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PubMed2004

Light induces phenylpropanoid metabolism in Arabidopsis roots.

Experiments have shown that many phenylpropanoid genes are highly expressed in light-grown Arabidopsis roots. Studies employing reporter gene constructs have indicated that the expression of these genes is localized not only to the lignifying root vasculature, but also to non-lignifying tissues, such as the root cortex, suggesting that the proteins encoded by these genes may be involved in aspects of phenylpropanoid metabolism other than lignification. Consistent with this hypothesis, roots of etiolated and soil-grown plants contain almost no soluble phenylpropanoids, but exposure to light leads to the accumulation of flavonoids, as well as high levels of coniferin and syringin (coniferyl and sinapyl-4-O-glycosides), compounds not previously reported to be accumulated in Arabidopsis. To elucidate the mechanism by which light induces root secondary metabolism, extracts of mutants defective in light perception and light responses were analyzed for phenylpropanoid content. The results of these assays showed that phytochrome (PHY)B and cryptochrome (CRY)2 are the primary photoreceptors involved in light-dependent phenylpropanoid accumulation, and that the hypocotyl elongated (HY5) transcription factor is also required for this response. The presence of phenylpropanoids in etiolated roots of cop (constitutively photomorphogenic)1, cop9, and det (de-etiolated)1 mutants indicate that the corresponding wild-type genes are required to repress root phenylpropanoid biosynthesis in the absence of light. Biochemical analysis of root cell walls and analysis of phenylpropanoid gene expression suggest that coniferin and syringin accumulation may be the result of both increased biosynthesis and decreased conversion of these compounds into other phenylpropanoid end products. Finally, our data suggest that the accumulation of coniferin, syringin, and flavonoids in Arabidopsis roots is a high-irradiance response (HIR), and suggest that comparative analysis of light- and dark-grown Arabidopsis roots may provide new insights into both phenylpropanoid biosynthesis and light signaling in plants.

Arabidopsis↗
PubMed2004

Phenotypic characterization of a photomorphogenic mutant.

Light is arguably the most important abiotic factor controlling plant growth and development throughout their life cycle. Plants have evolved sophisticated light-sensing mechanisms to monitor fluctuations in light quality, intensity, direction and periodicity (day length). In Arabidopsis, three families of photoreceptors have been identified by molecular genetic studies. The UV-A/blue light receptors cryptochromes and the red/far-red receptors phytochromes control an overlapping set of responses including photoperiodic flowering induction and de-etiolation. Phototropins are the primary photoreceptors for a set of specific responses to UV-A/blue light such as phototropism, chloroplast movement and stomatal opening. Mutants affecting a photoreceptor have a characteristic phenotype. It is therefore possible to determine the specific developmental responses and the photoreceptor pathway(s) affected in a mutant by performing an appropriate set of photobiological and genetic experiments. In this paper, we outline the principal and easiest experiments that can be performed to obtain a first indication about the nature of the photobiological defect in a given mutant.

Arabidopsis↗
PubMed2004

Hypocotyl growth orientation in blue light is determined by phytochrome A inhibition of gravitropism and phototropin promotion of phototropism.

How developing seedlings integrate gravitropic and phototropic stimuli to determine their direction of growth is poorly understood. In this study we tested whether blue light influences hypocotyl gravitropism in Arabidopsis. Phototropin1 (phot1) triggers phototropism under low fluence rates of blue light but, at least in the dark, has no effect on gravitropism. By analyzing the growth orientation of phototropism-deficient seedlings in response to gravitropic and phototropic stimulations we show that blue light not only triggers phototropism but also represses hypocotyl gravitropism. At low fluence rates of blue light phot1 mutants were agravitropic. In contrast, phyAphot1 double mutants grew exclusively according to gravity demonstrating that phytochrome A (phyA) is necessary to inhibit gravitropism. Analyses of phot1cry1cry2 triple mutants indicate that cryptochromes play a minor role in this response. Thus the optimal growth orientation of hypocotyls is determined by the action of phyA-suppressing gravitropism and the phototropin-triggering phototropism. It has long been known that phytochromes promote phototropism but the mechanism involved is still unknown. Our data show that by inhibiting gravitropism phyA acts as a positive regulator of phototropism.

Arabidopsis↗
PubMed2001

Molecular mechanisms of the biological clock in cultured fibroblasts.

In mammals, the central circadian pacemaker resides in the hypothalamic suprachiasmatic nucleus (SCN), but circadian oscillators also exist in peripheral tissues. Here, using wild-type and cryptochrome (mCry)-deficient cell lines derived from mCry mutant mice, we show that the peripheral oscillator in cultured fibroblasts is identical to the oscillator in the SCN in (i) temporal expression profiles of all known clock genes, (ii) the phase of the various mRNA rhythms (i.e., antiphase oscillation of Bmal1 and mPer genes), (iii) the delay between maximum mRNA levels and appearance of nuclear mPER1 and mPER2 protein, (iv) the inability to produce oscillations in the absence of functional mCry genes, and (v) the control of period length by mCRY proteins.

ARNTL Transcription Factors↗
PubMed2006

JETLAG resets the Drosophila circadian clock by promoting light-induced degradation of TIMELESS.

Organisms ranging from bacteria to humans synchronize their internal clocks to daily cycles of light and dark. Photic entrainment of the Drosophila clock is mediated by proteasomal degradation of the clock protein TIMELESS (TIM). We have identified mutations in jetlag-a gene coding for an F-box protein with leucine-rich repeats-that result in reduced light sensitivity of the circadian clock. Mutant flies show rhythmic behavior in constant light, reduced phase shifts in response to light pulses, and reduced light-dependent degradation of TIM. Expression of JET along with the circadian photoreceptor cryptochrome (CRY) in cultured S2R cells confers light-dependent degradation onto TIM, thereby reconstituting the acute response + of the circadian clock to light in a cell culture system. Our results suggest that JET is essential for resetting the clock by transmitting light signals from CRY to TIM.

Amino Acid Sequence↗
PubMed1999

Light-independent role of CRY1 and CRY2 in the mammalian circadian clock.

Cryptochrome (CRY), a photoreceptor for the circadian clock in Drosophila, binds to the clock component TIM in a light-dependent fashion and blocks its function. In mammals, genetic evidence suggests a role for CRYs within the clock, distinct from hypothetical photoreceptor functions. Mammalian CRY1 and CRY2 are here shown to act as light-independent inhibitors of CLOCK-BMAL1, the activator driving Per1 transcription. CRY1 or CRY2 (or both) showed light-independent interactions with CLOCK and BMAL1, as well as with PER1, PER2, and TIM. Thus, mammalian CRYs act as light-independent components of the circadian clock and probably regulate Per1 transcriptional cycling by contacting both the activator and its feedback inhibitors.

3T3 Cells↗
PubMed1999

Photic induction of mPer1 and mPer2 in cry-deficient mice lacking a biological clock.

Mice lacking mCry1 and mCry2 are behaviorally arrhythmic. As shown here, cyclic expression of the clock genes mPer1 and mPer2 (mammalian Period genes 1 and 2) in the suprachiasmatic nucleus and peripheral tissues is abolished and mPer1 and mPer2 mRNA levels are constitutively high. These findings indicate that the biological clock is eliminated in the absence of both mCRY1 and mCRY2 (mammalian cryptochromes 1 and 2) and support the idea that mammalian CRY proteins act in the negative limb of the circadian feedback loop. The mCry double-mutant mice retain the ability to have mPer1 and mPer2 expression induced by a brief light stimulus known to phase-shift the biological clock in wild-type animals. Thus, mCRY1 and mCRY2 are dispensable for light-induced phase shifting of the biological clock.

Animals↗
PubMed2006

Genome-wide analysis of light sensing in Prochlorococcus.

Prochlorococcus MED4 has, with a total of only 1,716 annotated protein-coding genes, the most compact genome of a free-living photoautotroph. Although light quality and quantity play an important role in regulating the growth rate of this organism in its natural habitat, the majority of known light-sensing proteins are absent from its genome. To explore the potential for light sensing in this phototroph, we measured its global gene expression pattern in response to different light qualities and quantities by using high-density Affymetrix microarrays. Though seven different conditions were tested, only blue light elicited a strong response. In addition, hierarchical clustering revealed that the responses to high white light and blue light were very similar and different from that of the lower-intensity white light, suggesting that the actual sensing of high light is mediated via a blue-light receptor. Bacterial cryptochromes seem to be good candidates for the blue-light sensors. The existence of a signaling pathway for the redox state of the photosynthetic electron transport chain was suggested by the presence of genes that responded similarly to red and blue light as well as genes that responded to the addition of DCMU [3-(3,4-dichlorophenyl)-1,1-N-N'-dimethylurea], a specific inhibitor of photosystem II-mediated electron transport.

Bacterial Proteins↗
PubMed2005

Coupling of human circadian and cell cycles by the timeless protein.

The Timeless protein is essential for circadian rhythm in Drosophila. The Timeless orthologue in mice is essential for viability and appears to be required for the maintenance of a robust circadian rhythm as well. We have found that the human Timeless protein interacts with both the circadian clock protein cryptochrome 2 and with the cell cycle checkpoint proteins Chk1 and the ATR-ATRIP complex and plays an important role in the DNA damage checkpoint response. Down-regulation of Timeless in human cells seriously compromises replication and intra-S checkpoints, indicating an intimate connection between the circadian cycle and the DNA damage checkpoints that is in part mediated by the Timeless protein.

Adaptor Proteins, Signal Transducing↗
PubMed2004

Light signal transduction in higher plants.

Plants utilize several families of photoreceptors to fine-tune growth and development over a large range of environmental conditions. The UV-A/blue light sensing phototropins mediate several light responses enabling optimization of photosynthetic yields. The initial event occurring upon photon capture is a conformational change of the photoreceptor that activates its protein kinase activity. The UV-A/blue light sensing cryptochromes and the red/far-red sensing phytochromes coordinately control seedling establishment, entrainment of the circadian clock, and the transition from vegetative to reproductive growth. In addition, the phytochromes control seed germination and shade-avoidance responses. The molecular mechanisms involved include light-regulated subcellular localization of the photoreceptors, a large reorganization of the transcriptional program, and light-regulated proteolytic degradation of several photoreceptors and signaling components.

Arabidopsis↗
PubMed2003

A fly's eye view of circadian entrainment.

The Drosophila circadian clock is an ideal model system for teasing out the molecular mechanisms of circadian behavior and the means by which animals synchronize to day-night cycles. The clock that drives behavioral rhythms, located in the lateral neurons in the central brain, consists of a feedback loop of the circadian genes period (per) and timeless (tim). The molecular cycle, roughly 24 h long, is constantly reset by the environment. This review focuses on the main input pathways of the dominant circadian zeitgeber, light. Light acts directly on the clock primarily through cryptochrome (cry), a deep brain blue-light photoreceptor. CRY activation causes rapid TIM degradation, which is a predicted means of resetting the clock both on a daily basis at dawn and on an acute basis following an entraining light pulse during the night hours. In the absence of cry, the clock can still be driven by photic input through the visual system, though the mechanisms underlying this entrainment are unclear. Temperature can also entrain the clock, although the mechanisms by which this occurs are also unclear.

Animals↗
PubMed2006

Immunoreactivities to three circadian clock proteins in two ground crickets suggest interspecific diversity of the circadian clock structure.

The closely related crickets Dianemobius nigrofasciatus and Allonemobius allardi exhibit similar circadian rhythms and photoperiodic responses, suggesting that they possess similar circadian and seasonal clocks. To verify this assumption, antisera to Period (PER), Doubletime (DBT), and Cryptochrome (CRY) were used to visualize circadian clock neurons in the cephalic ganglia. Immunoreactivities referred to as PER-ir, DBT-ir, and CRY-ir were distributed mainly in the optic lobes (OL), pars intercerebralis (PI), dorsolateral protocerebrum, and the subesophageal ganglion (SOG). A system of immunoreactive cells in the OL dominates in D. nigrofasciatus, while immunoreactivities in the PI and SOG prevail in A. allardi. Each OL of D. nigrofasciatus contains 3 groups of cells that coexpress PER-ir and DBT-ir and send processes over the frontal medulla face to the inner lamina surface, suggesting functional linkage to the compound eye. Only 2 pairs of PER-ir cells (no DBT-ir) were found in the OL of A. allardi. Several groups of PER-ir cells occur in the brain of both species. The PI also contains DBT-ir and CRY-ir cells, but in A. allardi, most of the DBT-ir is confined to the SOG. Most immunoreactive cells in the PI and in the dorsolateral brain send their fibers to the contralateral corpora cardiaca and corpora allata. The proximity and, in some cases, proven identity of the PER-ir, DBT-ir, and CRY-ir perikarya are consistent with presumed interactions between the examined clock components. The antigens were always found in the cytoplasm, and no diurnal oscillations in their amounts were detected. The photoperiod, which controls embryonic diapause, the rate of larval development, and the wing length of crickets, had no discernible effect on either distribution or the intensity of the immunostaining.

Animals↗
PubMed2006

Restoration of circadian rhythmicity in circadian clock-deficient mice in constant light.

In mammals, circadian rhythms in behavior and physiology are controlled by a central pacemaker, the SCN, and subordinated clocks throughout the body. On the molecular level, these clocks are based on transcriptional/translational feedback loops involving a set of clock genes that regulate their own transcription. Among the components driving the mammalian circadian clock are the Period 1 and 2 (Per1 and Per2) and Cryptochrome 1 and 2 (Cry1 and Cry2) genes. In the present study, the authors characterize the behavioral and molecular rhythms of Per2/Cry1 double mutant mice under 3 different lighting conditions. In an LD cycle, the activity of these animals is masked by light, while in DD, the mutants lose circadian rhythmicity but exhibit strong ultradian rhythms. In LL of higher intensity, circadian rhythms are restored on the behavioral level with a drastically shortened endogenous period. Furthermore, both in the SCN and in the periphery, clock gene rhythms are restored. Based on these observations and also on the fact that light-mediated induction of Per gene expression is preserved in these mutants, the authors propose a mechanism by which endogenous ultradian rhythms may relay timed light exposure to the SCN, leading to a reinitiation of self-sustained circadian rhythms in LL.

Animals↗
PubMed2006

Clock-gated photic stimulation of timeless expression at cold temperatures and seasonal adaptation in Drosophila.

Numerous lines of evidence indicate that the initial photoresponse of the circadian clock in Drosophila melanogaster is the light-induced degradation of TIMELESS (TIM). This posttranslational mechanism is in sharp contrast to the well-characterized pacemakers in mammals and Neurospora, where light evokes rapid changes in the transcriptional profiles of 1 or more clock genes. The authors show that light has novel effects on D. melanogaster circadian pacemakers, acutely stimulating the expression of tim at cold but not warm temperatures. This photoinduction occurs in flies defective for the classic visual phototransduction pathway or the circadian-relevant photoreceptor CRYPTOCHROME (CRY). Cold-specific stimulation of tim RNA abundance is regulated at the transcriptional level, and although numerous lines of evidence indicate that period (per) and tim expression are activated by the same mechanism, light has no measurable acute effect on per mRNA abundance. Moreover, light-induced increases in the levels of tim RNA are abolished or greatly reduced in the absence of functional CLOCK (CLK) or CYCLE (CYC) but not PER or TIM. These findings add to a growing number of examples where molecular and behavioral photoresponses in Drosophila are differentially influenced by "positive" (e.g., CLK and CYC) and "negative" (e.g., PER and TIM) core clock elements. The acute effects of light on tim expression are temporally gated, essentially restricted to the daily rising phase in tim mRNA levels. Because the start of the daily upswing in tim expression begins several hours after dawn in long photoperiods (day length), this gating mechanism likely ensures that sunrise does not prematurely stimulate tim expression during unseasonally cold spring/summer days. The results suggest that the photic stimulation of tim expression at low temperatures is part of a seasonal adaptive response that helps advance the phase of the clock on cold days, enabling flies to exhibit preferential daytime activity despite the (usually) earlier onset of dusk. Taken together with prior findings, the ability of temperature and photoperiod to adjust trajectories in the rising phases of 1 or more clock RNAs constitutes a major mechanism contributing to seasonal adaptation of clock function.

Adaptation, Physiological↗
PubMed2002

Three vitamins are involved in regulation of the circadian rhythm.

Photoreceptors in the eyes of mammals synchronize the innate circadian rhythm to the solar light-dark cycle. They differ from the photoreceptors serving vision in rods and cones of the retina and are located in the ganglion cells of the retina. They consist of melanopsin, a protein homologous to rhodopsin, with retinal as the light-sensitive prosthetic group; and cryptochromes 1 and 2, proteins combined with methenyltetrahydrofolate and flavin adenine dinucleotide, which function as blue light-sensitive photoreceptors.

Animals↗
PubMed2003

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.

Animals↗
PubMed2005

Regulation of gene expression by light.

Light signals perceived mainly by phytochromes and cryptochromes regulate plant growth and development by driving dramatic shifts of the transcriptome. Early light-responsive genes include a large proportion of transcripton factors of different DNA binding motifs. Mutations at loci encoding several transcriptional regulators, including some of those showing rapid changes in transcript levels, impair responses to light in Arabidopsis thaliana. Proteasome-mediated degradation involving the E3 ligase COP1 provides an additional layer of control of the levels of transcription factors. Some transcriptional regulators are shared by light, circadian and/or hormonal signalling circuits creating complex networks that interactively integrate environmental and endogenous cues.

Arabidopsis↗
PubMed2001

Dissection of the light signal transduction pathways regulating the two early light-induced protein genes in Arabidopsis.

The expression of light-regulated genes in plants is controlled by different classes of photoreceptors that act through a variety of signaling molecules. During photomorphogenesis, the early light-induced protein (Elip) genes are among the first to be induced. To understand the light signal transduction pathways that regulate Elip expression, the two Elip genes, Elip1 and Elip2, in Arabidopsis were studied, taking advantage of the genetic tools available for studying light signaling in Arabidopsis. Using two independent quantitative reverse transcriptase-PCR techniques, we found that red, far-red, and blue lights positively regulate expression of the Elip genes. Phytochrome A and phytochrome B are involved in this signaling. The cryptochrome or phototropin photoreceptors are not required for blue-light induction of either Elip gene, suggesting the involvement of an additional, unidentified, blue-light receptor. Although the COP9 signalosome, a downstream regulator, is involved in dark repression of both Elips, Elip1 and Elip2 show different expression patterns in the dark. The transcription factor HY5 promotes the light induction of Elip1, but not Elip2. A defect in photosystem II activity in greening of hy5 seedlings may result from the loss of Elip1. Heat shock positively controlled Elip1 and Elip2 in a light-independent fashion. This induction is independent of HY5, indicating that heat shock and light activate transcription of the Elip genes through independent pathways.

Arabidopsis↗