SPA Proteins Affect the Subcellular Localization of COP1 in the COP1/SPA Ubiquitin Ligase Complex during Photomorphogenesis

The E3 ubiquitin ligase COP1 regulates salt tolerance via GIGANTEA degradation in roots

Excess soil salinity significantly impairs plant growth and development. Our previous reports demonstrated that the core circadian clock oscillator GIGANTEA (GI) negatively regulates salt stress tolerance by sequestering the SALT OVERLY SENSITIVE (SOS) 2 kinase, an essential component of the SOS pathway. Salt stress induces calcium-dependent cytoplasmic GI degradation, resulting in activation of the SOS pathway; however, the precise molecular mechanism governing GI degradation during salt stress remains enigmatic. Here, we demonstrate that salt-induced calcium signals promote the cytoplasmic partitioning of CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), leading to the 26S proteasome-dependent degradation of GI exclusively in the roots. Salt stress-induced calcium signals accelerate the cytoplasmic localization of COP1 in the root cells, which targets GI for 26S proteasomal degradation. Align with this, the interaction between COP1 and GI is only observed in the roots, not the shoots, under salt-stress conditions. Notably, the gi-201 cop1-4 double mutant shows an enhanced tolerance to salt stress similar to gi-201, indicating that GI is epistatic to COP1 under salt-stress conditions. Taken together, our study provides critical insights into the molecular mechanisms governing the COP1-mediated proteasomal degradation of GI for salt stress tolerance, raising new possibilities for developing salt-tolerant crops.

Photoregulatory protein kinases fine-tune plant photomorphogenesis by directing a bifunctional phospho-code on HY5 in Arabidopsis

Photomorphogenesis is a light-dependent plant growth and development program. As the core regulator of photomorphogenesis, ELONGATED HYPOCOTYL 5 (HY5) is affected by dynamic changes in its transcriptional activity and protein stability; however, little is known about the mediators of these processes. Here, we identified PHOTOREGULATORY PROTEIN KINASE 1 (PPK1), which interacts with and phosphorylates HY5 in Arabidopsis, as one such mediator. The phosphorylation of HY5 by PPK1 is essential to establish high-affinity binding with B-BOX PROTEIN 24 (BBX24) and CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), which inhibit the transcriptional activity and promote the degradation of HY5, respectively. As such, PPKs regulate not only the binding of HY5 to its target genes under light conditions but also HY5 degradation when plants are transferred from light to dark. Our data identify a PPK-mediated phospho-code on HY5 that integrates the molecular mechanisms underlying the regulation of HY5 to precisely control plant photomorphogenesis.

What is going on inside of phytochrome B photobodies?

Plants exhibit an enormous phenotypic plasticity to adjust to changing environmental conditions. For this purpose, they have evolved mechanisms to detect and measure biotic and abiotic factors in their surroundings. Phytochrome B exhibits a dual function, since it serves as a photoreceptor for red and far-red light as well as a thermosensor. In 1999, it was first reported that phytochromes not only translocate into the nucleus but also form subnuclear foci upon irradiation by red light. It took more than 10 years until these phytochrome speckles received their name; these foci were coined photobodies to describe unique phytochrome-containing subnuclear domains that are regulated by light. Since their initial discovery, there has been much speculation about the significance and function of photobodies. Their presumed roles range from pure experimental artifacts to waste deposits or signaling hubs. In this review, we summarize the newest findings about the meaning of phyB photobodies for light and temperature signaling. Recent studies have established that phyB photobodies are formed by liquid-liquid phase separation via multivalent interactions and that they provide diverse functions as biochemical hotspots to regulate gene expression on multiple levels.

Phytochrome B photobody components

Phytochrome B (phyB) is a red and far-red photoreceptor that promotes light responses. Upon photoactivation, phyB enters the nucleus and forms a molecular condensate called a photobody through liquid-liquid phase separation. Phytochrome B photobody comprises phyB, the main scaffold molecule, and at least 37 client proteins. These clients belong to diverse functional categories enriched with transcription regulators, encompassing both positive and negative light signaling factors, with the functional bias toward the negative factors. The functionally diverse clients suggest that phyB photobody acts either as a trap to capture proteins, including negatively acting transcription regulators, for processes such as sequestration, modification, or degradation or as a hub where proteins are brought into close proximity for interaction in a light-dependent manner.

PH13 improves soybean shade traits and enhances yield for high-density planting at high latitudes

Shading in combination with extended photoperiods can cause exaggerated stem elongation (ESE) in soybean, leading to lodging and reduced yields when planted at high-density in high-latitude regions. However, the genetic basis of plant height in adaptation to these regions remains unclear. Here, through a genome-wide association study, we identify a plant height regulating gene on chromosome 13 (PH13) encoding a WD40 protein with three main haplotypes in natural populations. We find that an insertion of a Ty1/Copia-like retrotransposon in the haplotype 3 leads to a truncated PH13H3 with reduced interaction with GmCOP1s, resulting in accumulation of STF1/2, and reduced plant height. In addition, PH13H3 allele has been strongly selected for genetic improvement at high latitudes. Deletion of both PH13 and its paralogue PHP can prevent shade-induced ESE and allow high-density planting. This study provides insights into the mechanism of shade-resistance and offers potential solutions for breeding high-yielding soybean cultivar for high-latitude regions.

Molecular Bases of Signaling Processes Regulated by Cryptochrome Sensory Photoreceptors in Plants

The blue-light sensors, cryptochromes, compose the extensive class of flavoprotein photoreceptors, regulating signaling processes in plants underlying their development, growth, and metabolism. In several algae, cryptochromes may act not only as sensory photoreceptors but also as photolyases, catalyzing repair of the UV-induced DNA lesions. Cryptochromes bind FAD as the chromophore at the photolyase homologous region (PHR) domain and contain the cryptochrome C-terminal extension (CCE), which is absent in photolyases. Photosensory process in cryptochrome is initiated by photochemical chromophore conversions, including formation of the FAD redox forms. In the state with the chromophore reduced to neutral radical (FADH×), the photoreceptor protein undergoes phosphorylation, conformational changes, and disengagement from the PHR domain and CCE with subsequent formation of oligomers of cryptochrome molecules. Photooligomerization is a structural basis of the functional activities of cryptochromes, since it ensures formation of their complexes with a variety of signaling proteins, including transcriptional factors and regulators of transcription. Interactions in such complexes change the protein signaling activities, leading to regulation of gene expression and plant photomorphogenesis. In recent years, multiple papers, reporting novel, more detailed information about the molecular mechanisms of above-mentioned processes were published. The present review mainly focuses on analysis of the data contained in these publications, particularly regarding structural aspects of the cryptochrome transitions into photoactivated states and regulatory signaling processes mediated by the cryptochrome photoreceptors in plants.

Light Regulation of LoCOP1 and Its Role in Floral Scent Biosynthesis in Lilium 'Siberia'

Light is an important environmental signal that governs plant growth, development, and metabolism. Constitutive photomorphogenic 1 (COP1) is a light signaling component that plays a vital role in plant light responses. We isolated the COP1 gene (LoCOP1) from the petals of Lilium 'Siberia' and investigated its function. The LoCOP1 protein was found to be the most similar to Apostasia shenzhenica COP1. LoCOP1 was found to be an important factor located in the nucleus and played a negative regulatory role in floral scent production and emission using the virus-induced gene silencing (VIGS) approach. The yeast two-hybrid, β-galactosidase, and bimolecular fluorescence complementation (BiFC) assays revealed that LoCOP1 interacts with LoMYB1 and LoMYB3. Furthermore, light modified both the subcellular distribution of LoCOP1 and its interactions with LoMYB1 and MYB3 in onion cells. The findings highlighted an important regulatory mechanism in the light signaling system that governs scent emission in Lilium 'Siberia' by the ubiquitination and degradation of transcription factors via the proteasome pathway.

Signaling events for photomorphogenic root development

A germinating seedling incorporates environmental signals such as light into developmental outputs. Light is not only a source of energy, but also a central coordinative signal in plants. Traditionally, most research focuses on aboveground organs' response to light; therefore, our understanding of photomorphogenesis in roots is relatively scarce. However, root development underground is highly responsive to light signals from the shoot and understanding these signaling mechanisms will give a better insight into early seedling development. Here, we review the central light signaling hubs and their role in root growth promotion of Arabidopsis thaliana seedlings.

Abscisic acid-induced cytoplasmic translocation of constitutive photomorphogenic 1 enhances reactive oxygen species accumulation through the HY5-ABI5 pathway to modulate seed germination

Seed germination is a physiological process regulated by multiple factors. Abscisic acid (ABA) can inhibit seed germination to improve seedling survival under conditions of abiotic stress, and this process is often regulated by light signals. Constitutive photomorphogenic 1 (COP1) is an upstream core repressor of light signals and is involved in several ABA responses. Here, we demonstrate that COP1 is a negative regulator of the ABA-mediated inhibition of seed germination. Disruption of COP1 enhanced Arabidopsis seed sensitivity to ABA and increased reactive oxygen species (ROS) levels. In seeds, ABA induced the translocation of COP1 to the cytoplasm, resulting in enhanced ABA-induced ROS levels. Genetic evidence indicated that HY5 and ABI5 act downstream of COP1 in the ABA-mediated inhibition of seed germination. ABA-induced COP1 cytoplasmic localization increased HY5 and ABI5 protein levels in the nucleus, leading to increased expression of ABI5 target genes and ROS levels in seeds. Together, our results reveal that ABA-induced cytoplasmic translocation of COP1 activates the HY5-ABI5 pathway to promote the expression of ABA-responsive genes and the accumulation of ROS during ABA-mediated inhibition of seed germination. These findings enhance the role of COP1 in the ABA signal transduction pathway.

Signaling Mechanisms by Arabidopsis Cryptochromes

Cryptochromes (CRYs) are blue light photoreceptors that regulate growth, development, and metabolism in plants. In Arabidopsis thaliana (Arabidopsis), CRY1 and CRY2 possess partially redundant and overlapping functions. Upon exposure to blue light, the monomeric inactive CRYs undergo phosphorylation and oligomerization, which are crucial to CRY function. Both the N- and C-terminal domains of CRYs participate in light-induced interaction with multiple signaling proteins. These include the COP1/SPA E3 ubiquitin ligase, several transcription factors, hormone signaling intermediates and proteins involved in chromatin-remodeling and RNA N6 adenosine methylation. In this review, we discuss the mechanisms of Arabidopsis CRY signaling in photomorphogenesis and the recent breakthroughs in Arabidopsis CRY research.

Phytochrome A Mediates the Disassembly of Processing Bodies in Far-Red Light

Phytochromes are red- and far-red light receptors that control the growth and development of plants, enabling them to respond adequately to changing light conditions. It has been shown that halted mRNAs stored in RNA granules called processing bodies are released upon light perception and contribute to the adaptation to the light environment. However, the photophysiological background of this process is largely unknown. We found that light of different wavelengths can trigger the disassembly of processing bodies in a dose- and time-dependent manner. We show that phytochromes control this process in red- and far-red light and that cytoplasmic phytochrome A is sufficient and necessary for the far-red light-induced disassembly of processing bodies. This adds a novel, unexpected cytoplasmic function to the processes controlled by phytochrome A. Overall, our findings suggest a role of phytochromes in the control of translationally halted mRNAs that are stored in processing bodies. We expect our findings to facilitate understanding of how light and environmental cues control the assembly and disassembly of processing bodies, which could have broader implications for the regulation of non-membranous organelles in general.

Functions of COP1/SPA E3 Ubiquitin Ligase Mediated by MpCRY in the Liverwort Marchantia polymorpha under Blue Light

COP1/SPA1 complex in Arabidopsis inhibits photomorphogenesis through the ubiquitination of multiple photo-responsive transcription factors in darkness, but such inhibiting function of COP1/SPA1 complex would be suppressed by cryptochromes in blue light. Extensive studies have been conducted on these mechanisms in Arabidopsis whereas little attention has been focused on whether another branch of land plants bryophyte utilizes this blue-light regulatory pathway. To study this problem, we conducted a study in the liverwort Marchantia polymorpha and obtained a MpSPA knock-out mutant, in which Mpspa exhibits the phenotype of an increased percentage of individuals with asymmetrical thallus growth, similar to MpCRY knock-out mutant. We also verified interactions of MpSPA with MpCRY (in a blue light-independent way) and with MpCOP1. Concomitantly, both MpSPA and MpCOP1 could interact with MpHY5, and MpSPA can promote MpCOP1 to ubiquitinate MpHY5 but MpCRY does not regulate the ubiquitination of MpHY5 by MpCOP1/MpSPA complex. These data suggest that COP1/SPA ubiquitinating HY5 is conserved in Marchantia polymorpha, but dissimilar to CRY in Arabidopsis, MpCRY is not an inhibitor of this process under blue light.

Light-induced degradation of SPA2 via its N-terminal kinase domain is required for photomorphogenesis

Arabidopsis (Arabidopsis thaliana) CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) and members of the SUPPRESSOR OF PHYTOCHROMEA-105 (SPA) protein family form an E3 ubiquitin ligase that suppresses light signaling in darkness by polyubiquitinating positive regulators of the light response. COP1/SPA is inactivated by light to allow photomorphogenesis to proceed. Mechanisms of inactivation include light-induced degradation of SPA1 and, in particular, SPA2, corresponding to a particularly efficient inactivation of COP1/SPA2 by light. Here, we show that SPA3 and SPA4 proteins are stable in the light, indicating that light-induced destabilization is specific to SPA1 and SPA2, possibly related to the predominant function of SPA1 and SPA2 in dark-grown etiolating seedlings. SPA2 degradation involves cullin and the COP10-DEETIOLATED-DAMAGED-DNA BINDING PROTEIN (DDB1) CDD complex, besides COP1. Consistent with this finding, light-induced SPA2 degradation required the DDB1-interacting Trp-Asp (WD)-repeat domain of SPA2. Deletion of the N-terminus of SPA2 containing the kinase domain led to strong stabilization of SPA2 in darkness and fully abolished light-induced degradation of SPA2. This prevented seedling de-etiolation even in very strong far-red and blue light and reduced de-etiolation in red light, indicating destabilization of SPA2 through its N-terminal domain is essential for light response. SPA2 is exclusively destabilized by phytochrome A in far-red and blue light. However, deletion of the N-terminal domain of SPA2 did not abolish SPA2-phytochrome A interaction in yeast nor in vivo. Our domain mapping suggests there are two SPA2-phytochrome A interacting domains, the N-terminal domain and the WD-repeat domain. Conferring a light-induced SPA2-phyA interaction only via the WD-repeat domain may thus not lead to COP1/SPA2 inactivation.

Direct phosphorylation of HY5 by SPA kinases to regulate photomorphogenesis in Arabidopsis

Elongated hypocotyl5 (HY5) is a key transcription factor that promotes photomorphogenesis. Constitutive photomorphogenic1 (COP1)-Suppressor of phytochrome A-105 (SPA) E3 ubiquitin ligase complex promotes ubiquitination and degradation of HY5 to repress photomorphogenesis in darkness. HY5 is also regulated by phosphorylation at serine 36 residue. However, the kinase responsible for phosphorylation of HY5 remains unknown. Here, using extensive in vitro and in vivo biochemical, genetic, and photobiological techniques, we have identified a new kinase that phosphorylates HY5 and demonstrated the significance of phosphorylation of HY5 in Arabidopsis thaliana. We show that SPA proteins are the missing kinases necessary for HY5 phosphorylation. SPAs can directly phosphorylate HY5 in vitro, and the phosphorylated HY5 is absent in the spaQ background in vivo. We also demonstrate that the unphosphorylated HY5 interacts strongly with both COP1 and SPA1 and is the preferred substrate for degradation, whereas the phosphorylated HY5 is more stable in the dark. In addition, the unphosphorylated HY5 actively binds to the target promoters and is the physiologically more active form. Consistently, the transgenic plants expressing the unphosphorylated form of HY5 display enhanced photomorphogenesis. Collectively, our study revealed the missing kinase responsible for direct phosphorylation of HY5 that fine-tunes its stability and activity to regulate photomorphogenesis.

Direct phosphorylation of HY5 by SPA kinases to regulate photomorphogenesis in Arabidopsis

Elongated hypocotyl5 (HY5) is a key transcription factor that promotes photomorphogenesis. Constitutive photomorphogenic1 (COP1)-Suppressor of phytochrome A-105 (SPA) E3 ubiquitin ligase complex promotes ubiquitination and degradation of HY5 to repress photomorphogenesis in darkness. HY5 is also regulated by phosphorylation at serine 36 residue. However, the kinase responsible for phosphorylation of HY5 remains unknown. Here, using extensive in vitro and in vivo biochemical, genetic, and photobiological techniques, we have identified a new kinase that phosphorylates HY5 and demonstrated the significance of phosphorylation of HY5 in Arabidopsis thaliana. We show that SPA proteins are the missing kinases necessary for HY5 phosphorylation. SPAs can directly phosphorylate HY5 in vitro, and the phosphorylated HY5 is absent in the spaQ background in vivo. We also demonstrate that the unphosphorylated HY5 interacts strongly with both COP1 and SPA1 and is the preferred substrate for degradation, whereas the phosphorylated HY5 is more stable in the dark. In addition, the unphosphorylated HY5 actively binds to the target promoters and is the physiologically more active form. Consistently, the transgenic plants expressing the unphosphorylated form of HY5 display enhanced photomorphogenesis. Collectively, our study revealed the missing kinase responsible for direct phosphorylation of HY5 that fine-tunes its stability and activity to regulate photomorphogenesis.

Transcriptome and Metabolomic Analyses Reveal Regulatory Networks Controlling Maize Stomatal Development in Response to Blue Light

(1) Background: Blue light is important for the formation of maize stomata, but the signal network remains unclear. (2) Methods: We replaced red light with blue light in an experiment and provided a complementary regulatory network for the stomatal development of maize by using transcriptome and metabolomics analysis. (3) Results: Exposure to blue light led to 1296 differentially expressed genes and 419 differential metabolites. Transcriptome comparisons and correlation signaling network analysis detected 55 genes, and identified 6 genes that work in the regulation of the HY5 module and MAPK cascade, that interact with PTI1, COI1, MPK2, and MPK3, in response to the substitution of blue light in environmental adaptation and signaling transduction pathways. Metabolomics analysis showed that two genes involved in carotenoid biosynthesis and starch and sucrose metabolism participate in stomatal development. Their signaling sites located on the PHI1 and MPK2 sites of the MAPK cascade respond to blue light signaling. (4) Conclusions: Blue light remarkably changed the transcriptional signal transduction and metabolism of metabolites, and eight obtained genes worked in the HY5 module and MAPK cascade.

Transcriptome and Metabolomic Analyses Reveal Regulatory Networks Controlling Maize Stomatal Development in Response to Blue Light

(1) Background: Blue light is important for the formation of maize stomata, but the signal network remains unclear. (2) Methods: We replaced red light with blue light in an experiment and provided a complementary regulatory network for the stomatal development of maize by using transcriptome and metabolomics analysis. (3) Results: Exposure to blue light led to 1296 differentially expressed genes and 419 differential metabolites. Transcriptome comparisons and correlation signaling network analysis detected 55 genes, and identified 6 genes that work in the regulation of the HY5 module and MAPK cascade, that interact with PTI1, COI1, MPK2, and MPK3, in response to the substitution of blue light in environmental adaptation and signaling transduction pathways. Metabolomics analysis showed that two genes involved in carotenoid biosynthesis and starch and sucrose metabolism participate in stomatal development. Their signaling sites located on the PHI1 and MPK2 sites of the MAPK cascade respond to blue light signaling. (4) Conclusions: Blue light remarkably changed the transcriptional signal transduction and metabolism of metabolites, and eight obtained genes worked in the HY5 module and MAPK cascade.

COP1 promotes ABA-induced stomatal closure by modulating the abundance of ABI/HAB and AHG3 phosphatases

Plant stomata play a crucial role in leaf function, controlling water transpiration in response to environmental stresses and modulating the gas exchange necessary for photosynthesis. The phytohormone abscisic acid (ABA) promotes stomatal closure and inhibits light-induced stomatal opening. The Arabidopsis thaliana E3 ubiquitin ligase COP1 functions in ABA-mediated stomatal closure. However, the underlying molecular mechanisms are still not fully understood. Yeast two-hybrid assays were used to identify ABA signaling components that interact with COP1, and biochemical, molecular and genetic studies were carried out to elucidate the regulatory role of COP1 in ABA signaling. The cop1 mutants are hyposensitive to ABA-triggered stomatal closure under light and dark conditions. COP1 interacts with and ubiquitinates the Arabidopsis clade A type 2C phosphatases (PP2Cs) ABI/HAB group and AHG3, thus triggering their degradation. Abscisic acid enhances the COP1-mediated degradation of these PP2Cs. Mutations in ABI1 and AHG3 partly rescue the cop1 stomatal phenotype and the phosphorylation level of OST1, a crucial SnRK2-type kinase in ABA signaling. Our data indicate that COP1 is part of a novel signaling pathway promoting ABA-mediated stomatal closure by regulating the stability of a subset of the Clade A PP2Cs. These findings provide novel insights into the interplay between ABA and the light signaling component in the modulation of stomatal movement.

Illuminating the COP1/SPA Ubiquitin Ligase: Fresh Insights Into Its Structure and Functions During Plant Photomorphogenesis

CONSTITUTIVE PHOTOMORPHOGENIC 1 functions as an E3 ubiquitin ligase in plants and animals. Discovered originally in Arabidopsis thaliana, COP1 acts in a complex with SPA proteins as a central repressor of light-mediated responses in plants. By ubiquitinating and promoting the degradation of several substrates, COP1/SPA regulates many aspects of plant growth, development and metabolism. In contrast to plants, human COP1 acts as a crucial regulator of tumorigenesis. In this review, we discuss the recent important findings in COP1/SPA research including a brief comparison between COP1 activity in plants and humans.

COLD REGULATED 27 and 28 are targets of CONSTITUTIVELY PHOTOMORPHOGENIC 1 and negatively affect phytochrome B signalling

Phytochromes are red/far-red light receptors in plants involved in the regulation of growth and development. Phytochromes can sense the light environment and contribute to measuring day length; thereby, they allow plants to respond and adapt to changes in the ambient environment. Two well-characterized signalling pathways act downstream of phytochromes and link light perception to the regulation of gene expression. The CONSTITUTIVELY PHOTOMORPHOGENIC 1/SUPPRESSOR OF PHYA-105 (COP1/SPA) E3 ubiquitin ligase complex and the PHYTOCHROME INTERACTING FACTORs (PIFs) are key components of these pathways and repress light responses in the dark. In light-grown seedlings, phytochromes inhibit COP1/SPA and PIF activity and thereby promote light signalling. In a yeast-two-hybrid screen for proteins binding to light-activated phytochromes, we identified COLD-REGULATED GENE 27 (COR27). COR27 and its homologue COR28 bind to phyA and phyB, the two primary phytochromes in seed plants. COR27 and COR28 have been described previously with regard to a function in the regulation of freezing tolerance, flowering and the circadian clock. Here, we show that COR27 and COR28 repress early seedling development in blue, far-red and in particular red light. COR27 and COR28 contain a conserved Val-Pro (VP)-peptide motif, which mediates binding to the COP1/SPA complex. COR27 and COR28 are targeted for degradation by COP1/SPA and mutant versions with a VP to AA amino acid substitution in the VP-peptide motif are stabilized. Overall, our data suggest that COR27 and COR28 accumulate in light but act as negative regulators of light signalling during early seedling development, thereby preventing an exaggerated response to light.

Molecular mechanisms suppressing COP1/SPA E3 ubiquitin ligase activity in blue light

Arabidopsis CONSTITUTIVE PHOTOMORPHOGENIC1/SUPPRESSOR OF PHYA-105 (COP1/SPA) is an E3 ubiquitin ligase complex that prevents photomorphogenesis in darkness by ubiquitinating and subsequently degrading light-responsive transcription factors. Upon light perception, photoreceptors directly interact with the COP1/SPA complex to suppress its activity. In blue light (450-500 nm of visible spectrum), COP1/SPA activity is inhibited by the cryptochrome photoreceptors (CRY1 and CRY2), FKF1 from the ZEITLUPE family as well as phytochrome A. Together, these photoreceptors regulate vital aspects of plant growth and development from seedling stage to the induction of flowering. This review presents and discusses the recent advances in blue light-mediated suppression of COP1/SPA activity.

CONSTITUTIVELY PHOTOMORPHOGENIC1 promotes ABA-mediated inhibition of post-germination seedling establishment

Under acute stress conditions, precocious seedling development may result in the premature death of young seedlings, before they switch to autotrophic growth. The phytohormone abscisic acid (ABA) inhibits seed germination and post-germination seedling establishment under unfavorable conditions. Various environmental signals interact with the ABA pathway to optimize these early developmental events under stress. Here, we show that light availability critically influences ABA sensitivity during early seedling development. In dark conditions, the ABA-mediated inhibition of post-germination seedling establishment is strongly enhanced. COP1, a central regulator of seedling development in the dark, is necessary for this enhanced post-germination ABA sensitivity in darkness. Despite their slower germination, cop1 seedlings establish faster than wild type in the presence of ABA in both light and dark. PHY and CRY photoreceptors that inhibit COP1 activity in light modulate ABA-mediated inhibition of seedling establishment in light. Genetically, COP1 acts downstream to ABI5, a key transcriptional regulator of ABA signaling, and does not influence the transcriptional and protein levels of ABI5 during the early post-germination stages. COP1 promotes post-germination growth arrest independent of the antagonistic interaction between ABA and cytokinin signaling pathways. COP1 facilitates the binding of ABI5 on its target promoters and the ABA-mediated upregulation of these target genes is reduced in cop1-4. Together, our results suggest that COP1 positively regulates ABA signaling to inhibit post-germination seedling establishment under stress.

The Photomorphogenic Central Repressor COP1: Conservation and Functional Diversification during Evolution

Green plants on the earth have evolved intricate mechanisms to acclimatize to and utilize sunlight. In Arabidopsis, light signals are perceived by photoreceptors and transmitted through divergent but overlapping signaling networks to modulate plant photomorphogenic development. COP1 (CONSTITUTIVE PHOTOMORPHOGENIC 1) was first cloned as a central repressor of photomorphogenesis in higher plants and has been extensively studied for over 30 years. It acts as a RING E3 ubiquitin ligase downstream of multiple photoreceptors to target key light-signaling regulators for degradation, primarily as part of large protein complexes. The mammalian counterpart of COP1 is a pluripotent regulator of tumorigenesis and metabolism. A great deal of information on COP1 has been derived from whole-genome sequencing and functional studies in lower green plants, which enables us to illustrate its evolutionary history. Here, we review the current understanding about COP1, with a focus on the conservation and functional diversification of COP1 and its signaling partners in different taxonomic clades.

Cryptochrome 2 competes with COP1 substrates to repress COP1 ubiquitin ligase activity during Arabidopsis photomorphogenesis

In plants, the cryptochrome photoreceptors suppress the activity of the COP1/SPA ubiquitin ligase to initiate photomorphogenesis in blue light. Both CRY1 and CRY2 interact with the COP1/SPA complex in a blue light-dependent manner. The mechanisms underlying the inhibition of COP1 activity through direct interactions with photoactivated CRYs are not fully understood. Here we tested the hypothesis that CRY2 inhibits COP1 by displacing the degradation substrates from COP1. To this end, we analyzed the role of a conserved valine-proline (VP) motif in the C-terminal domain of CRY2 (CCT2), which resembles the core COP1-WD40-binding sequences present in the substrates of COP1. We show that the VP motif in CRY2 is essential for the interaction of CRY2 with COP1 in yeast two-hybrid assays and in planta Mutations in the VP motif of CRY2 abolished the CRY2 activity in photomorphogenesis, indicating the importance of VP. The interaction between COP1 and its VP-containing substrate PAP2 was prevented in the presence of coexpressed CRY2, but not in the presence of CRY2 carrying a VP mutation. Thus, since both PAP2 and CRY2 engage VP motifs to bind to COP1, these results demonstrate that CRY2 outcompetes PAP2 for binding to COP1. We further found that the previously unknown interaction between SPA1-WD and CCT2 occurs via the VP motif in CRY2, suggesting structural similarities in the VP-binding pockets of COP1-WD40 and SPA1-WD40 domains. A VP motif present in CRY1 is also essential for binding to COP1. Thus, CRY1 and CRY2 might share this mechanism of COP1 inactivation.

Characterization of spa mutants in the moss Physcomitrella provides evidence for functional divergence of SPA genes during the evolution of land plants

The Arabidopsis COP1/SPA complex is a key repressor of photomorphogenesis that suppresses light signaling in the dark. Both COP1 and SPA proteins are essential components of this complex. Although COP1 also exists in humans, SPA genes are specific to the green lineage. To elucidate the evolution of SPA genes we analyzed SPA functions in the moss Physcomitrella patens by characterizing knockout mutants in the two Physcomitrella SPA genes PpSPAa and PpSPAb. Light-grown PpspaAB double mutants exhibit smaller gametophores than the wild-type. In the dark, PpspaAB mutant gametophores show enhanced continuation of growth but etiolate normally. Gravitropism in the dark is reduced in PpspaAB mutant protonemata. The expression of light-regulated genes is mostly not constitutive in PpspaAB mutants. PpSPA and PpCOP1 interact; PpCOP1 also interacts with the transcription factor PpHY5 and, indeed, PpHY5 is destabilized in dark-grown Physcomitrella. Degradation of PpHY5 in darkness, however, does not require PpSPAa and PpSPAb. The data suggest that COP1/SPA-mediated light signaling is only partially conserved between Arabidopsis and Physcomitrella. Whereas COP1/SPA interaction and HY5 degradation in darkness is conserved, the role of SPA proteins appears to have diverged. PpSPA genes, unlike their Arabidopsis counterparts, are only required to suppress a subset of light responses in darkness.

UVR8 disrupts stabilisation of PIF5 by COP1 to inhibit plant stem elongation in sunlight

Alterations in light quality significantly affect plant growth and development. In canopy shade, phytochrome photoreceptors perceive reduced ratios of red to far-red light (R:FR) and initiate stem elongation to enable plants to overtop competitors. This shade avoidance response is achieved via the stabilisation and activation of PHYTOCHROME INTERACTING FACTORs (PIFs) which elevate auxin biosynthesis. UV-B inhibits shade avoidance by reducing the abundance and activity of PIFs, yet the molecular mechanisms controlling PIF abundance in UV-B are unknown. Here we show that the UV-B photoreceptor UVR8 promotes rapid PIF5 degradation via the ubiquitin-proteasome system in a response requiring the N terminus of PIF5. In planta interactions between UVR8 and PIF5 are not observed. We further demonstrate that PIF5 interacts with the E3 ligase COP1, promoting PIF5 stabilisation in light-grown plants. Binding of UVR8 to COP1 in UV-B disrupts this stabilisation, providing a mechanism to rapidly lower PIF5 abundance in sunlight.

UV-B light suppresses the shade avoidance response in plants by reducing the abundance of PIF transcription factors by an undefined mechanism. Here the authors show that UV-B perceived by the UVR8 receptor inhibits the shade avoidance response by preventing stabilisation of PIF5 by COP1.

A phyB-PIF1-SPA1 kinase regulatory complex promotes photomorphogenesis in Arabidopsis

CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) is a highly conserved E3 ubiquitin ligase from plants to animals and acts as a central repressor of photomorphogenesis in plants. SUPPRESSOR OF PHYA-105 1 family members (SPA1-SPA4) directly interact with COP1 and enhance COP1 activity. Despite the presence of a kinase domain at the N-terminus, no COP1-independent role of SPA proteins has been reported. Here we show that SPA1 acts as a serine/threonine kinase and directly phosphorylates PIF1 in vitro and in vivo. SPAs are necessary for the light-induced phosphorylation, ubiquitination and subsequent degradation of PIF1. Moreover, the red/far-red light photoreceptor phyB interacts with SPA1 through its C-terminus and enhances the recruitment of PIF1 for phosphorylation. These data provide a mechanistic view on how the COP1-SPA complexes serve as an example of a cognate kinase-E3 ligase complex that selectively triggers rapid phosphorylation and removal of its substrates, and how phyB modulates this process to promote photomorphogenesis.

SPA proteins repress plant photomorphogenesis by promoting the E3 ligase activity of COP1. Here the authors show that SPAs also act as serine/threonine kinase and are required for phyB-mediated light-dependent phosphorylation and degradation of the PIF1 transcription factor.

A transient expression system in soybean mesophyll protoplasts reveals the formation of cytoplasmic GmCRY1 photobody-like structures

Soybean (Glycine max (L.) Merr.), grown for its plant oils and proteins, is one of the most important crops throughout the world. Generating stable and heritable transgenic soybeans is relatively inefficient; therefore, there is an urgent need for a simple and high-efficient transient transformation method by which to enable the investigation of gene functions in soybeans, which will facilitate the elucidation and improvement of the molecular mechanisms regulating the associated agronomic traits. We established a system of transient expression in soybean mesophyll protoplasts and obtained a high level of protoplast transfection efficiency (up to 83.5%). The subcellular activity of the protoplasts was well preserved, as demonstrated by the dynamic formation of GmCRY nucleus photobodies (NPs) and/or cytoplasmic photobody-like structures (CPs) in response to blue light. In addition, we showed that GmCRY1b CPs colocalized with GmCOP1b, a co-ortholog of Arabidopsis thaliana CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), which provided new insight into the potential roles of GmCRY1s in the cytoplasm.

Feeling the Heat: Searching for Plant Thermosensors

To draw the complete picture of plant thermal signaling, it is important to find the missing links between the temperature cue, the actual sensing, and the subsequent response. In this context, several plant thermosensors have been proposed. Here, we compare these with thermosensors in various other organisms, put them in the context of thermosensing in plants, and suggest a set of criteria to which a thermosensor must adhere. Finally, we propose that more emphasis should be given to structural analysis of DNA, RNA, and proteins in light of the activity of potential thermosensors.

The pseudokinase TRIB1 toggles an intramolecular switch to regulate COP1 nuclear export

COP1 is a highly conserved ubiquitin ligase that regulates diverse cellular processes in plants and metazoans. Tribbles pseudokinases, which only exist in metazoans, act as scaffolds that interact with COP1 and its substrates to facilitate ubiquitination. Here, we report that, in addition to this scaffolding role, TRIB1 promotes nuclear localization of COP1 by disrupting an intramolecular interaction between the WD40 domain and a previously uncharacterized regulatory site within COP1. This site, which we have termed the pseudosubstrate latch (PSL), resembles the consensus COP1-binding motif present in known COP1 substrates. Our findings support a model in which binding of the PSL to the WD40 domain stabilizes a conformation of COP1 that is conducive to CRM1-mediated nuclear export, and TRIB1 displaces this intramolecular interaction to induce nuclear retention of COP1. Coevolution of Tribbles and the PSL in metazoans further underscores the importance of this role of Tribbles in regulating COP1 function.

Photoreceptor-mediated regulation of the COP1/SPA E3 ubiquitin ligase

Plants have evolved specific photoreceptors that capture informational cues from sunlight. The phytochrome, cryptochrome, and UVR8 photoreceptors perceive red/far-red, blue/UV-A, and UV-B light, respectively, and control overlapping photomorphogenic responses important for plant growth and development. A major repressor of such photomorphogenic responses is the E3 ubiquitin ligase formed by CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and SUPPRESSOR OF PHYA-105 (SPA) proteins, which acts by regulating the stability of photomorphogenesis-promoting transcription factors. The direct interaction of light-activated photoreceptors with the COP1/SPA complex represses its activity via nuclear exclusion of COP1, disruption of the COP1-SPA interaction, and/or SPA protein degradation. This process enables plants to integrate different light signals at the level of the COP1/SPA complex to enact appropriate photomorphogenic responses according to the light environment.

Phytochrome B Requires PIF Degradation and Sequestration to Induce Light Responses across a Wide Range of Light Conditions

Phytochrome B (phyB) inhibits the function of phytochrome-interacting factors (PIFs) by inducing their degradation and sequestration, but the relative physiological importance of these two phyB activities is unclear. In an analysis of published Arabidopsis thaliana phyB mutations, we identified a point mutation in the N-terminal half of phyB (phyB^G111D) that abolishes its PIF sequestration activity without affecting its PIF degradation activity. We also identified a point mutation in the phyB C-terminal domain, which, when combined with a deletion of the C-terminal end (phyB990^G767R), does the opposite; it blocks PIF degradation without affecting PIF sequestration. The resulting phyB proteins, phyB990^G767R and phyB^G111D, are equally capable of inducing light responses under continuous red light. However, phyB^G111D, which exhibits only the PIF degradation activity, induces stronger light responses than phyB990^G767R under white light with prolonged dark periods (i.e., diurnal cycles). In contrast, phyB990^G767R, which exhibits only the PIF sequestration activity, induces stronger light responses in flickering light (a condition that mimics sunflecks). Together, our results indicate that both of these separable phyB activities are required for light responses in varying light conditions.

The blue light-induced interaction of cryptochrome 1 with COP1 requires SPA proteins during Arabidopsis light signaling

Plants constantly adjust their growth, development and metabolism to the ambient light environment. Blue light is sensed by the Arabidopsis photoreceptors CRY1 and CRY2 which subsequently initiate light signal transduction by repressing the COP1/SPA E3 ubiquitin ligase. While the interaction between cryptochromes and SPA is blue light-dependent, it was proposed that CRY1 interacts with COP1 constitutively, i.e. also in darkness. Here, our in vivo co-immunoprecipitation experiments suggest that CRY1 and CRY2 form a complex with COP1 only after seedlings were exposed to blue light. No association between COP1 and CRY1 or CRY2 was observed in dark-grown seedlings. Thus, our results suggest that cryptochromes bind the COP1/SPA complex after photoactivation by blue light. In a spa quadruple mutant that is devoid of all four SPA proteins, CRY1 and COP1 did not interact in vivo, neither in dark-grown nor in blue light-grown seedlings. Hence, SPA proteins are required for the high-affinity interaction between CRY1 and COP1 in blue light. Yeast three-hybrid experiments also show that SPA1 enhances the CRY1-COP1 interaction. The coiled-coil domain of SPA1 which is responsible for COP1-binding was necessary to mediate a CRY1-SPA1 interaction in vivo, implying that-in turn-COP1 may be necessary for a CRY1-SPA1 complex formation. Hence, SPA1 and COP1 may act cooperatively in recognizing and binding photoactivated CRY1. In contrast, the blue light-induced association between CRY2 and COP1 was not dependent on SPA proteins in vivo. Similarly, ΔCC-SPA1 interacted with CRY2, though with a much lower affinity than wild-type SPA1. In total, our results demonstrate that CRY1 and CRY2 strongly differ in their blue light-induced interaction with the COP1/SPA complex.

The blue light-induced interaction of cryptochrome 1 with COP1 requires SPA proteins during Arabidopsis light signaling

Plants constantly adjust their growth, development and metabolism to the ambient light environment. Blue light is sensed by the Arabidopsis photoreceptors CRY1 and CRY2 which subsequently initiate light signal transduction by repressing the COP1/SPA E3 ubiquitin ligase. While the interaction between cryptochromes and SPA is blue light-dependent, it was proposed that CRY1 interacts with COP1 constitutively, i.e. also in darkness. Here, our in vivo co-immunoprecipitation experiments suggest that CRY1 and CRY2 form a complex with COP1 only after seedlings were exposed to blue light. No association between COP1 and CRY1 or CRY2 was observed in dark-grown seedlings. Thus, our results suggest that cryptochromes bind the COP1/SPA complex after photoactivation by blue light. In a spa quadruple mutant that is devoid of all four SPA proteins, CRY1 and COP1 did not interact in vivo, neither in dark-grown nor in blue light-grown seedlings. Hence, SPA proteins are required for the high-affinity interaction between CRY1 and COP1 in blue light. Yeast three-hybrid experiments also show that SPA1 enhances the CRY1-COP1 interaction. The coiled-coil domain of SPA1 which is responsible for COP1-binding was necessary to mediate a CRY1-SPA1 interaction in vivo, implying that-in turn-COP1 may be necessary for a CRY1-SPA1 complex formation. Hence, SPA1 and COP1 may act cooperatively in recognizing and binding photoactivated CRY1. In contrast, the blue light-induced association between CRY2 and COP1 was not dependent on SPA proteins in vivo. Similarly, ΔCC-SPA1 interacted with CRY2, though with a much lower affinity than wild-type SPA1. In total, our results demonstrate that CRY1 and CRY2 strongly differ in their blue light-induced interaction with the COP1/SPA complex.