The Arabidopsis PHYTOCHROME KINASE SUBSTRATE2 protein is a phototropin signaling element that regulates leaf flattening and leaf positioning

Phytochrome-Interacting Proteins

Phytochromes are photoreceptors of plants, fungi, slime molds bacteria and heterokonts. These biliproteins sense red and far-red light and undergo light-induced changes between the two spectral forms, Pr and Pfr. Photoconversion triggered by light induces conformational changes in the bilin chromophore around the ring C-D-connecting methine bridge and is followed by conformational changes in the protein. For plant phytochromes, multiple phytochrome interacting proteins that mediate signal transduction, nuclear translocation or protein degradation have been identified. Few interacting proteins are known as bacterial or fungal phytochromes. Here, we describe how the interacting partners were identified, what is known about the different interactions and in which context of signal transduction these interactions are to be seen. The three-dimensional arrangement of these interacting partners is not known. Using an artificial intelligence system-based modeling software, a few predicted and modulated examples of interactions of bacterial phytochromes with their interaction partners are interpreted.

Light and temperature regulation of leaf morphogenesis in Arabidopsis

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

Air channels create a directional light signal to regulate hypocotyl phototropism

In plants, light direction is perceived by the phototropin photoreceptors, which trigger directional growth responses known as phototropism. The formation of a phototropin activation gradient across a photosensitive organ initiates this response. However, the optical tissue properties that functionally contribute to phototropism remain unclear. In this work, we show that intercellular air channels limit light transmittance through various organs in several species. Air channels enhance light scattering in Arabidopsis hypocotyls, thereby steepening the light gradient. This is required for an efficient phototropic response in Arabidopsis and Brassica. We identified an embryonically expressed ABC transporter required for the presence of air channels in seedlings and a structure surrounding them. Our work provides insights into intercellular air space development or maintenance and identifies a mechanism of directional light sensing in plants.

Different LED light spectra's and nano-chelated potassium affect the quality traits of Dolce Vita cut roses in soilless culture condition

Roses are classified as neutral day plants, but high light and cool temperatures produce high quality flowers in roses. As light quantity, the light quality and its special spectra can affect the flower yield and quality. This research aimed to study of the effect of LED light (control (sunlight), blue and red spectra's) and nano-chelated potassium at three levels (0, 1.5 and 3 g/l) on some morphophysiological and biochemical traits of Rosa hybrida cv. Dolce Vita. Light and nano-chelated potassium treatments have a significant effect on most traits measured in the present study. According to the results, the use of red light and nano-chelated potassium in rose cultivation improved the quality characteristics and increased vase life. The highest fresh and dry weight of flowering branch and plant height was observed in red light treatment and the concentration of 3 g/l nano-chelated potassium. Biochemical parameters such as phenolic compounds, leaf and petal flavonoids, petal anthocyanin content, antioxidant capacity and vase life were also significantly increased under red light and with the concentration of 3 g/l nano-chelated potassium compared to the control. In general, it can be said that the use of red light and a concentration of 3 g/l nano-chelated potassium, can be effective in improving the quality of rose flowers, especially in low light condition.

LED omics in Rocket Salad (Diplotaxis tenuifolia): Comparative Analysis in Different Light-Emitting Diode (LED) Spectrum and Energy Consumption

By applying three different LED light treatments, designated as blue (B), red (R)/blue (B), red (R) and white (W) light, as well as the control, the effect on Diplotaxis tenuifolia phenotype (yield and quality), and physiological, biochemical, and molecular status, as well as growing system resource use efficiency, was examined. We observed that basic leaf characteristics, such as leaf area, leaf number, relative chlorophyll content, as well as root characteristics, such as total root length and root architecture, remained unaffected by different LEDs. Yield expressed in fresh weight was slightly lower in LED lights than in the control (1113 g m^-2), with R light producing the least (679 g m^-2). However, total soluble solids were significantly affected (highest, 5.5° Brix, in R light) and FRAP was improved in all LED lights (highest, 191.8 μg/g FW, in B) in comparison to the control, while the nitrate content was less (lowest, 949.2 μg/g FW, in R). Differential gene expression showed that B LED light affected more genes in comparison to R and R/B lights. Although total phenolic content was improved under all LED lights (highest, 1.05 mg/g FW, in R/B), we did not detect a significant amount of DEGs in the phenylpropanoid pathway. R light positively impacts the expression of the genes encoding for photosynthesis components. On the other hand, the positive impact of R light on SSC was possibly due to the expression of key genes being induced, such as SUS1. In summary, this research is an integrative and innovative study, where the exploration of the effect of different LED lights on rocket growing under protected cultivation, in a closed chamber cultivation system, was performed at multiple levels.

The phosphorylation status of NONPHOTOTROPIC HYPOCOTYL3 affects phot2-dependent phototropism in Arabidopsis

The blue light photoreceptors, phototropin 1 (phot1) and phot2, and their signal transducer, NONPHOTOTROPIC HYPOCOTYL3 (NPH3), are activators of the phototropic responses of Arabidopsis hypocotyls. In a recent study, we reported that the control of NPH3 phosphorylation at serine 7 (S7: or S5), S213, S223, S237, S467, S474 (or S476), and S722 (or S723) contributes to the photosensory adaptation of phot1 signaling during the phototropic response. Phosphomimetic NPH3^SE mutant and unphosphorylatable NPH3^SA mutant on those serine residues function efficiently under blue light conditions at fluence rates of 10^-5 µmol m^-2 s^-1 and 10^-3 µmol m^-2 s^-1 or more, respectively. We here demonstrate that phosphomimetic NPH3^SE, but not unphosphorylatable NPH3^SA, promotes phot2-dependent phototropism under blue light condition at 100 µmol m^-2 s^-1. This result suggests that phot1 negatively controls phot2 signaling through the dephosphorylation of NPH3 at those residues and that the hyperactivation of phot1- and phot2-NPH3 complexes does not occur at the same time under high intensity blue light. We hypothesize that the dephosphorylation of NPH3 on those serine residues suppresses both phot1 and phot2 signaling, which results in different impacts on phot1- and phot2-dependent hypocotyl phototropism due to the differences in the photosensitivity and activation levels of phot1 and phot2.

The action of enhancing weak light capture via phototropic growth and chloroplast movement in plants

To cope with fluctuating light conditions, terrestrial plants have evolved precise regulation mechanisms to help optimize light capture and increase photosynthetic efficiency. Upon blue light-triggered autophosphorylation, activated phototropin (PHOT1 and PHOT2) photoreceptors function solely or redundantly to regulate diverse responses, including phototropism, chloroplast movement, stomatal opening, and leaf positioning and flattening in plants. These responses enhance light capture under low-light conditions and avoid photodamage under high-light conditions. NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3) and ROOT PHOTOTROPISM 2 (RPT2) are signal transducers that function in the PHOT1- and PHOT2-mediated response. NPH3 is required for phototropism, leaf expansion and positioning. RPT2 regulates chloroplast accumulation as well as NPH3-mediated responses. NRL PROTEIN FOR CHLOROPLAST MOVEMENT 1 (NCH1) was recently identified as a PHOT1-interacting protein that functions redundantly with RPT2 to mediate chloroplast accumulation. The PHYTOCHROME KINASE SUBSTRATE (PKS) proteins (PKS1, PKS2, and PKS4) interact with PHOT1 and NPH3 and mediate hypocotyl phototropic bending. This review summarizes advances in phototropic growth and chloroplast movement induced by light. We also focus on how crosstalk in signaling between phototropism and chloroplast movement enhances weak light capture, providing a basis for future studies aiming to delineate the mechanism of light-trapping plants to improve light-use efficiency.

Physiological and Transcriptomic Analysis Revealed the Molecular Mechanism of Pinus koraiensis Responses to Light

Korean pine (Pinus koraiensis Sieb. et Zucc.), as the main tree species in northeast China, has important economic and ecological values. Currently, supplementary light has been widely used in plant cultivation projects. However, the studies about different supplementary light sources on the growth and development of Korean pine are few. In this study, the one with no supplementary light was used as the control, and two kinds of light sources were set up: light-emitting diode (LED) and incandescent lamp, to supplement light treatment of Korean pine. The spectrum and intensity of these two light sources were different. The results showed that the growth and physiological-biochemical indicators were significantly different under different supplementary light treatments. The biomass of supplementary light treatment was significantly lower than the control. Compared with the control, IAA and GA were lower, and JA, ABA, ZT, and ETH were higher under supplementary light conditions. Photosynthetic parameters in supplementary light conditions were significantly lower than the control. Supplemental light induces chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid accumulation. From RNA-seq data, differentially expressed genes (DEGs) were observed in all the comparison groups, and there were 487 common DEGs. The expression levels of DEGs encoding transcription factors were also changed. According to GO and KEGG analysis, the plant hormone signal transduction, circadian rhythm-plant, and flavonoid biosynthesis pathways were the most enriched. These results provided a theoretical basis for the response of Korean pine to different supplementary lights.

A tonoplast-localized magnesium transporter is crucial for stomatal opening in Arabidopsis under high Mg2+ conditions

Plant stomata play an important role in CO₂ uptake for photosynthesis and transpiration, but the mechanisms underlying stomatal opening and closing under changing environmental conditions are still not completely understood. Through large-scale genetic screening, we isolated an Arabidopsis mutant (closed stomata2 (cst2)) that is defective in stomatal opening. We cloned the causal gene (MGR1/CST2) and functionally characterized this gene. The mutant phenotype was caused by a mutation in a gene encoding an unknown protein with similarities to the human magnesium (Mg²⁺ ) efflux transporter ACDP/CNNM. MGR1/CST2 was localized to the tonoplast and showed transport activity for Mg²⁺ . This protein was constitutively and highly expressed in guard cells. Knockout of this gene resulted in stomatal closing, decreased photosynthesis and growth retardation, especially under high Mg²⁺ conditions, while overexpression of this gene increased stomatal opening and tolerance to high Mg²⁺ concentrations. Furthermore, guard cell-specific expression of MGR1/CST2 in the mutant partially restored its stomatal opening. Our results indicate that MGR1/CST2 expression in the leaf guard cells plays an important role in maintaining cytosolic Mg²⁺ concentrations through sequestering Mg²⁺ into vacuoles, which is required for stomatal opening, especially under high Mg²⁺ conditions.

Auxin and cytokinin control fate determination of cotyledons in the one-leaf plant Monophyllaea glabra

One-leaf plants in the Gesneriaceae family initially have two cotyledons of identical size; one cotyledon stops growing shortly after germination, whereas the other continues indeterminate growth. Factors involved in the unequal growth have been investigated, and a competitive relationship between the two cotyledons was previously proposed. However, questions regarding the fate determination of the two cotyledons remain: Why does only one cotyledon grow indeterminately while the other stops; is the fate of the cotyledons reversible; and what role does light quality play in the fate determination of the cotyledons? In this study, physiological experiments using the one-leaf plant species Monophyllaea glabra suggest that a biased auxin concentration between the two cotyledons and subsequent cytokinin levels may determine the fate of the cotyledons. In addition, observation of relatively mature individuals without hormone treatment and younger individuals with cytokinin treatment under laboratory growth conditions revealed that the fate determination of the microcotyledon is reversible. Although light quality has been suggested to be important for the determination of cotyledon fate in Streptocarpus rexii, an anisocotylous species, we conclude that light quality is not important in M. glabra.

Shade suppresses wound-induced leaf repositioning through a mechanism involving PHYTOCHROME KINASE SUBSTRATE (PKS) genes

Shaded plants challenged with herbivores or pathogens prioritize growth over defense. However, most experiments have focused on the effect of shading light cues on defense responses. To investigate the potential interaction between shade-avoidance and wounding-induced Jasmonate (JA)-mediated signaling on leaf growth and movement, we used repetitive mechanical wounding of leaf blades to mimic herbivore attacks. Phenotyping experiments with combined treatments on Arabidopsis thaliana rosettes revealed that shade strongly inhibits the wound effect on leaf elevation. By contrast, petiole length is reduced by wounding both in the sun and in the shade. Thus, the relationship between the shade and wounding/JA pathways varies depending on the physiological response, implying that leaf growth and movement can be uncoupled. Using RNA-sequencing, we identified genes with expression patterns matching the hyponastic response (opposite regulation by both stimuli, interaction between treatments with shade dominating the wound signal). Among them were genes from the PKS (Phytochrome Kinase Substrate) family, which was previously studied for its role in phototropism and leaf positioning. Interestingly, we observed reduced shade suppression of the wounding effect in pks2pks4 double mutants while a PKS4 overexpressing line showed constitutively elevated leaves and was less sensitive to wounding. Our results indicate a trait-specific interrelationship between shade and wounding cues on Arabidopsis leaf growth and positioning. Moreover, we identify PKS genes as integrators of external cues in the control of leaf hyponasty further emphasizing the role of these genes in aerial organ positioning.

Plants face different types of stressful situations without the ability to relocate to favorable environments. For example, increasing plant density reduces access to sunlight as plants start to shade each other. Foliar shading represents a stress that many plants cope with by changing their morphology. This includes elongation of stem-like structures and repositioning of leaves to favor access to unfiltered sunlight. Plants also defend themselves against various pathogens including herbivores. Defense mechanisms include the production of deterrent chemical and morphological adaptations such as stunted growth and downwards leaf repositioning. Here we studied the morphological response of plants when simultaneously facing shade and herbivore stress. When facing both stresses petiole growth was intermediate between the shade-enhanced and wound-repressed response. In contrast, the shade cue overrides the wounding cue leading to a similar upwards leaf repositioning in the combined treatments or in the response to shade alone. Using gene expression analyses and genetics we identified two members of the Phytochrome Kinase Substrate family as playing a signal integration role when plants simultaneously faced both stresses. This contributes to our understanding of the mechanisms underlying plant morphological adaptations when facing multiple stresses.

Deciphering the Molecular Mechanisms of Chilling Tolerance in Lsi1-Overexpressing Rice

Improving tolerance to low-temperature stress during the rice seedling stage is of great significance in agricultural science. In this study, using the low silicon gene 1 (Lsi1)-overexpressing (Dular-OE) and wild-type rice (Dular-WT), we showed that Lsi1 overexpression enhances chilling tolerance in Dular-OE. The overexpression of the Lsi1 increases silicon absorption, but it was not the main reason for chilling tolerance in Dular-OE. Instead, our data suggest that the overexpression of a Lsi1-encoding NIP and its interaction with key proteins lead to chilling tolerance in Dular-OE. Additionally, we show that the high-mobility group protein (HMG1) binds to the promoter of Lsi1, positively regulating its expression. Moreover, Nod26-like major intrinsic protein (NIP)’s interaction with α and β subunits of ATP synthase and the 14-3-3f protein was validated by co-immunoprecipitation (Co-IP), bimolecular fluorescent complementary (BiFC), and GST-pulldown assays. Western blotting revealed that the overexpression of NIP positively regulates the ATP-synthase subunits that subsequently upregulate calcineurin B-like interacting protein kinases (CIPK) negatively regulating 14-3-3f. Overall, these NIP-mediated changes trigger corresponding pathways in an orderly manner, enhancing chilling tolerance in Dular-OE.

Physiological Analysis of Phototropic Responses to Blue and Red Light in Arabidopsis

Plants utilize light as sole energy source. To maximize light capture, they are able to detect the light direction and orient themselves toward the light source. This phototropic response is mediated by the plant blue-light photoreceptors phototropin1 and phototropin2 (phot1 and phot2). Although fully differentiated plants also exhibit this response, it can be best observed in etiolated seedlings. Differences in light between the illuminated and shaded site of a seedling stem lead to changes in the auxin distribution, resulting in cell elongation on the shaded site. Since phototropism connects light perception, signaling, and auxin transport, it is of great interest to analyze this response with a fast and simple method. Moreover, pre-exposure to red light enhances the phototropic response via phytochrome A (phyA) and phyB action. Here we describe a method to analyze the phototropic response of Arabidopsis seedlings to blue light and the enhanced response with a red-light pretreatment. With numerous mutants available, its fast germination, and its small size, Arabidopsis is well suited for this analysis. Different genotypes can be simultaneously probed in less than a week.

Blue light promotes vascular reconnection, while red light boosts the physiological response and quality of grafted watermelon seedlings

The wound inflicted during grafting of watermelon seedlings requires rapid and sufficient vascular development which is affected by light quality. Our objective was to investigate the effect of light spectra emitted by light-emitting diodes (LEDs) during healing of grafted watermelon (Citrullus lanatus) seedlings on their vascular development, physiological and phytohormonal profile, and root architecture. Three LEDs emitting red (R), blue (B), and RB with 12% blue (12B) were tested in a healing chamber. During the first three days, the photosynthetic apparatus portrayed by PI_ABS, φ_P0, ψ_E0, and ΔV_IP was less damaged and faster repaired in B-treated seedlings. B and 12B promoted vascular reconnection and root development (length, surface area and volume). This was the result of signaling cascade between phytohormones such as indole-3-acetic acid and others. After vascular reconnection the seedlings switched lights for 3 more days and the picture was reversed. Seedlings treated with B for the first 3 days and R for days 4 to 6 had better photosynthetic characteristics, root system development, morphological, shoot and root biomass, and quality (i.e. Dickson’s quality index) characteristics. We concluded that blue light is important during the first 3 days of healing, while the presence of red is necessary after vascular reconnection.

Phototropin-mediated perception of light direction in leaves regulates blade flattening

One conserved feature among angiosperms is the development of flat thin leaves. This developmental pattern optimizes light capture and gas exchange. The blue light (BL) receptors phototropins are required for leaf flattening, with the null phot1phot2 mutant showing curled leaves in Arabidopsis (Arabidopsis thaliana). However, key aspects of their function in leaf development remain unknown. Here, we performed a detailed spatiotemporal characterization of phototropin function in Arabidopsis leaves. We found that phototropins perceive light direction in the blade, and, similar to their role in hypocotyls, they control the spatial pattern of auxin signaling, possibly modulating auxin transport, to ultimately regulate cell expansion. Phototropin signaling components in the leaf partially differ from hypocotyls. Moreover, the light response on the upper and lower sides of the leaf blade suggests a partially distinct requirement of phototropin signaling components on each side. In particular, NON PHOTOTROPIC HYPOCOTYL 3 showed an adaxial-specific function. In addition, we show a prominent role of PHYTOCHROME KINASE SUBSTRATE 3 in leaf flattening. Among auxin transporters, PIN-FORMED 3,4,7 and AUXIN RESISTANT 1 (AUX1)/LIKE AUXIN RESISTANT 1 (LAX1) are required for the response while ABCB19 has a regulatory role. Overall, our results show that directional BL perception by phototropins is a key aspect of leaf development, integrating endogenous and exogenous signals.

Effects of UV radiation on transcript and metabolite accumulation are dependent on monochromatic light background in cucumber

During recent years, we have advanced our understanding of plant molecular responses to ultraviolet radiation (UV, 280-400 nm); however, how plants respond to UV radiation under different spectral light qualities is poorly understood. In this study, cucumber plants (Cucumis sativus "Lausanna RZ F1") were grown under monochromatic blue, green, red, and broadband white light in combination with UV radiation. The effects of light quality and UV radiation on acclimatory responses were assessed by measuring transcript accumulation of ELONGATED HYPOCOTYL 5 (HY5), CHALCONE SYNTHASE 2 (CHS2), and LIGHT HARVESTING COMPLEX II (LHCII), and the accumulation of flavonoids and hydroxycinnamic acids in the leaves. The growth light backgrounds differentially regulated gene expression and metabolite accumulation. While HY5 and CHS2 transcripts were induced by blue and white light, LHCII was induced by white and red light. Furthermore, UV radiation antagonized the effects of blue, red, green, and white light on transcript accumulation in a gene-dependent manner. Plants grown under blue light with supplementary UV radiation increased phenylalanine, flavonol disaccharide I and caffeic acid contents compared to those exposed only to blue light. UV radiation also induced the accumulation of flavonol disaccharide I and II, ferulic acid hexose and coumaric acid hexose in plants grown under green light. Our findings provide a further understanding of plant responses to UV radiation in combination with different light spectra and contribute to the design of light recipes for horticultural practices that aim to modify plant metabolism and ultimately improve crop quality.

Light-triggered and phosphorylation-dependent 14-3-3 association with NON-PHOTOTROPIC HYPOCOTYL 3 is required for hypocotyl phototropism

NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3) is a key component of the auxin-dependent plant phototropic growth response. We report that NPH3 directly binds polyacidic phospholipids, required for plasma membrane association in darkness. We further demonstrate that blue light induces an immediate phosphorylation of a C-terminal 14-3-3 binding motif in NPH3. Subsequent association of 14-3-3 proteins is causal for the light-induced release of NPH3 from the membrane and accompanied by NPH3 dephosphorylation. In the cytosol, NPH3 dynamically transitions into membraneless condensate-like structures. The dephosphorylated state of the 14-3-3 binding site and NPH3 membrane recruitment are recoverable in darkness. NPH3 variants that constitutively localize either to the membrane or to condensates are non-functional, revealing a fundamental role of the 14-3-3 mediated dynamic change in NPH3 localization for auxin-dependent phototropism. This regulatory mechanism might be of general nature, given that several members of the NPH3-like family interact with 14-3-3 via a C-terminal motif.

NPH3 is required for auxin-dependent plant phototropism. Here Reuter et al. show that NPH3 is a plasma membrane-bound phospholipid-binding protein and that in response to blue light, NPH3 is phosphorylated and associates with 14-3-3 proteins which leads to dissociation from the plasma membrane.

Phosphorylation of NONPHOTOTROPIC HYPOCOTYL3 affects photosensory adaptation during the phototropic response

Photosensory adaptation, which can be classified as sensor or effector adaptation, optimizes the light sensing of living organisms by tuning their sensitivity to changing light conditions. During the phototropic response in Arabidopsis (Arabidopsis thaliana), the light-dependent expression controls of blue-light (BL) photoreceptor phototropin 1 (phot1) and its modulator ROOT PHOTOTROPISM2 (RPT2) are known as the molecular mechanisms underlying sensor adaptation. However, little is known about effector adaption in plant phototropism. Here, we show that control of the phosphorylation status of NONPHOTOTROPIC HYPOCOTYL3 (NPH3) leads to effector adaptation in hypocotyl phototropism. We generated unphosphorable and phosphomimetic NPH3 proteins on seven phosphorylation sites in the etiolated seedlings of Arabidopsis. Unphosphorable NPH3 showed a shortening of its retention time in the cytosol and caused an inability to adapt to very low fluence rates of BL (∼10-5 µmol m-2 s-1) during the phototropic response. In contrast, the phosphomimetic NPH3 proteins had a lengthened retention time in the cytosol and could not enable the adaptation to BL at fluence rates of 10-3 µmol m-2 s-1 or more. Our results indicate that the activation level of phot1 and the corresponding phosphorylation level of NPH3 determine the dissociation rate and the reassociation rate of NPH3 on the plasma membrane, respectively. These mechanisms may moderately maintain the active state of phot1 signaling across a broad range of BL intensities and contribute to the photosensory adaptation of phot1 signaling during the phototropic response in hypocotyls.

Phototropin 1 Mediates High-Intensity Blue Light-Induced Chloroplast Accumulation Response in a Root Phototropism 2-Dependent Manner in Arabidopsis phot2 Mutant Plants

Phototropins, namely, phototropin 1 (phot1) and phototropin 2 (phot2), mediate chloroplast movement to maximize photosynthetic efficiency and prevent photodamage in plants. Phot1 primarily functions in chloroplast accumulation process, whereas phot2 mediates both chloroplast avoidance and accumulation responses. The avoidance response of phot2-mediated chloroplasts under high-intensity blue light (HBL) limited the understanding of the function of phot1 in the chloroplast accumulation process at the HBL condition. In this study, we showed that the phot2 mutant exhibits a chloroplast accumulation response under HBL, which is defective when the root phototropism 2 (RPT2) gene is mutated in the phot2 background, mimicking the phenotype of the phot1 phot2 double mutant. A further analysis revealed that the expression of RPT2 was induced by HBL and the overexpression of RPT2 could partially enhance the chloroplast accumulation response under HBL. These results confirmed that RPT2 also participates in regulating the phot1-mediated chloroplast accumulation response under HBL. In contrast, RPT2 functions redundantly with neural retina leucine zipper (NRL) protein for chloroplast movement 1 (NCH1) under low-light irradiation. In addition, no chloroplast accumulation response was detected in the phot2 jac1 double mutant under HBL, which has been previously observed in phot2 rpt2 and phot1 phot2 double mutants. Taken together, our results indicated that phot1 mediates the HBL-induced chloroplast accumulation response in an RPT2-dependent manner and is also regulated by j-domain protein required for chloroplast accumulation response 1 (JAC1).

Phototropin2-mediated hypocotyl phototropism is negatively regulated by JAC1 and RPT2 in Arabidopsis

Hypocotyl phototropism is redundantly mediated by phot1 and phot2, two blue light receptor phototropins, under the intensity of blue light>1 μmol m^-2 s^-1. As light intensity increases, phot1 inhibits the phot2-mediated response. To date, only Arabidopsis Root Phototropism2 (RPT2) has been shown to participate in phot1-mediated inhibition of phototropism. To dissect the signaling network that underlies phot1-mediated inhibition, we carried out a yeast two-hybrid (Y2H) screening assay for RPT2 interacting proteins and identified J-domain protein required for chloroplast accumulation response 1 (JAC1). The interaction between JAC1 and RPT2 was verified by bimolecular fluorescence complementation and Co-IP assays. JAC1 is expressed mainly in cotyledons and hypocotyls. Like RPT2, JAC1 can be induced by blue light, suggesting that it may function similarly to RPT2 in the inhibition of phototropism. Genetic analysis showed that jac1 mutation significantly enhanced the hypocotyl bending of phot1 mutants towards intermediate-intensity blue light, and this effect was inhibited by the constitutive expression of JAC1 in the phot1 jac1 mutant. The phot1 rpt2 double mutant also exhibited enhanced phototropism compared with the phot1 mutant. Taken together, our data clearly demonstrate that JAC1 cooperates with RPT2 to negatively regulate hypocotyl phototropism in plants and may act either downstream of or in parallel with phot1.

Unveiling the shade nature of cyanic leaves: A view from the "blue absorbing side" of anthocyanins

Anthocyanins have long been suggested as having great potential in offering photoprotection to plants facing high light irradiance. Nonetheless, their effective ability in protecting the photosynthetic apparatus from supernumerary photons has been questioned by some authors, based upon the inexact belief that anthocyanins almost exclusively absorb green photons, which are poorly absorbed by chlorophylls. Here we focus on the blue light absorbing features of anthocyanins, a neglected issue in anthocyanin research. Anthocyanins effectively absorb blue photons: the absorbance of blue relative to green photons increases from tri- to mono-hydroxy B-ring substituted structures, reaching up to 50% of green photons absorption. We offer a comprehensive picture of the molecular events activated by low blue-light availability, extending our previous analysis in purple and green basil, which we suggest to be responsible for the "shade syndrome" displayed by cyanic leaves. While purple leaves display overexpression of genes promoting chlorophyll biosynthesis and light harvesting, in green leaves it is the genes involved in the stability/repair of photosystems that are largely overexpressed. As a corollary, this adds further support to the view of an effective photoprotective role of anthocyanins. We discuss the profound morpho-anatomical adjustments imposed by the epidermal anthocyanin shield, which reflect adjustments in light harvesting capacity under imposed shade and make complex the analysis of the photosynthetic performance of cyanic versus acyanic leaves.

The ectopic expression of Arabidopsis glucosyltransferase UGT74D1 affects leaf positioning through modulating indole-3-acetic acid homeostasis

Leaf angle is an important agronomic trait affecting photosynthesis efficiency and crop yield. Although the mechanisms involved in the leaf angle control are intensively studied in monocots, factors contribute to the leaf angle in dicots are largely unknown. In this article, we explored the physiological roles of an Arabidopsis glucosyltransferase, UGT74D1, which have been proved to be indole-3-acetic acid (IAA) glucosyltransferase in vitro. We found that UGT74D1 possessed the enzymatic activity toward IAA glucosylation in vivo and its expression was induced by auxins. The ectopically expressed UGT74D1 obviously reduced the leaf angle with an altered IAA level, auxin distribution and cell size in leaf tissues. The expression of several key genes involved in the leaf shaping and leaf positioning, including PHYTOCHROME KINASE SUBSTRATE (PKS) genes and TEOSINTE BRANCHED1, CYCLOIDEA, and PCF (TCP) genes, were dramatically changed by ectopic expression of UGT74D1. In addition, clear transcription changes of YUCCA genes and other auxin related genes can be observed in overexpression lines. Taken together, our data indicate that glucosyltransferase UGT74D1 could affect leaf positioning through modulating auxin homeostasis and regulating transcription of PKS and TCP genes, suggesting a potential new role of UGT74D1 in regulation of leaf angle in dicot Arabidopsis.

A comprehensive review on carotenoids in foods and feeds: status quo, applications, patents, and research needs

Carotenoids are isoprenoids widely distributed in foods that have been always part of the diet of humans. Unlike the other so-called food bioactives, some carotenoids can be converted into retinoids exhibiting vitamin A activity, which is essential for humans. Furthermore, they are much more versatile as they are relevant in foods not only as sources of vitamin A, but also as natural pigments, antioxidants, and health-promoting compounds. Lately, they are also attracting interest in the context of nutricosmetics, as they have been shown to provide cosmetic benefits when ingested in appropriate amounts. In this work, resulting from the collaborative work of participants of the COST Action European network to advance carotenoid research and applications in agro-food and health (EUROCAROTEN, www.eurocaroten.eu, https://www.cost.eu/actions/CA15136/#tabs|Name:overview) research on carotenoids in foods and feeds is thoroughly reviewed covering aspects such as analysis, carotenoid food sources, carotenoid databases, effect of processing and storage conditions, new trends in carotenoid extraction, daily intakes, use as human, and feed additives are addressed. Furthermore, classical and recent patents regarding the obtaining and formulation of carotenoids for several purposes are pinpointed and briefly discussed. Lastly, emerging research lines as well as research needs are highlighted.

An ATP-Binding Cassette Transporter, ABCB19, Regulates Leaf Position and Morphology during Phototropin1-Mediated Blue Light Responses

Blue light regulates multiple processes that optimize light capture and gas exchange in plants, including chloroplast movement, changes in stomatal conductance, and altered organ positioning. In Arabidopsis (Arabidopsis thaliana), these processes are primarily modulated by the blue light phototropin photoreceptors phot1 and phot2. Changes in leaf positioning and shape involve several signaling components that include NON-PHOTOTROPIC HYPOCOTYL3, PHYTOCHROME KINASE SUBSTRATE, ROOT PHOTOTROPISM2, and alterations in localized auxin streams. Direct phosphorylation of the auxin transporter ATP-BINDING CASSETTE subfamily B19 (ABCB19) by phot1 in phototropic seedlings suggests that phot1 may directly regulate ABCB19 to adjust auxin-dependent leaf responses. Here, abcb19 mutants were analyzed for fluence and blue light-dependent changes in leaf positioning and morphology. abcb19 displays upright petiole angles that remain unchanged in response to red and blue light. Similarly, abcb19 mutants develop irregularly wavy rosette leaves that are less sensitive to blue light-mediated leaf flattening. Visualization of auxin distribution, measurement of auxin transport in protoplasts, and direct quantification of free auxin levels suggest these irregularities are caused by misregulation of ABCB19-mediated auxin distribution in addition to light-dependent auxin biosynthesis.

CIPK23 regulates blue light-dependent stomatal opening in Arabidopsis thaliana

Phototropins (phot1 and phot2) are plant blue light receptor kinases that function to mediate phototropism, chloroplast movement, leaf flattening, and stomatal opening in Arabidopsis. Considerable progress has been made in understanding the mechanisms associated with phototropin receptor activation by light. However, the identities of phototropin signaling components are less well understood by comparison. In this study, we specifically searched for protein kinases that interact with phototropins by using an in vitro screening method (AlphaScreen) to profile interactions against an Arabidopsis protein kinase library. We found that CBL-interacting protein kinase 23 (CIPK23) interacts with both phot1 and phot2. Although these interactions were verified by in vitro pull-down and in vivo bimolecular fluorescence complementation assays, CIPK23 was not phosphorylated by phot1, as least in vitro. Mutants lacking CIPK23 were found to exhibit impaired stomatal opening in response to blue light but no deficits in other phototropin-mediated responses. We further found that blue light activation of inward-rectifying K⁺ (K⁺ _in ) channels was impaired in the guard cells of cipk23 mutants, whereas activation of the plasma membrane H⁺ -ATPase was not. The blue light activation of K⁺ _in channels was also impaired in the mutant of BLUS1, which is one of the phototropin substrates in guard cells. We therefore conclude that CIPK23 promotes stomatal opening through activation of K⁺ _in channels most likely in concert with BLUS1, but through a mechanism other than activation of the H⁺ -ATPase. The role of CIPK23 as a newly identified component of phototropin signaling in stomatal guard cells is discussed.

Interactive Effect of Light and CdO Nanoparticles on Dodonaea viscosa Morphological, Antioxidant, and Phytochemical Properties

Cadmium nanoparticles (NPs) used in semiconducting devices are photosensitive and optically active. The objective of this study was to investigate the interactive effect of different spectral lights and CdO NPs on morphological, antioxidant, and phytochemical characteristics of Dodonaea viscosa. The plants were grown on media in the presence of green and chemically synthesized CdO NPs and under red, yellow, green, blue, and white light intensities. Results illustrated that plant morphological parameters changed in the presence of different spectral lights and NPs behaved differentially under different spectral lights. Fresh and dry weights of plants decreased in the presence of NPs in the media; however, the concentration and route of synthesis of NPs have a significant effect on these parameters. The same was observed in the case of shoot and root lengths; however, green synthesized NPs were found to be less toxic under different spectral lights. The total antixodant response increased under yellow, blue, and white lights, while the total reducing potential of plant extracts significantly varied depending upon the NP concentration and light spectrum. Different spectral lights significantly influenced the syntheses of phenolics and flavonoids under CdO NP stress and light regimes. It is concluded that toxicity of NPs also depends upon the wavelength of striking light that varies the morphological, biochemical, and antioxidative response of the plants. Furthermore, the white light might have synergistic effects of different wavelengths.

Domain Organization in Plant Blue-Light Receptor Phototropin2 of Arabidopsis thaliana Studied by Small-Angle X-ray Scattering

Phototropin2 (phot2) is a blue-light (BL) receptor protein that regulates the BL-dependent activities of plants for efficient photosynthesis. Phot2 is composed of two light-oxygen-voltage sensing domains (LOV1 and LOV2) to absorb BL, and a kinase domain. Photo-activated LOV domains, especially LOV2, play a major role in photo-dependent increase in the phosphorylation activity of the kinase domain. The atomic details of the overall structure of phot2 and the intramolecular mechanism to convert BL energy to a phosphorylation signal remain unknown. We performed structural studies on the LOV fragments LOV1, LOV2, LOV2-linker, and LOV2-kinase, and full-length phot2, using small-angle X-ray scattering (SAXS). The aim of the study was to understand structural changes under BL irradiation and discuss the molecular mechanism that enhance the phosphorylation activity under BL. SAXS is a suitable technique for visualizing molecular structures of proteins in solution at low resolution and is advantageous for monitoring their structural changes in the presence of external physical and/or chemical stimuli. Structural parameters and molecular models of the recombinant specimens were obtained from SAXS profiles in the dark, under BL irradiation, and after dark reversion. LOV1, LOV2, and LOV2-linker fragments displayed minimal structural changes. However, BL-induced rearrangements of functional domains were noted for LOV2-kinase and full-length phot2. Based on the molecular model together with the absorption measurements and biochemical assays, we discuss the intramolecular interactions and domain motions necessary for BL-enhanced phosphorylation activity of phot2.

Impairment of chloroplast movement reduces growth and delays reproduction of Arabidopsis thaliana in natural and controlled conditions

PREMISE

The importance of chloroplast movement for plant growth in constant, controlled light and of nonphotochemical quenching (NPQ) in variable, natural light are known. Here we concurrently investigated growth and reproduction of several Arabidopsis thaliana mutants to assess the relative importance of photoprotection via chloroplast movement and NPQ.

METHODS

Plants were grown outdoors (natural conditions) or in a growth chamber with variable light and chilling temperatures (controlled conditions). Phenotypic growth and reproductive variables were determined at set times before maturity in wild-type (WT) and phot1, phot2, phot1phot2 (e.g., impaired chloroplast movement, stomatal conductance, leaf flattening), chup1 (impaired chloroplast movement), and npq1 (reduced NPQ) plants.

RESULTS

Mutants were most adversely affected in natural conditions, with phot1phot2 and chup1 most severely impacted. These mutants bolted later and produced fewer leaves and siliques, less leaf biomass, and fewer secondary inflorescences than WT. In controlled conditions, leaf traits of these mutants were unaffected, but phot1phot2 bolted later and produced fewer secondary inflorescences and siliques than WT. For most variables, there were significant interactions between growth conditions and plant genotype. Many variables were correlated, but those relationships changed with growth conditions and genotype.

CONCLUSIONS

Phenotypic variables at the time of the harvest were strongly affected by growth conditions and genotype. In natural conditions, phot1phot2 and chup1 mutants were most adversely affected, demonstrating the importance of chloroplast movement. In controlled conditions, only phot1phot2 was consistently affected, also emphasizing the important, pleiotropic effects of phototropins. In both conditions, NPQ was less important.

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.

Phototropin2 Contributes to the Chloroplast Avoidance Response at the Chloroplast-Plasma Membrane Interface

Blue-light-induced chloroplast movements play an important role in maximizing light utilization for photosynthesis in plants. Under a weak light condition, chloroplasts accumulate to the cell surface to capture light efficiently (chloroplast accumulation response). Conversely, chloroplasts escape from strong light and move to the side wall to reduce photodamage (chloroplast avoidance response). The blue light receptor phototropin (phot) regulates these chloroplast movements and optimizes leaf photosynthesis by controlling other responses in addition to chloroplast movements. Seed plants such as Arabidopsis (Arabidopsis thaliana) have phot1 and phot2. They redundantly mediate phototropism, stomatal opening, leaf flattening, and the chloroplast accumulation response. However, the chloroplast avoidance response is induced by strong blue light and regulated primarily by phot2. Phots are localized mainly on the plasma membrane. However, a substantial amount of phot2 resides on the chloroplast outer envelope. Therefore, differentially localized phot2 might have different functions. To determine the functions of plasma membrane- and chloroplast envelope-localized phot2, we tethered it to these structures with their respective targeting signals. Plasma membrane-localized phot2 regulated phototropism, leaf flattening, stomatal opening, and chloroplast movements. Chloroplast envelope-localized phot2 failed to mediate phototropism, leaf flattening, and the chloroplast accumulation response but partially regulated the chloroplast avoidance response and stomatal opening. Based on the present and previous findings, we propose that phot2 localized at the interface between the plasma membrane and the chloroplasts is required for the chloroplast avoidance response and possibly for stomatal opening as well.

Cryptochrome-mediated hypocotyl phototropism was regulated antagonistically by gibberellic acid and sucrose in Arabidopsis

Both phototropins (phot1 and phot2) and cryptochromes (cry1 and cry2) were proven as the Arabidopsis thaliana blue light receptors. Phototropins predominately function in photomovement, and cryptochromes play a role in photomorphogenesis. Although cryptochromes have been proposed to serve as positive modulators of phototropic responses, the underlying mechanism remains unknown. Here, we report that depleting sucrose from the medium or adding gibberellic acids (GAs) can partially restore the defects in phototropic curvature of the phot1 phot2 double mutants under high-intensity blue light; this restoration does not occur in phot1 phot2 cry1 cry2 quadruple mutants and nph3 (nonphototropic hypocotyl 3) mutants which were impaired phototropic response in sucrose-containing medium. These results indicate that GAs and sucrose antagonistically regulate hypocotyl phototropism in a cryptochromes dependent manner, but it showed a crosstalk with phototropin signaling on NPH3. Furthermore, cryptochromes activation by blue light inhibit GAs synthesis, thus stabilizing DELLAs to block hypocotyl growth, which result in the higher GAs content in the shade side than the lit side of hypocotyl to support the asymmetric growth of hypocotyl. Through modulation of the abundance of DELLAs by sucrose depletion or added GAs, it revealed that cryptochromes have a function in mediating phototropic curvature.

Corrigendum: The Arabidopsis ATP-BINDING CASSETTE Transporter ABCB21 Regulates Auxin Levels in Cotyledons, the Root Pericycle, and Leaves

[This corrects the article DOI: 10.3389/fpls.2019.00806.].

Raf-like kinases CBC1 and CBC2 negatively regulate stomatal opening by negatively regulating plasma membrane H+-ATPase phosphorylation in Arabidopsis

Stomatal pores, which are surrounded by pairs of guard cells in the plant epidermis, regulate gas exchange between plants and the atmosphere, thereby controlling photosynthesis and transpiration. Blue light works as a signal to guard cells, to induce intracellular signaling and open stomata. Blue light receptor phototropins (phots) are activated by blue light; phot-mediated signals promote plasma membrane (PM) H+-ATPase activity via C-terminal Thr phosphorylation, serving as the driving force for stomatal opening in guard cells. However, the details of this signaling process are not fully understood. In this study, through an in vitro screening of phot-interacting protein kinases, we obtained the CBC1 and CBC2 that had been reported as signal transducers in stomatal opening. Promoter activities of CBC1 and CBC2 indicated that both genes were expressed in guard cells. Single and double knockout mutants of CBC1 and CBC2 showed no lesions in the context of phot-mediated phototropism, chloroplast movement, or leaf flattening. In contrast, the cbc1cbc2 double mutant showed larger stomatal opening under both dark and blue light conditions. Interestingly, the level of phosphorylation of C-terminal Thr of PM H+-ATPase was higher in double mutant guard cells. The larger stomatal openings of the double mutant were effectively suppressed by the phytohormone abscisic acid (ABA). CBC1 and CBC2 interacted with BLUS1 and PM H+-ATPase in vitro. From these results, we conclude that CBC1 and CBC2 act as negative regulators of stomatal opening, probably via inhibition of PM H+-ATPase activity.

The continuing arc toward phototropic enlightenment

Phototropism represents a simple physiological mechanism-differential growth across the growing organ of a plant-to respond to gradients of light and maximize photosynthetic light capture (in aerial tissues) and water/nutrient acquisition (in roots). The phototropin blue light receptors, phot1 and phot2, have been identified as the essential sensors for phototropism. Additionally, several downstream signal/response components have been identified, including the phot-interacting proteins NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3) and PHYTOCHROME SUBSTRATE 4 (PKS4). While the structural and photochemical properties of the phots are quite well understood, much less is known about how the phots signal through downstream regulators. Recent advances have, however, provided some intriguing clues. It appears that inactive receptor phot1 is found dispersed in a monomeric form at the plasma membrane in darkness. Upon light absorption dimerizes and clusters in sterol-rich microdomains where it is signal active. Additional studies showed that the phot-regulated phosphorylation status of both NPH3 and PKS4 is linked to phototropic responsiveness. While PKS4 can function as both a positive (in low light) and a negative (in high light) regulator of phototropism, NPH3 appears to function solely as a key positive regulator. Ultimately, it is the subcellular localization of NPH3 that appears crucial, an aspect regulated by its phosphorylation status. While phot1 activation promotes dephosphorylation of NPH3 and its movement from the plasma membrane to cytoplasmic foci, phot2 appears to modulate relocalization back to the plasma membrane. Together these findings are beginning to illuminate the complex biochemical and cellular events, involved in adaptively modifying phototropic responsiveness under a wide varying range of light conditions.

Photoresponse to different lighting strategies during red leaf lettuce growth

The objective of the study was to investigate the effects of growth-stage specific lighting for the physiological homeostasis of red leaf lettuce (Lactuca sativa L. cv. Red Cos), by measuring the productivity of photosynthesis and primary metabolism. In the experiments, the main photosynthetic photon flux consisted of red (R) and blue (B) light, supplemented with blue, green (G) or UV-A wavelengths. Decrease of fructose, accompanied by significant decrease of stomatal conductance (gs), the ratio of intracellular to ambient CO2 concentration (Ci/Ca), photosynthetic rate (Pr), light adapted actual quantum yield of PSII photochemistry (ΦPSII), biomass formation and significant increase of transpiration rate (Tr) suggest that supplemental UV-A during maturity stage, after supplemental green irradiation during seedling stage (BRG to BRUV) was the least favourable condition for red leaf lettuce. However, constant irradiation with supplemental green (BRG) or supplemental green irradiation after increased blue exposure (B↑R to BRG) resulted in significant increase of Pr, gs, Ci/Ca, and light use efficiency(LUE), and decrease of Tr and Water use efficiency (WUE). Significant increase of leaf area was observed under supplemental green in both seedlings (BR; BRG) and matured plants (B↑R to BRG). Significant increase of specific leaf area was found under supplemental green (BRG) for seedlings and under increased blue (B↑R) for matured plants. Accordingly, the most favourable growth-stage specific lighting spectrum strategy for red leaf lettuce, based on photosynthetic and primary metabolite response, is supplemental green irradiation after increased blue exposure (B↑R to BRG), whereas, the most favourable condition for seedlings is BRG. According to the PCA correlation matrix, associations among the measured data indicate that WUE negatively correlated with gs and Ci/Ca, while LUE positively correlated with gs and Pr. However, weak correlations between ФPSII, LUE and photochemical reflectance index (PRI) suggest that selected light conditions were not optimal for red leaf lettuce.

Spectral Composition of Light Affects Sensitivity to UV-B and Photoinhibition in Cucumber

Ultraviolet B (UV-B) (280-315 nm) and ultraviolet A (UV-A) (315-400 nm) radiation comprise small portions of the solar radiation but regulate many aspects of plant development, physiology and metabolism. Until now, how plants respond to UV-B in the presence of different light qualities is poorly understood. This study aimed to assess the effects of a low UV-B dose (0.912 ± 0.074 kJ m^-2 day^-1, at a 6 h daily UV exposure) in combination with four light treatments (blue, green, red and broadband white at 210 μmol m^-2 s^-1 Photosynthetically active radiation [PAR]) on morphological and physiological responses of cucumber (Cucumis sativus cv. "Lausanna RZ F1"). We explored the effects of light quality backgrounds on plant morphology, leaf gas exchange, chlorophyll fluorescence, epidermal pigment accumulation, and on acclimation ability to saturating light intensity. Our results showed that supplementary UV-B significantly decreased biomass accumulation in the presence of broad band white, blue and green light, but not under red light. UV-B also reduced the photosynthetic efficiency of CO₂ fixation (α) when combined with blue light. These plants, despite showing high accumulation of anthocyanins, were unable to cope with saturating light conditions. No significant effects of UV-B in combination with green light were observed for gas exchange and chlorophyll fluorescence parameters, but supplementary UV-B significantly increased chlorophyll and flavonol contents in the leaf epidermis. Plants grown under red light and UV-B significantly increased maximum photosynthetic rate and dark respiration compared to pure red light. Additionally, red and UV-B treated plants exposed to saturating light intensity showed higher quantum yield of photosystem II (PSII), fraction of open PSII centres and electron transport rate and showed no effect on the apparent maximum quantum efficiency of PSII photochemistry (F_v/F_m) or non-photochemical quenching, in contrast to solely red-light conditions. These findings provide new insights into how plants respond to UV-B radiation in the presence of different light spectra.

Small RNAs as plant morphogens

The coordination of cell fate decisions within complex multicellular structures rests on intercellular communication. To generate ordered patterns, cells need to know their relative positions within the growing structure. This is commonly achieved via the production and perception of mobile signaling molecules. In animal systems, such positional signals often act as morphogens and subdivide a field of cells into domains of discrete cell identities using a threshold-based readout of their mobility gradient. Reflecting the independent origin of multicellularity, plants evolved distinct signaling mechanisms to drive cell fate decisions. Many of the basic principles underlying developmental patterning are, however, shared between animals and plants, including the use of signaling gradients to provide positional information. In plant development, small RNAs can act as mobile instructive signals, and similar to classical morphogens in animals, employ a threshold-based readout of their mobility gradient to generate precisely defined cell fate boundaries. Given the distinctive nature of peptide morphogens and small RNAs, how might mechanisms underlying the function of traditionally morphogens be adapted to create morphogen-like behavior using small RNAs? In this review, we highlight the contributions of mobile small RNAs to pattern formation in plants and summarize recent studies that have advanced our understanding regarding the formation, stability, and interpretation of small RNA gradients.

Photoreaction Dynamics of Full-Length Phototropin from Chlamydomonas reinhardtii

Phototropin (phot) is a blue light sensor involved in the light responses of several species from green algae to higher plants. Phot consists of two photoreceptive domains (LOV1 and LOV2) and a Ser/Thr kinase domain. These domains are connected by a hinge and a linker domain. So far, studies on the photochemical reaction dynamics of phot have been limited to short fragments, and the reactions of intact phot have not been well elucidated. Here, the photoreactions of full-length phot and of several mutants from Chlamydomonas reinhardtii (Cr) were investigated by the transient grating and circular dichroism (CD) methods. Full-length Cr phot is in monomeric form in both dark and light states and shows conformational changes upon photoexcitation. When LOV1 is excited, the hinge helix unfolds with a time constant of 77 ms. Upon excitation of LOV2, the linker helix unfolds initially followed by a tertiary structural change of the kinase domain with a time constant of 91 ms. The quantum yield of conformational change after adduct formation of LOV2 is much smaller than that of LOV1, indicating that reactive and nonreactive forms exist. The conformational changes associated with the excitations of LOV1 and LOV2 occur independently and additively, even when they are excited simultaneously. Hence, the role of LOV1 is not to enhance the kinase activity in addition to LOV2 function; we suggest LOV1 has different functions such as regulation of intermolecular interactions.

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.

Measurements of Photoreaction and Kinase Activity of Phototropin, a Photoreceptor Protein for Tropic Response in Plants: Involvement of Kinase Activity in the Photosensitivity of Tropic Response

Phototropin is a photoreceptor protein responsible for phototropic responses in plants. A phototropin molecule has two photoreceptive domains named LOV1 and LOV2 in the N-terminal region. Blue light absorbed by a chromophore in these domains triggers conformational changes in the protein moiety. The C-terminal region of phototropin forms a Ser/Thr kinase that is activated by these conformational changes. The activated phototropin kinase transmits signals downstream leading to tropic responses. The lifetime of the activated state may concern the sensitivity of the tropic responses to light. Thus, spectrophotometric and kinase activity analyses of phototropin are important to understand the light signaling processes related to the photosensitivity. The preparation of polypeptide samples of Arabidopsis phototropin and the methods of spectroscopic measurements and kinase assay of these samples are shown in this chapter.

Adding Blue to Red Supplemental Light Increases Biomass and Yield of Greenhouse-Grown Tomatoes, but Only to an Optimum

Greenhouse crop production in northern countries often relies heavily on supplemental lighting for year-round yield and product quality. Among the different spectra used in supplemental lighting, red is often considered the most efficient, but plants do not develop normally when grown solely under monochromatic red light ("red light syndrome"). Addition of blue light has been shown to aid normal development, and typical lighting spectra in greenhouse production include a mixture of red and blue light. However, it is unclear whether sunlight, as part of the light available to plants in the greenhouse, may be sufficient as a source of blue light. In a greenhouse high-wire tomato (Solanum lycopersicum), we varied the percentage of blue supplemental light (in a red background) as 0, 6, 12, and 24%, while keeping total photosynthetically active radiation constant. Light was supplied as a mixture of overhead (99 μmol m-2 s-1) and intracanopy (48 μmol m-2 s-1) LEDs, together with sunlight. Averaged over the whole experiment (111 days), sunlight comprised 58% of total light incident onto the crop. Total biomass, yield and number of fruits increased with the addition of blue light to an optimum, suggesting that both low (0%) and high (24%) blue light intensities were suboptimal for growth. Stem and internode lengths, as well as leaf area, decreased with increases in blue light percentage. While photosynthetic capacity increased linearly with increases in blue light percentage, photosynthesis in the low blue light treatment (0%) was not low enough to suggest the occurrence of the red light syndrome. Decreased biomass at low (0%) blue light was likely caused by decreased photosynthetic light use efficiency. Conversely, decreased biomass at high (24%) blue light was likely caused by reductions in canopy light interception. We conclude that while it is not strictly necessary to add blue light to greenhouse supplemental red light to obtain a functional crop, adding some (6-12%) blue light is advantageous for growth and yield while adding 24% blue light is suboptimal for growth.

The Arabidopsis ATP-BINDING CASSETTE Transporter ABCB21 Regulates Auxin Levels in Cotyledons, the Root Pericycle, and Leaves

The phytohormone auxin plays significant roles in regulating plant growth and development. In Arabidopsis, a subset of ATP-BINDING CASSETTE subfamily B (ABCB) transporters participate in polar movement of auxin by exclusion from and prevention of reuptake at the plasma membrane. A previous analysis identified ABCB21 as a conditional auxin uptake/efflux transporter that regulates cellular auxin levels, but clear physiological roles for ABCB21 in planta remain unknown. Here we show that ABCB21 maintains the acropetal auxin transport stream by regulating auxin levels in the pericycle. Loss of ABCB21 reduces rootward auxin transport and delays lateral root emergence. In seedling shoots, ABCB21 regulates mobilization of auxin from the photosynthetic cotyledons that is important for phototropic bending. In rosette leaves ABCB21 contributes to lateral auxin distribution. These results support a primary role for ABCB21 in regulating auxin distribution supplementary to the primary ABCB auxin transporters ABCB1 and 19.

Transcriptomic insights into the blue light-induced female floral sex expression in cucumber (Cucumis sativus L.)

In cucurbitaceous crops, sex differentiation of flower buds is a crucial developmental process that directly affects fruit yield. Here we showed that the induction of female flower was the highest in the blue light-treated monoecious cucumber plants compared with that in other light qualities (white, green and red). High-throughput RNA-Seq analysis of the shoot apexes identified a total of 74 differently-expressed genes (DEGs), in which 52 up-regulated and 22 down-regulated under the blue light compared with that in white light. The DEGs were mainly involved in metabolic pathways, biosynthesis of secondary metabolites, plant hormone signal transduction, starch and sucrose metabolism and phenylpropanoid biosynthesis. While the  ethylene and gibberellins synthesis and signaling related genes were down-regulated, the abscisic acid and auxin signal transduction pathways were up-regulated by the blue light treatment. Furthermore, the blue light treatment up-regulated the transcription of genes relating to photosynthesis, starch and sucrose metabolism. Meanwhile, the blue light suppressed the GA₃ concentration but promoted the concentrations of auxin and photosynthetic pigments. Taken together, the results suggest that the blue light-induced female floral sex expression is closely associated with the blue light-induced changes in abscisic acid, auxin, gibberellins, photosynthesis, starch and sucrose metabolism pathways, which is potentially different from the traditional ethylene-dependent pathway.

Phot2-regulated relocation of NPH3 mediates phototropic response to high-intensity blue light in Arabidopsis thaliana

Two redundant blue-light receptors, known as phototropins (phot1 and phot2), influence a variety of physiological responses, including phototropism, chloroplast positioning, and stomatal opening in Arabidopsis thaliana. Whereas phot1 functions in both low- and high-intensity blue light (HBL), phot2 functions primarily in HBL. Here, we aimed to elucidate phot2-specific functions by screening for HBL-insensitive mutants among mutagenized Arabidopsis phot1 mutants. One of the resulting phot2 signaling associated (p2sa) double mutants, phot1 p2sa2, exhibited phototropic defects that could be restored by constitutively expressing NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3), indicating that P2SA2 was allelic to NPH3. It was observed that NPH3-GFP signal mainly localized to and clustered on the plasma membrane in darkness. This NPH3 clustering on the plasma membrane was not affected by mutations in genes encoding proteins that interact with NPH3, including PHOT1, PHOT2 and ROOT PHOTOTROPISM 2 (RPT2). However, the HBL irradiation-mediated release of NPH3 proteins into the cytoplasm was inhibited in phot1 mutants and enhanced in phot2 and rpt2-2 mutants. Furthermore, HBL-induced hypocotyl phototropism was enhanced in phot1 mutants and inhibited in the phot2 and rpt2-2 mutants. Our findings indicate that phot1 regulates the dissociation of NPH3 from the plasma membrane, whereas phot2 mediates the stabilization and relocation of NPH3 to the plasma membrane to acclimate to HBL.

A phosphorylation switch turns a positive regulator of phototropism into an inhibitor of the process

Phototropins are light-activated protein kinases, which contribute to photosynthesis optimization both through enhancement of photon absorption when light is limiting and avoidance responses in high light. This duality is in part endowed by the presence of phototropins with different photosensitivity (phot1 and phot2). Here we show that phot1, which senses low light to promote positive phototropism (growth towards the light), also limits the response in high light. This response depends in part on phot1-mediated phosphorylation of Phytochrome Kinase Substrate 4 (PKS4). This light-regulated phosphorylation switch changes PKS4 from a phototropism enhancer in low light to a factor limiting the process in high light. In such conditions phot1 and PKS4 phosphorylation prevent phototropic responses to shallow light gradients and limit phototropism in a natural high light environment. Hence, by modifying PKS4 activity in high light the phot1-PKS4 regulon enables appropriate physiological adaptations over a range of light intensities.

Light conditions modify plant growth and development via photoreceptors such as phototropins. Here the authors show that while phot1 promotes phototropism under low light, it can act to suppress phototropism in high-light environments through phosphorylation of PKS4.

A chemical genetic approach to engineer phototropin kinases for substrate labeling

Protein kinases (PKs) control many aspects of plant physiology by regulating signaling networks through protein phosphorylation. Phototropins (phots) are plasma membrane-associated serine/threonine PKs that control a range of physiological processes that collectively serve to optimize photosynthetic efficiency in plants. These include phototropism, leaf positioning and flattening, chloroplast movement, and stomatal opening. Despite their identification over two decades ago, only a handful of substrates have been identified for these PKs. Progress in this area has been hampered by the lack of a convenient means to confirm the identity of potential substrate candidates. Here we demonstrate that the kinase domain of Arabidopsis phot1 and phot2 can be successfully engineered to accommodate non-natural ATP analogues by substituting the bulky gatekeeper residue threonine for glycine. This approach circumvents the need for radioactivity to track phot kinase activity and follow light-induced receptor autophosphorylation in vitro by incorporating thiophosphate from N⁶-benzyl-ATPγS. Consequently, thiophosphorylation of phot substrate candidates can be readily monitored when added or co-expressed with phots in vitro Furthermore, gatekeeper-modified phot1 retained its functionality and its ability to accommodate N⁶-benzyl-ATPγS as a phosphodonor when expressed in Arabidopsis We therefore anticipate that this chemical genetic approach will provide new opportunities for labeling and identifying substrates for phots and other related AGC kinases under in vitro and near-native in vivo conditions.

Shining Light on the Function of NPH3/RPT2-Like Proteins in Phototropin Signaling

NRL proteins coordinate different aspects of phototropin signaling through signaling processes that are conserved in land plants and algae.

Functional Analysis of MAX2 in Phototropins-Mediated Cotyledon Flattening in Arabidopsis

Phototropins (phot1 and phot2) are blue-light receptors that control cotyledon flattening and positioning under strong light; however, their functional redundancy restricts our understanding of the specific roles of phot2. To identify the factors responsible for phot2-dependent cotyledon flattening and growth, we screened for light-insensitive mutants among mutagenized phot1 mutants in Arabidopsis thaliana. The double mutant phot1 lea1 (leaf expansion associated 1), which is defective in cotyledon flattening and positioning but not the phototropic response was selected. This mutant phenotype could be alleviated by constitutively expressing MORE AXILLARY GROWTH 2 (MAX2), indicating that LEA1 was allelic to MAX2. The max2 mutants (max2-2 and max2-3) are defective in cotyledon flattening, which is similar to that of the phot1 phot2 mutants. Moreover, the amounts of MAX2 transcripts are inhibited in leaves of phot1 mutant. However, the additional disruption of PHOT1 gene in max2-2 or max2-3 did not affect their phenotype, including MAX2-mediated inhibition of hypocotyl elongation. By contrast, phototropins-mediated hypocotyl phototropism was not regulated by MAX2. Together, these results suggest that cotyledon flattening was mediated by both phototropins and MAX2 signaling, but the relationship between two pathways need further study.

Blue light-excited LOV1 and LOV2 domains cooperatively regulate the kinase activity of full-length phototropin2 from Arabidopsis

Phototropin2 (phot2) is a blue-light (BL) receptor that regulates BL-dependent activities for efficient photosynthesis in plants. phot2 comprises two BL-receiving light-oxygen-voltage-sensing domains (LOV1 and LOV2) and a kinase domain. BL-excited LOV2 is thought to be primarily responsible for the BL-dependent activation of the kinase. However, the molecular mechanisms by which small BL-induced conformational changes in the LOV2 domain are transmitted to the kinase remain unclear. Here, we used full-length wild-type and mutant phot2 proteins from Arabidopsis to study their molecular properties in the dark and under BL irradiation. Phosphorylation assays and absorption measurements indicated that the LOV1 domain assists the thermal relaxation of BL-excited LOV2 and vice versa. Using small-angle X-ray scattering and electron microscopy, we observed that phot2 forms a dimer and has a rod shape with a maximum length of 188 Å and a radius of gyration of 44 Å. Under BL, phot2 displayed large conformational changes that bent the rod shape. By superimposing the crystal structures of the LOV1 dimer, LOV2, and a homology model of the kinase to the observed changes, we inferred that the BL-dependent change consisted of positional shifts of both LOV2 and the kinase relative to LOV1. Furthermore, phot2 mutants lacking the photocycle in LOV1 or LOV2 still exhibited conformational changes under BL, suggesting that LOV1 and LOV2 cooperatively contribute to the conformational changes that activate the kinase. These results suggest that BL-activated LOV1 contributes to the kinase activity of phot2. We discuss the possible intramolecular interactions and signaling mechanisms in phot2.