ALAN should be used with caution as alternative LED colours in coastal areas, where there might be a need to strike a balance between the effects of green and red light on marine primary producers with the benefit they appear to bring to other organisms.

C-Terminal Extension of a Plant Cryptochrome Dissociates from the β-Sheet of the Flavin-Binding Domain

Plant cryptochromes are central blue light receptors in land plants and algae. Photoreduction of the flavin bound to the photolyase homology region (PHR) causes a dissociation of the C-terminal extension (CCT) as effector via an unclear pathway. We applied the recently developed in-cell infrared difference (ICIRD) spectroscopy to study the response of the full-length pCRY from Chlamydomonas reinhardtii in living bacterial cells, because the receptor degraded upon isolation. We demonstrate a stabilization of the flavin neutral radical as photoproduct and of the resulting β-sheet reorganization by binding of cellular ATP. Comparison between light-induced structural responses of full-length pCRY and PHR reveals a downshift in frequency of the β-sheet signal, implying an association of the CCT close to the only β-sheet of the PHR in the dark. We provide a missing link in activation of plant cryptochromes after flavin photoreduction by indicating that β-sheet reorganization causes the CCT release and restructuring.

Improving the Predictive Value of Phytochrome Photoequilibrium: Consideration of Spectral Distortion Within a Leaf

The ratio of active phytochrome (Pfr) to total phytochrome (Pr + Pfr), called phytochrome photo-equilibrium (PPE; also called phytochrome photostationary state, PSS) has been used to explain shade avoidance responses in both natural and controlled environments. PPE is commonly estimated using measurements of the spectral photon distribution (SPD) above the canopy and photoconversion coefficients. This approach has effectively predicted morphological responses when only red and far-red (FR) photon fluxes have varied, but controlled environment research often utilizes unique ratios of wavelengths so a more rigorous evaluation of the predictive ability of PPE on morphology is warranted. Estimations of PPE have rarely incorporated the optical effects of spectral distortion within a leaf caused by pigment absorbance and photon scattering. We studied stem elongation rate in the model plant cucumber under diverse spectral backgrounds over a range of one to 45% FR (total photon flux density, 400-750 nm, of 400 μmol m^-2 s^-1) and found that PPE was not predictive when blue and green varied. Preferential absorption of red and blue photons by chlorophyll results in an SPD that is relatively enriched in green and FR at the phytochrome molecule within a cell. This can be described by spectral distortion functions for specific layers of a leaf. Multiplying the photoconversion coefficients by these distortion functions yields photoconversion weighting factors that predict phytochrome conversion at the site of photon perception within leaf tissue. Incorporating spectral distortion improved the predictive value of PPE when phytochrome was assumed to be homogeneously distributed within the whole leaf. In a supporting study, the herbicide norflurazon was used to remove chlorophyll in seedlings. Using distortion functions unique to either green or white cotyledons, we came to the same conclusions as with whole plants in the longer-term study. Leaves of most species have similar spectral absorbance so this approach for predicting PPE should be broadly applicable. We provide a table of the photoconversion weighting factors. Our analysis indicates that the simple, intuitive ratio of FR (700-750 nm) to total photon flux (far-red fraction) is also a reliable predictor of morphological responses like stem length.

The Magnetic Compass of Birds: The Role of Cryptochrome

The geomagnetic field provides directional information for birds. The avian magnetic compass is an inclination compass that uses not the polarity of the magnetic field but the axial course of the field lines and their inclination in space. It works in a flexible functional window, and it requires short-wavelength light. These characteristics result from the underlying sensory mechanism based on radical pair processes in the eyes, with cryptochrome suggested as the receptor molecule. The chromophore of cryptochrome, flavin adenine dinucleotide (FAD), undergoes a photocycle, where radical pairs are formed during photo-reduction as well as during re-oxidation; behavioral data indicate that the latter is crucial for detecting magnetic directions. Five types of cryptochromes are found in the retina of birds: cryptochrome 1a (Cry1a), cryptochrome 1b, cryptochrome 2, cryptochrome 4a, and cryptochrome 4b. Because of its location in the outer segments of the ultraviolet cones with their clear oil droplets, Cry1a appears to be the most likely receptor molecule for magnetic compass information.

Engineered Illumination Devices for Optogenetic Control of Cellular Signaling Dynamics

Spatially and temporally varying patterns of morphogen signals during development drive cell fate specification at the proper location and time. However, current in vitro methods typically do not allow for precise, dynamic spatiotemporal control of morphogen signaling and are thus insufficient to readily study how morphogen dynamics affect cell behavior. Here, we show that optogenetic Wnt/β-catenin pathway activation can be controlled at user-defined intensities, temporal sequences, and spatial patterns using engineered illumination devices for optogenetic photostimulation and light activation at variable amplitudes (LAVA). By patterning human embryonic stem cell (hESC) cultures with varying light intensities, LAVA devices enabled dose-responsive control of optoWnt activation and Brachyury expression. Furthermore, time-varying and spatially localized patterns of light revealed tissue patterning that models the embryonic presentation of Wnt signals in vitro. LAVA devices thus provide a low-cost, user-friendly method for high-throughput and spatiotemporal optogenetic control of cell signaling for applications in developmental and cell biology.

Cryptochromes: Photochemical and structural insight into magnetoreception

Cryptochromes (CRYs) function as blue light photoreceptors in diverse physiological processes in nearly all kingdoms of life. Over the past several decades, they have emerged as the most likely candidates for light-dependent magnetoreception in animals, however, a long history of conflicts between in vitro photochemistry and in vivo behavioral data complicate validation of CRYs as a magnetosensor. In this review, we highlight the origins of conflicts regarding CRY photochemistry and signal transduction, and identify recent data that provides clarity on potential mechanisms of signal transduction in magnetoreception. The review primarily focuses on examining differences in photochemistry and signal transduction in plant and animal CRYs, and identifies potential modes of convergent evolution within these independent lineages that may identify conserved signaling pathways.

Cryptochromes and the Circadian Clock: The Story of a Very Complex Relationship in a Spinning World

Cryptochromes are flavin-containing blue light photoreceptors, present in most kingdoms, including archaea, bacteria, plants, animals and fungi. They are structurally similar to photolyases, a class of flavoproteins involved in light-dependent repair of UV-damaged DNA. Cryptochromes were first discovered in Arabidopsis thaliana in which they control many light-regulated physiological processes like seed germination, de-etiolation, photoperiodic control of the flowering time, cotyledon opening and expansion, anthocyanin accumulation, chloroplast development and root growth. They also regulate the entrainment of plant circadian clock to the phase of light-dark daily cycles. Here, we review the molecular mechanisms by which plant cryptochromes control the synchronisation of the clock with the environmental light. Furthermore, we summarise the circadian clock-mediated changes in cell cycle regulation and chromatin organisation and, finally, we discuss a putative role for plant cryptochromes in the epigenetic regulation of genes.

Does Green Really Mean Go? Increasing the Fraction of Green Photons Promotes Growth of Tomato but Not Lettuce or Cucumber

The photon flux in the green wavelength region is relatively enriched in shade and the photon flux in the blue region is selectively filtered. In sole source lighting environments, increasing the fraction of blue typically decreases stem elongation and leaf expansion, and smaller leaves reduce photon capture and yield. Photons in the green region reverse these blue reductions through the photoreceptor cryptochrome in Arabidopsis thaliana, but studies in other species have not consistently shown the benefits of photons in the green region on leaf expansion and growth. Spectral effects can interact with total photon flux. Here, we report the effect of the fraction of photons in the blue (10 to 30%) and green (0 to 50%) regions at photosynthetic photon flux densities of 200 and 500 µmol m^-2 s^-1 in lettuce, cucumber and tomato. As expected, increasing the fraction of photons in the blue region consistently decreased leaf area and dry mass. By contrast, large changes in the fraction of photons in the green region had minimal effects on leaf area and dry mass in lettuce and cucumber. Photons in the green region were more potent at a lower fraction of photons in the blue region. Photons in the green region increased stem and petiole length in cucumber and tomato, which is a classic shade avoidance response. These results suggest that high-light crop species might respond to the fraction of photons in the green region with either shade tolerance (leaf expansion) or shade avoidance (stem elongation).

Express Arabidopsis Cryptochrome in Sf9 Insect Cells Using the Baculovirus Expression System

The Bac-to-Bac^® Baculovirus Expression System provides a rapid and efficient method to generate recombinant cryptochrome (CRY) proteins with chromophore flavin (FAD), which showed blue light response in vitro.

Light Emitting Diodes (LEDs) as Agricultural Lighting: Impact and Its Potential on Improving Physiology, Flowering, and Secondary Metabolites of Crops

A reduction in crop productivity in cultivable land and challenging environmental factors have directed advancement in indoor cultivation systems, such that the yield parameters are higher in outdoor cultivation systems. In wake of this situation, light emitting diode (LED) lighting has proved to be promising in the field of agricultural lighting. Properties such as energy efficiency, long lifetime, photon flux efficacy and flexibility in application make LEDs better suited for future agricultural lighting systems over traditional lighting systems. Different LED spectrums have varied effects on the morphogenesis and photosynthetic responses in plants. LEDs have a profound effect on plant growth and development and also control key physiological processes such as phototropism, the immigration of chloroplasts, day/night period control and the opening/closing of stomata. Moreover, the synthesis of bioactive compounds and antioxidants on exposure to LED spectrum also provides information on the possible regulation of antioxidative defense genes to protect the cells from oxidative damage. Similarly, LEDs are also seen to escalate the nutrient metabolism in plants and flower initiation, thus improving the quality of the crops as well. However, the complete management of the irradiance and wavelength is the key to maximize the economic efficacy of crop production, quality, and the nutrition potential of plants grown in controlled environments. This review aims to summarize the various advancements made in the area of LED technology in agriculture, focusing on key processes such as morphological changes, photosynthetic activity, nutrient metabolism, antioxidant capacity and flowering in plants. Emphasis is also made on the variation in activities of different LED spectra between different plant species. In addition, research gaps and future perspectives are also discussed of this emerging multidisciplinary field of research and its development.

Resolving Structural Changes of Photoreceptors in Living Escherichia coli via In-cell Infrared Difference Spectroscopy

Several in-cell spectroscopic techniques have been developed recently to investigate the structure and mechanism of proteins in their native environment. Conditions in vivo differ dramatically from those selected for in vitro experiments. Accordingly, the cellular environment can affect the protein mechanism for example by molecular crowding or binding of small molecules. Fourier transform infrared (FTIR) difference spectroscopy is a well-suited method to study the light-induced structural responses of photoreceptors including changes in cofactor, side chains and secondary structure. Here, we describe a protocol to study the response of cofactor and protein in living E. coli cells via in-cell infrared difference (ICIRD) spectroscopy using the attenuated total reflection (ATR) configuration. Proteins are overexpressed in E. coli, the cells are transferred into saline solution and the copy number per cell is determined using fluorescence spectroscopy. The suspension is centrifuged and the concentrated cells transferred onto the ATR cell inside the FTIR spectrometer. The thermostatted cell is sealed and illuminated from the top with an LED. Intensity spectra are recorded before and after illumination to generate the difference spectrum of the receptor inside the living cell. With ICIRD spectroscopy, structural changes of soluble photoreceptors are resolved in a near-native environment. The approach works in H₂O at ambient conditions, is label free, without any limitations in protein size and does not require any purification step. Graphic abstract: In-cell infrared difference spectroscopy on photoreceptors in living E. coli using attenuated total reflection.

Structural insights into photoactivation of plant Cryptochrome-2

Cryptochromes (CRYs) are evolutionarily conserved photoreceptors that mediate various light-induced responses in bacteria, plants, and animals. Plant cryptochromes govern a variety of critical growth and developmental processes including seed germination, flowering time and entrainment of the circadian clock. CRY's photocycle involves reduction of their flavin adenine dinucleotide (FAD)-bound chromophore, which is completely oxidized in the dark and semi to fully reduced in the light signaling-active state. Despite the progress in characterizing cryptochromes, important aspects of their photochemistry, regulation, and light-induced structural changes remain to be addressed. In this study, we determine the crystal structure of the photosensory domain of Arabidopsis CRY2 in a tetrameric active state. Systematic structure-based analyses of photo-activated and inactive plant CRYs elucidate distinct structural elements and critical residues that dynamically partake in photo-induced oligomerization. Our study offers an updated model of CRYs photoactivation mechanism as well as the mode of its regulation by interacting proteins.

Structural insights into the photoactivation of Arabidopsis CRY2

The blue-light receptor cryptochrome (CRY) in plants undergoes oligomerization to transduce blue-light signals after irradiation, but the corresponding molecular mechanism remains poorly understood. Here, we report the cryogenic electron microscopy structure of a blue-light-activated CRY2 tetramer at a resolution of 3.1 Å, which shows how the CRY2 tetramer assembles. Our study provides insights into blue-light-mediated activation of CRY2 and a theoretical basis for developing regulators of CRYs for optogenetic manipulation.

The Universally Conserved Residues Are Not Universally Required for Stable Protein Expression or Functions of Cryptochromes

Universally conserved residues (UCRs) are invariable amino acids evolutionarily conserved among members of a protein family across diverse kingdoms of life. UCRs are considered important for stability and/or function of protein families, but it has not been experimentally examined systematically. Cryptochromes are photoreceptors in plants or light-independent components of the circadian clocks in mammals. We experimentally analyzed 51 UCRs of Arabidopsis cryptochrome 2 (CRY2) that are universally conserved in eukaryotic cryptochromes from Arabidopsis to human. Surprisingly, we found that UCRs required for stable protein expression of CRY2 in plants are not similarly required for stable protein expression of human hCRY1 in human cells. Moreover, 74% of the stably expressed CRY2 proteins mutated in UCRs retained wild-type-like activities for at least one photoresponses analyzed. Our finding suggests that the evolutionary mechanisms underlying conservation of UCRs or that distinguish UCRs from non-UCRs determining the same functions of individual cryptochromes remain to be investigated.

Shades of green: untying the knots of green photoperception

The development of economical LED technology has enabled the application of different light qualities and quantities to control plant growth. Although we have a comprehensive understanding of plants' perception of red and blue light, the lack of a dedicated green light sensor has frustrated our utilization of intermediate wavelengths, with many contradictory reports in the literature. We discuss the contribution of red and blue photoreceptors to green light perception and highlight how green light can be used to improve crop quality. Importantly, our meta-analysis demonstrates that green light perception should instead be considered as a combination of distinct 'green' and 'yellow' light-induced responses. This distinction will enable clearer interpretation of plants' behaviour in response to green light as we seek to optimize plant growth and nutritional quality in horticultural contexts.

In-cell infrared difference spectroscopy of LOV photoreceptors reveals structural responses to light altered in living cells

Proteins are usually studied in well-defined buffer conditions, which differ substantially from those within a host cell. In some cases, the intracellular environment has an impact on the mechanism, which might be missed by in vitro experiments. IR difference spectroscopy previously has been applied to study the light-induced response of photoreceptors and photoenzymes in vitro Here, we established the in-cell IR difference (ICIRD) spectroscopy in the transmission and attenuated total reflection configuration to investigate the light-induced response of soluble proteins in living bacterial cells. ICIRD spectroscopy on the light, oxygen, or voltage (LOV) domains of the blue light receptors aureochrome and phototropin revealed a suppression of the response of specific secondary structure elements, indicating that the intracellular environment affects LOV photoreceptor mechanisms in general. Moreover, in-cell fluorescence spectroscopy disclosed that the intracellular environment slows down the recovery of the light-induced flavin adduct. Segment-resolved ICIRD spectroscopy on basic-region leucine zipper (bZIP)-LOV of aureochrome 1a from the diatom Phaeodactylum tricornutum indicated a signal progression from the LOV sensor to the bZIP effector independent of unfolding of the connecting A'α-helix, an observation that stood in contrast to in vitro results. This deviation was recapitulated in vitro by emulating the intracellular environment through the addition of the crowding agent BSA, but not by sucrose polymers. We conclude that ICIRD spectroscopy is a noninvasive, label-free approach for assessing conformational changes in receptors in living cells at ambient conditions. As demonstrated, these near-native responses may deviate from the mechanisms established under in vitro conditions.

The photoreceptor UVR8 mediates the perception of both UV-B and UV-A wavelengths up to 350 nm of sunlight with responsivity moderated by cryptochromes

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

Genetic diversity in developmental responses to light spectral quality in barley (Hordeum vulgare L.)

BACKGROUND

Plants use light wavelength, intensity, direction and duration to predict imminent seasonal changes and to determine when to initiate physiological and developmental processes. Among them, crop responses to light are not fully understood. Here, we study how light quality affects barley development, using two broad-spectrum light sources, metal halide (M) and fluorescent (F) lamps. Eleven varieties with known allelic variants for the major flowering time genes were evaluated under controlled conditions (long days, same light intensity). Two experiments were carried out with fully-vernalized plants: 1) control treatments (M, F); 2) shifting chambers 10 days after the start of the experiment (MF, FM).

RESULTS

In general, varieties developed faster under longer exposure to M conditions. The greatest differences were due to a delay promoted by F light bulbs, especially in the time to first node appearance and until the onset of stem elongation. Yield related-traits as the number of seeds were also affected by the conditions experienced. However, not each variety responded equally, and they could be classified in insensitive and sensitive to light quality. Expression levels of flowering time genes HvVRN1, HvFT1 and PPD-H1 were high in M, while HvFT3 and HvVRN2 were higher under F conditions. The expression under shift treatments revealed also a high correlation between HvVRN1 and PPD-H1 transcript levels.

CONCLUSIONS

The characterization of light quality effects has highlighted the important influence of the spectrum on early developmental stages, affecting the moment of onset of stem elongation, and further consequences on the morphology of the plant and yield components. We suggest that light spectra control the vernalization and photoperiod genes probably through the regulation of upstream elements of signalling pathways. The players behind the different responses to light spectra found deserve further research, which could help to optimize breeding strategies.

Structural insights into BIC-mediated inactivation of Arabidopsis cryptochrome 2

Cryptochromes (CRYs) are blue-light receptors in plants that harbor FAD as a cofactor and regulate various physiological responses. Photoactivated CRYs undergo oligomerization, which increases the binding affinity to downstream signaling partners. Despite decades of research on the activation of CRYs, little is known about how they are inactivated. Binding of blue-light inhibitors of cryptochromes (BICs) to CRY2 suppresses its photoactivation, but the underlying mechanism remains unknown. Here, we report crystal structures of CRY2N (CRY2 PHR domain) and the BIC2-CRY2N complex with resolutions of 2.7 and 2.5 Å, respectively. In the BIC2-CRY2N complex, BIC2 exhibits an extremely extended structure that sinuously winds around CRY2N. In this way, BIC2 not only restrains the transfer of electrons and protons from CRY2 to FAD during photoreduction but also interacts with the CRY2 oligomer to return it to the monomer form. Uncovering the mechanism of CRY2 inactivation lays a solid foundation for the investigation of cryptochrome protein function.

Quantification of Spectral Perception of Plants with Light Absorption of Photoreceptors

Although plant responses to artificial lighting spectra often produce abnormal morphogenesis and reduced productivity, no quantification method to determine how plants perceive and respond to light has been available. Our objective in this study was to test whether a plant's spectral perception can be quantified using the light absorption of its major photoreceptors, phytochrome, cryptochrome, and phototropin. We developed an artificial solar lamp and three different light sources, based on a high-pressure sodium lamp, a fluorescent lamp, and red and blue light-emitting diodes, whose absorption by photoreceptors was equal to that of the standard solar spectrum. Cucumber plants grown under the artificial solar and developed light sources showed normal photomorphogenesis and were indistinguishable from each other. Plants grown under unmodified commercial light sources had abnormal photomorphogenesis that made them short and small. The photosynthetic rate was higher under the unmodified light sources; however, dry masses were highest under the artificial solar and modified light sources, indicating that the cucumber plants are optimized to the solar spectrum. Our results clearly demonstrate that the spectral perceptions of plants can be quantified using the light absorption of their photoreceptors, not visual color or spectra. We expect that our findings will contribute to a better understanding of plant perceptions of and responses to light quality, and improve the productivity of plants cultivated under artificial light.

Contributions of cryptochromes and phototropins to stomatal opening through the day

The UV-A/blue photoreceptors phototropins and cryptochromes are both known to contribute to stomatal opening (Δgs) in blue light. However, their relative contributions to the maintenance of gs in blue light through the whole photoperiod remain unknown. To elucidate this question, Arabidopsis phot1 phot2 and cry1 cry2 mutants (MTs) and their respective wild types (WTs) were irradiated with 200 μmolm-2s-1 of blue-, green- or red-light (BL, GL or RL) throughout a 11-h photoperiod. Stomatal conductance (gs) was higher under BL than under RL or GL. Under RL, gs was not affected by either of the photoreceptor mutations, but under GL gs was slightly lower in cry1 cry2 than its WT. Under BL, the presence of phototropins was essential for rapid stomatal opening at the beginning of the photoperiod, and maximal stomatal opening beyond 3 h of irradiation required both phototropins and cryptochromes. Time courses of whole-plant net carbon assimilation rate (Anet) and the effective quantum yield of PSII photochemistry (ΦPSII) were consistent with an Anet-independent contribution of BL on gs both in phot1 phot2 and cry1 cry2 mutants. The changing roles of phototropins and cryptochromes through the day may allow more flexible coordination between gs and Anet.

Cryptochrome mediated magnetic sensitivity in Arabidopsis occurs independently of light-induced electron transfer to the flavin

Arabidopsis cryptochrome-dependent magnetosensitivity occurs via a reaction that does not require light. This excludes radical pairs formed during light-triggered electron transfer to the flavin.

Cryptochromes integrate green light signals into the circadian system

Plants are acutely sensitive of their light environment, adapting their growth habit and prioritizing developmental decisions to maximize fecundity. In addition to providing an energy source and directional information, light quality also contributes to entrainment of the circadian system, an endogenous timing mechanism that integrates endogenous and environmental signalling cues to promote growth. Whereas plants' perception of red and blue portions of the spectrum are well defined, green light sensitivity remains enigmatic. In this study, we show that low fluence rates of green light are sufficient to entrain and maintain circadian rhythms in Arabidopsis and that cryptochromes contribute to this response. Importantly, green light responses are distinguishable from low blue light-induced phenotypes. These data suggest a distinct signalling mechanism enables entrainment of the circadian system in green light-enriched environments, such as those found in undergrowth and in densely planted monoculture.

Floral Induction in the Short-Day Plant Chrysanthemum Under Blue and Red Extended Long-Days

Shorter photoperiod and lower daily light integral (DLI) limit the winter greenhouse production. Extending the photoperiod by supplemental light increases biomass production but inhibits flowering in short-day plants such as Chrysanthemum morifolium. Previously, we reported that flowering in growth-chamber grown chrysanthemum with red (R) and blue (B) LED-light could also be induced in long photoperiods by applying only blue light during the last 4h of 15h long-days. This study investigates the possibility to induce flowering by extending short-days in greenhouses with 4h of blue light. Furthermore, flower induction after 4h of red light extension was tested after short-days RB-LED light in a growth-chamber and after natural solar light in a greenhouse. Plants were grown at 11h of sole source RB light (60:40) in a growth-chamber or solar light in the greenhouse (short-days). Additionally, plants were grown under long-days, which either consisted of short-days as described above extended with 4h of B or R light to long-days or of 15h continuous RB light or natural solar light. Flower initiation and normal capitulum development occurred in the blue-extended long-days in the growth-chamber after 11h of sole source RB, similarly as in short-days. However, when the blue extension was applied after 11h of full-spectrum solar light in a greenhouse, no flower initiation occurred. With red-extended long-days after 11h RB (growth-chamber) flower initiation occurred, but capitulum development was hindered. No flower initiation occurred in red-extended long-days in the greenhouse. These results indicate that multiple components of the daylight spectrum influence different phases in photoperiodic flowering in chrysanthemum in a time-dependent manner. This research shows that smart use of LED-light can open avenues for a more efficient year-round cultivation of chrysanthemum by circumventing the short-day requirement for flowering when applied in emerging vertical farm or plant factories that operate without natural solar light. In current year-round greenhouses' production, however, extension of the natural solar light during the first 11 h of the photoperiod with either red or blue sole LED light, did inhibit flowering.

Interconnection of the Antenna Pigment 8-HDF and Flavin Facilitates Red-Light Reception in a Bifunctional Animal-like Cryptochrome

Cryptochromes are ubiquitous flavin-binding light sensors closely related to DNA-repairing photolyases. The animal-like cryptochrome CraCRY from the green alga Chlamydomonas reinhardtii challenges the paradigm of cryptochromes as pure blue-light receptors by acting as a (6-4) photolyase, using 8-hydroxy-5-deazaflavin (8-HDF) as a light-harvesting antenna with a 17.4 Å distance to flavin and showing spectral sensitivity up to 680 nm. The expanded action spectrum is attributed to the presence of the flavin neutral radical (FADH^•) in the dark, despite a rapid FADH^• decay observed in vitro in samples exclusively carrying flavin. Herein, the red-light response of CraCRY carrying flavin and 8-HDF was studied, revealing a 3-fold prolongation of the FADH^• lifetime in the presence of 8-HDF. Millisecond time-resolved ultraviolet-visible spectroscopy showed the red-light-induced formation and decay of an absorbance band at 458 nm concomitant with flavin reduction. Time-resolved Fourier transform infrared (FTIR) spectroscopy and density functional theory attributed these changes to the deprotonation of 8-HDF, challenging the paradigm of 8-HDF being permanently deprotonated in photolyases. FTIR spectra showed changes in the hydrogen bonding network of asparagine 395, a residue suggested to indirectly control flavin protonation, indicating the involvement of N395 in the stabilization of FADH^•. Fluorescence spectroscopy revealed a decrease in the energy transfer efficiency of 8-HDF upon flavin reduction, possibly linked to 8-HDF deprotonation. The discovery of the interdependence of flavin and 8-HDF beyond energy transfer processes highlights the essential role of the antenna, introducing a new concept enabling CraCRY and possibly other bifunctional cryptochromes to fulfill their dual function.

The Arabidopsis cryptochrome 2 I404F mutant is hypersensitive and shows flavin reduction even in the absence of light

The cryptochrome photoreceptor mutant cry2I404F exhibits hyperactivity in the dark, hypersensitivity in different light conditions, and in contrast to the wild-type protein, its flavin chromophore is reducible even in the absence of light. Plant cryptochromes (cry) are blue-light photoreceptors involved in multiple signaling pathways and various photomorphogenic responses. One biologically hyperactive mutant of a plant cryptochrome that was previously characterized is Arabidopsis cry1L407F (Exner et al. in Plant Physiol 154:1633-1645, 2010). Protein sequence alignments of different cryptochromes revealed that L407 in cry1 corresponds to I404 in cry2. Point mutation of Ile to Phe in cry2 in this position created a novel mutant. The present study provided a baseline data on the elucidation of the properties of cry2I404F. This mutant was still able to bind ATP-triggering conformational changes, as confirmed by partial tryptic digestion and thermo-FAD assays. Surprisingly, the FAD cofactor of cry2I404F was reduced by the addition of reductant even in the absence of light. In vivo, cry2I404F exhibited a cop phenotype in the dark and hypersensitivity to various light conditions compared to cry2 wild type. Overall, these data suggest that the hypersensitivity to red and blue light and hyperactivity of this novel mutant in the dark can be mostly accounted to structural alterations brought forth by the Ile to Phe mutation at position 404 that allows reduction of the flavin chromophore even in the absence of light.

Plasticity of photosynthetic processes and the accumulation of secondary metabolites in plants in response to monochromatic light environments: A review

Light spectra significantly influence plant metabolism, growth and development. Here, we review the effects of monochromatic blue, red and green light compared to those of multispectral light sources on the morpho-anatomical, photosynthetic and molecular traits of herbaceous plants. Emphasis is given to the effect of light spectra on the accumulation of secondary metabolites, which are important bioactive phytochemicals that determine the nutritional quality of vegetables. Overall, blue light may promote the accumulation of phenylpropanoid-based compounds without substantially affecting plant morpho-anatomical traits compared to the effects of white light. Red light, conversely, strongly alters plant morphology and physiology compared to that under white light without showing a consistent positive effect on secondary metabolism. Due to species-specific effects and the small shifts in the spectral band within the same color that can substantially affect plant growth and metabolism, it is conceivable that monochromatic light significantly affects not only plant photosynthetic performance but also the "quality" of plants by modulating the biosynthesis of photoprotective compounds.

Theoretical insights into the formation and stability of radical oxygen species in cryptochromes

Cryptochrome is a blue-light absorbing flavoprotein containing a flavin adenine dinucleotide (FAD) cofactor. FAD can accept up to two electrons and two protons, which can be subsequently transferred to substrates present in the binding pocket. It is well known that reactive oxygen species are generated when triplet molecular oxygen is present in the cavity. Here, we investigate the formation and stability of radical oxygen species in Drosophila melanogaster cryptochrome using molecular dynamics simulations and electronic structure calculations. We find that the superoxide and hydroxyl radicals in doublet spin states are stabilized in the pocket due to the attractive electrostatic interactions and hydrogen bonding with partially reduced FAD. These findings validate from a molecular dynamics perspective that [FAD˙--HO2˙] or [FADH˙-O2˙-] can be alternative radical pairs at the origin of magnetoreception.

Chemical and structural analysis of a photoactive vertebrate cryptochrome from pigeon

Computational and biochemical studies implicate the blue-light sensor cryptochrome (CRY) as an endogenous light-dependent magnetosensor enabling migratory birds to navigate using the Earth's magnetic field. Validation of such a mechanism has been hampered by the absence of structures of vertebrate CRYs that have functional photochemistry. Here we present crystal structures of Columba livia (pigeon) CRY4 that reveal evolutionarily conserved modifications to a sequence of Trp residues (Trp-triad) required for CRY photoreduction. In ClCRY4, the Trp-triad chain is extended to include a fourth Trp (W369) and a Tyr (Y319) residue at the protein surface that imparts an unusually high quantum yield of photoreduction. These results are consistent with observations of night migratory behavior in animals at low light levels and could have implications for photochemical pathways allowing magnetosensing.

How do cryptochromes and UVR8 interact in natural and simulated sunlight?

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

Time-Resolved Infrared and Visible Spectroscopy on Cryptochrome aCRY: Basis for Red Light Reception

Cryptochromes function as flavin-binding photoreceptors in bacteria, fungi, algae, land plants, and insects. The discovery of an animal-like cryptochrome in the green alga Chlamydomonas reinhardtii has expanded the spectral range of sensitivity of these receptors from ultraviolet A/blue light to almost the complete visible spectrum. The broadened light response has been explained by the presence of the flavin neutral radical as a chromophore in the dark. Concomitant with photoconversion of the flavin, an unusually long-lived tyrosyl radical with a red-shifted ultraviolet-visible spectrum is formed, which is essential for the function of the receptor. In this study, the microenvironment of this key residue, tyrosine 373, was scrutinized using time-resolved Fourier transform infrared spectroscopy on several variants of animal-like cryptochrome and density functional theory for band assignment. The reduced tyrosine takes on distinct hydrogen bond scenarios depending on the presence of the C-terminal extension and of a neighboring cysteine. Upon radical formation, all variants showed a signal at 1400 cm^-1, which we assigned to the ν7'a marker band of the CO stretching mode. The exceptionally strong downshift of this band cannot be attributed to a loss of hydrogen bonding only. Time-resolved ultraviolet-visible spectroscopy on W322F, a mutant of the neighboring tryptophan residue, revealed a decrease of the tyrosyl radical lifetime by almost two orders of magnitude, along with a shift of the absorbance maximum from 416 to 398 nm. These findings strongly support the concept of a π-π stacking as an apolar interaction between Y373 and W322 to be responsible for the characteristics of the tyrosyl radical. This concept of radical stabilization has been unknown to cryptochromes so far but might be highly relevant for other homologs with a tetrad of tryptophans and tyrosines as electron donors.

Regulation of flowering by green light depends on its photon flux density and involves cryptochromes

Photoperiodic lighting can promote flowering of long-day plants (LDPs) and inhibit flowering of short-day plants (SDPs). Red (R) and far-red (FR) light regulate flowering through phytochromes, whereas blue light does so primarily through cryptochromes. In contrast, the role of green light in photoperiodic regulation of flowering has been inconsistent in previous studies. We grew four LDP species (two petunia cultivars, ageratum, snapdragon and Arabidopsis) and two SDP species (three chrysanthemum cultivars and marigold) in a greenhouse under truncated 9-h short days with or without 7-h day-extension lighting from green light (peak = 521 nm) at 0, 2, 13 or 25 μmol m^-2  s^-1 or R + white (W) + FR light at 2 μmol m^-2  s^-1 . Increasing the green photon flux density from 0 to 25 μmol m^-2  s^-1 accelerated flowering of all LDPs and delayed flowering of all SDPs. Petunia flowered similarly fast under R + W + FR light and moderate green light but was shorter and developed more branches under green light. To be as effective as R + W + FR light, saturation green photon flux densities were 2 μmol m^-2  s^-1 for LDP ageratum and SDP marigold and 13 μmol m^-2  s^-1 for LDP petunia. Snapdragon was the least sensitive to green light. In Arabidopsis, cryptochrome 2 mediated promotion of flowering under moderate green light, whereas both phytochrome B and cryptochrome 2 mediated that under R + W + FR light. We conclude that 7-h day-extension lighting from green light-emitting diodes can control flowering of photoperiodic ornamentals and that in Arabidopsis, cryptochrome 2 mediates promotion of flowering under green light.