Regulation of Plant Photoresponses by Protein Kinase Activity of Phytochrome A.

Extensive research has been conducted for decades to elucidate the molecular and regulatory mechanisms for phytochrome-mediated light signaling in plants. As a result, tens of downstream signaling components that physically interact with phytochromes are identified, among which negative transcription factors for photomorphogenesis, PHYTOCHROME-INTERACTING FACTORs (PIFs), are well known to be regulated by phytochromes. In addition, phytochromes are also shown to inactivate an important E3 ligase complex consisting of CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and SUPPRESSORs OF phyA-105 (SPAs). This inactivation induces the accumulation of positive transcription factors for plant photomorphogenesis, such as ELONGATED HYPOCOTYL 5 (HY5). Although many downstream components of phytochrome signaling have been studied thus far, it is not fully elucidated which intrinsic activity of phytochromes is necessary for the regulation of these components. It should be noted that phytochromes are autophosphorylating protein kinases. Recently, the protein kinase activity of phytochrome A (phyA) has shown to be important for its function in plant light signaling using Avena sativa phyA mutants with reduced or increased kinase activity. In this review, we highlight the function of phyA as a protein kinase to explain the regulation of plant photoresponses by phyA.

Plant Phytochromes and their Phosphorylation.

Extensive research over several decades in plant light signaling mediated by photoreceptors has identified the molecular mechanisms for how phytochromes regulate photomorphogenic development, which includes degradation of phytochrome-interacting factors (PIFs) and inactivation of COP1-SPA complexes with the accumulation of master transcription factors for photomorphogenesis, such as HY5. However, the initial biochemical mechanism for the function of phytochromes has not been fully elucidated. Plant phytochromes have long been known as phosphoproteins, and a few protein phosphatases that directly interact with and dephosphorylate phytochromes have been identified. However, there is no report thus far of a protein kinase that acts on phytochromes. On the other hand, plant phytochromes have been suggested as autophosphorylating serine/threonine protein kinases, proposing that the kinase activity might be important for their functions. Indeed, the autophosphorylation of phytochromes has been reported to play an important role in the regulation of plant light signaling. More recently, evidence that phytochromes function as protein kinases in plant light signaling has been provided using phytochrome mutants displaying reduced kinase activities. In this review, we highlight recent advances in the reversible phosphorylation of phytochromes and their functions as protein kinases in plant light signaling.

Regulation of Photomorphogenic Development by Plant Phytochromes.

Photomorphogenesis and skotomorphogenesis are two key events that control plant development, from seed germination to flowering and senescence. A group of wavelength-specific photoreceptors, E3 ubiquitin ligases, and various transcription factors work together to regulate these two critical processes. Phytochromes are the main photoreceptors in plants for perceiving red/far-red light and transducing the light signals to downstream factors that regulate the gene expression network for photomorphogenic development. In this review, we highlight key developmental stages in the life cycle of plants and how phytochromes and other components in the phytochrome signaling pathway play roles in plant growth and development.

A CRY-BIC negative-feedback circuitry regulating blue light sensitivity of Arabidopsis.

Cryptochromes are blue light receptors that regulate various light responses in plants. Arabidopsis cryptochrome 1 (CRY1) and cryptochrome 2 (CRY2) mediate blue light inhibition of hypocotyl elongation and long-day (LD) promotion of floral initiation. It has been reported recently that two negative regulators of Arabidopsis cryptochromes, Blue light Inhibitors of Cryptochromes 1 and 2 (BIC1 and BIC2), inhibit cryptochrome function by blocking blue light-dependent cryptochrome dimerization. However, it remained unclear how cryptochromes regulate the BIC gene activity. Here we show that cryptochromes mediate light activation of transcription of the BIC genes, by suppressing the activity of CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), resulting in activation of the transcription activator ELONGATED HYPOCOTYL 5 (HY5) that is associated with chromatins of the BIC promoters. These results demonstrate a CRY-BIC negative-feedback circuitry that regulates the activity of each other. Surprisingly, phytochromes also mediate light activation of BIC transcription, suggesting a novel photoreceptor co-action mechanism to sustain blue light sensitivity of plants under the broad spectra of solar radiation in nature.

A Fungus-Inducible Pepper Carboxylesterase Exhibits Antifungal Activity by Decomposing the Outer Layer of Fungal Cell Walls.

Colletotrichum species are major fungal pathogens that cause devastating anthracnose diseases in many economically important crops. In this study, we observed the hydrolyzing activity of a fungus-inducible pepper carboxylesterase (PepEST) on cell walls of C. gloeosporioides, causing growth retardation of the fungus by blocking appressorium formation. To determine the cellular basis for the growth inhibition, we observed the localization of PepEST on the fungus and found the attachment of the protein on surfaces of conidia and germination tubes. Moreover, we examined the decomposition of cell-wall materials from the fungal surface after reaction with PepEST, which led to the identification of 1,2-dithiane-4,5-diol (DTD) by gas chromatography mass spectrometry analysis. Exogenous DTD treatment did not elicit expression of defense-related genes in the host plant but did trigger the necrosis of C. gloeosporioides. Furthermore, the DTD compound displayed protective effects on pepper fruits and plants against C. gloeosporioides and C. coccodes, respectively. In addition, DTD was also effective in preventing other diseases, such as rice blast, tomato late blight, and wheat leaf rust. Therefore, our results provide evidence that PepEST is involved in hydrolysis of the outmost layer of the fungal cell walls and that DTD has antifungal activity, suggesting an alternative strategy to control agronomically important phytopathogens.

New Constitutively Active Phytochromes Exhibit Light-Independent Signaling Activity.

Plant phytochromes are photoreceptors that mediate a variety of photomorphogenic responses. There are two spectral photoisomers, the red light-absorbing Pr and far-red light-absorbing Pfr forms, and the photoreversible transformation between the two forms is important for the functioning of phytochromes. In this study, we isolated a Tyr-268-to-Val mutant of Avena sativa phytochrome A (AsYVA) that displayed little photoconversion. Interestingly, transgenic plants of AsYVA showed light-independent phytochrome signaling with a constitutive photomorphogenic (cop) phenotype that is characterized by shortened hypocotyls and open cotyledons in the dark. In addition, the corresponding Tyr-303-to-Val mutant of Arabidopsis (Arabidopsis thaliana) phytochrome B (AtYVB) exhibited nuclear localization and interaction with phytochrome-interacting factor 3 (PIF3) independently of light, conferring a constitutive photomorphogenic development to its transgenic plants, which is comparable to the first constitutively active version of phytochrome B (YHB; Tyr-276-to-His mutant). We also found that chromophore ligation was required for the light-independent interaction of AtYVB with PIF3. Moreover, we demonstrated that AtYVB did not exhibit phytochrome B activity when it was localized in the cytosol by fusion with the nuclear export signal and that AsYVA exhibited the full activity of phytochrome A when localized in the nucleus by fusion with the nuclear localization signal. Furthermore, the corresponding Tyr-269-to-Val mutant of Arabidopsis phytochrome A (AtYVA) exhibited similar cop phenotypes in transgenic plants to AsYVA. Collectively, these results suggest that the conserved Tyr residues in the chromophore-binding pocket play an important role during the Pr-to-Pfr photoconversion of phytochromes, providing new constitutively active alleles of phytochromes by the Tyr-to-Val mutation.

Characterization of a small constitutive promoter from Arabidopsis translationally controlled tumor protein (AtTCTP) gene for plant transformation.

KEY MESSAGE: A plant-derived 0.3 kb constitutive promoter was obtained from AtTCTP expression analysis, and successfully applied to the expression of a selectable marker gene for production of transgenic creeping bentgrass plants. The isolation and use of an efficient promoter is essential to develop a vector system for efficient genetic transformation of plants, and constitutive promoters are particularly useful for the expression of selectable marker genes. In this study, we characterized a small size of the constitutive promoter from the expression analysis of Arabidopsis thaliana translationally controlled tumor protein (AtTCTP) gene. Histochemical and fluorometric GUS analyses revealed that a 303 bp upstream region from the start codon of the AtTCTP gene showed strong GUS expression throughout all plant tissues, which is approximately 55 % GUS activity compared with the cauliflower mosaic virus 35S promoter (35Spro). To examine the possible application of this promoter for the development of genetically engineered crops, we introduced pCAMBIA3301 vector harboring the 0.3 kb promoter of AtTCTP (0.3kbpro) that was fused to the herbicide resistance BAR gene (0.3kb pro ::BAR) into creeping bentgrass. Our transformation results demonstrate that transgenic creeping bentgrass plants with herbicide resistance were successfully produced using 0.3kb pro ::BAR as a selectable marker. Northern blot analysis revealed that the transgenic plants with 0.3kb pro ::BAR showed reduced but comparable expression levels of BAR to those with 35S pro ::BAR. Moreover, the transcription activity of the 0.3 kb promoter could be increased by the fusion of an enhancer sequence. These results indicate that the 0.3 kb AtTCTP promoter can be used as a plant-derived constitutive promoter for the expression of selectable marker genes, which facilitates its use as an alternative to the 35S promoter for developing genetically engineered crops.

Phytochrome phosphorylation in plant light signaling.

Plant phytochromes, renowned phosphoproteins, are red and far-red photoreceptors that regulate growth and development in response to light signals. Studies on phytochrome phosphorylation postulate that the N-terminal extension (NTE) and hinge region between N- and C-domains are sites of phosphorylation. Further studies have demonstrated that phosphorylation in the hinge region is important for regulating protein-protein interactions with downstream signaling partners, and phosphorylation in the NTE partakes in controlling phytochrome activity for signal attenuation and nuclear import. Moreover, phytochrome-associated protein phosphatases have been reported, indicating a role of reversible phosphorylation in phytochrome regulation. Furthermore, phytochromes exhibit serine/threonine kinase activity with autophosphorylation, and studies on phytochrome mutants with impaired or increased kinase activity corroborate that they are functional protein kinases in plants. In addition to the autophosphorylation, phytochromes negatively regulate PHYTOCHROME-INTERACTING FACTORs (PIFs) in a light-dependent manner by phosphorylating them as kinase substrates. Very recently, a few protein kinases have also been reported to phosphorylate phytochromes, suggesting new views on the regulation of phytochrome via phosphorylation. Using these recent advances, this review details phytochrome regulation through phosphorylation and highlights their significance as protein kinases in plant light signaling.

Overexpression of phytochrome A and its hyperactive mutant improves shade tolerance and turf quality in creeping bentgrass and zoysiagrass.

Phytochrome A (phyA) in higher plants is known to function as a far-red/shade light-sensing photoreceptor in suppressing shade avoidance responses (SARs) to shade stress. In this paper, the Avena PHYA gene was introduced into creeping bentgrass (Agrostis stolonifera L.) and zoysiagrass (Zoysia japonica Steud.) to improve turf quality by suppressing the SARs. In addition to wild-type PHYA, a hyperactive mutant gene (S599A-PHYA), in which a phosphorylation site involved in light-signal attenuation was removed, was also transformed into the turfgrasses. Phenotypic traits of the transgenic plants were compared to assess the suppression of SARs under a simulated shade condition and outdoor field conditions after three growth seasons. Under the shade condition, the S599A-PhyA transgenic creeping bentgrass plants showed shade avoidance-suppressing phenotypes with a 45 % shorter leaf lengths, 24 % shorter internode lengths, and twofold increases in chlorophyll concentrations when compared with control plants. Transgenic zoysiagrass plants overexpressing S599A-PHYA also showed shade-tolerant phenotypes under the shade condition with reductions in leaf length (15 %), internode length (30 %), leaf length/width ratio (19 %) and leaf area (22 %), as well as increases in chlorophyll contents (19 %) and runner lengths (30 %) compared to control plants. The phenotypes of transgenic zoysiagrass were also investigated in dense field habitats, and the transgenic turfgrass exhibited shade-tolerant phenotypes similar to those observed under laboratory shade conditions. Therefore, the present study suggests that the hyperactive phyA is effective for the development of shade-tolerant plants, and that the shade tolerance nature is sustained under field conditions.

Overexpression of an Arabidopsis ß-glucosidase gene enhances drought resistance with dwarf phenotype in creeping bentgrass.

UNLABELLED: An Arabidopsis ß-glucosidase, AtBG1 is known to hydrolyze glucose-conjugated, biologically inactive abscisic acid (ABA) to produce active ABA, which increases the level of ABA in plants. Since an increase of ABA in plants confers tolerance against abiotic stress such as drought, we introduced the pCAMBIA3301 vector harboring the AtBG1 gene into creeping bentgrass through Agrobacterium-mediated transformation. After transformation, putative transgenic plants were selected using the BASTA resistance assay at a concentration of 0.8%. Genomic integration of the AtBG1 gene was confirmed by genomic PCR and Southern blot analysis, and gene expression was validated by Northern blot and Western blot analyses. Interestingly, the transgenic bentgrass plants overexpressing AtBG1 had a dwarf phenotype with reduced growth rates when compared to wild-type creeping bentgrass. In addition, the transgenic plants accumulated higher ABA levels and displayed enhanced drought tolerance. These results suggest that the expression of AtBG1 in plants induces the accumulation of higher ABA levels, which results in the formation of dwarf creeping bentgrass and enhances the survival in water-limiting environments. KEY MESSAGE: We used an Arabidopsis ß-glucosidase AtBG1 to engineer a crop with elevated active ABA levels, and developed transgenic creeping bentgrass with enhanced drought tolerance and dwarf phenotype.

A small GTPase activator protein interacts with cytoplasmic phytochromes in regulating root development.

Phytochromes enable plants to sense light information and regulate developmental responses. Phytochromes interact with partner proteins to transmit light signals to downstream components for plant development. PIRF1 (phytochrome-interacting ROP guanine-nucleotide exchange factor (RopGEF 1)) functions as a light-signaling switch regulating root development through the activation of ROPs (Rho-like GTPase of plant) in the cytoplasm. In vitro pulldown and yeast two-hybrid assays confirmed the interaction between PIRF1 and phytochromes. PIRF1 interacted with the N-terminal domain of phytochromes through its conserved PRONE (plant-specific ROP nucleotide exchanger) region. PIRF1 also interacted with ROPs and activated them in a phytochrome-dependent manner. The Pr form of phytochrome A enhanced the RopGEF activity of PIRF1, whereas the Pfr form inhibited it. A bimolecular fluorescence complementation analysis demonstrated that PIRF1 was localized in the cytoplasm and bound to the phytochromes in darkness but not in light. PIRF1 loss of function mutants (pirf1) of Arabidopsis thaliana showed a longer root phenotype in the dark. In addition, both PIRF1 overexpression mutants (PIRF1-OX) and phytochrome-null mutants (phyA-211 and phyB-9) showed retarded root elongation and irregular root hair formation, suggesting that PIRF1 is a negative regulator of phytochrome-mediated primary root development. We propose that phytochrome and ROP signaling are interconnected through PIRF1 in regulating the root growth and development in Arabidopsis.

Photochemical characterization of phytochrome missense mutants.

Phytochromes are photoreceptors that regulate many aspects of plant growth and development in response to red/far-red light signals from the environment. In this study, we analyzed chromophore ligation and photochromism of missense phytochrome mutants in the Per-Arnt-Sim (PAS)-related domain (PRD). Among the 14 mutants analyzed, the Gly768Asp mutant of Avena phytochrome A showed aberrant photochromism and dark reversion, suggesting that amino acid residues in the C-terminal domain affect the photochemical properties of the photosensory N-terminal domain.

Protein Kinase Activity of Phytochrome A Positively Correlates With Photoresponses in Arabidopsis.

Plant phytochromes are known as autophosphorylating serine/threonine protein kinases. However, the functional importance of their kinase activity is not fully elucidated. Previously, the kinase activity is shown to be necessary for the function of Avena sativa phytochrome A (AsphyA) using transgenic plants with mutants displaying reduced kinase activity, such as K411L and T418D. In this study, we isolated and analyzed two AsphyA mutants, K411R and T418V, that showed increased kinase activity. Transgenic phyA-201 plants with these mutants showed hypersensitive responses to far-red (FR) light, such as shorter hypocotyls and more expanded cotyledons than those of control plant (i.e., transgenic phyA-201 plant with wild-type AsphyA). Contrary to the mutants with reduced kinase activity, these mutants accelerated FR-induced phosphorylation and subsequent degradation of phytochrome-interacting factor 3 (PIF3) in Arabidopsis. Moreover, elongated hypocotyl 5 (HY5), a critical positive regulator of photoresponses in plants, accumulated in higher amounts in the transgenic plants under FR light than in the control plant. In addition, PIF1 degradation was accelerated in the transgenic plants. Consequently, the transgenic plants exhibit higher germination frequencies than the control plant. Collectively, our results demonstrate that the AsphyA mutants with increased kinase activity are hyperactive in plants, supporting a positive relationship between the kinase activity of phytochromes and photoresponses in plants.

Suppression of Phytochrome-Interacting Factors Enhances Photoresponses of Seedlings and Delays Flowering With Increased Plant Height in Brachypodium distachyon.

Phytochromes are red and far-red photoreceptors that regulate plant growth and development under ambient light conditions. During phytochrome-mediated photomorphogenesis, phytochrome-interacting factors (PIFs) are the most important signaling partners that regulate the expression of light-responsive genes. However, the function of PIFs in monocots has not been studied well. In this study, using RNA interference (RNAi), we investigated the functions of BdPIL1 and BdPIL3, two PIF-like genes identified in Brachypodium distachyon, which are closely related to Arabidopsis PIF1 and PIF3. The expression of their genes is light-inducible, and both BdPIL1 and BdPIL3 proteins interact with phytochromes in an active form-specific manner. Transgenic Brachypodium seedlings with the RNAi constructs of BdPIL1 and BdPIL3 showed decreased coleoptile lengths and increased leaf growth when exposed to both red and far-red light. In addition, the transgenic plants were taller with elongated internodes than wild-type Bd21-3 plant, exhibiting late flowering. Moreover, RNA-seq analysis revealed downregulation of many genes in the transgenic plants, especially those related to the regulation of cell number, floral induction, and chlorophyll biosynthesis, which were consistent with the phenotypes of increased plant height, delayed flowering, and pale green leaves. Furthermore, we demonstrated the DNA-binding ability of BdPIL1 and BdPIL3 to the putative target promoters and that the DNA-binding was inhibited in the presence of phytochromes. Therefore, this study determines a molecular mechanism underlying phytochrome-mediated PIF regulation in Brachypodium, i.e., sequestration, and also elucidates the functions of BdPIL1 and BdPIL3 in the growth and development of the monocot plant.

Functional characterization of phytochrome autophosphorylation in plant light signaling.

Plant phytochromes, molecular light switches that regulate various aspects of plant growth and development, are phosphoproteins that are also known to be autophosphorylating serine/threonine kinases. Although a few protein phosphatases that directly interact with and dephosphorylate phytochromes have been identified, no protein kinase that acts on phytochromes has been reported thus far, and the exact site of phytochrome autophosphorylation has not been identified. In this study, we investigated the functional role of phytochrome autophosphorylation. We first mapped precisely the autophosphorylation sites of oat phytochrome A (phyA), and identified Ser8 and Ser18 in the 65 amino acid N-terminal extension (NTE) region as being the autophosphorylation sites. The in vivo functional roles of phytochrome autophosphorylation were examined by introducing autophosphorylation site mutants into phyA-deficient Arabidopsis thaliana. We found that all the transgenic plants expressing the autophosphorylation site mutants exhibited hypersensitive light responses, indicating an increase in phyA activity. Further analysis showed that these phyA mutant proteins were degraded at a significantly slower rate than wild-type phyA under light conditions, which suggests that the increased phyA activity of the mutants is related to their increased protein stability. In addition, protoplast transfection analyses with green fluorescent protein (GFP)-fused phyA constructs showed that the autophosphorylation site mutants formed sequestered areas of phytochrome (SAPs) in the cytosol much more slowly than did wild-type phyA. These results suggest that the autophosphorylation of phyA plays an important role in the regulation of plant phytochrome signaling through the control of phyA protein stability.

Production of purple-colored creeping bentgrass using maize transcription factor genes Pl and Lc through Agrobacterium-mediated transformation.

Purple-colored transgenic creeping bentgrass (Agrostis stolonifera L.) plants were developed for ornamental purpose by means of Agrobacterium-mediated transformation. Embryogenic creeping bentgrass calli were transformed with the pCAMBIA 3301 vector harboring maize (Zea mays) flavonoid/anthocyanin biosynthetic pathway transcription factor genes, Lc (Leaf color) and Pl (Purple leaf), individually and in combination, and three types of putative transgenic plants (Lc, Pl, and Lc + Pl) were generated. Genomic integration and expression of the transgenes were confirmed by Southern and northern blot analyses, respectively. The transgenic creeping bentgrass plants expressing both Lc and Pl genes were entirely purple, whereas those expressing Pl alone had purple stems and those expressing Lc alone lacked purple pigmentation in adult plants. The anthocyanin content was estimated in all the three types of transgenic plant and correlated well with the degree of purple coloration observed. These results suggest that both Lc and Pl genes are necessary and sufficient to confer purple coloration to creeping bentgrass.

A novel protein phosphatase indirectly regulates phytochrome-interacting factor 3 via phytochrome.

Light signal transduction in plants involves an intricate series of pathways which is finely regulated by interactions between specific signalling proteins, as well as by protein modifications such as phosphorylation and ubiquitination. The identification of novel phytochrome-interacting proteins and the precise signalling mechanisms that they mediate is still ongoing. In our present study, we show that the newly identified putative phytochrome-associated protein, PAPP2C (phytochrome-associated protein phosphatase type 2C), interacts in the nucleus with phyA (phytochrome A) and phyB, both in vitro and in vivo. Moreover, the phosphatase activity of PAPP2C and its association with phytochromes were found to be enhanced by red light, indicating that it plays a role in mediating phytochrome signalling. In particular, PAPP2C specifically binds to the N-terminal PHY domain of the phytochromes. We thus speculate that this interaction reflects a unique regulatory function of this phosphatase toward established phytochrome-associated proteins. We also show that PAPP2C effectively dephosphorylates phytochromes in vitro. Interestingly, PAPP2C indirectly mediates the dephosphorylation of PIF3 (phytochrome-interacting factor 3) in vitro. Taken together, we suggest that PAPP2C functions as a regulator of PIF3 by dephosphorylating phytochromes in the nucleus.

Expression, Purification, and Spectral Characterization of Phytochromes.

Expression and purification of recombinant proteins are important for the structure-function study of phytochromes. However, it is difficult to purify phytochrome proteins from natural sources or using a bacterial expression system, due to the presence of multiple phytochrome species and low expression and solubility, respectively. Here we describe the expression of recombinant full-length plant phytochromes in the yeast Pichia pastoris, and the spectral analysis of chromophore-assembled phytochromes before and after the purification by streptavidin affinity chromatography.

Overexpression of Arabidopsis thaliana brassinosteroid-related acyltransferase 1 gene induces brassinosteroid-deficient phenotypes in creeping bentgrass.

Brassinosteroids (BRs) are naturally occurring steroidal hormones that play diverse roles in various processes during plant growth and development. Thus, genetic manipulation of endogenous BR levels might offer a way of improving the agronomic traits of crops, including plant architecture and stress tolerance. In this study, we produced transgenic creeping bentgrass (Agrostis stolonifera L.) overexpressing a BR-inactivating enzyme, Arabidopsis thaliana BR-related acyltransferase 1 (AtBAT1), which is known to catalyze the conversion of BR intermediates to inactive acylated conjugates. After putative transgenic plants were selected using herbicide resistance assay, genomic integration of the AtBAT1 gene was confirmed by genomic PCR and Southern blot analysis, and transgene expression was validated by northern blot analysis. The transgenic creeping bentgrass plants exhibited BR-deficient phenotypes, including reduced plant height with shortened internodes (i.e., semi-dwarf), reduced leaf growth rates with short, wide, and thick architecture, high chlorophyll contents, decreased numbers of vascular bundles, and large lamina joint bending angles (i.e., erect leaves). Subsequent analyses showed that the transgenic plants had significantly reduced amounts of endogenous BR intermediates, including typhasterol, 6-deoxocastasterone, and castasterone. Moreover, the AtBAT1 transgenic plants displayed drought tolerance as well as delayed senescence. Therefore, the results of the present study demonstrate that overexpression of an Arabidopsis BR-inactivating enzyme can reduce the endogenous levels of BRs in creeping bentgrass resulting in BR-deficient phenotypes, indicating that the AtBAT1 gene from a dicot plant is also functional in the monocot crop.

Photoactivation and inactivation of Arabidopsis cryptochrome 2.

Cryptochromes are blue-light receptors that regulate development and the circadian clock in plants and animals. We found that Arabidopsis cryptochrome 2 (CRY2) undergoes blue light-dependent homodimerization to become physiologically active. We identified BIC1 (blue-light inhibitor of cryptochromes 1) as an inhibitor of plant cryptochromes that binds to CRY2 to suppress the blue light-dependent dimerization, photobody formation, phosphorylation, degradation, and physiological activities of CRY2. We hypothesize that regulated dimerization governs homeostasis of the active cryptochromes in plants and other evolutionary lineages.