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

CONSTANS, a HUB for all seasons: How photoperiod pervades plant physiology regulatory circuits

How does a plant detect the changing seasons and make important developmental decisions accordingly? How do they incorporate daylength information into their routine physiological processes? Photoperiodism, or the capacity to measure the daylength, is a crucial aspect of plant development that helps plants determine the best time of the year to make vital decisions, such as flowering. The protein CONSTANS (CO) constitutes the central regulator of this sensing mechanism, not only activating florigen production in the leaves but also participating in many physiological aspects in which seasonality is important. Recent discoveries place CO in the center of a gene network that can determine the length of the day and confer seasonal input to aspects of plant development and physiology as important as senescence, seed size, or circadian rhythms. In this review, we discuss the importance of CO protein structure, function, and evolutionary mechanisms that embryophytes have developed to incorporate annual information into their physiology.

The dual-action mechanism of Arabidopsis cryptochromes

Photoreceptor cryptochromes (CRYs) mediate blue-light regulation of plant growth and development. It has been reported that Arabidopsis CRY1and CRY2 function by physically interacting with at least 84 proteins, including transcription factors or co-factors, chromatin regulators, splicing factors, messenger RNA methyltransferases, DNA repair proteins, E3 ubiquitin ligases, protein kinases and so on. Of these 84 proteins, 47 have been reported to exhibit altered binding affinity to CRYs in response to blue light, and 41 have been shown to exhibit condensation to CRY photobodies. The blue light-regulated composition or condensation of CRY complexes results in changes of gene expression and developmental programs. In this mini-review, we analyzed recent studies of the photoregulatory mechanisms of Arabidopsis CRY complexes and proposed the dual mechanisms of action, including the "Lock-and-Key" and the "Liquid-Liquid Phase Separation (LLPS)" mechanisms. The dual CRY action mechanisms explain, at least partially, the structural diversity of CRY-interacting proteins and the functional diversity of the CRY photoreceptors.

Light signaling in plants-a selective history

In addition to providing the radiant energy that drives photosynthesis, sunlight carries signals that enable plants to grow, develop and adapt optimally to the prevailing environment. Here we trace the path of research that has led to our current understanding of the cellular and molecular mechanisms underlying the plant's capacity to perceive and transduce these signals into appropriate growth and developmental responses. Because a fully comprehensive review was not possible, we have restricted our coverage to the phytochrome and cryptochrome classes of photosensory receptors, while recognizing that the phototropin and UV classes also contribute importantly to the full scope of light-signal monitoring by the plant.

Diurnal Rhythms in the Red Seaweed Gracilariopsis chorda are Characterized by Unique Regulatory Networks of Carbon Metabolism

Cellular and physiological cycles are driven by endogenous pacemakers, the diurnal and circadian rhythms. Key functions such as cell cycle progression and cellular metabolism are under rhythmic regulation, thereby maintaining physiological homeostasis. The photoreceptors phytochrome and cryptochrome, in response to light cues, are central input pathways for physiological cycles in most photosynthetic organisms. However, among Archaeplastida, red algae are the only taxa that lack phytochromes. Current knowledge about oscillatory rhythms is primarily derived from model species such as Arabidopsis thaliana and Chlamydomonas reinhardtii in the Viridiplantae, whereas little is known about these processes in other clades of the Archaeplastida, such as the red algae (Rhodophyta). We used genome-wide expression profiling of the red seaweed Gracilariopsis chorda and identified 3,098 rhythmic genes. Here, we characterized possible cryptochrome-based regulation and photosynthetic/cytosolic carbon metabolism in this species. We found a large family of cryptochrome genes in G. chorda that display rhythmic expression over the diurnal cycle and may compensate for the lack of phytochromes in this species. The input pathway gates regulatory networks of carbon metabolism which results in a compact and efficient energy metabolism during daylight hours. The system in G. chorda is distinct from energy metabolism in most plants, which activates in the dark. The green lineage, in particular, land plants, balance water loss and CO2 capture in terrestrial environments. In contrast, red seaweeds maintain a reduced set of photoreceptors and a compact cytosolic carbon metabolism to thrive in the harsh abiotic conditions typical of intertidal zones.

The mechanism of low blue light-induced leaf senescence mediated by GmCRY1s in soybean

Leaf senescence is a crucial trait that has a significant impact on crop quality and yield. Previous studies have demonstrated that light is a key factor in modulating the senescence process. However, the precise mechanism by which plants sense light and control senescence remains largely unknown, particularly in crop species. In this study, we reveal that the reduction in blue light under shading conditions can efficiently induce leaf senescence in soybean. The blue light receptors GmCRY1s rather than GmCRY2s, primarily regulate leaf senescence in response to blue light signals. Our results show that GmCRY1s interact with DELLA proteins under light-activated conditions, stabilizing them and consequently suppressing the transcription of GmWRKY100 to delay senescence. Conversely, LBL reduces the interaction between GmCRY1s and the DELLA proteins, leading to their degradation and premature senescence of leaves. Our findings suggest a GmCRY1s-GmDELLAs-GmWRKY100 regulatory cascade that is involved in mediating LBL-induced leaf senescence in soybean, providing insight into the mechanism of how light signals regulate leaf senescence. Additionally, we generate GmWRKY100 knockout soybeans that show delayed leaf senescence and improved yield under natural field conditions, indicating potential applications in enhancing soybean production by manipulating the leaf senescence trait.

The blue light signal transduction module FaCRY1-FaCOP1-FaHY5 regulates anthocyanin accumulation in cultivated strawberry

Anthocyanins have important physiological functions and are beneficial to the improvement of fruit quality in strawberry. Light is important for anthocyanin biosynthesis, and specific light quality was identified to promote anthocyanin accumulation in many fruits. However, research on the molecular mechanisms of anthocyanin accumulation regulated by light quality in strawberry remains limited. Here we described the effects of red- and blue-light irradiation on anthocyanin accumulation in strawberry. The results showed that blue light, rather than red light, could lead to the rapid accumulation of anthocyanins after exposure to light for 48 hours. The transcriptional levels of anthocyanin structural and regulatory genes displayed similar trend to the anthocyanin content. To investigate the mechanism of blue light-induced anthocyanin accumulation, the homologs of Arabidopsis blue light signal transduction components, including the blue light photoreceptor FaCRY1, an E3 ubiquitin ligase FaCOP1 and light-responsive factor FaHY5, were cloned from the strawberry cultivar 'Benihoppe'. The protein-protein interaction of FaCRY1-FaCOP1-FaHY5 was revealed by yeast two-hybrid and fluorescence signal assays. Functional complementation analysis showed that overexpression of either FaCOP1 or FaHY5 restored the anthocyanin content and hypocotyl length in corresponding Arabidopsis mutants under blue light. Moreover, dual-luciferase assays showed that FaHY5 could increase the activity of FaRAP (anthocyanin transport gene) promoter and that this function relied on other, likely B-box protein FaBBX22, factors. The overexpression of FaHY5-VP16 (chimeric activator form of FaHY5) and FaBBX22 promoted the accumulation of anthocyanins in transgenic strawberry plants. Further, transcriptomic profiling indicated that the genes involved in the phenylpropanoid biosynthesis pathway were enriched in both FaHY5-VP16-OX and FaBBX22-OX strawberry plants. In summary, our findings provide insights into a mechanism involving the regulation of blue light-induced anthocyanin accumulation via a FaCRY1-FaCOP1-FaHY5 signal transduction module in strawberry.

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.

Photoperiodic flowering in Arabidopsis: Multilayered regulatory mechanisms of CONSTANS and the florigen FLOWERING LOCUS T

The timing of flowering affects the success of sexual reproduction. This developmental event also determines crop yield, biomass, and longevity. Therefore, this mechanism has been targeted for improvement along with crop domestication. The underlying mechanisms of flowering are highly conserved in angiosperms. Central to these mechanisms is how environmental and endogenous conditions control transcriptional regulation of the FLOWERING LOCUS T (FT) gene, which initiates floral development under long-day conditions in Arabidopsis. Since the identification of FT as florigen, efforts have been made to understand the regulatory mechanisms of FT expression. Although many transcriptional regulators have been shown to directly influence FT, the question of how they coordinately control the spatiotemporal expression patterns of FT still requires further investigation. Among FT regulators, CONSTANS (CO) is the primary one whose protein stability is tightly controlled by phosphorylation and ubiquitination/proteasome-mediated mechanisms. In addition, various CO interaction partners, some of them previously identified as FT transcriptional regulators, positively or negatively modulate CO protein activity. The FT promoter possesses several transcriptional regulatory "blocks," highly conserved regions among Brassicaceae plants. Different transcription factors bind to specific blocks and affect FT expression, often causing topological changes in FT chromatin structure, such as the formation of DNA loops. We discuss the current understanding of the regulation of FT expression mainly in Arabidopsis and propose future directions related to this topic.

Arabidopsis phytochromes A and B synergistically repress SPA1 under blue light

In Arabidopsis, although studies have demonstrated that phytochrome A (phyA) and phyB are involved in blue light signaling, how blue light-activated phytochromes modulate the activity of the CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1)-SUPPRESSOR OF PHYA-105 (SPA1) E3 complex remains largely unknown. Here, we show that phyA responds to early and weak blue light, whereas phyB responds to sustainable and strong blue light. Activation of both phyA and phyB by blue light inhibits SPA1 activity. Specifically, blue light irradiation promoted the nuclear import of both phytochromes to stimulate their binding to SPA1, abolishing SPA1's interaction with LONG HYPOCOTYL 5 (HY5) to release HY5, which promoted seedling photomorphogenesis.

Co-action of COP1, SPA and cryptochrome in light signal transduction and photomorphogenesis of the moss Physcomitrium patens

The Arabidopsis COP1/SPA ubiquitin ligase suppresses photomorphogenesis in darkness. In the light, photoreceptors inactivate COP1/SPA to allow a light response. While SPA genes are specific to the green lineage, COP1 also exists in humans. This raises the question of when in evolution plant COP1 acquired the need for SPA accessory proteins. We addressed this question by generating Physcomitrium Ppcop1 mutants and comparing their visible and molecular phenotypes with those of Physcomitrium Ppspa mutants. The phenotype of Ppcop1 nonuple mutants resembles that of Ppspa mutants. Most importantly, both mutants produce green chloroplasts in complete darkness. They also exhibit dwarfed gametophores, disturbed branching of protonemata and absent gravitropism. RNA-sequencing analysis indicates that both mutants undergo weak constitutive light signaling in darkness. PpCOP1 and PpSPA proteins form a complex and they interact via their WD repeat domains with the VP motif of the cryptochrome CCE domain in a blue light-dependent manner. This resembles the interaction of Arabidopsis SPA proteins with Arabidopsis CRY1, and is different from that with Arabidopsis CRY2. Taken together, the data indicate that PpCOP1 and PpSPA act together to regulate growth and development of Physcomitrium. However, in contrast to their Arabidopsis orthologs, PpCOP1 and PpSPA proteins execute only partial suppression of light signaling in darkness. Hence, additional repressors may exist that contribute to the repression of a light response in dark-exposed Physcomitrium.

Shade avoidance in the context of climate change

When exposed to changes in the light environment caused by neighboring vegetation, shade-avoiding plants modify their growth and/or developmental patterns to access more sunlight. In Arabidopsis (Arabidopsis thaliana), neighbor cues reduce the activity of the photosensory receptors phytochrome B (phyB) and cryptochrome 1, releasing photoreceptor repression imposed on PHYTOCHROME INTERACTING FACTORs (PIFs) and leading to transcriptional reprogramming. The phyB-PIF hub is at the core of all shade-avoidance responses, whilst other photosensory receptors and transcription factors contribute in a context-specific manner. CONSTITUTIVELY PHOTOMORPHOGENIC1 is a master regulator of this hub, indirectly stabilizing PIFs and targeting negative regulators of shade avoidance for degradation. Warm temperatures reduce the activity of phyB, which operates as a temperature sensor and further increases the activities of PIF4 and PIF7 by independent temperature sensing mechanisms. The signaling network controlling shade avoidance is not buffered against climate change; rather, it integrates information about shade, temperature, salinity, drought, and likely flooding. We, therefore, predict that climate change will exacerbate shade-induced growth responses in some regions of the planet while limiting the growth potential in others.

Divergence of flowering-related genes to control flowering in five Euphorbiaceae genomes

Reproductive growth and vegetative growth are a pair of main contradictions in the process of plant growth. Flowering, as part of reproductive growth, is a key switch in the life cycle of higher plants, which affects the yield and economic benefits of plants to a certain extent. The Euphorbiaceae species, including castor bean (Ricinus communis), physic nut (Jatropha curcas), tung tree (Vernicia fordii), cassava (Manihot esculenta), and rubber tree (Hevea brasiliensis), have important economic values because they are raw materials for the production of biodiesel, rubber, etc. The flowering mechanisms are still excluded in the Euphorbiaceae species. The flowering-related genes of Arabidopsis thaliana (Arabidopsis) were used as a reference to determine the orthologs of these genes in Euphorbiaceae genomes. The result showed that 146, 144, 114, 114, and 149 of 207 A. thaliana genes were respectively matched to R. communis, V. fordii, J. curcas, H. brasiliensis, and M. esculenta. These identified genes were clustered into seven pathways including gibberellins, floral meristem identity (FMI), vernalization, photoperiod, floral pathway integrators (FPIs), and autonomous pathways. Then, some key numbers of flowering-related genes are widely conserved in the Euphorbiaceae genomes including but not limited to FPI genes LFY, SOC1, FT, and FMI genes AG, CAL, and FUL. However, some genes, including FRI, FLC, and GO, were missing in several or all five Euphorbiaceae species. In this study, we proposed the putative mechanisms of flowering-related genes to control flowering and provided new candidate flowering genes for using marker-assisted breeding to improve variety quality.

Blue light-induced phosphorylation of Arabidopsis cryptochrome 1 is essential for its photosensitivity

Plants possess two cryptochrome photoreceptors, cryptochrome 1 (CRY1) and cryptochrome 2 (CRY2), that mediate overlapping and distinct physiological responses. Both CRY1 and CRY2 undergo blue light-induced phosphorylation, but the molecular details of CRY1 phosphorylation remain unclear. Here we identify 19 in vivo phosphorylation sites in CRY1 using mass spectrometry and systematically analyze the physiological and photobiochemical activities of CRY1 variants with phosphosite substitutions. We demonstrate that nonphosphorylatable CRY1 variants have impaired phosphorylation, degradation, and physiological functions, whereas phosphomimetic variants mimic the physiological functions of phosphorylated CRY1 to constitutively inhibit hypocotyl elongation. We further demonstrate that phosphomimetic CRY1 variants exhibit enhanced interaction with the E3 ubiquitin ligase COP1 (CONSTITUTIVELY PHOTOMORPHOGENIC 1). This finding is consistent with the hypothesis that phosphorylation of CRY1 is required for COP1-dependent signaling and regulation of CRY1. We also determine that PHOTOREGULATORY PROTEIN KINASEs (PPKs) phosphorylate CRY1 in a blue light-dependent manner and that this phosphorylation is critical for CRY1 signaling and regulation. These results indicate that, similar to CRY2, blue light-dependent phosphorylation of CRY1 determines its photosensitivity.

UBP12 and UBP13 deubiquitinases destabilize the CRY2 blue light receptor to regulate Arabidopsis growth

Light is a crucial exogenous signal sensed by cryptochrome (CRY) blue light receptors to modulate growth and the circadian clock in plants and animals. However, how CRYs interpret light quantity to regulate growth in plants remains poorly understood. Furthermore, CRY2 protein levels and activity are tightly regulated in light to fine-tune hypocotyl growth; however, details of the mechanisms that explain precise control of CRY2 levels are not fully understood. We show that in Arabidopsis, UBP12 and UBP13 deubiquitinases physically interact with CRY2 in light. UBP12/13 negatively regulates CRY2 by promoting its ubiquitination and turnover to modulate hypocotyl growth. Growth and development were explicitly affected in blue light when UBP12/13 were disrupted or overexpressed, indicating their role alongside CRY2. UBP12/13 also interacted with and stabilized COP1, which is partially required for CRY2 turnover. Our combined genetic and molecular data support a mechanistic model in which UBP12/13 interact with CRY2 and COP1, leading to the stabilization of COP1. Stabilized COP1 then promotes the ubiquitination and degradation of CRY2 under blue light. Despite decades of studies on deubiquitinases, the knowledge of how their activity is regulated is limited. Our study provides insight into how exogenous signals and ligands, along with their receptors, regulate deubiquitinase activity by protein-protein interaction. Collectively, our results provide a framework of cryptochromes and deubiquitinases to detect and interpret light signals to control plant growth at the most appropriate time.

Arabidopsis cryptochrome 1 undergoes COP1 and LRBs-dependent degradation in response to high blue light

Arabidopsis cryptochrome 1 (CRY1) is an important blue light photoreceptor that promotes photomorphogenesis under blue light. The blue light photoreceptors CRY2 and phototropin 1, and the red/far-red light photoreceptors phytochromes B and A undergo degradation in response to blue and red light, respectively. This study investigated whether and how CRY1 might undergo degradation in response to high-intensity blue light (HBL). We demonstrated that CRY1 is ubiquitinated and degraded through the 26S proteasome pathway in response to HBL. We found that the E3 ubiquitin ligase constitutive photomorphogenic 1 (COP1) is involved in mediating HBL-induced ubiquitination and degradation of CRY1. We also found that the E3 ubiquitin ligases LRBs physically interact with CRY1 and are also involved in mediating CRY1 ubiquitination and degradation in response to HBL. We further demonstrated that blue-light inhibitor of cryptochromes 1 interacts with CRY1 in a blue-light-dependent manner to inhibit CRY1 dimerization/oligomerization, leading to the repression of HBL-induced degradation of CRY1. Our findings indicate that the regulation of CRY1 stability in HBL is coordinated by COP1 and LRBs, which provides a mechanism by which CRY1 attenuates its own signaling and optimizes photomorphogenesis under HBL.

Arabidopsis SPA2 represses seedling de-etiolation under multiple light conditions

In Arabidopsis, phytochrome (phy) A, phyB, and cryptochrome 1 (cry1) are representative far-red, red, and blue light photoreceptors, respectively. Members of the SUPPRESSOR OF PHYA-105 (SPA) protein family (SPA1-SPA4) form E3 ubiquitin ligase complexes with CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), which mediates the degradation of photomorphogenesis-promoting factors to desensitize light signaling. SPA2 has been reported to promote seedling etiolation in the dark. However, the unique roles of SPA2 and its three functional domains in suppressing photomorphogenesis under different light conditions are largely unknown. Here, we demonstrate that overexpression of the full-length or the central coiled-coil and C-terminal WD-repeat domains of SPA2 cause hyper-etiolation phenotypes under several light conditions. The SPA2 central coiled-coil and C-terminal WD-repeat domains are necessary and sufficient for repressing seedling de-etiolation, cotyledon unfolding, and promoting hypocotyl negative gravitropism under several light conditions. Furthermore, phyA, phyB, cry1, and COP1 repress protein accumulation or nuclear translocation of SPA2 through direct interactions with its kinase-like and coiled-coil domains located in the N-terminus in response to far-red, red, and blue light treatments, respectively. Taken together, our results demonstrate that SPA2 functions under multiple light conditions; moreover, light-activated photoreceptors rapidly suppress SPA2 activity via direct interactions in response to different light treatments.

Differential photoregulation of the nuclear and cytoplasmic CRY1 in Arabidopsis

Arabidopsis cryptochrome 1 (CRY1) is a blue light receptor distributed in the nucleus and cytoplasm. The nuclear CRY1, but not cytoplasmic CRY1, mediates blue light inhibition of hypocotyl elongation. However, the photobiochemical mechanisms distinguishing the CRY1 protein in the two subcellular compartments remains unclear. Here we show that the nuclear CRY1, but not the cytoplasmic CRY1, is regulated by phosphorylation, polyubiquitination and 26S proteasome-dependent proteolysis in response to blue light. The blue light-dependent CRY1 degradation is observed only under high fluences of blue light. The nuclear specificity and high fluence dependency of CRY1 explain why this photochemical regulatory mechanism of CRY1 was not observed previously and it further supports the hypothesis that CRY1 is a high light receptor regulating photomorphogenesis. We further show that the nuclear CRY1, but not cytoplasmic CRY1, undergoes blue light-dependent phosphorylation by photoregulatory protein kinase 1 (PPK1) followed by polyubiquitination by the E3 ubiquitin ligase Cul4^COP1/SPAs , resulting in the blue light-dependent proteolysis. Both phosphorylation and ubiquitination of nuclear CRY1 are inhibited by blue-light inhibitor of cryptochromes 1 (BIC1), demonstrating the involvement of photo-oligomerization of the nuclear CRY1. These finding reveals a photochemical mechanism that differentially regulates the physiological activity of the CRY1 photoreceptor in distinct subcellular compartments.

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.

Arabidopsis cryptochrome 1 promotes stomatal development through repression of AGB1 inhibition of SPEECHLESS DNA-binding activity

Cryptochromes are blue light photoreceptors that mediate various light responses in plants and mammals. The heterotrimeric G-protein is known to regulate various physiological processes in plants and mammals. In Arabidopsis, cryptochrome 1 (CRY1) and the G-protein β subunit AGB1 act antagonistically to regulate stomatal development. The molecular mechanism by which CRY1 and AGB1 regulate this process remains unknown. Here, we show that Arabidopsis CRY1 acts partially through AGB1, and AGB1 acts through SPEECHLESS (SPCH), a master transcription factor that drives stomatal initiation and proliferation, to regulate stomatal development. We demonstrate that AGB1 physically interacts with SPCH to block the bHLH DNA-binding domain of SPCH and inhibit its DNA-binding activity. Moreover, we demonstrate that photoexcited CRY1 represses the interaction of AGB1 with SPCH to release AGB1 inhibition of SPCH DNA-binding activity, leading to the expression of SPCH-target genes promoting stomatal development. Taken together, our results suggest that the mechanism by which CRY1 promotes stomatal development involves positive regulation of the DNA-binding activity of SPCH mediated by CRY1 inhibition of the AGB1-SPCH interaction. We propose that the antagonistic regulation of SPCH DNA-binding activity by CRY1 and AGB1 may allow plants to balance light and G-protein signaling and optimize stomatal density and pattern.

Functional comparison of the WD-repeat domains of SPA1 and COP1 in suppression of photomorphogenesis

The Arabidopsis COP1/SPA complex acts as a cullin4-based E3 ubiquitin ligase to suppress photomorphogenesis in darkness. It is a tetrameric complex of two COP1 and two SPA proteins. Both COP1 and SPA are essential for the activity of this complex, and they both contain a C-terminal WD-repeat domain responsible for substrate recruitment and binding of DDB1. Here, we used a WD domain swap-approach to address the cooperativity of COP1 and SPA proteins. We found that expression of a chimeric COP1 carrying the WD-repeat domain of SPA1 mostly complemented the cop1-4-mutant phenotype in darkness, indicating that the WD repeat of SPA1 can replace the WD repeat of COP1. In the light, SPA1-WD partially substituted for COP1-WD. In contrast, expression of a chimeric SPA1 protein carrying the WD repeat of COP1 did not rescue the spa-mutant phenotype. Together, our findings demonstrate that a SPA1-type WD repeat is essential for COP1/SPA activity, while a COP1-type WD is in part dispensible. Moreover, a complex with four SPA1-WDs is more active than a complex with only two SPA1-WDs. A homology model of SPA1-WD based on the crystal structure of COP1-WD uncovered two insertions and several amino acid substitutions at the predicted substrate-binding pocket of SPA1-WD.

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.

Arabidopsis cryptochrome 1 controls photomorphogenesis through regulation of H2A.Z deposition

Light is a key environmental cue that fundamentally regulates plant growth and development, which is mediated by the multiple photoreceptors including the blue light (BL) photoreceptor cryptochrome 1 (CRY1). The signaling mechanism of Arabidopsis thaliana CRY1 involves direct interactions with CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1)/SUPPRESSOR OF PHYA-105 1 and stabilization of COP1 substrate ELONGATED HYPOCOTYL 5 (HY5). H2A.Z is an evolutionarily conserved histone variant, which plays a critical role in transcriptional regulation through its deposition in chromatin catalyzed by SWR1 complex. Here we show that CRY1 physically interacts with SWC6 and ARP6, the SWR1 complex core subunits that are essential for mediating H2A.Z deposition, in a BL-dependent manner, and that BL-activated CRY1 enhances the interaction of SWC6 with ARP6. Moreover, HY5 physically interacts with SWC6 and ARP6 to direct the recruitment of SWR1 complex to HY5 target loci. Based on previous studies and our findings, we propose that CRY1 promotes H2A.Z deposition to regulate HY5 target gene expression and photomorphogenesis in BL through the enhancement of both SWR1 complex activity and HY5 recruitment of SWR1 complex to HY5 target loci, which is likely mediated by interactions of CRY1 with SWC6 and ARP6, and CRY1 stabilization of HY5, respectively.

Perception of solar UV radiation by plants: photoreceptors and mechanisms

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

The involvement of the N-terminal PHR domain of Arabidopsis cryptochromes in mediating light signaling

Light is a key environmental cue that fundamentally regulates all aspects of plant growth and development, which is mediated by the multiple photoreceptors including the blue light photoreceptors cryptochromes (CRYs). In Arabidopsis, there are two well-characterized homologous CRYs, CRY1 and CRY2. Whereas CRYs are flavoproteins, they lack photolyase activity and are characterized by an N-terminal photolyase-homologous region (PHR) domain and a C-terminal extension domain. It has been established that the C-terminal extension domain of CRYs is involved in mediating light signaling through direct interactions with the master negative regulator of photomorphogenesis, COP1. Recent studies have revealed that the N-terminal PHR domain of CRYs is also involved in mediating light signaling. In this review, we mainly summarize and discuss the recent advances in CRYs signaling mediated by the N-terminal PHR domain, which involves the N-terminal PHR domain-mediated dimerization/oligomerization of CRYs and physical interactions with the pivotal transcription regulators in light and phytohormone signaling.

Regulation of Arabidopsis photoreceptor CRY2 by two distinct E3 ubiquitin ligases

Cryptochromes (CRYs) are photoreceptors or components of the molecular clock in various evolutionary lineages, and they are commonly regulated by polyubiquitination and proteolysis. Multiple E3 ubiquitin ligases regulate CRYs in animal models, and previous genetics study also suggest existence of multiple E3 ubiquitin ligases for plant CRYs. However, only one E3 ligase, Cul4^COP1/SPAs, has been reported for plant CRYs so far. Here we show that Cul3^LRBs is the second E3 ligase of CRY2 in Arabidopsis. We demonstrate the blue light-specific and CRY-dependent activity of LRBs (Light-Response Bric-a-Brack/Tramtrack/Broad 1, 2 & 3) in blue-light regulation of hypocotyl elongation. LRBs physically interact with photoexcited and phosphorylated CRY2, at the CCE domain of CRY2, to facilitate polyubiquitination and degradation of CRY2 in response to blue light. We propose that Cul4^COP1/SPAs and Cul3^LRBs E3 ligases interact with CRY2 via different structure elements to regulate the abundance of CRY2 photoreceptor under different light conditions, facilitating optimal photoresponses of plants grown in nature.

Leaf elongation response to blue light is mediated by stomatal-induced variations in transpiration in Festuca arundinacea

Reduced blue light irradiance is known to enhance leaf elongation rate (LER) in grasses, but the mechanisms involved have not yet been elucidated. We investigated whether leaf elongation response to reduced blue light could be mediated by stomata-induced variations of plant transpiration. Two experiments were carried out on tall fescue in order to monitor LER and transpiration under reduced blue light irradiance. Additionally, LER dynamics were compared with those observed in the response to vapour pressure deficit (VPD)-induced variations of transpiration. Finally, we developed a model of water flow within a tiller to simulate the observed short-term response of LER to various transpiration regimes. LER dramatically increased in response to blue light reduction and then reached new steady states, which remained higher than the control. Reduced blue light triggered a simultaneous stomatal closure which induced an immediate decrease of leaf transpiration. The hydraulic model of leaf elongation accurately predicted the LER response to blue light and VPD, resulting from an increase in the growth-induced water potential gradient in the leaf growth zone. Our results suggest that the blue light signal is sensed by stomata of expanded leaves and transduced to the leaf growth zone through the hydraulic architecture of the tiller.

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.

Molecular pathways regulating elongation of aerial plant organs: a focus on light, the circadian clock, and temperature

Organs such as hypocotyls and petioles rapidly elongate in response to shade and temperature cues, contributing to adaptive responses that improve plant fitness. Growth plasticity in these organs is achieved through a complex network of molecular signals. Besides conveying information from the environment, this signaling network also transduces internal signals, such as those associated with the circadian clock. A number of studies performed in Arabidopsis hypocotyls, and to a lesser degree in petioles, have been informative for understanding the signaling networks that regulate elongation of aerial plant organs. In particular, substantial progress has been made towards understanding the molecular mechanisms that regulate responses to light, the circadian clock, and temperature. Signals derived from these three stimuli converge on the BAP module, a set of three different types of transcription factors that interdependently promote gene transcription and growth. Additional key positive regulators of growth that are also affected by environmental cues include the CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) and SUPPRESSOR OF PHYA-105 (SPA) E3 ubiquitin ligase proteins. In this review we summarize the key signaling pathways that regulate the growth of hypocotyls and petioles, focusing specifically on molecular mechanisms important for transducing signals derived from light, the circadian clock, and temperature. While it is clear that similarities abound between the signaling networks at play in these two organs, there are also important differences between the mechanisms regulating growth in hypocotyls and petioles.

Low Blue Light Enhances Phototropism by Releasing Cryptochrome1-Mediated Inhibition of PIF4 Expression

Shade-avoiding plants, including Arabidopsis (Arabidopsis thaliana), display a number of growth responses, such as elongation of stem-like structures and repositioning of leaves, elicited by shade cues, including a reduction in the blue and red portions of the solar spectrum and a low-red to far-red ratio. Shade also promotes phototropism of de-etiolated seedlings through repression of phytochrome B, presumably to enhance capture of unfiltered sunlight. Here we show that both low blue light and a low-red to far-red light ratio are required to rapidly enhance phototropism in Arabidopsis seedlings. However, prolonged low blue light treatments are sufficient to promote phototropism through reduced cryptochrome1 (cry1) activation. The enhanced phototropic response of cry1 mutants in the lab and in response to natural canopies depends on PHYTOCHROME INTERACTING FACTORs (PIFs). In favorable light conditions, cry1 limits the expression of PIF4, while in low blue light, PIF4 expression increases, which contributes to phototropic enhancement. The analysis of quantitative DII-Venus, an auxin signaling reporter, indicates that low blue light leads to enhanced auxin signaling in the hypocotyl and, upon phototropic stimulation, a steeper auxin signaling gradient across the hypocotyl. We conclude that phototropic enhancement by canopy shade results from the combined activities of phytochrome B and cry1 that converge on PIF regulation.

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.

Genetic and molecular basis of floral induction in Arabidopsis thaliana

Many plants synchronize their life cycles in response to changing seasons and initiate flowering under favourable environmental conditions to ensure reproductive success. To confer a robust seasonal response, plants use diverse genetic programmes that integrate environmental and endogenous cues and converge on central floral regulatory hubs. Technological advances have allowed us to understand these complex processes more completely. Here, we review recent progress in our understanding of genetic and molecular mechanisms that control flowering in Arabidopsis thaliana.

Cryptochrome-mediated blue-light signalling modulates UVR8 photoreceptor activity and contributes to UV-B tolerance in Arabidopsis

UV-B constitutes a critical part of the sunlight reaching the earth surface. The homodimeric plant UV-B photoreceptor UV RESISTANCE LOCUS 8 (UVR8) monomerizes in response to UV-B and induces photomorphogenic responses, including UV-B acclimation and tolerance. REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1 (RUP1) and RUP2 are negative feedback regulators that operate by facilitating UVR8 ground state reversion through re-dimerization. Here we show that RUP1 and RUP2 are transcriptionally induced by cryptochrome photoreceptors in response to blue light, which is dependent on the bZIP transcriptional regulator ELONGATED HYPOCOTYL 5 (HY5). Elevated RUP1 and RUP2 levels under blue light enhance UVR8 re-dimerization, thereby negatively regulating UVR8 signalling and providing photoreceptor pathway cross-regulation in a polychromatic light environment, as is the case in nature. We further show that cryptochrome 1, as well as the red-light photoreceptor phytochrome B, contribute to UV-B tolerance redundantly with UVR8. Thus, photoreceptors for both visible light and UV-B regulate UV-B tolerance through an intricate interplay allowing the integration of diverse sunlight signals.

The Arabidopsis UVR8 photoreceptor is a dimer that monomerizes in response to UV-B. Here the authors show that cryptochromes contribute to UV tolerance and facilitate UVR8 redimerization via induction of RUP proteins in response to blue light, modifying UV-B signalling in polychromatic light environments.

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.

Developmental Plasticity at High Temperature

Molecular mechanisms controlling the thermal response in Arabidopsis.

Plant photoreceptors and their signaling components compete for COP1 binding via VP peptide motifs

Plants sense different parts of the sun's light spectrum using distinct photoreceptors, which signal through the E3 ubiquitin ligase COP1. Here, we analyze why many COP1‐interacting transcription factors and photoreceptors harbor sequence‐divergent Val‐Pro (VP) motifs that bind COP1 with different binding affinities. Crystal structures of the VP motifs of the UV‐B photoreceptor UVR8 and the transcription factor HY5 in complex with COP1, quantitative binding assays, and reverse genetic experiments together suggest that UVR8 and HY5 compete for COP1. Photoactivation of UVR8 leads to high‐affinity cooperative binding of its VP motif and its photosensing core to COP1, preventing COP1 binding to its substrate HY5. UVR8–VP motif chimeras suggest that UV‐B signaling specificity resides in the UVR8 photoreceptor core. Different COP1–VP peptide motif complexes highlight sequence fingerprints required for COP1 targeting. The blue‐light photoreceptors CRY1 and CRY2 also compete with transcription factors for COP1 binding using similar VP motifs. Thus, our work reveals that different photoreceptors and their signaling components compete for COP1 via a conserved mechanism to control different light signaling cascades.

Exploring the protein-protein interaction landscape in plants

Protein-protein interactions (PPIs) represent an essential aspect of plant systems biology. Identification of key protein players and their interaction networks provide crucial insights into the regulation of plant developmental processes and into interactions of plants with their environment. Despite the great advance in the methods for the discovery and validation of PPIs, still several challenges remain. First, the PPI networks are usually highly dynamic, and the in vivo interactions are often transient and difficult to detect. Therefore, the properties of the PPIs under study need to be considered to select the most suitable technique, because each has its own advantages and limitations. Second, besides knowledge on the interacting partners of a protein of interest, characteristics of the interaction, such as the spatial or temporal dynamics, are highly important. Hence, multiple approaches have to be combined to obtain a comprehensive view on the PPI network present in a cell. Here, we present the progress in commonly used methods to detect and validate PPIs in plants with a special emphasis on the PPI features assessed in each approach and how they were or can be used for the study of plant interactions with their environment.

The SUMO Conjugation Complex Self-Assembles into Nuclear Bodies Independent of SIZ1 and COP1

Attachment of the small ubiquitin-like modifier (SUMO) to substrate proteins modulates their turnover, activity, or interaction partners. However, how this SUMO conjugation activity concentrates the proteins involved and the substrates into uncharacterized nuclear bodies (NBs) remains poorly understood. Here, we characterized the requirements for SUMO NB formation and for their subsequent colocalization with the E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a master regulator of plant growth. COP1 activity results in degradation of transcription factors, which primes the transcriptional response that underlies elongation growth induced by darkness and high ambient temperatures (skoto- and thermomorphogenesis, respectively). SUMO conjugation activity alone was sufficient to target the SUMO machinery into NBs. Colocalization of these bodies with COP1 required, in addition to SUMO conjugation activity, a SUMO acceptor site in COP1 and the SUMO E3 ligase SAP and Miz 1 (SIZ1). We found that SIZ1 docks in the substrate-binding pocket of COP1 via two valine-proline peptide motifs, which represent a known interaction motif of COP1 substrates. The data reveal that SIZ1 physically connects COP1 and SUMO conjugation activity in the same NBs that can also contain the blue-light receptors CRYPTOCHROME 1 and CRYPTOCHROME 2. Our findings thus suggest that sumoylation stimulates COP1 activity within NBs. Moreover, the presence of SIZ1 and SUMO in these NBs explains how both the timing and amplitude of the high-temperature growth response is controlled. The strong colocalization of COP1 and SUMO in these NBs might also explain why many COP1 substrates are sumoylated.

Photoexcited CRYPTOCHROME 1 Interacts Directly with G-Protein β Subunit AGB1 to Regulate the DNA-Binding Activity of HY5 and Photomorphogenesis in Arabidopsis

Light and the heterotrimeric G-protein are known to antagonistically regulate photomorphogenesis in Arabidopsis. However, whether light and G-protein coordinate the regulation of photomorphogenesis is largely unknown. Here we show that the blue light photoreceptor cryptochrome 1 (CRY1) physically interacts with the G-protein β subunit, AGB1, in a blue light-dependent manner. We also show that AGB1 directly interacts with HY5, a basic leucine zipper transcriptional factor that acts as a critical positive regulator of photomorphogenesis, to inhibit its DNA-binding activity. Genetic studies suggest that CRY1 acts partially through AGB1, and AGB1 acts partially through HY5 to regulate photomorphogenesis. Moreover, we demonstrate that blue light-triggered interaction of CRY1 with AGB1 promotes the dissociation of HY5 from AGB1. Our results suggest that the CRY1 signaling mechanism involves positive regulation of the DNA-binding activity of HY5 mediated by the CRY1-AGB1 interaction, which inhibits the association of AGB1 with HY5. We propose that the antagonistic regulation of HY5 DNA-binding activity by CRY1 and AGB1 may allow plants to balance light and G-protein signaling and optimize photomorphogenesis.

Beyond the photocycle-how cryptochromes regulate photoresponses in plants?

Cryptochromes (CRYs) are blue light receptors that mediate light regulation of plant growth and development. Land plants possess various numbers of cryptochromes, CRY1 and CRY2, which serve overlapping and partially redundant functions in different plant species. Cryptochromes exist as physiologically inactive monomers in darkness; photoexcited cryptochromes undergo homodimerization to increase their affinity to the CRY-signaling proteins, such as CIBs (CRY2-interacting bHLH), PIFs (Phytochrome-Interacting Factors), AUX/IAA (Auxin/INDOLE-3-ACETIC ACID), and the COP1-SPAs (Constitutive Photomorphogenesis 1-Suppressors of Phytochrome A) complexes. These light-dependent protein-protein interactions alter the activity of the CRY-signaling proteins to change gene expression and developmental programs in response to light. In the meantime, photoexcitation also changes the affinity of cryptochromes to the CRY-regulatory proteins, such as BICs (Blue-light Inhibitors of CRYs) and PPKs (Photoregulatory Protein Kinases), to modulate the activity, modification, or abundance of cryptochromes and photosensitivity of plants in response to the changing light environment.

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.