Repressor Of Photomorphogenesis Research Articles

Shining light on molecular details of SPA2, a repressor of photomorphogenesis

Shining light on molecular details of SPA2, a repressor of photomorphogenesis

The blue light receptor CRY1 interacts with GID1 and DELLA proteins to repress GA signaling during photomorphogenesis in Arabidopsis

Light is a critical environmental cue that regulates a variety of diverse plant developmental processes. Cryptochrome 1 (CRY1) is the major photoreceptor that mediates blue light-dependent photomorphogenic responses such as the inhibition of hypocotyl elongation. Gibberellin (GA) participates in the repression of photomorphogenesis and promotes hypocotyl elongation. However, the antagonistic interaction between blue light and GA is not well understood. Here, we report that blue light represses GA-induced degradation of the DELLA proteins (DELLAs), which are key negative regulators in the GA signaling pathway, via CRY1, thereby inhibiting the GA response during hypocotyl elongation. Both in vitro and in vivo biochemical analyses demonstrated that CRY1 physically interacts with GA receptors—GA-INSENSITIVE DWARF 1 proteins (GID1s)—and DELLAs in a blue light-dependent manner. Furthermore, we showed that CRY1 inhibits the association between GID1s and DELLAs. Genetically, CRY1 antagonizes the function of GID1s to repress the expression of cell elongation-related genes and thus hypocotyl elongation. Taken together, our findings demonstrate that CRY1 coordinates blue light and GA signaling for plant photomorphogenesis by stabilizing DELLAs through the binding and inactivation of GID1s, providing new insights into the mechanism by which blue light antagonizes the function of GA in photomorphogenesis.

AtHDA15 binds directly to COP1 positively regulating photomorphogenesis

Reversible histone acetylation and deacetylation play crucial roles in modulating light-regulated gene expression during seedling development. However, it remains largely unknown how histone-modifying enzymes interpose within the molecular framework of light signaling network. In this study, we show that AtHDA15 positively regulates photomorphogenesis by directly binding to COP1, a master regulator in the repression of photomorphogenesis. hda15 T-DNA knock-out and RNAi lines exhibited light hyposensitivity with reduced HY5 and PIF3 protein levels leading to long hypocotyl phenotypes in the dark while its overexpression leads to increased HY5 concentrations and short hypocotyl phenotypes. In vivo and in vitro binding assays show that HDA15 directly interacts with COP1 inside the nucleus modulating COP1’s repressive activities. As COP1 is established to act within the nucleus to regulate specific transcription factors associated with growth and development in skotomorphogenesis, the direct binding by HDA15 is predicted to abrogate activities of COP1 in the presence of light and modulate its repressive activities in the dark. Our results append the mounting evidence for the role of HDACs in post-translational regulation in addition to their well-known histone modifying functions.

B-Box Containing Proteins BBX30 and BBX31, Acting Downstream of HY5, Negatively Regulate Photomorphogenesis in Arabidopsis.

Light-mediated seedling development is coordinately controlled by a variety of key regulators. Here, we identified two B-box (BBX)-containing proteins, BBX30 and BBX31, as repressors of photomorphogenesis. ELONGATED HYPOCOTYL5, a central regulator of light signaling, directly binds to the G-box cis-element present in the promoters of BBX30 and BBX31 and negatively controls their transcription levels in the light. Seedlings with mutations in BBX30 or BBX31 are hypersensitive to light, whereas the overexpression of BBX30 or BBX31 leads to hypo-photomorphogenic growth in the light. Furthermore, transgenic and phenotypic analysis revealed that the B-box domain of BBX30 or BBX31 is essential for their respective functioning in the regulation of photomorphogenic development in plants. In conclusion, BBX30 and BBX31 act as key negative regulators of light signaling, and their transcription is repressed by ELONGATED HYPOCOTYL5 through directly associating with their promoters.

Photoexcited CRYPTOCHROME1 Interacts with Dephosphorylated BES1 to Regulate Brassinosteroid Signaling and Photomorphogenesis in Arabidopsis.

Cryptochromes (CRYs) are blue light photoreceptors that mediate a variety of light responses in plants and animals, including photomorphogenesis, flowering, and circadian rhythms. The signaling mechanism by which Arabidopsis thaliana cryptochromes CRY1 and CRY2 promote photomorphogenesis involves direct interactions with COP1, a RING motif-containing E3 ubiquitin ligase, and its enhancer SPA1. Brassinosteroid (BR) is a key phytohormone involved in the repression of photomorphogenesis, and here, we show that the signaling mechanism of Arabidopsis CRY1 involves the inhibition of BR signaling. CRY1 and CRY2 physically interact with BES1-INTERACTING MYC-LIKE1 (BIM1), a basic helix-loop-helix protein. BIM1, in turn, interacts with and enhances the activity of BRI1-EMS SUPPRESSOR1 (BES1), a master transcription factor in the BR signaling pathway. In addition, CRY1 and CRY2 interact specifically with dephosphorylated BES1, the physiologically active form of BES1 that is activated by BR in a blue light-dependent manner. The CRY1-BES1 interaction leads to both the inhibition of BES1 DNA binding activity and the repression of its target gene expression. Our study suggests that the blue light-dependent, BR-induced interaction of CRY1 with BES1 is a tightly regulated mechanism by which plants optimize photomorphogenesis according to the availability of external light and internal BR signals.

SEUSS and PIF4 Coordinately Regulate Light and Temperature Signaling Pathways to Control Plant Growth

Plants continuously monitor environmental conditions (such as light and temperature) and adjust their growth and development accordingly. The transcription factor PHYTOCHROME-INTERACTING FACTOR4 (PIF4) regulates both light and temperature signaling pathways. Here, we identified ENHANCED PHOTOMORPHOGENIC2 (EPP2) as a new repressor of photomorphogenesis in red, far-red, and blue light. Map-based cloning revealed that EPP2 encodes the SEUSS (SEU) transcription regulator. The C terminus of SEU has transcriptional activation activity, and SEU physically interacts with PIF4. Moreover, SEU promotes the expression of many genes, including auxin biosynthetic and responsive genes, and regulates IAA levels in plants. SEU associates with the regulatory regions of INDOLE-3-ACETIC ACID INDUCIBLE6 (IAA6) and IAA19 in a PIF4-independent manner, whereas the binding of PIF4 to these genes requires SEU. Furthermore, mutations in SEU affect H3K4me3 methylation at IAA6 and IAA19, and SEU positively regulates warm temperature-mediated hypocotyl growth together with PIF4. Collectively, our results reveal that SEU acts as a central regulator integrating light and temperature signals to control plant growth by coordinating with PIF4.

COP1, a negative regulator of photomorphogenesis, positively regulates plant disease resistance via double-stranded RNA binding proteins.

The E3 ubiquitin ligase COP1 (Constitutive Photomorphogenesis 1) is a well known component of the light-mediated plant development that acts as a repressor of photomorphogenesis. Here we show that COP1 positively regulates defense against turnip crinkle virus (TCV) and avrRPM1 bacteria by contributing to stability of resistance (R) protein HRT and RPM1, respectively. HRT and RPM1 levels and thereby pathogen resistance is significantly reduced in the cop1 mutant background. Notably, the levels of at least two double-stranded RNA binding (DRB) proteins DRB1 and DRB4 are reduced in the cop1 mutant background suggesting that COP1 affects HRT stability via its effect on the DRB proteins. Indeed, a mutation in either drb1 or drb4 resulted in degradation of HRT. In contrast to COP1, a multi-subunit E3 ligase encoded by anaphase-promoting complex (APC) 10 negatively regulates DRB4 and TCV resistance but had no effect on DRB1 levels. We propose that COP1-mediated positive regulation of HRT is dependent on a balance between COP1 and negative regulators that target DRB1 and DRB4.

Characterization of Phytochrome Interacting Factors from the Moss Physcomitrella patens Illustrates Conservation of Phytochrome Signaling Modules in Land Plants.

Across the plant kingdom, phytochrome (PHY) photoreceptors play an important role during adaptive and developmental responses to light. In Arabidopsis thaliana, light-activated PHYs accumulate in the nucleus, where they regulate downstream signaling components, such as phytochrome interacting factors (PIFs). PIFs are transcription factors that act as repressors of photomorphogenesis; their inhibition by PHYs leads to substantial changes in gene expression. The nuclear function of PHYs, however, has so far been investigated in only a few non-seed plants. Here, we identified putative target genes of PHY signaling in the moss Physcomitrella patens and found light-regulated genes that are putative orthologs of PIF-controlled genes in Arabidopsis. Phylogenetic analyses revealed that an ancestral PIF-like gene was already present in streptophyte algae, i.e., before the water-to-land transition of plants. The PIF homologs in the genome of P. patens resemble Arabidopsis PIFs in their protein domain structure, molecular properties, and physiological effects, albeit with notable differences in the motif-dependent PHY interaction. Our results suggest that P. patens PIFs are involved in PHY signaling. The PHY-PIF signaling node that relays light signals to target genes has been largely conserved during land plant evolution, with evidence of lineage-specific diversification.

Chloroplast Biogenesis-Associated Nuclear Genes: Control by Plastid Signals Evolved Prior to Their Regulation as Part of Photomorphogenesis.

The assembly of photosynthetically competent chloroplasts occurs in angiosperm seedlings when first exposed to light, and is due to the control by light of photosynthesis-associated nuclear genes (PhANGs), also dependent upon plastid-to-nucleus “biogenic” communication signals. The relationship between light- and plastid signal-regulation of PhANGs is close but poorly understood. In contrast, many conifers green in the dark and the promoter of a pine PhANG, Lhcb, is active in the dark in tobacco. Here, we show that the activity of this promoter in tobacco is sensitive to plastid photobleaching, or to the inhibition of plastid translation in the light or the dark, and the same interventions reduce expression of the native gene in pine seedlings, demonstrating classic plastid biogenic signaling in gymnosperms. Furthermore, Arabidopsis mutations causing defective plastid biogenesis suppress the effect in darkness of mutations in COP1 and DET1, repressors of photomorphogenesis, for the expression of several PhANGs but not a photosynthesis-unrelated, light-regulated gene. GLK transcriptional regulators mediate the response of LHCB but not of other tested PhANGs. We propose the ability to suppress PhANG response to positive plastid biogenic signals in the dark may have contributed to the evolution of light-controlled chloroplast biogenesis.

CONSTITUTIVE PHOTOMORPHOGENIC 10 (COP10) Contributes to Floral Repression under Non-Inductive Short Days in Arabidopsis.

In Arabidopsis, CONSTITUTIVE PHOTOMORPHOGENIC/DE-ETIOLATED/FUSCA (COP/DET/FUS) genes act in repression of photomorphogenesis in darkness, and recent reports revealed that some of these genes, such as COP1 and DET1, also have important roles in controlling flowering time and circadian rhythm. The COP/DET/FUS protein COP10 interacts with DET1 and DNA DAMAGE-BINDING PROTEIN 1 (DDB1) to form a CDD complex and represses photomorphogenesis in darkness. The cop10-4 mutants flower normally in inductive long days (LD) but early in non-inductive short days (SD) compared with wild type (WT); however, the role of COP10 remains unknown. Here, we investigate the role of COP10 in SD-dependent floral repression. Reverse transcription-quantitative PCR revealed that in SD, expression of the LD-dependent floral inducers GI, FKF1, and FT significantly increased in cop10-4 mutants, compared with WT. This suggests that COP10 mainly regulates FT expression in a CO-independent manner. We also show that COP10 interacts with GI in vitro and in vivo, suggesting that COP10 could also affect GI function at the posttranslational level. Moreover, FLC expression was repressed drastically in cop10-4 mutants and COP10 interacts with MULTICOPY SUPPRESSOR OF IRA1 4 (MSI4)/FVE (MSI4/FVE), which epigenetically inhibits FLC expression. These data suggest that COP10 contributes to delaying flowering in the photoperiod and autonomous pathways by downregulating FT expression under SD.

Arabidopsis DE-ETIOLATED1 represses photomorphogenesis by positively regulating phytochrome-interacting factors in the dark.

Arabidopsis thaliana seedlings undergo photomorphogenic development even in darkness when the function of DE-ETIOLATED1 (DET1), a repressor of photomorphogenesis, is disrupted. However, the mechanism by which DET1 represses photomorphogenesis remains unclear. Our results indicate that DET1 directly interacts with a group of transcription factors known as the phytochrome-interacting factors (PIFs). Furthermore, our results suggest that DET1 positively regulates PIF protein levels primarily by stabilizing PIF proteins in the dark. Genetic analysis showed that each pif single mutant could enhance the det1-1 phenotype, and ectopic expression of each PIF in det1-1 partially suppressed the det1-1 phenotype, based on hypocotyl elongation and cotyledon opening angles observed in darkness. Genomic analysis also revealed that DET1 may modulate the expression of light-regulated genes to mediate photomorphogenesis partially through PIFs. The observed interaction and regulation between DET1 and PIFs not only reveal how DET1 represses photomorphogenesis, but also suggest a possible mechanism by which two groups of photomorphogenic repressors, CONSTITUTIVE PHOTOMORPHOGENESIS/DET/FUSCA and PIFs, work in concert to repress photomorphogenesis in darkness.

LeMYC2 acts as a negative regulator of blue light mediated photomorphogenic growth, and promotes the growth of adult tomato plants.

BackgroundArabidopsis ZBF1/MYC2bHLH transcription factor is a repressor of photomorphogenesis, and acts as a point of cross talk in light, abscisic acid (ABA) and jasmonic acid (JA) signaling pathways. MYC2 also functions as a positive regulator of lateral root development and flowering time under long day conditions. However, the function of MYC2 in growth and development remains unknown in crop plants.ResultsHere, we report the functional analyses of LeMYC2 in tomato (Lycopersicon esculentum). The amino acid sequence of LeMYC2 showed extensive homology with Arabidopsis MYC2, containing the conserved bHLH domain. To study the function of LeMYC2 in tomato, overexpression and RNA interference (RNAi) LeMYC2 tomato transgenic plants were generated. Examination of seedling morphology, physiological responses and light regulated gene expression has revealed that LeMYC2 works as a negative regulator of blue light mediated photomorphogenesis. Furthermore, LeMYC2 specifically binds to the G-box of LeRBCS-3A promoter. Overexpression of LeMYC2 has led to increased root length with more number of lateral roots. The tomato plants overexpressing LeMYC2 have reduced internode distance with more branches, and display the opposite morphology to RNAi transgenic lines. Furthermore, this study shows that LeMYC2 promotes ABA and JA responsiveness.ConclusionsCollectively, this study highlights that working in light, ABA and JA signaling pathways LeMYC2 works as an important regulator for growth and development in tomato plants.

Crystal structure and versatile functional roles of the COP9 signalosome subunit 1

The constitutive photomorphogenesis 9 (COP9) signalosome (CSN) plays key roles in many biological processes, such as repression of photomorphogenesis in plants and protein subcellular localization, DNA-damage response, and NF-κB activation in mammals. It is an evolutionarily conserved eight-protein complex with subunits CSN1 to CSN8 named following the descending order of molecular weights. Here, we report the crystal structure of the largest CSN subunit, CSN1 from Arabidopsis thaliana (atCSN1), which belongs to the Proteasome, COP9 signalosome, Initiation factor 3 (PCI) domain containing CSN subunit family, at 2.7 Å resolution. In contrast to previous predictions and distinct from the PCI-containing 26S proteasome regulatory particle subunit Rpn6 structure, the atCSN1 structure reveals an overall globular fold, with four domains consisting of helical repeat-I, linker helix, helical repeat-II, and the C-terminal PCI domain. Our small-angle X-ray scattering envelope of the CSN1-CSN7 complex agrees with the EM structure of the CSN alone (apo-CSN) and suggests that the PCI end of each molecule may mediate the interaction. Fitting of the CSN1 structure into the CSN-Skp1-Cul1-Fbox (SCF) EM structure shows that the PCI domain of CSN1 situates at the hub of the CSN for interaction with several other subunits whereas the linker helix and helical repeat-II of CSN1 contacts SCF using a conserved surface patch. Furthermore, we show that, in human, the C-terminal tail of CSN1, a segment not included in our crystal structure, interacts with IκBα in the NF-κB pathway. Therefore, the CSN complex uses multiple mechanisms to hinder NF-κB activation, a principle likely to hold true for its regulation of many other targets and pathways.

ArabidopsisChromatin Remodeling Factor PICKLE Interacts with Transcription Factor HY5 to Regulate Hypocotyl Cell Elongation

Photomorphogenesis is a critical plant developmental process that involves light-mediated transcriptome changes, histone modifications, and inhibition of hypocotyl growth. However, the chromatin-based regulatory mechanism underlying this process remains largely unknown. Here, we identify ENHANCED PHOTOMORPHOGENIC1 (EPP1), previously known as PICKLE (PKL), an ATP-dependent chromatin remodeling factor of the chromodomain/helicase/DNA binding family, as a repressor of photomorphogenesis in Arabidopsis thaliana. We show that PKL/EPP1 expression is repressed by light in the hypocotyls in a photoreceptor-dependent manner. Furthermore, we reveal that the transcription factor ELONGATED HYPOCOTYL5 (HY5) binds to the promoters of cell elongation-related genes and recruits PKL/EPP1 through their physical interaction. PKL/EPP1 in turn negatively regulates HY5 by repressing trimethylation of histone H3 Lys 27 at the target loci, thereby regulating the expression of these genes and, thus, hypocotyl elongation. We also show that HY5 possesses transcriptional repression activity. Our study reveals a crucial role for a chromatin remodeling factor in repressing photomorphogenesis and demonstrates that transcription factor-mediated recruitment of chromatin-remodeling machinery is important for plant development in response to changing light environments.

Interaction of Arabidopsis DET1 with CCA1 and LHY in Mediating Transcriptional Repression in the Plant Circadian Clock

The COP10-DET1-DDB1 (CDD) complex is an evolutionarily conserved protein complex discovered for its role in the repression of photomorphogenesis in Arabidopsis. It is important in many cellular and developmental processes in both plants and animals, but its molecular mode of action remains poorly understood. Here, we show that the CDD component DET1 possesses transcriptional repression activity and physically interacts with two closely related MYB transcription factors, CCA1 and LHY, which are core components of the plant circadian clock. DET1 associates with the promoter of CCA1/LHY target genes, such as TOC1, in a CCA1/LHY-dependent manner and is required for their repression, suggesting a recruitment of DET1 by the central oscillator components to regulate the clock. Our results reveal DET1 as a core transcriptional repression factor important for clock progression. Overall, the CDD complex may function as a transcriptional corepressor in diverse processes through direct interaction with distinct transcription factors.

Phosphorylation by CK2 Enhances the Rapid Light-induced Degradation of Phytochrome Interacting Factor 1 in Arabidopsis

The phytochrome family of sensory photoreceptors interacts with phytochrome interacting factors (PIFs), repressors of photomorphogenesis, in response to environmental light signals and induces rapid phosphorylation and degradation of PIFs to promote photomorphogenesis. However, the kinase that phosphorylates PIFs is still unknown. Here we show that CK2 directly phosphorylates PIF1 at multiple sites. α1 and α2 subunits individually phosphorylated PIF1 weakly in vitro. However, each of four β subunits strongly stimulated phosphorylation of PIF1 by α1 or α2. Mapping of the phosphorylation sites identified seven Ser/Thr residues scattered throughout PIF1. Ser/Thr to Ala scanning mutations at all seven sites eliminated CK2-mediated phosphorylation of PIF1 in vitro. Moreover, the rate of degradation of the Ser/Thr to Ala mutant PIF1 was significantly reduced compared with wild-type PIF1 in transgenic plants. In addition, hypocotyl lengths of the mutant PIF1 transgenic plants were much longer than the wild-type PIF1 transgenic plants under light, suggesting that the mutant PIF1 is suppressing photomorphogenesis. Taken together, these data suggest that CK2-mediated phosphorylation enhances the light-induced degradation of PIF1 to promote photomorphogenesis.

The Early Dark-Response in Arabidopsis thaliana Revealed by cDNA Microarray Analysis

Despite intense research on light responses in plants, the consequences of a simple shift from light to darkness remain poorly characterized. We have examined the transcriptome of Arabidopsis thaliana seedling leaves upon a shift from constant light to darkness for between 1 and 8 h, while excluding most effects associated with circadian oscillation. Expression clustering and gene ontology analyses identified about 790 responsive genes implicated in diverse cellular processes. Compared to the better-studied long-term dark adaptation response, the early response to darkness is partially overlapping yet clearly distinct, encompassing early transient, early sustained, and late response clusters. The repressor of photomorphogenesis, COP1 (constitutive photomorphogenic 1), is not a chief regulator of the early response to darkness, in contrast to its well-established role during long-term dark adaptation and etiolation. Only part of the early dark response can be understood as the opposite of the response following a dark-to-light transition and as a response to sugar deprivation. Bioinformatic comparisons with published microarray datasets further suggest that abscisic acid (ABA) signaling plays a prominent role in the early response to darkness, although this effect is not mediated by an increase in the ABA level. The potential basis for the co-regulation by darkness and ABA is discussed in light of sugar and redox signaling.

A role for Arabidopsis cryptochromes and COP1 in the regulation of stomatal opening

Cryptochromes (CRY) are blue light photoreceptors that mediate various light-induced responses in plants and animals. Arabidopsis CRY (CRY1 and CRY2) functions through negatively regulating constitutive photomorphogenic (COP) 1, a repressor of photomorphogenesis. Water evaporation and photosynthesis are regulated by the stomatal pores in plants, which are closed in darkness but open in response to blue light. There is evidence only for the phototropin blue light receptors (PHOT1 and PHOT2) in mediating blue light regulation of stomatal opening. Here, we report a previously uncharacterized role for Arabidopsis CRY and COP1 in the regulation of stomatal opening. Stomata of the cry1 cry2 double mutant showed reduced blue light response, whereas those of the CRY1-overexpressing plants showed hypersensitive response to blue light. In addition, stomata of the phot1 phot2 double mutant responded to blue light, but those of the cry1 cry2 phot1 phot2 quadruple mutant hardly responded. Strikingly, stomata of the cop1 mutant were constitutively open in darkness and stomata of the cry1 cry2 cop1 and phot1 phot2 cop1 triple mutants were open as wide as those of the cop1 single mutant under blue light. These results indicate that CRY functions additively with PHOT in mediating blue light-induced stomatal opening and that COP1 is a repressor of stomatal opening and likely acts downstream of CRY and PHOT signaling pathways.

The COP9 signalosome.

The COP9 signalosome (CSN) is composed of eight distinct subunits and is highly homologous to the lid sub-complex of the 26S proteasome. CSN was initially defined as a repressor of photomorphogenesis in Arabidopsis, and it has now been found to participate in diverse cellular and developmental processes in various eukaryotic organisms. Recently, CSN was revealed to have a metalloprotease activity centered in the CSN5/Jab1 subunit, which removes the post-translational modification of a ubiquitin-like protein, Nedd8/Rub1, from the cullin component of SCF ubiquitin E3 ligase (i.e., de-neddylation). In addition, CSN is associated with de-ubiquitination activity and protein kinase activities capable of phosphorylating important signaling regulators. The involvement of CSN in a number of cellular and developmental processes has been attributed to its control over ubiquitin-proteasome-mediated protein degradation.

The COP9 signalosome: regulating plant development through the control of proteolysis.

The COP9 signalosome (CSN) is a multiprotein complex that was initially identified in plants as a repressor of photomorphogenesis. It is now known to play major roles in several other developmental pathways, from auxin response to flower development. Furthermore, the COP9 signalosome shares homologies with the lid sibcomplex of the proteasome and is evolutionarily conserved from fission yeast to humans. It is important for the proper development of virtually all higher eukaryotes. In recent years, significant progress has been made in unraveling the molecular, cellular, and physiological mode of action of the COP9 signalosome. This review discusses our current understanding of the COP9 signalosome function with particular emphasis on its recently defined role in modulating a wide variety of cellular processes by regulating specific protein degradation events.