Role In Photomorphogenesis Research Articles

SlCPK27 cross-links SlHY5 and SlPIF4 in brassinosteroid-dependent photo- and thermo-morphogenesis in tomato.

ELONGATED HYPOCOTOYL5 (HY5) and PHYTOCHROME INTERACTING FACTORs (PIFs) are two types of important light-related regulators of plant growth, however, their interplay remains elusive. Here, we report that the activated tomato (Solanum lycopersicum) HY5 (SlHY5) triggers the transcription of a Calcium-dependent Protein Kinase SlCPK27. SlCPK27 interacts with and phosphorylates SlPIF4 at Ser-252 and Ser-308 phosphosites to promote its degradation. SlPIF4 promotes hypocotyl elongation mainly by activating the transcription of SlDWF, a key gene in brassinosteroid (BR) biosynthesis. Such a SlHY5-SlCPK27-SlPIF4-BR cascade not only plays a crucial role in photomorphogenesis but also regulates thermomorphogenesis. Our results uncover a previously unidentified mechanism that integrates Ca2+ signaling with the light signaling pathways to regulate plant growth by modulating BR biosynthesis in response to changes in ambient light and temperature.

Biological Importance, Pharmacological Activities, and Nutraceutical Potential of Capsanthin: A Review of Capsicum Plant Capsaicinoids.

Carotenoids are natural hydrocarbons that play an important role in photomorphogenesis, photosynthesis, photoprotection, development, and defense mechanism of plants. Carotenoids have good anti-oxidants and provitamin A contents with their additional colorant nature, which are indispensable to plants and human diets. Capsicum species are well known for their culinary uses worldwide; they are not only cultivated as vegetables but used in numerous medicinal preparations as well due to their medicinal aspects. This article aims to collect data on the beneficial aspects of capsaicinoids with a major emphasis on capsanthin. In order to instigate the biological potential and therapeutic benefit of capsanthin in medicine, in the present work, scientific research data on capsanthin were collected from different literature sources and analyzed. The biological potential of Capsicum annuum in medicine was also investigated through literature data analysis of different scientific research work. Scientific data on capsanthin were collected from Google, Google Scholar, PubMed, Science Direct, and Scopus using the term capsanthin and capsicum in the present work. Detailed pharmacological activities of capsanthin were presented and discussed in the present work through scientific data analysis of research work. Analytical techniques for the separation, isolation, and identification of capsanthin were taken into consideration in this work. Scientific data analysis revealed the biological importance and therapeutic benefit of capsanthin and capsicum in medicine. Capsicum annuum is a member of the Solanaceae family, which is one of the most cultivated spices worldwide. Capsaicinoids are one of the main classes of phytochemicals found in chili peppers, i.e., Capsicum annuum, and are mainly responsible for the pungent and spicy flavor of chili peppers. Capsanthin is a crystalline red color pigment found as the main component of Capsicum annuum fruits during ripening. Capsanthin is also found in Lilium, Aesculus, Berberis, and Asparagus officinalis. Chemically, capsanthin contains a cyclopentane ring, 11 conjugated double bonds, and a conjugated keto group. Capsanthin is a powerful antioxidant, exhibits anti-tumor activities, attenuates obesity-induced inflammation, and raises plasma HDL cholesterol levels. Scientific studies have proven the pharmacological benefits of capsanthin in medicine as it is helpful in pain relief, cardioprotection, weight loss, and body temperature regulation. Moreover, it also has anti-inflammatory, anticancer, antioxidant, and antimicrobial activities. In the literature database, numerous extraction and isolation techniques have been documented for capsanthin. In addition, the analytical techniques and other bioanalytical tools for the isolation and identification of capsanthin were also discussed in the present article. Medicinal importance and pharmacological activities of capsanthin were reviewed and discussed in this paper. This review aimed to highlight the literature on capsanthin in drug discoveries with their analytical development.

Two types of GLR channels cooperate differently in light and dark growth of Arabidopsis seedlings

BackgroundGLutamate Receptor-like (GLR) channels are multimeric, ionotropic, ligand-gated plant transmembrane receptors. They are homologous to mammalian glutamate receptors, iGLuRs, which are critical to neuronal function. GLRs have been reported several times to play a role in photomorphogenesis. However, to date, no study has looked at the mechanism of their involvement in this process. Here we focused on examining the impact of GLRs on the regulation of early seedling growth in blue light, red light, and in the dark.ResultsWild type and six photoreceptor mutant seedlings were grown on media supplemented with known iGLuR/GLR channel antagonists: MK-801, which non-competitively blocks NMDA channels in mammalian cells, and CNQX, known for competitive blocking of AMPA channels in mammalian cells. The lengths of hypocotyls and roots were measured in seedlings of phyA, phyB, phot1, phot2, cry1, and cry2 mutants after 7 days of in vitro culture. Changes in growth parameters, both in light and in darkness upon application of chemical antagonists, show that both types of GLR channels, NMDA-like and AMPA-like, are involved in the regulation of seedling growth irrespective of light conditions. Analysis of seedling growth of photoreceptor mutants indicates that the channels are influenced by signaling from phot1, phot2, and cry1. To extend our analysis, we also evaluated the elicitation of a calcium wave, which is likely to be partially driven by GLRs, in Arabidopsis seedlings. The changes in cellobiose-induced calcium waves observed after applying GLR inhibitors suggest that both types of channels likely cooperate in shaping Arabidopsis seedling growth and development.ConclusionsOur work provides the first experimental evidence that two types of GLR channels function in plants: NMDA-like and AMPA-like. We also demonstrate that the channels are involved in seedling growth and development, at least partially through modulation of calcium signaling, but they are unlikely to play a major role in photomorphogenesis.

A base substitution in OsphyC disturbs its Interaction with OsphyB and affects flowering time and chlorophyll synthesis in rice

BackgroundPhytochromes are important photoreceptors in plants, and play essential roles in photomorphogenesis. The functions of PhyA and PhyB in plants have been fully analyzed, while those of PhyC in plant are not well understood.ResultsA rice mutant, late heading date 3 (lhd3), was characterized, and the gene LHD3 was identified with a map-based cloning strategy. LHD3 encodes phytochrome C in rice. Animo acid substitution in OsphyC disrupted its interaction with OsphyB or itself, restraining functional forms of homodimer or heterodimer formation. Compared with wild-type plants, the lhd3 mutant exhibited delayed flowering under both LD (long-day) and SD (short-day) conditions, and delayed flowering time was positively associated with the day length via the Ehd1 pathway. In addition, lhd3 showed a pale-green-leaf phenotype and a slower chlorophyll synthesis rate during the greening process. The transcription patterns of many key genes involved in photoperiod-mediated flowering and chlorophyll synthesis were altered in lhd3.ConclusionThe dimerization of OsPhyC is important for its functions in the regulation of chlorophyll synthesis and heading. Our findings will facilitate efforts to further elucidate the function and mechanism of OsphyC and during light signal transduction in rice.

Light Quality-Mediated Influence of Morphogenesis in Micropropagated Horticultural Crops: A Comprehensive Overview.

In plants, light quality plays significant roles in photomorphogenesis and photosynthesis. Efficient in vitro plant propagation techniques involve tailoring of various environmental cues and culture media according to the plant species. Plant tissue culture consists of several applications in scientific research, agriculture, biotechnology, and commercial industrial purposes. Utilization of light to enhance the quality of the in vitro raised plants have been evidenced by numerous researchers in plant tissue culture. The advent of light-emitting diode- (LED-) based artificial lighting systems in plant tissue culture for micropropagation has enhanced callus induction, shoot and root organogenesis, and acclimatization of in vitro propagated plants. Plants tend to perceive the light spectra present in the photosynthetically active region (PAR) ranging from 400 to 700 nm; this includes blue and red light wavelengths. Although the influence of spectral quality is being investigated in diverse plant species, particularly, its importance in in vitro propagated horticultural crops is gaining notable interest among researchers. In recent days, the application of LEDs provides better amenability according to the plant species of interest for efficient plant regeneration. Considering the growing necessity and emerging applications of LED supplemental lights for propagation of plants in in vitro, the present review summarizes the outcomes of various research studies dealing with LEDs in plant tissue culture. Moreover, the present endeavor has provided a comprehensive overview on the effects of LEDs in the morphogenesis of plants cultured in vitro.

A highly efficient and suitable spectral profile Cr3+-doped garnet near-infrared emitting phosphor for regulating photomorphogenesis of plants

Far-red/phytochrome (PFR) plays a key role in photomorphogenesis of plants. However, how to obtain a near-infrared (NIR) emitting phosphor with high external quantum efficiency (EQE), suitable spectral profile, and low thermal quenching remains a huge challenge. Herein, a NIR phosphor, Gd2.4Lu0.6Ga4AlO12:Cr3+,H3BO3 (GLGA:Cr3+) was developed via regulating the crystal field environment and adding fluxes, which exhibits a peak maximum at 728 nm with a relatively narrow full-width at half maximum (FWHM) of 107 nm, matching well with the absorption band of PFR. Upon 450 nm excitation, the internal quantum efficiency (IQE) and EQE of the optimal phosphor are 90.3% and 32.0%, respectively. At 423 K, the integrated emission intensity of the investigated phosphor is about 75% of that at room temperature. Benefiting from the excellent optical performance, a NIR phosphor-converted light-emitting diode (pc-LED) was fabricated, which shows a NIR output power of 505.99 mW and photoelectric conversion efficiency of 11.24% at 300 mA. Moreover, plant growth experiments demonstrate that the biomass of pea seedlings is increased by 67.72% under supplementary NIR light irradiation. The findings of this research will motivate further research on new Cr3+-doped NIR phosphors for regulating photomorphogenesis of plants.

Chaperone-like protein DAY plays critical roles in photomorphogenesis

Photomorphogenesis, light-mediated development, is an essential feature of all terrestrial plants. While chloroplast development and brassinosteroid (BR) signaling are known players in photomorphogenesis, proteins that regulate both pathways have yet to be identified. Here we report that DE-ETIOLATION IN THE DARKANDYELLOWING IN THE LIGHT (DAY), a membrane protein containing DnaJ-like domain, plays a dual-role in photomorphogenesis by stabilizing the BR receptor, BRI1, as well as a key enzyme in chlorophyll biosynthesis, POR. DAY localizes to both the endomembrane and chloroplasts via its first transmembrane domain and chloroplast transit peptide, respectively, and interacts with BRI1 and POR in their respective subcellular compartments. Using genetic analysis, we show that DAY acts independently on BR signaling and chlorophyll biogenesis. Collectively, this work uncovers DAY as a factor that simultaneously regulates BR signaling and chloroplast development, revealing a key regulator of photomorphogenesis that acts across cell compartments.

Phytochrome Photobodies in Mammalian Cells

Photobodies play a significant role in photomorphogenesis and scotomorphogenesis in higher plants. Assemble/disassemble of these membrane-less organelles (MLOs) are induced by light photoconvertion of phytochromes and cryptochromes, which are their scaffold proteins. For example, the formation of phytochromes-based photobodies often are caused by interaction of active phytochrome B (phyB) Pfr conformers with phytochrome-interacting factors (PIFs). We showed that MLOs based on plant phyB and PIFs can arise in mammalian cells (HEK 293) and that red/far-red cell irradiation induces the increase/decrease of the size of these photobodies.

Characterization of phytochrome C functions in the control of de-etiolation and agronomic traits in rice

Although phytochrome A (phyA) and phyB have been functionally characterized, functions of phyC in rice growth and development have remained elusive because of the functional dependency of phyC on the phyB protein. In this study, we introduced PHYB(C364A), in which the chromophore attachment site cysteine 364 was converted to alanine, into the phyAphyB double mutant (aabb) and the phyAphyBphyC triple mutant (aabbcc) to produce PHYB(C364A)/aabb lines and PHYB(C364A)/aabbcc lines, respectively. PHYB(C364A)/aabbcc lines were insensitive to red light (R) and far-red light (FR), suggesting that PHYB(C364A) protein was biologically inactive. Functions of phyC were characterized using the PHYB(C364A)/aabb lines, without the functional interference of phyA or phyB. Phytochrome C responded to R and FR to trigger de-etiolation in the very-low-fluence response and low-fluence response in the PHYB(C364A)/aabb lines. Compared with the aabb mutant, seedlings of PHYB(C364A)/aabb lines showed higher chlorophyll content and reduced leaf angle. The PHYB(C364A)/aabb lines also showed a delayed heading date under long-day conditions. Phytochrome C-regulated agronomic traits were measured at the mature stage. The PHYB(C364A)/aabb lines showed significantly increased plant height, panicle length, grain number per main panicle, seed-setting rate, grain size, and grain weight, compared with those of the aabb mutant. Taken together, the present findings confirm that phyC perceives R and FR, and plays an important role in photomorphogenesis and yield determination in rice.

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.

Functional Characterization of the Maize Phytochrome-Interacting Factors PIF4 and PIF5.

Phytochrome-interacting factors (PIFs) play important roles in photomorphogenesis, the shade avoidance response, and other aspects of plant growth and development. PIF family proteins have been well-studied in Arabidopsis thaliana, but little is known about their physiological functions and molecular mechanisms in maize (Zea mays). In this study, we investigated the physiological functions of ZmPIF4 and ZmPIF5, two highly conserved members of the PIF gene family. RT-qPCR and western blot analyses revealed that ZmPIF4 and ZmPIF5 expression and ZmPIF4 and ZmPIF5 levels peak at night and remain low during the day. Overexpression of ZmPIF4 and ZmPIF5 in Arabidopsis partially rescued the reduced hypocotyl elongation and defective response to gravity in pif1 pif3 pif4 pif5 quadruple mutants (pifq). In addition, under high red: far-red light conditions, Arabidopsis lines overexpressing ZmPIF4 exhibited a constitutive shade avoidance response, including early flowering, slender leaves and inflorescences, plant lodging and precocious leaf senescence. Furthermore, ZmPIF4 physically interacted with the Arabidopsis DELLA protein REPRESSOR OF GA1-3 (RGA), indicating a potential interaction between ZmPIF4 and gibberellin signaling pathway on plant growth. Taken together, our results revealed that ZmPIF4 and ZmPIF5 are functionally conserved proteins that may play conserved roles in the response to phytochrome signaling in plants.Highlights:In this study, the functions of ZmPIF4 and ZmPIF5 were characterized by expression in Arabidopsis, revealing conserved roles of PIF family proteins in photomorphogenesis and the shade avoidance response in land plants.

Arabidopsis MKK10-MPK6 mediates red-light-regulated opening of seedling cotyledons through phosphorylation of PIF3

Photomorphogenesis is an important process in which seedlings emerge from soil and begin autotrophic growth. Mechanisms of photomorphogenesis include light signal perception, signal transduction, and the modulation of expression of light-responsive genes, ultimately leading to cellular and developmental changes. Phytochrome-interacting factors (PIFs) play negative regulatory roles in photomorphogenesis. Light-induced activation of phytochromes triggers rapid phosphorylation and degradation of PIFs, but the kinases responsible for the phosphorylation of PIFs are largely unknown. Here, we show that Arabidopsis MPK6 is a kinase involved in phosphorylating PIF3 and regulating red light-induced cotyledon opening, a crucial process during seedling photomorphogenesis. MPK6 was activated by red light, and the cotyledon opening angle in red light was reduced in mpk6 seedlings. MKK10, a MAPKK whose function is currently unclear, appears to act as a kinase upstream of MPK6 in regulating cotyledon opening. Activation of MPK6 by MKK10 led to the phosphorylation of PIF3 and accelerated its turnover in transgenic seedlings. Accordingly, the overexpression of PIF3 suppressed MKK10-induced cotyledon opening. MKK10 and MPK6 function downstream of phyB in regulating seedling cotyledon opening in red light. Therefore, the MKK10-MPK6 cascade appears to mediate the regulation of red-light-controlled seedling photomorphogenesis via a mechanism that might involve the phosphorylation of PIF3.

Fungal Histidine Phosphotransferase Plays a Crucial Role in Photomorphogenesis and Pathogenesis in Magnaporthe oryzae.

Two-component signal transduction (TCST) pathways play crucial roles in many cellular functions such as stress responses, biofilm formation, and sporulation. The histidine phosphotransferase (HPt), which is an intermediate phosphotransfer protein in a two-component system, transfers a phosphate group to a phosphorylatable aspartate residue in the target protein(s), and up-regulates stress-activated MAP kinase cascades. Most fungal genomes carry a single copy of the gene coding for HPt, which are potential antifungal targets. However, unlike the histidine kinases (HK) or the downstream response regulators (RR) in two-component system, the HPts have not been well-studied in phytopathogenic fungi. In this study, we investigated the role of HPt in the model rice-blast fungal pathogen Magnaporthe oryzae. We found that in M. oryzae an additional isoform of the HPT gene YPD1 was expressed specifically in response to light. Further, the expression of light-regulated genes such as those encoding envoy and blue-light-harvesting protein, and PAS domain containing HKs was significantly reduced upon down-regulation of YPD1 in M. oryzae. Importantly, down-regulation of YPD1 led to a significant decrease in the ability to penetrate the host cuticle and in light-dependent conidiation in M. oryzae. Thus, our results indicate that Ypd1 plays an important role in asexual development and host invasion, and suggest that YPD1 isoforms likely have distinct roles to play in the rice-blast pathogen M. oryzae.

NUCLEAR FACTOR Y, Subunit C (NF-YC) Transcription Factors Are Positive Regulators of Photomorphogenesis in Arabidopsis thaliana.

Recent reports suggest that NF-Y transcription factors are positive regulators of skotomorphogenesis in Arabidopsis thaliana. Three NF-YC genes (NF-YC3, NF-YC4, and NF-YC9) are known to have overlapping functions in photoperiod dependent flowering and previous studies demonstrated that they interact with basic leucine zipper (bZIP) transcription factors. This included ELONGATED HYPOCOTYL 5 (HY5), which has well-demonstrated roles in photomorphogenesis. Similar to hy5 mutants, we report that nf-yc3 nf-yc4 nf-yc9 triple mutants failed to inhibit hypocotyl elongation in all tested light wavelengths. Surprisingly, nf-yc3 nf-yc4 nf-yc9 hy5 mutants had synergistic defects in light perception, suggesting that NF-Ys represent a parallel light signaling pathway. As with other photomorphogenic transcription factors, nf-yc3 nf-yc4 nf-yc9 triple mutants also partially suppressed the short hypocotyl and dwarf rosette phenotypes of CONSTITUTIVE PHOTOMORPHOGENIC 1 (cop1) mutants. Thus, our data strongly suggest that NF-Y transcription factors have important roles as positive regulators of photomorphogenesis, and in conjunction with other recent reports, implies that the NF-Y are multifaceted regulators of early seedling development.

Isolation of three B-box zinc finger proteins that interact with STF1 and COP1 defines a HY5/COP1 interaction network involved in light control of development in soybean

LONG HYPOCOTYL5 (HY5) and STF1 (Soybean TGACG-motif binding Factor 1) are two related bZIP transcription factors that play a positive role in photomorphogenesis and hormonal signaling. In this study, we compared full length STF1 and truncated STF1 overexpression lines and found that the C-terminal 133 amino acids (194–306) possess all the HY5-like function in Arabidopsis. The STF1-DC1 mutant (1–306), with a 20 amino acid deletion at the carboxy terminus, failed to complement the hy5 mutant phenotype, which suggests an intact C-terminus is required for STF1 function. To understand the role of the C-terminal domain in photomorphogenesis we used a yeast two-hybrid screen to isolate proteins that bind to the STF1 C-terminus. We isolated three soybean cDNAs encoding the zinc-finger proteins GmSTO, GmSTH, and GmSTH2, which interact with STF1. These proteins belong to a family of B-box zinc finger proteins that include Arabidopsis SALT TOLERANCE (STO) and STO HOMOLOG (STH) and STH2, which play a role in light-dependent development and gene expression. The C-terminal 63 amino acids of STF1, containing a leucine zipper and the two N-terminal B-boxes, contains the domain involved in interactions between STF1 and GmSTO. In addition, we identified an interaction between soybean COP1 (GmCOP1) and GmSTO and GmSTH, as well as STF1, which strongly suggests the presence of a similar regulatory circuit for light signaling in soybean as in Arabidopsis. This study shows that photomorphogenic control requires complex molecular interactions among several different classes of transcription factors such as bZIP, B-box factors, and COP1, a ubiquitin ligase.

COP1 E3 ligase protects HYL1 to retain microRNA biogenesis

Constitutive photomorphogenic 1 (COP1) is a RING-finger E3 ligase that plays a central role in photomorphogenesis by destabilizing many light-regulated transcription factors and photoreceptors. Here, we reveal a novel function for COP1 E3 ligase in controlling global miRNA biogenesis in Arabidopsis thaliana. In cop1 mutants, the level of miRNAs is dramatically reduced because of the diminution of HYPONASTIC LEAVES 1 (HYL1), an RNA-binding protein required for precise miRNA processing. HYL1 is destabilized by an unidentified protease, which we tentatively call protease X, that specifically cleaves the N-terminal region from HYL1, thus neutralizing its function. Our results further show that the cytoplasmic partitioning of COP1 under light is essential to protect HYL1 against protease X. Taken together, we suggest a novel regulatory network involving HYL1, protease X, COP1 and light signalling that is indispensable for miRNA biogenesis in Arabidopsis thaliana.

Effect of Blue Light on Endogenous Isopentenyladenine and Endoreduplication during Photomorphogenesis and De-Etiolation of Tomato (Solanum lycopersicum L.) Seedlings

Light is one of the most important factor influencing plant growth and development all through their life cycle. One of the well-known light-regulated processes is de-etiolation, i.e. the switch from skotomorphogenesis to photomorphogenesis. The hormones cytokinins (CKs) play an important role during the establishment of photomorphogenesis as exogenous CKs induced photomorphogenesis of dark-grown seedlings. Most of the studies are conducted on the plant model Arabidopsis, but no or few information are available for important crop species, such as tomato (Solanum lycopersicum L.). In our study, we analyzed for the first time the endogenous CKs content in tomato hypocotyls during skotomorphogenesis, photomorphogenesis and de-etiolation. For this purpose, two tomato genotypes were used: cv. Rutgers (wild-type; WT) and its corresponding mutant (7B-1) affected in its responses to blue light (BL). Using physiological and molecular approaches, we identified that the skotomorphogenesis is characterized by an endoreduplication-mediated cell expansion, which is inhibited upon BL exposure as seen by the accumulation of trancripts encoding CycD3, key regulators of the cell cycle. Our study showed for the first time that iP (isopentenyladenine) is the CK accumulated in the tomato hypocotyl upon BL exposure, suggesting its specific role in photomorphogenesis. This result was supported by physiological experiments and gene expression data. We propose a common model to explain the role and the relationship between CKs, namely iP, and endoreduplication during de-etiolation and photomorphogenesis.

The COP9 Signalosome: Its Regulation of Cullin-Based E3 Ubiquitin Ligases and Role in Photomorphogenesis

Plants are sessile organisms that have evolved an extraordinary phenotypic and developmental plasticity to optimally adapt to the ambient environment. Light is one of the most critical environmental factors, since it is a key signal for regulating a wide range of developmental and physiological

Light-dependent, Dark-promoted Interaction between Arabidopsis Cryptochrome 1 and Phytochrome B Proteins

Plant photoreceptors transduce environmental light cues to downstream signaling pathways, regulating a wide array of processes during growth and development. Two major plant photoreceptors with critical roles in photomorphogenesis are phytochrome B (phyB), a red/far-red absorbing photoreceptor, and cryptochrome 1 (CRY1), a UV-A/blue photoreceptor. Despite substantial genetic evidence for cross-talk between phyB and CRY1 pathways, a direct interaction between these proteins has not been observed. Here, we report that Arabidopsis phyB interacts directly with CRY1 in a light-dependent interaction. Surprisingly, the interaction is light-dissociated; CRY1 interacts specifically with the dark/far-red (Pr) state of phyB, but not with the red light-activated (Pfr) or the chromophore unconjugated form of the enzyme. The interaction is also regulated by light activation of CRY1; phyB Pr interacts only with the unstimulated form of CRY1 but not with the photostimulated protein. Further studies reveal that a small domain extending from the photolyase homology region (PHR) of CRY1 regulates the specificity of the interaction with different conformational states of phyB. We hypothesize that in plants, the phyB/CRY1 interaction may mediate cross-talk between the red/far-red- and blue/UV-sensing pathways, enabling fine-tuning of light responses to different spectral inputs.

Ubiquitin ligase switch in plant photomorphogenesis: A hypothesis

The E3 ubiquitin ligase COP1 (CONSTITUTIVE PHOTOMORPHOGENIC1) plays a key role in the repression of the plant photomorphogenic development in darkness. In the presence of light, COP1 is inactivated by a mechanism which is not completely understood. This leads to accumulation of COP1’s target transcription factors, which initiates photomorphogenesis, resulting in dramatic changes of the seedling’s physiology.Here we use a mathematical model to explore the possible mechanism of COP1 modulation upon dark/light transition in Arabidopsis thaliana based upon data for two COP1 target proteins: HY5 and HFR1, which play critical roles in photomorphogenesis. The main reactions in our model are the inactivation of COP1 by a proposed photoreceptor-related inhibitor I and interactions between COP1 and a CUL4 (CULLIN4)-based ligase. For building and verification of the model, we used the available published and our new data on the kinetics of HY5 and HFR1 together with the data on COP1 abundance. HY5 has been shown to accumulate at a slower rate than HFR1. To describe the observed differences in the timecourses of the “slow” target HY5 and the “fast” target HFR1, we hypothesize a switch between the activities of COP1 and CUL4 ligases upon dark/light transition, with COP1 being active mostly in darkness and CUL4 in light. The model predicts a bi-phasic kinetics of COP1 activity upon the exposure of plants to light, with its restoration after the initial decline and the following slow depletion of the total COP1 content. CUL4 activity is predicted to increase in the presence of light. We propose that the ubiquitin ligase switch is important for the complex regulation of multiple transcription factors during plants development. In addition, this provides a new mechanism for sensing the duration of light period, which is important for seasonal changes in plant development.