The Cape Verde Islands allele of cryptochrome 2 enhances cotyledon unfolding in the absence of blue light in Arabidopsis

We analyzed the natural genetic variation between Landsburg erecta (Ler) and Cape Verde Islands (Cvi) accessions by studying 105 recombinant inbred lines to search for players in the regulation of sensitivity to light signals perceived by phytochromes in etiolated seedlings of Arabidopsis. In seedlings grown under hourly pulses of far-red (FR) light, we identified three quantitative trait loci (QTLs; VLF3, VLF4, and VLF5) for hypocotyl growth inhibition and three different QTLs (VLF6, VLF7, and VLF1) for cotyledon unfolding. This indicates that different physiological outputs have selective regulation of sensitivity during de-etiolation. Ler alleles, compared with Cvi alleles, of VLF3, VLF4, VLF5, VLF7, and VLF1 enhanced, whereas the Ler allele of VLF6 reduced, the response to pulses of FR. We confirmed and narrowed down the position of some QTLs by using near-isogenic lines. VLF6 mapped close to the CRY2 (cryptochrome 2) gene. Transgenic Ler seedlings expressing the Cvi allele of CRY2 showed enhanced cotyledon unfolding under hourly pulses of FR compared with the wild type or transgenics expressing the CRY2-Ler allele. This response required phytochrome A. The cry1 cry2 double mutant lacking both cryptochromes showed reduced cotyledon unfolding under FR pulses. Because the CRY2-Cvi is a gain-of-function allele compared with CRY2-Ler, cryptochrome activity correlates positively with cotyledon unfolding under FR pulses. We conclude that the blue light photoreceptor cryptochrome 2 can modulate seedling photomorphogenesis in the absence of blue light. In addition to the nuclear loci, we identified cytoplasmic effects on seedling de-etiolation.

A detailed predictive model of the mammalian circadian clock

Experimental data on the circadian (approximately 24-h) clock in mammalian cells are vast, diverse, and detailed. Mathematical models are therefore needed to piece these data together and to study overall clock behavior. Previous models have focused on Neurospora or Drosophila or can be converted to a Drosophila model simply by renaming variables. Those models used Hill-type terms for transcription regulation and Michaelis-Menten type or delay terms for posttranslation regulation. Recent mammalian experimental data call into question some of the assumptions in these approaches. Moreover, gene duplication has led to more proteins in the mammalian system than in lower organisms. Here we develop a detailed distinctly mammalian model by using mass action kinetics. Parameters for our model are found from experimental data by using a coordinate search method. The model accurately predicts the phase of entrainment, amplitude of oscillation, and shape of time profiles of clock mRNAs and proteins and is also robust to parameter changes and mutations.

Functional analysis and intracellular localization of rice cryptochromes

Blue-light-receptor cryptochrome (CRY), which mediates cotyledon expansion, increased accumulation of anthocyanin, and inhibition of hypocotyl elongation, was first identified in Arabidopsis. Two Arabidopsis cryptochromes (AtCRY1 and AtCRY2) have been reported to be localized to the nucleus. However, there is no information on the cryptochromes in monocotyledons. In this study, we isolated two cryptochrome cDNAs, OsCRY1 and OsCRY2, from rice (Oryza sativa) plants. The deduced amino acid sequences of OsCRY1 and OsCRY2 have a photolyase-like domain in their N termini and are homologous to AtCRY1. To investigate the function of OsCRY1, we overexpressed a green fluorescence protein-OsCRY1 fusion gene in Arabidopsis and assessed the phenotypes of the resulting transgenic plants. When the seedlings were germinated in the dark, no discernible effect was observed. However, light-germinated seedlings showed pronounced inhibition of hypocotyl elongation and increased accumulation of anthocyanin. These phenotypes were induced in a blue-light-dependent manner, indicating that OsCRY1 functions as a blue-light-receptor cryptochrome. We also examined the intracellular localization of green fluorescence protein-OsCRY1 in the transgenic plants. It was localized to both the nucleus and the cytoplasm. We identified two nuclear localization domains in the primary structure of OsCRY1. We discuss the relationship between the function and intracellular localization of rice cryptochromes by using additional data obtained with OsCRY2.

Requirement of mammalian Timeless for circadian rhythmicity

Despite a central circadian role in Drosophila for the transcriptional regulator Timeless (dTim), the relevance of mammalian Timeless (mTim) remains equivocal. Conditional knockdown of mTim protein expression in the rat suprachiasmatic nucleus (SCN) disrupted SCN neuronal activity rhythms, and altered levels of known core clock elements. Full-length mTim protein (mTIM-fl) exhibited a 24-hour oscillation, where as a truncated isoform (mTIM-s) was constitutively expressed. mTIM-fl associated with the mammalian clock Period proteins (mPERs) in oscillating SCN cells. These data suggest that mTim is required for rhythmicity and is a functional homolog of dTim on the negative-feedback arm of the mammalian molecular clockwork.

A molecular mechanism regulating circadian expression of vascular endothelial growth factor in tumor cells

Because angiogenesis is essential for tumor growth and metastasis, inhibition of angiogenesis has emerged as a new therapy to treat cancers. Hypoxia-induced expression of vascular endothelial growth factor (VEGF) plays a central role in tumor-induced angiogenesis. In this study, we found that expression of VEGF in hypoxic tumor cells was affected by the circadian organization of molecular clockwork. The core circadian oscillator is composed of an autoregulatory transcription-translation feedback loop in which CLOCK and BMAL1 are positive regulators, and Period and Cryptochrome genes act as negative ones. The levels of VEGF mRNA in tumor cells implanted in mice rose substantially in response to hypoxia, but the levels fluctuated rhythmically in a circadian fashion. Luciferase reporter gene analysis revealed that Period2 and Cryptochrome1, whose expression in the implanted tumor cells showed a circadian oscillation, inhibited the hypoxia-induced VEGF promoter activity. These results suggest that the negative limbs of the molecular loop periodically inhibit the hypoxic induction of VEGF transcription, resulting in the circadian fluctuation of its mRNA expression. We also showed that the antitumor efficacy of antiangiogenic agents could be enhanced by administering the drugs at the time when VEGF production increased. These findings support the notion that monitoring of the circadian rhythm in VEGF production is useful for choosing the most appropriate time of day for administration of antiangiogenic agents.

Control mechanism of the circadian clock for timing of cell division in vivo

Cell division in many mammalian tissues is associated with specific times of day, but just how the circadian clock controls this timing has not been clear. Here, we show in the regenerating liver (of mice) that the circadian clock controls the expression of cell cycle-related genes that in turn modulate the expression of active Cyclin B1-Cdc2 kinase, a key regulator of mitosis. Among these genes, expression of wee1 was directly regulated by the molecular components of the circadian clockwork. In contrast, the circadian clockwork oscillated independently of the cell cycle in single cells. Thus, the intracellular circadian clockwork can control the cell-division cycle directly and unidirectionally in proliferating cells.

Circadian rhythms. Liver regeneration clocks on

In some simple protozoans and unicellular algae, the cell cycle clock is strongly influenced by the circadian clock, such that mitotic cell division takes place only at certain times of the day. Now, as Schibler reports in his Perspective, new work on regenerating liver cells in mice (Matsuo et al.) reveals that the cell cycle clock of mammalian cells is also under the yoke of the master circadian oscillator.

Cloning and study of the expression of a novel cry1Ia-type gene from Bacillus thuringiensis subsp. kurstaki

AIMS

Cloning and expression of a new cry1Ia-type gene of Bacillus thuringiensis.

METHODS AND RESULTS

PCR amplification, using gene cry1I-specific primers revealed the presence of such a gene in the strain BNS3 of Bacillus thuringiensis subsp. kurstaki. The cloning and sequencing from BNS3 of the cry1Ia-type gene, called crybns3-3, showed an open reading frame of 2160-bp, encoding a protein of 719 amino acid residues. Both nucleotide and amino acid sequences similarity analysis revealed that the crybns3-3 is a new cry1Ia-type gene, presenting several differences from the cry1Ia-type genes. The study of the expression of crybns3-3 by Northern blot and RT-PCR showed that it was transcribed. The expression of crybns3-3 under the control of BtI and BtII promoters revealed that Crybns3-3 would co-crystallize with the endogenous delta-endotoxins.

CONCLUSIONS

crybns3-3 is a novel cry1Ia gene isolated from B. thuringiensis subsp. kurstaki strain BNS3.

SIGNIFICANCE AND IMPACT OF THE STUDY

The characteristics of crybns3-3 indicate that it is a new cry1Ia-type gene. Amino acid residue substitutions presented in Crybns3-3 could be exploited for both toxicity and specificity studies. Crybns3-3 would interact and co-crystallize at least partially with the endogenous delta-endotoxins of BNS3, and then participate in the formation of the parasporal crystal inclusions.

Purification and characterization of three members of the photolyase/cryptochrome family blue-light photoreceptors from Vibrio cholerae

The sequence of Vibrio cholerae genome revealed three genes belonging to the photolyase/cryptochrome blue-light photoreceptor family. The proteins encoded by the three genes were purified and characterized. All three proteins contain folate and flavin cofactors and have absorption peaks in the range of 350-500 nm. Only one of the three, VcPhr, is a photolyase specific for cyclobutane pyrimidine dimers. The other two are cryptochromes and were designated VcCry1 and VcCry2, respectively. Mutation of phr abolishes photoreactivation of UV-induced killing, whereas mutations in cry1 and cry2 do not affect photorepair activity. VcCry1 exhibits some unique features. Of all cryptochromes characterized to date, it is the only one that contains stoichiometric amounts of both chromophores and retains its flavin cofactor in the two-electron reduced FADH2 form. In addition, VcCry1 exhibits RNA binding activity and co-purifies with an RNA of 60-70 nucleotides in length.

Drosophila free-running rhythms require intercellular communication

Robust self-sustained oscillations are a ubiquitous characteristic of circadian rhythms. These include Drosophila locomotor activity rhythms, which persist for weeks in constant darkness (DD). Yet the molecular oscillations that underlie circadian rhythms damp rapidly in many Drosophila tissues. Although much progress has been made in understanding the biochemical and cellular basis of circadian rhythms, the mechanisms that underlie the differences between damped and self-sustaining oscillations remain largely unknown. A small cluster of neurons in adult Drosophila brain, the ventral lateral neurons (LN(v)s), is essential for self-sustained behavioral rhythms and has been proposed to be the primary pacemaker for locomotor activity rhythms. With an LN(v)-specific driver, we restricted functional clocks to these neurons and showed that they are not sufficient to drive circadian locomotor activity rhythms. Also contrary to expectation, we found that all brain clock neurons manifest robust circadian oscillations of timeless and cryptochrome RNA for many days in DD. This persistent molecular rhythm requires pigment-dispersing factor (PDF), an LN(v)-specific neuropeptide, because the molecular oscillations are gradually lost when Pdf(01) mutant flies are exposed to free-running conditions. This observation precisely parallels the previously reported effect on behavioral rhythms of the Pdf(01) mutant. PDF is likely to affect some clock neurons directly, since the peptide appears to bind to the surface of many clock neurons, including the LN(v)s themselves. We showed that the brain circadian clock in Drosophila is clearly distinguishable from the eyes and other rapidly damping peripheral tissues, as it sustains robust molecular oscillations in DD. At the same time, different clock neurons are likely to work cooperatively within the brain, because the LN(v)s alone are insufficient to support the circadian program. Based on the damping results with Pdf(01) mutant flies, we propose that LN(v)s, and specifically the PDF neuropeptide that it synthesizes, are important in coordinating a circadian cellular network within the brain. The cooperative function of this network appears to be necessary for maintaining robust molecular oscillations in DD and is the basis of sustained circadian locomotor activity rhythms.

Genomic and physiological studies of early cryptochrome 1 action demonstrate roles for auxin and gibberellin in the control of hypocotyl growth by blue light

Blue light inhibits elongation of etiolated Arabidopsis thaliana hypocotyls during the first 30 min of irradiation by a mechanism that depends on the phototropin 1 (phot1) photoreceptor. The cryptochrome 1 (cry1) photoreceptor begins to exert control after 30 min. To identify genes responsible for the cry1 phase of growth inhibition, mRNA expression profiles of cry1 and wild-type seedlings were compared using DNA microarrays. Of the roughly 420 genes found to be differentially expressed at the point of cry1 response incipience, approximately half were expressed higher and half lower in cry1 relative to the wild type. Many of the cry1-dependent genes encoded kinases, transcription factors, cell cycle regulators, cell wall metabolism enzymes, gibberellic acid (GA) biosynthesis enzymes, and auxin response factors. High-resolution growth studies supported the hypothesis that genes in the last two categories were indeed relevant to cry1-mediated growth control. Inhibiting GA4 biosynthesis with a 3beta-hydroxylase inhibitor (Ca-prohexadione) restored wild-type response kinetics in cry1 and completely suppressed its long-hypocotyl phenotype in blue light. Co-treatment of cry1 seedlings with Ca-prohexadione plus GA4 completely reversed the effects of the inhibitor, restoring the long-hypocotyl phenotype typical of the mutant. Treatment of wild-type seedlings with GA4 was not sufficient to phenocopy cry1 seedlings, but co-treatment with IAA plus GA4 produced cry1-like growth kinetics for a period of approximately 5 h. The genomic and physiological data together indicate that blue light acting through cry1 quickly affects the expression of many genes, a subset of which suppresses stem growth by repressing GA and auxin levels and/or sensitivity.

Light-induced fluorescence changes in Phycomyces: evidence for blue light-receptor associated flavo-semiquinones

Light-induced fluorescence changes (LIFCs) were detected in sporangiophores of the blue-light-sensitive fungus Phycomyces blakesleeanus (Burgeff). The LIFCs can be utilized as a spectrophotometric assay for blue-light photoreceptors and for the in vivo characterization of their photochemical primary reactions. Blue-light irradiation of sporangiophores elicited a transient decrease and subsequent regeneration of flavin-like fluorescence emission at 525 nm. The signals recovered in darkness in about 120 min. In contrast to blue light, near-UV (370 nm) caused an increase in the fluorescence emission at 525 nm. Because the LIFCs were altered in a light-insensitive madC mutant with a defective photoreceptor, the fluorescence changes must be associated with early photochemical events of the transduction chain. Action spectra for the fluorescence changes at 525 nm showed major peaks near 470 and 600 nm. Double-pulse experiments involving two consecutive pulses of either blue and near-UV, blue and red, or near-UV and red showed that the responses depended on the sequence in which the different wavelengths were applied. The results indicate a blue-light receptor with intermediates in the near-UV, blue and red spectral regions. We explain the results in the framework of a general model, in which the three redox states of the flavin photoreceptor, the oxidized flavin (Fl), the flavo-semiquinone (FlH*), and the flavo-hydroquinone (FlH2) are each acting as chromophores with their own characteristic photochemical primary reactions. These consist of the photoreduction of the oxidized flavin generating semiquinone, the photoreduction of the semiquinone generating hydroquinone, and the photooxidation of the flavo-hydroquinone regenerating the pool of oxidized flavins. The proposed mechanism represents a photocycle in which two antagonistic photoreceptor forms, Fl and FlH2, determine the pool size of the biological effector molecule, the flavo-semiquinone. The redox changes that are associated with the photocycle are maintained by redox partners, pterins, that function in the near-UV as secondary chromophores.

Blue light-dependent in vivo and in vitro phosphorylation of Arabidopsis cryptochrome 1

Cryptochromes are photolyase-like blue/UV-A light receptors that regulate various light responses in animals and plants. Arabidopsis cryptochrome 1 (cry1) is the major photoreceptor mediating blue light inhibition of hypocotyl elongation. The initial photochemistry underlying cryptochrome function and regulation remain poorly understood. We report here a study of the blue light-dependent phosphorylation of Arabidopsis cry1. Cry1 is detected primarily as unphosphorylated protein in etiolated seedlings, but it is phosphorylated in plants exposed to blue light. Cry1 phosphorylation increases in response to increased fluence of blue light, whereas the phosphorylated cry1 disappears rapidly when plants are transferred from light to dark. Light-dependent cry1 phosphorylation appears specific to blue light, because little cry1 phosphorylation is detected in seedlings treated with red light or far-red light, and it is largely independent from phytochrome actions, because no phytochrome mutants tested significantly affect cry1 phosphorylation. The Arabidopsis cry1 protein expressed and purified from insect cells is phosphorylated in vitro in a blue light-dependent manner, consistent with cry1 undergoing autophosphorylation. To determine whether cry1 phosphorylation is associated with its function or regulation, we isolated and characterized missense cry1 mutants that express full-length CRY1 apoprotein. Mutant residues are found throughout the CRY1 coding sequence, but none of these inactive cry1 mutant proteins shows blue light-induced phosphorylation. These results demonstrate that blue light-dependent cry1 phosphorylation is closely associated with the function or regulation of the photoreceptor and that the overall structure of cry1 is critical to its phosphorylation.

Expression of the Blue-Light Receptor Cryptochrome in the Human Retina

PURPOSE

To analyze the patterns of expression of the cryptochromes, CRY1 and CRY2, in the human retina and to correlate expression of these putative blue-light receptors with nonvisual photoreceptor localization.

METHODS

CRY1 and CRY2 mRNA expression was analyzed in 4-mm diameter punches of macula and midperipheral human retina by quantitative RT-PCR. CRY2 protein expression was examined by immunohistochemistry in cross sections of human retina, and its subcellular localization was determined by immunoblot analysis of fractionated human retinal extracts.

RESULTS

CRY2 mRNA was 11 times more abundant than CRY1 throughout adult human retina. CRY2 immunoreactivity was detected in most cells in the ganglion cell layer (GCL) and in a subset of cells in the inner nuclear layer (INL) in both the macula and periphery. Immunoperoxidase staining further revealed that CRY2 was localized throughout the cytoplasm of cells in the GCL as well as within nuclei. This intracellular localization of CRY2 was confirmed by immunoblot analysis of fractionated human retinal extracts.

CONCLUSIONS

Photopigments governing circadian photoreception have been localized to the inner retina. The relative abundance of CRY2 transcripts, coupled with CRY2 localization to the inner retina, supports a photoreceptive role for CRY2 in human retina. Furthermore, the discovery that CRY2 is also localized within the cytoplasm of some cells in the GCL, suggests it may perform a function separate from its known nuclear role in the transcriptional feedback loop underlying the molecular circadian clock.

Light perception and signalling in higher plants

Plants monitor changes in the ambient light environment using sensory photoreceptor families: the phototropins and cryptochromes, which absorb UV-A or blue light; the phytochromes, which sense red/far-red light; and the UV-B photoreceptors, which have not yet been identified. Recent advances suggest that photoreceptor-induced signalling cascades regulate light-modulated gene expression. These regulatory networks interact at the levels of transcription, posttranslational modification and nucleo-cytoplasmic compartmentalisation.

Structural basis of a phototropin light switch

Phototropins are light-activated kinases important for plant responses to blue light. Light initiates signaling in these proteins by generating a covalent protein-flavin mononucleotide (FMN) adduct within sensory Per-ARNT-Sim (PAS) domains. We characterized the light-dependent changes of a phototropin PAS domain by solution nuclear magnetic resonance spectroscopy and found that an alpha helix located outside the canonical domain plays a key role in this activation process. Although this helix associates with the PAS core in the dark, photoinduced changes in the domain structure disrupt this interaction. We propose that this mechanism couples light-dependent bond formation to kinase activation and identifies a signaling pathway conserved among PAS domains.

Functional and structural analyses of cryptochrome. Vertebrate CRY regions responsible for interaction with the CLOCK:BMAL1 heterodimer and its nuclear localization

Mouse mCRY1 and zebrafish zCRY1a and zCRY3 belong to the DNA photolyase/Cryptochrome family. mCRY1 and zCRY1a repress CLOCK:BMAL1-mediated transcription, whereas zCRY3 does not. Reciprocal chimeras between zCRY1a and zCRY3 were generated to determine the zCRY1a regions responsible for nuclear translocation, interaction with the CLOCK:BMAL1 heterodimer, and repression of CLOCK:BMAL1-mediated transcription. Three regions, RD-2a-(126-196), RD-1-(197-263), and RD-2b-(264-293), were identified. Proteins in this family consist of an N-terminal alpha/beta domain and a C-terminal helical domain connected by an interdomain loop. RD-2a is within this loop, RD-1 is at the N-terminal 50 amino acids, and RD-2b at the following 31 amino acid residues of the helical domain. Either RD-2a or RD-1 is required for interaction with the CLOCK: BMAL1 heterodimer, and either RD-1 or RD-2b is required for the nuclear translocation of CRY. Both of these functions are prerequisites for the transcriptional repressor activity. The functional nuclear localizing signal in the RD-2b region also was identified. The sequence is well conserved among repressor-type CRYs, including mCRY1. Mutations in the nuclear localizing signal of mCRY1 reduce the extent of its nuclear localization. These findings show that both nuclear localization and interaction with the CLOCK:BMAL heterodimer are essential for transcriptional repression by CRY.

Photoperiod differentially regulates circadian oscillators in central and peripheral tissues of the Syrian hamster

In many seasonally breeding rodents, reproduction and metabolism are activated by long summer days (LD) and inhibited by short winter days (SD). After several months of SD, animals become refractory to this inhibitory photoperiod and spontaneously revert to LD-like physiology. The suprachiasmatic nuclei (SCN) house the primary circadian oscillator in mammals. Seasonal changes in photic input to this structure control many annual physiological rhythms via SCN-regulated pineal melatonin secretion, which provides an internal endocrine signal representing photoperiod. We compared LD- and SD-housed animals and show that the waveform of SCN expression for three circadian clock genes (Per1, Per2, and Cry2) is modified by photoperiod. In SD-refractory (SD-R) animals, SCN and melatonin rhythms remain locked to SD, reflecting ambient photoperiod, despite LD-like physiology. In peripheral oscillators, Per1 and Dbp rhythms are also modified by photoperiod but, in contrast to the SCN, revert to LD-like, high-amplitude rhythms in SD-R animals. Our data suggest that circadian oscillators in peripheral organs participate in photoperiodic time measurement in seasonal mammals; however, circadian oscillators operate differently in the SCN. The clear dissociation between SCN and peripheral oscillators in refractory animals implicates intermediate factor(s), not directly driven by the SCN or melatonin, in entrainment of peripheral clocks.

Mutant analyses define multiple roles for phytochrome C in Arabidopsis photomorphogenesis

The analysis of Arabidopsis mutants deficient in the A, B, D, and E phytochromes has revealed that each of these phytochrome isoforms has both distinct and overlapping roles throughout plant photomorphogenesis. Although overexpression studies of phytochrome C (phyC) have suggested photomorphogenic roles for this receptor, conclusive evidence of function has been lacking as a result of the absence of mutants in the PHYC locus. Here, we describe the isolation of a T-DNA insertion mutant of phyC (phyC-1), the subsequent creation of mutant lines deficient in multiple phytochrome combinations, and the physiological characterization of these lines. In addition to operating as a weak red light sensor, phyC may perform a significant role in the modulation of other photoreceptors. phyA and phyC appear to act redundantly to modulate the phyB-mediated inhibition of hypocotyl elongation in red light and to function together to regulate rosette leaf morphology. In addition, phyC performs a significant role in the modulation of blue light sensing. Several of these phenotypes are supported by the parallel analysis of a quadruple mutant deficient in phytochromes A, B, D, and E, which thus contains only active phyC. Together, these data suggest that phyC has multiple functions throughout plant development that may include working as a coactivator with other phytochromes and the cryptochrome blue light receptors.

Circadian clock genes and photoperiodism: Comprehensive analysis of clock gene expression in the mediobasal hypothalamus, the suprachiasmatic nucleus, and the pineal gland of Japanese Quail under various light schedules

In birds, the mediobasal hypothalamus (MBH) including the infundibular nucleus, inferior hypothalamic nucleus, and median eminence is considered to be an important center that controls the photoperiodic time measurement. Here we show expression patterns of circadian clock genes in the MBH, putative suprachiasmatic nucleus (SCN), and pineal gland, which constitute the circadian pacemaker under various light schedules. Although expression patterns of clock genes were different between long and short photoperiod in the SCN and pineal gland, the results were not consistent with those under night interruption schedule, which causes testicular growth. These results indicate that different expression patterns of the circadian clock genes in the SCN and pineal gland are not an absolute requirement for encoding and decoding of seasonal information. In contrast, expression patterns of clock genes in the MBH were stable under various light conditions, which enables animals to keep a steady-state photoinducible phase.

Making (a) sense of non-visual ocular photoreception

A subset of intrinsically photosensitive retinal ganglion cells transduce information about ambient lighting conditions to areas of the brain involved in tasks including entrainment of the circadian clock, pupillary light reflexes and melatonin synthesis. The phototransduction system(s) utilized by these cells are unknown. Melanopsin and cryptochromes have been proposed as candidate photopigments for this system. Recent analyses of retinal degenerate mice lacking melanopsin or cryptochromes indicates that outer and inner photoreceptors can both contribute to non-visual photoresponses, and that both melanopsin and cryptochromes play important roles in this process.

From seed to seed: the role of photoreceptors in Arabidopsis development

As sessile organisms, plants have evolved a multitude of developmental responses to cope with the ever-changing environmental conditions that challenge the plant throughout its life cycle. Of the many environmental cues that regulate plant development, light is probably the most important. From determining the developmental pattern of the emerging seedling, to influencing the organization of organelles to best maximize energy available for photosynthesis, light has dramatic effects on development during all stages of plant life. In plants, three classes of photoreceptors that mediate light perception have been characterized at the molecular level. The phytochromes recognize light in the red portion of the spectrum, while cryptochromes and phototropins perceive blue and UVA light. In this review, we discuss the different aspects of development that are regulated by these photoreceptors in the model plant species Arabidopsis thaliana and how the phytochromes, cryptochromes, and phototropins bring about changes in development seen in the growing plant.