Effects of synergistic signaling by phytochrome A and cryptochrome1 on circadian clock-regulated catalase expression

Protective effects of pretreatment with Fe2+, Cu2+, and Rb+ on phoxim poisoning in silkworm, Bombyx mori

BACKGROUND

Phoxim is a widely used organophosphorus pesticide in agriculture. People are paying more and more attention to its toxicity. At present, there is no appropriate way to solve the phoxim poisoning of silkworm, which severely affected the development of sericulture. Fe²⁺, Cu²⁺, Rb⁺ exerted their biological effects through various forms in vivo.

METHODS

To evaluate the effect of Fe²⁺/Cu²⁺/Rb⁺ on phoxim poisoning in silkworm, Bombyx mori were treated with fresh mulberry leaves soaked in 2.5 mg/L phoxim for 2 min with 50 mg/L FeCl₂, 150 mg/L CuCl₂, or 0.5 mg/L RbCl from 5 days of the fifth-instar silkworm.

RESULTS

Fe²⁺, Cu²⁺, and Rb⁺ pretreatments significantly inhibited the phoxim-induced reduction of survival rate and alleviated the phoxim-induced poisoning symptoms. The protective effects of Fe²⁺, Cu²⁺, and Rb⁺ on phoxim poisoning might be due to their enhancement of superoxide dismutase (SOD), catalase (CAT), and carboxylesterase (CarE) in the hemolymph and fat body of silkworm. This enhancement might reduce reactive oxygen species (ROS) accumulation and oxidative stress (OS) caused by phoxim poisoning. Thereby it reduced the damage to silkworm tissues and cells.

CONCLUSIONS

These results showed that Fe²⁺, Cu²⁺, and Rb⁺ treatments protected the silkworm from phoxim poisoning by directly enhancing the activity of SOD, CAT, and CarE enzymes and reducing oxidative stress, but not dependent on the high expression of CYP genes. The use of Fe²⁺, Cu²⁺, and Rb⁺ to enhance the activity of SOD, CAT, and CarE enzymes may be an underlying effective way to solve phoxim poisoning in the silkworm industry.

LED light mediates phenolic accumulation and enhances antioxidant activity in Melissa officinalis L. under drought stress condition

The popularity of lemon balm in conventional medicine is suggested by the existence of two groups of compounds, namely essential oil and phenylpropanoids pathway derivatives. One of the promising approaches to improve tolerance to drought stress induced oxidative damage in seedlings grown in greenhouses and plant growth chambers is replacing the traditional and high-cost light technologies by recently developed light emitting diodes (LED). In this experiment, we analyzed the role of various LED lights including red (R), blue (B), red (70%) + blue (30%) (RB), and white (W) as well as normal greenhouse light (as control) to stimulate defense mechanisms against drought stress in two genotypes of Melissa officinalis L. The present study demonstrates that pre-treatment with LEDs with high-intensity output for 4 weeks alleviated harmful effects of drought stress in the two genotypes. Under drought stress, RB LED pre-treated plantlets of the two genotypes exhibited the highest relative growth index of shoot and root and total phenolic and anthocyanin content compared to those irradiated with other LEDs and greenhouse lights. The highest amount of malondialdehyde level was detected in R LED exposed plants. In response to drought, LED-exposed plants especially RB light-irradiated plants of the two genotypes maintained significantly higher antioxidant and phenylalanine ammonia-lyase (PAL) enzyme activities and higher expression level of the PAL1 and 4CL-1 genes compared to those irradiated with greenhouse light. We concluded that RB LED light provides a better growth condition and resistance to drought stress for the two genotypes of lemon balm by the highest antioxidant activity and the least amount of damage to the cell membranes. Our data suggest that LED light pre-treatments as moderate stress activate antioxidant systems, enhance the scavenging of ROS and induce drought stress tolerance in the two genotypes of lemon balm plants.

Molecular tools enabling pennycress (Thlaspi arvense) as a model plant and oilseed cash cover crop

Thlapsi arvense L. (pennycress) is being developed as a profitable oilseed cover crop for the winter fallow period throughout the temperate regions of the world, controlling soil erosion and nutrients run-off on otherwise barren farmland. We demonstrate that pennycress can serve as a user-friendly model system akin to Arabidopsis that is well-suited for both laboratory and field experimentation. We sequenced the diploid genome of the spring-type Spring 32-10 inbred line (1C DNA content of 539 Mb; 2n = 14), identifying variation that may explain phenotypic differences with winter-type pennycress, as well as predominantly a one-to-one correspondence with Arabidopsis genes, which makes translational research straightforward. We developed an Agrobacterium-mediated floral dip transformation method (0.5% transformation efficiency) and introduced CRISPR-Cas9 constructs to produce indel mutations in the putative FATTY ACID ELONGATION1 (FAE1) gene, thereby abolishing erucic acid production and creating an edible seed oil comparable to that of canola. We also stably transformed pennycress with the Euonymus alatus diacylglycerol acetyltransferase (EaDAcT) gene, producing low-viscosity acetyl-triacylglycerol-containing seed oil suitable as a diesel-engine drop-in fuel. Adoption of pennycress as a model system will accelerate oilseed-crop translational research and facilitate pennycress' rapid domestication to meet the growing sustainable food and fuel demands.

Beyond Transcription: Fine-Tuning of Circadian Timekeeping by Post-Transcriptional Regulation

The circadian clock is an important endogenous timekeeper, helping plants to prepare for the periodic changes of light and darkness in their environment. The clockwork of this molecular timer is made up of clock proteins that regulate transcription of their own genes with a 24 h rhythm. Furthermore, the rhythmically expressed clock proteins regulate time-of-day dependent transcription of downstream genes, causing messenger RNA (mRNA) oscillations of a large part of the transcriptome. On top of the transcriptional regulation by the clock, circadian rhythms in mRNAs rely in large parts on post-transcriptional regulation, including alternative pre-mRNA splicing, mRNA degradation, and translational control. Here, we present recent insights into the contribution of post-transcriptional regulation to core clock function and to regulation of circadian gene expression in Arabidopsis thaliana.

Interdependence of tetrapyrrole metabolism, the generation of oxidative stress and the mitigative oxidative stress response

Tetrapyrroles are involved in light harvesting and light perception, electron-transfer reactions, and as co-factors for key enzymes and sensory proteins. Under conditions in which cells exhibit stress-induced imbalances of photosynthetic reactions, or light absorption exceeds the ability of the cell to use photoexcitation energy in synthesis reactions, redox imbalance can occur in photosynthetic cells. Such conditions can lead to the generation of reactive oxygen species (ROS) associated with alterations in tetrapyrrole homeostasis. ROS accumulation can result in cellular damage and detrimental effects on organismal fitness, or ROS molecules can serve as signals to induce a protective or damage-mitigating oxidative stress signaling response in cells. Induced oxidative stress responses include tetrapyrrole-dependent and -independent mechanisms for mitigating ROS generation and/or accumulation. Thus, tetrapyrroles can be contributors to oxidative stress, but are also essential in the oxidative stress response to protect cells by contributing to detoxification of ROS. In this review, we highlight the interconnection and interdependence of tetrapyrrole metabolism with the occurrence of oxidative stress and protective oxidative stress signaling responses in photosynthetic organisms.

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Tetrapyrroles are involved in light sensing and oxidative stress mitigation.

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Reactive oxygen species (ROS) can form upon light exposure of free tetrapyrroles.

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Tetrapyrrole homeostasis must be tightly regulated to avoid oxidative stress.

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ROS can result in cellular damage or oxidative stress signaling in cells.

Circadian rhythms and post-transcriptional regulation in higher plants

The circadian clock of plants allows them to cope with daily changes in their environment. This is accomplished by the rhythmic regulation of gene expression, in a process that involves many regulatory steps. One of the key steps involved at the RNA level is post-transcriptional regulation, which ensures a correct control on the different amounts and types of mRNA that will ultimately define the current physiological state of the plant cell. Recent advances in the study of the processes of regulation of pre-mRNA processing, RNA turn-over and surveillance, regulation of translation, function of lncRNAs, biogenesis and function of small RNAs, and the development of bioinformatics tools have helped to vastly expand our understanding of how this regulatory step performs its role. In this work we review the current progress in circadian regulation at the post-transcriptional level research in plants. It is the continuous interaction of all the information flow control post-transcriptional processes that allow a plant to precisely time and predict daily environmental changes.

RNA around the clock - regulation at the RNA level in biological timing

The circadian timing system in plants synchronizes their physiological functions with the environment. This is achieved by a global control of gene expression programs with a considerable part of the transcriptome undergoing 24-h oscillations in steady-state abundance. These circadian oscillations are driven by a set of core clock proteins that generate their own 24-h rhythm through periodic feedback on their own transcription. Additionally, post-transcriptional events are instrumental for oscillations of core clock genes and genes in clock output. Here we provide an update on molecular events at the RNA level that contribute to the 24-h rhythm of the core clock proteins and shape the circadian transcriptome. We focus on the circadian system of the model plant Arabidopsis thaliana but also discuss selected regulatory principles in other organisms.

Natural variation reveals that intracellular distribution of ELF3 protein is associated with function in the circadian clock

Natural selection of variants within the Arabidopsis thaliana circadian clock can be attributed to adaptation to varying environments. To define a basis for such variation, we examined clock speed in a reporter-modified Bay-0 x Shakdara recombinant inbred line and localized heritable variation. Extensive variation led us to identify EARLY FLOWERING3 (ELF3) as a major quantitative trait locus (QTL). The causal nucleotide polymorphism caused a short-period phenotype under light and severely dampened rhythm generation in darkness, and entrainment alterations resulted. We found that ELF3-Sha protein failed to properly localize to the nucleus, and its ability to accumulate in darkness was compromised. Evidence was provided that the ELF3-Sha allele originated in Central Asia. Collectively, we showed that ELF3 protein plays a vital role in defining its light-repressor action in the circadian clock and that its functional abilities are largely dependent on its cellular localization.

DOI:http://dx.doi.org/10.7554/eLife.02206.001

Life on Earth tends to follow a daily rhythm: some animals are awake during the day and asleep at night, whilst others are more active at night, or during the twilight around dawn and dusk. For many living things, these cycles of activity are driven by an internal body clock that helps the organism to adapt to the daily cycle of light and dark—and similar internal clocks also exist in plants.

These internal clocks define daily—or circadian—cycles whereby multiple genes are switched ‘on’ or ‘off’ at different time points in every 24-hr period. And, because light and ambient temperatures also vary with time of the day, many organisms use these external signals as cues to reset their own internal clocks. Moreover, the hours of daylight and temperature vary around the world, and also with the seasons, so plants and animals must be able to change how these external signals influence their internal clocks so that they stay in tune with the day/night cycle. However, it is not clear how they do this.

To explore this question, Anwer et al. grew plants that were from a cross between two types of the model plant Arabidopsis thaliana from different environments: one from Germany, and the other from Tajikistan in Central Asia. These offspring were also genetically engineered so that an enzyme that could give off light was produced under the control of the internal clock. Anwer et al. found that the plants continued to glow and fade with an almost daily rhythm even after external cues, such as changes in temperature or light, had been removed.

Different offspring plants consistently glowed and faded with different rhythms such that some had, for example, a 21-hr day and others a 28-hr day. These differences were caused by many genes that differed from the original German and Tajikistan parent plants, and Anwer et al. ‘mapped’ one of these genetic differences to a single gene. Offspring that inherited a version of a gene called ELF3 from the Tajikistan parent had internal clocks that ran faster when the plant was under the light. These plants also gradually stopped glowing as brightly as the German parent when they were kept in the dark, suggesting that their internal clocks were ‘ticking more softly’. It was already known that the ELF3 gene affected the circadian clock in plants, and Anwer et al. thus concluded that the plants with Tajikistan version of this gene, called ELF3-Sha, were also less able to reset their internal clocks to synchronize in response to external cues.

Anwer et al. also showed that the normal ELF3 protein is more likely to be found in the nucleus of a plant cell than the ELF3-Sha version, which might suggest that this protein is involved in switching genes off. Further research is now needed to uncover exactly how the ELF3 protein does this to keep the plant's internal clock ‘ticking’ correctly.

DOI:http://dx.doi.org/10.7554/eLife.02206.002

Emerging roles for post-transcriptional regulation in circadian clocks

Circadian clocks temporally organize behavior and physiology across the 24-h day. Great progress has been made in understanding the molecular basis of timekeeping, with a focus on transcriptional feedback networks that are post-translationally modulated. Yet emerging evidence indicates an important role for post-transcriptional regulation, from splicing, polyadenylation and mRNA stability to translation and non-coding functions exemplified by microRNAs. This level of regulation affects virtually all aspects of circadian systems, from the core timing mechanism and input pathways that synchronize clocks to the environment and output pathways that control overt rhythmicity. We hypothesize that post-transcriptional control confers on circadian clocks enhanced robustness as well as the ability to adapt to different environments. As much of what is known derives from nonneural cells or tissues, future work will be required to investigate the role of post-transcriptional regulation in neural clocks.

The role of phytochrome in stress tolerance

It is well-documented that phytochromes can control plant growth and development from germination to flowering. Additionally, these photoreceptors have been shown to modulate both biotic and abiotic stress. This has led to a series of studies exploring the molecular and biochemical basis by which phytochromes modulate stresses, such as salinity, drought, high light or herbivory. Evidence for a role of phytrochromes in plant stress tolerance is explored and reviewed.

The role of phytochrome in stress tolerance

It is well-documented that phytochromes can control plant growth and development from germination to flowering. Additionally, these photoreceptors have been shown to modulate both biotic and abiotic stress. This has led to a series of studies exploring the molecular and biochemical basis by which phytochromes modulate stresses, such as salinity, drought, high light or herbivory. Evidence for a role of phytrochromes in plant stress tolerance is explored and reviewed.

RNA-based regulation in the plant circadian clock

The circadian clock is an endogenous, approximately 24-h timer that enables plants to anticipate daily changes in their environment and regulates a considerable fraction of the transcriptome. At the core of the circadian system is the oscillator, made up of interconnected feedback loops, involving transcriptional regulation of clock genes and post-translational modification of clock proteins. Recently, it has become clear that post-transcriptional events are also critical for shaping rhythmic mRNA and protein profiles. This review covers regulation at the RNA level of both the core clock and output genes in Arabidopsis (Arabidopsis thaliana), with comparisons with other model organisms. We discuss the role of splicing, mRNA decay and translational regulation as well as recent insights into rhythms of noncoding regulatory RNAs.

Type II protein arginine methyltransferase 5 (PRMT5) is required for circadian period determination in Arabidopsis thaliana

Posttranslational modification is an important element in circadian clock function from cyanobacteria through plants and mammals. For example, a number of key clock components are phosphorylated and thereby marked for subsequent ubiquitination and degradation. Through forward genetic analysis we demonstrate that protein arginine methyltransferase 5 (PRMT5; At4g31120) is a critical determinant of circadian period in Arabidopsis. PRMT5 is coregulated with a set of 1,253 genes that shows alterations in phase of expression in response to entrainment to thermocycles versus photocycles in constant temperature. PRMT5 encodes a type II protein arginine methyltransferase that catalyzes the symmetric dimethylation of arginine residues (Rsme2). Rsme2 modification has been observed in many taxa, and targets include histones, components of the transcription complex, and components of the spliceosome. Neither arginine methylation nor PRMT5 has been implicated previously in circadian clock function, but the period lengthening associated with mutational disruption of prmt5 indicates that Rsme2 is a decoration important for the Arabidopsis clock and possibly for clocks in general.

Functional comparison of catalase genes in the elimination of photorespiratory H2O2 using promoter- and 3'-untranslated region exchange experiments in the Arabidopsis cat2 photorespiratory mutant

Photorespiration-associated production of H(2) O(2) accounts for the majority of total H(2) O(2) in leaves of C(3) plants and is mainly eliminated by catalases. In Arabidopsis, lack of CAT2, but not CAT1 or CAT3, results in growth suppression and a marked accumulation of H(2) O(2) in leaves. To evaluate the contribution of individual catalase genes and their promoters to catalase function, we investigated the growth suppression and H(2) O(2) accumulation phenotypes of Arabidopsis derivatives expressing catalase genes from heterologous CAT promoters in a cat2 mutant background. The expression of CAT2 from the CAT2 promoter restored the wild-type phenotype in a cat2-1 mutant, while CAT1 and CAT3 promoter-driven expression of CAT2 did not. Ectopic expression of CAT3 from the CAT2 promoter also restored the normal phenotype, unlike that of CAT1 which required replacement of the CAT1 3'-untranslated region (UTR) with that of CAT2. These results demonstrated that the photorespiratory role of CAT2 is determined mainly by the regulation of its promoter activity. The 3'-UTR of CAT2 was vital for controlling CAT2 protein levels under photorespiratory conditions. Identification of component of heterotetramers catalase isoforms suggested that there is some functional redundancy between CAT2 and CAT1 and CAT3.

CRYPTOCHROME 1 is implicated in promoting R protein-mediated plant resistance to Pseudomonas syringae in Arabidopsis

Plants have evolved complex mechanisms to defend themselves against pathogens. It has been shown that several defense responses are influenced by light, and the red/far-red light photoreceptor phytochromes (PHY) modulate plant defense responses in Arabidopsis. Blue light receptor cryptochromes (CRY) work together with PHY to regulate many light-controlled responses, including photomorphogenesis, floral induction, and entrainment of the circadian clock. We report here that the Arabidopsis blue light photoreceptor CRY1 positively regulates inducible resistance to Pseudomonas syringae under continuous light conditions. By challenging plants with P. syringae pv. tomato (Pst.) DC3000 carrying avrRpt2, we demonstrate that effector-triggered local resistance is down-regulated in the cry1 mutant, leading to more pathogen multiplication. In plants overexpressing CRY1 (CRY1-ovx), however, local resistance is significantly up-regulated. We also show that systemic acquired resistance (SAR) is positively regulated by CRY1, and that salicylic acid (SA)-induced pathogenesis-related gene PR-1 expression is reduced in the cry1 mutant, but enhanced in CRY1-ovx plants. However, our results indicate that CRY1 only modestly influences SA accumulation and has no effect on hypersensitive cell death. These results suggest that CRY1 may positively regulate R protein-mediated resistance to P. syringae with increased PR gene expression.

The 5'UTR of CCA1 includes an autoregulatory cis element that segregates between light and circadian regulation of CCA1 and LHY

The transcription factor CCA1 (CIRCADIAN CLOCK ASSOCIATED 1) participates in both light and circadian clock regulation in Arabidopsis. Two sets of transgenic plants in which GFP was fused to the CCA1 promoter with (1.3-kb fragment) or without (1.01-kb fragment) its 5'UTR were engineered. The transgenic plants transformed with the promoter including the 5'UTR had altered circadian regulation resulting in elongated hypocotyls, a bushy appearance and delayed flowering. In contrast, the transgenic plants transformed with the promoter without the 5'UTR showed earlier flowering than the wild type. Changes in CCA1, LHY and TOC1 gene expression were investigated under light-dark (L:D) fluctuations, continuous darkness (D:D) and continuous light (L:L). The circadian expression of CCA1 was altered in both sets of transgenic plants, being repressed in the plants transformed with the 1.01-kb fragment and constitutively overexpressed in those transformed with the 1.3-kb fragment. Under L:D conditions, regulation of LHY and TOC1 expression was separated from CCA1 regulation in both sets of transgenic plants, with intact rhythmic expression of both LHY and TOC1. Under D:D conditions, the rhythmic expression of LHY and TOC1 was lost in the 1.3 plants but retained with some erratic pattern under L:L conditions. In the 1.01 plants, under both D:D and L:L conditions the rhythmic expression was retained. These results indicate separate light-induced signal-transmission pathways for LHY and CCA1.

Photorespiration

Photorespiration is initiated by the oxygenase activity of ribulose-1,5-bisphosphate-carboxylase/oxygenase (RUBISCO), the same enzyme that is also responsible for CO(2) fixation in almost all photosynthetic organisms. Phosphoglycolate formed by oxygen fixation is recycled to the Calvin cycle intermediate phosphoglycerate in the photorespiratory pathway. This reaction cascade consumes energy and reducing equivalents and part of the afore fixed carbon is again released as CO(2). Because of this, photorespiration was often viewed as a wasteful process. Here, we review the current knowledge on the components of the photorespiratory pathway that has been mainly achieved through genetic and biochemical studies in Arabidopsis. Based on this knowledge, the energy costs of photorespiration are calculated, but the numerous positive aspects that challenge the traditional view of photorespiration as a wasteful pathway are also discussed. An outline of possible alternative pathways beside the major pathway is provided. We summarize recent results about photorespiration in photosynthetic organisms expressing a carbon concentrating mechanism and the implications of these results for understanding Arabidopsis photorespiration. Finally, metabolic engineering approaches aiming to improve plant productivity by reducing photorespiratory losses are evaluated.

Phytochrome B is involved in mediating red light-induced stomatal opening in Arabidopsis thaliana

The stomatal pores of higher plants enable gaseous exchange into and out of leaves for photosynthesis and evaporation. Stomatal opening is induced by both blue and red lights. It is shown that blue light-induced stomatal opening is mediated by the blue light receptor phototropins (PHOT1 and PHOT2) and cryptochromes (CRY1 and CRY2). However, whether phytochrome B (phyB) is involved in red light regulation of stomatal opening remains largely unclear. Here, we report a positive role for Arabidopsis (Arabidopsis thaliana) phyB in the regulation of red light-induced stomatal opening. The phyB mutant stomata displayed a reduced red light response, whereas stomata of the phyB-overexpressing plants displayed a hypersensitive response to red light. In addition, stomata of the cry1 cry2 phyB, phot1 phot2 phyB, and cry1 phyA phyB triple mutant plants showed more reduced light response than those of the single or double mutant plants under white light, implying that phyB acts in concert with phyA, CRY, and PHOT in light regulation of stomatal opening. Stomata of phyB cop1 mutant opened less wide than those of the cop1 mutant, and stomata of the pif3 pif4 mutant opened wider than those of the wild-type, indicating that COP1, together with the PIFs (phytochrome interacting factors), may act downstream of PHYB in regulating stomatal opening. Furthermore, quantitative RT-PCR analysis showed that the expression of MYB60 was reduced in the cry1 cry2 and phyA phyB mutants under blue and red lights, respectively, but induced in the CRY1- and phyB-overexpressing plants. These results demonstrate that phyB and CRY might regulate stomatal opening, at least in part, by regulating MYB60 expression.

Circadian timekeeping during early Arabidopsis development

The circadian coordination of organismal biology with the local temporal environment has consequences for fitness that may become manifest early in development. We directly explored the development of the Arabidopsis (Arabidopsis thaliana) clock in germinating seedlings by monitoring expression of clock genes. Clock function is detected within 2 d of imbibition (hydration of the dried seed). Imbibition is sufficient to synchronize individuals in a population in the absence of entraining cycles of light-dark or temperature, although light-dark and temperature cycles accelerate the appearance of rhythmicity and improve synchrony among individuals. Oscillations seen during the first 2 d following imbibition are dependent on the clock genes LATE ELONGATED HYPOCOTYL, TIMING OF CAB EXPRESSION1, ZEITLUPE, GIGANTEA, PSEUDO-RESPONSE REGULATOR7 (PRR7), and PRR9, although later circadian oscillations develop in mutants defective in each of these genes. In contrast to circadian rhythmicity, which developed under all conditions, amplitude was the only circadian parameter that demonstrated a clear response to the light environment; clock amplitude is low in the dark and high in the light. A circadian clock entrainable by temperature cycles in germinating etiolated seedlings may synchronize the buried seedling with the local daily cycles before emergence from the soil and exposure to light.

Circadian control of messenger RNA stability. Association with a sequence-specific messenger RNA decay pathway

Transcriptional and posttranscriptional regulation are well-established mechanisms for circadian gene expression. Among the latter, differential messenger RNA (mRNA) stability has been hypothesized to control gene expression in response to the clock. However, direct proof that the rate of mRNA turnover can be regulated by the clock is lacking. Previous microarray expression data for unstable mRNAs in Arabidopsis (Arabidopsis thaliana) revealed that mRNA instability is associated with a group of genes controlled by the circadian clock. Here, we show that CCR-LIKE (CCL) and SENESCENCE ASSOCIATED GENE 1 transcripts are differentially regulated at the level of mRNA stability at different times of day. In addition, the changes in CCL mRNA stability continue under free-running conditions, indicating that it is controlled by the Arabidopsis circadian clock. Furthermore, we show that these mRNAs are targets of the mRNA degradation pathway mediated by the downstream (DST) instability determinant. Disruption of the DST-mediated decay pathway in the dst1 mutant leads to aberrant circadian mRNA oscillations that correlate with alterations of the half-life of CCL mRNA relative to parental plants in the morning and afternoon. That this is due to an effect on the circadian control is evidenced by mRNA decay experiments carried out in continuous light. Finally, we show that the defects exhibited by dst mutants are reflected by an impact on circadian regulation at the whole plant level. Together, these results demonstrate that regulation of mRNA stability is important for clock-controlled expression of specific genes in Arabidopsis. Moreover, these data uncover a connection between circadian rhythms and a sequence-specific mRNA decay pathway.

N-terminal domain-mediated homodimerization is required for photoreceptor activity of Arabidopsis CRYPTOCHROME 1

Cryptochromes (CRY) are blue light receptors that share sequence similarity with photolyases, flavoproteins that catalyze the repair of UV light-damaged DNA. Transgenic Arabidopsis thaliana seedlings expressing the C-terminal domains of the Arabidopsis CRY fused to beta-glucuronidase (GUS) display a constitutive photomorphogenic (COP) phenotype, indicating that the signaling mechanism of Arabidopsis CRY is mediated through the C-terminal domain. The role of the Arabidopsis CRY N-terminal photolyase-like domain in CRY action remains poorly understood. Here, we report the essential role of the Arabidopsis CRY1 N-terminal domain (CNT1) in the light activation of CRY1 photoreceptor activity. Yeast two-hybrid assay, in vitro binding, in vivo chemical cross-linking, gel filtration, and coimmunoprecipitation studies indicate that CRY1 homodimerizes in a light-independent manner. Mutagenesis and transgenic studies demonstrate that CNT1-mediated dimerization is required for light activation of the C-terminal domain of CRY1 (CCT1). Transgenic data and native gel electrophoresis studies suggest that multimerization of GUS is both responsible and required for mediating a COP phenotype on fusion to CCT1. These results indicate that the properties of the GUS multimer are analogous to those of the light-modified CNT1 dimer. Irradiation with blue light modifies the properties of the CNT1 dimer, resulting in a change in CCT1, activating CCT1, and eventually triggering the CRY1 signaling pathway.

Circadian rhythms, oxidative stress, and antioxidative defense mechanisms

Endogenous circadian and exogenously driven daily rhythms of antioxidative enzyme activities and of low molecular weight antioxidants (LMWAs) are described in various phylogenetically distant organisms. Substantial amplitudes are detected in several cases, suggesting the significance of rhythmicity in avoiding excessive oxidative stress. Mammalian and/or avian glutathione peroxidase and, as a consequence, glutathione reductase activities follow the rhythm of melatonin. Another hint for an involvement of melatonin in the control of redox processes is seen in its high-affinity binding to cytosolic quinone reductase 2, previously believed to be a melatonin receptor. Although antioxidative protection by pharmacological doses of melatonin is repeatedly reported, explanations of these findings are still insufficient and their physiological and chronobiological relevance is not yet settled. Recent data indicate a role of melatonin in the avoidance of mitochondrial radical formation, a function which may prevail over direct scavenging. Rhythmic changes in oxidative damage of protein and lipid molecules are also reported. Enhanced oxidative protein modification accompanied by a marked increase in the circadian amplitude of this parameter is detected in the Drosophila mutant rosy, which is deficient in the LMWA urate. Preliminary evidence for the significance of circadian rhythmicity in diminishing oxidative stress comes from clock mutants. In Drosophila, moderately enhanced protein damage is described for the arrhythmic and melatonin null mutant per0, but even more elevated, periodic damage is found in the short-period mutant per(s), synchronized to LD 12:12. Remarkably large increases in oxidative protein damage, along with impairment of tissue integrity and--obviously insufficient--compensatory elevations in protective enzymes are observed in a particularly vulnerable organ, the Harderian gland, of another short-period mutant tau, in the Syrian hamster. Mice deficient in the per2 gene homolog are reported to be cancer-prone, a finding which might also relate to oxidative stress. In the dinoflagellate Lingulodinium polyedrum [Gonyaulax polyedra], various treatments that cause oxidative stress result in strong suppressions of melatonin and its metabolite 5-methoxytryptamine (5-MT) and to secondary effects on overt rhythmicity. The glow maximum, depending on the presence of elevated 5-MT at the end of subjective night, decreases in a dose-dependent manner already under moderate, non-lethal oxidative stress, but is restored by replenishing melatonin. Therefore, a general effect of oxidative stress may consist in declines of easily oxidizable signaling molecules such as melatonin, and this can have consequences on the circadian intraorganismal organization and expression of overt rhythms. Recent findings on a redox-sensitive input into the core oscillator via modulation of NPAS2/BMAL1 or CLK/BMAL1 heterodimer binding to DNA indicate a direct influence of cellular redox balance, including oxidative stress, on the circadian clock.

Arabidopsis NDK1 is a Component of ROS Signaling by Interacting with Three Catalases

Plants sense various environmental stimuli and have specific signaling pathways to respond to these cues. We focused on light responsive components and found that NDKs were phosphorylated specifically after red light irradiation in Pisum sativum [Tanaka et al. (1998) J. Photochem. Photobiol. B 45: 113] and after blue light irradiation in Neurospora crassa [Oda and Hasunuma (1997) Mol. Gen. Genet. 256: 593, Ogura et al. (2001) J. Biol. Chem. 276: 21228]. We performed yeast two-hybrid screening using AtNDK1, the counterpart of NDK-P1 (Pisum sativum NDK1) in Arabidopsis, as bait, and isolated catalase3 (AtCat3). Interactions between AtNDK1-AtCAT1 and AtNDK1-AtCAT2 were also detected with the two-hybrid system. Non-denaturing two-dimensional gel electrophoresis of crude extracts from plants revealed that catalase and NDK activities co-migrated in the same area of the gel. Transgenic plants expressing AtNDK1 under control of the CaMV 35S promoter exhibited tolerance to paraquat and high ability to eliminate exogenous H2O2. These results indicate that AtNDK1 has a role in ROS response.

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.

Enhancer trapping reveals widespread circadian clock transcriptional control in Arabidopsis

The circadian clock synchronizes the internal biology of an organism with the environment and has been shown to be widespread among organisms. Microarray experiments have shown that the circadian clock regulates mRNA abundance of about 10% of the transcriptome in plants, invertebrates, and mammals. In contrast, the circadian clock regulates the transcription of the virtually all cyanobacterial genes. To determine the extent to which the circadian clock controls transcription in Arabidopsis, we used in vivo enhancer trapping. We found that 36% of our enhancer trap lines display circadian-regulated transcription, which is much higher than estimates of circadian regulation based on analysis of steady-state mRNA abundance. Individual lines identified by enhancer trapping exhibit peak transcription rates at circadian phases spanning the complete circadian cycle. Flanking genomic sequence was identified for 23 enhancer trap lines to identify clock-controlled genes (CCG-ETs). Promoter analysis of CCG-ETs failed to predict new circadian clock response elements (CCREs), although previously defined CCREs, the CCA1-binding site, and the evening element were identified. However, many CCGs lack either the CCA1-binding site or the evening element; therefore, the presence of these CCREs is insufficient to confer circadian regulation, and it is clear that additional elements play critical roles.

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.

The Arabidopsis SRR1 gene mediates phyB signaling and is required for normal circadian clock function

Plants possess several photoreceptors to sense the light environment. In Arabidopsis cryptochromes and phytochromes play roles in photomorphogenesis and in the light input pathways that synchronize the circadian clock with the external world. We have identified SRR1 (sensitivity to red light reduced), a gene that plays an important role in phytochrome B (phyB)-mediated light signaling. The recessive srr1 null allele and phyB mutants display a number of similar phenotypes indicating that SRR1 is required for normal phyB signaling. Genetic analysis suggests that SRR1 works both in the phyB pathway but also independently of phyB. srr1 mutants are affected in multiple outputs of the circadian clock in continuous light conditions, including leaf movement and expression of the clock components, CCA1 and TOC1. Clock-regulated gene expression is also impaired during day-night cycles and in constant darkness. The circadian phenotypes of srr1 mutants in all three conditions suggest that SRR1 activity is required for normal oscillator function. The SRR1 gene was identified and shown to code for a protein conserved in numerous eukaryotes including mammals and flies, implicating a conserved role for this protein in both the animal and plant kingdoms.

Cryptochrome structure and signal transduction

Cryptochromes are photosensory receptors mediating light regulation of growth and development in plants. Since the isolation of the Arabidopsis CRY1 gene in 1993, cryptochromes have been found in every multicellular eukaryote examined. Most plant cryptochromes have a chromophore-binding domain that shares similar structure with DNA photolyase, and a carboxyl terminal extension that contains a DQXVP-acidic-STAES (DAS) domain conserved from moss, to fern, to angiosperm. In Arabidopsis, cryptochromes are nuclear proteins that mediate light control of stem elongation, leaf expansion, photoperiodic flowering, and the circadian clock. Cryptochromes may act by interacting with proteins such as phytochromes, COP1, and clock proteins, or/and chromatin and DNA. Recent studies suggest that cryptochromes undergo a blue light-dependent phosphorylation that affects the conformation, intermolecular interactions, physiological activities, and protein abundance of the photoreceptors.

Phase-specific circadian clock regulatory elements in Arabidopsis

We have defined a minimal Arabidopsis CATALASE 3 (CAT3) promoter sufficient to drive evening-specific circadian transcription of a LUCIFERASE reporter gene. Deletion analysis and site-directed mutagenesis reveal a circadian response element, the evening element (EE: AAAATATCT), that is necessary for evening-specific transcription. The EE differs only by a single base pair from the CIRCADIAN CLOCK ASSOCIATED 1-binding site (CBS: AAAAAATCT), which is important for morning-specific transcription. We tested the hypothesis that the EE and the CBS specify circadian phase by site-directed mutagenesis to convert the CAT3 EE into a CBS. Changing the CAT3 EE to a CBS changes the phase of peak transcription from the evening to the morning in continuous dark and in light-dark cycles, consistent with the specification of phase by the single base pair that distinguishes these elements. However, rhythmicity of the CBS-containing CAT3 promoter is dramatically compromised in continuous light. Thus, we conclude that additional information normally provided in the context of a morning-specific promoter is necessary for full circadian activity of the CBS.

The out of phase 1 mutant defines a role for PHYB in circadian phase control in Arabidopsis

Arabidopsis displays circadian rhythms in stomatal aperture, stomatal conductance, and CO(2) assimilation, each of which peaks around the middle of the day. The rhythmic opening and closing of stomata confers a rhythm in sensitivity and resistance, respectively, to the toxic gas sulfur dioxide. Using this physiological assay as a basis for a mutant screen, we isolated mutants with defects in circadian timing. Here, we characterize one mutant, out of phase 1 (oop1), with the circadian phenotype of altered phase. That is, the timing of the peak (acrophase) of multiple circadian rhythms (leaf movement, CO(2) assimilation, and LIGHT-HARVESTING CHLOROPHYLL a/b-BINDING PROTEIN transcription) is early with respect to wild type, although all circadian rhythms retain normal period length. This is the first such mutant to be characterized in Arabidopsis. oop1 also displays a strong photoperception defect in red light characteristic of phytochrome B (phyB) mutants. The oop1 mutation is a nonsense mutation of PHYB that results in a truncated protein of 904 amino acids. The defect in circadian phasing is seen in seedlings entrained by a light-dark cycle but not in seedlings entrained by a temperature cycle. Thus, PHYB contributes light information critical for proper determination of circadian phase.

Tales from the crypt(ochromes)

Cryptochromes are a family of flavoproteins found in organisms ranging from Arabidopsis to man. Across phylogeny, these proteins have been used for pleiotropic functions ranging from blue-light-dependent development in plants and blue-light-mediated phase shifting of the circadian clock in insects to a core circadian clock component in mammals. Review of the roles of cryptochromes in model organisms reveals several common themes: Multiple cryptochrome family members within individual organisms have redundant functions; cryptochromes used in photic entrainment pathways of the circadian clock are partially redundant with other photopigments; and cryptochromes may function in circadian phototransduction and core clock mechanisms in the same organism, with different functions in different tissues. The present review summarizes recent research on the functions of cryptochrome in the circadian timekeeping and photic entrainment pathways.

The Arabidopsis circadian system

Rhythms with periods of approximately 24 hr are widespread in nature. Those that persist in constant conditions are termed circadian rhythms and reflect the activity of an endogenous biological clock. Plants, including Arabidopsis, are richly rhythmic. Expression analysis, most recently on a genomic scale, indicates that the Arabidopsis circadian clock regulates a number of key metabolic pathways and stress responses. A number of sensitive and high-throughput assays have been developed to monitor the Arabidopsis clock. These assays have facilitated the identification of components of plant circadian systems through genetic and molecular biological studies. Although much remains to be learned, the framework of the Arabidopsis circadian system is coming into focus.DedicationThis review is dedicated to the memory of DeLill Nasser, a wonderful mentor and an unwavering advocate of both Arabidopsis and circadian rhythms research.

Identification of a light-regulated protein kinase activity from seedlings of Arabidopsis thaliana

Protein kinase transduction pathways are thought to be involved in light signaling in plants, but other than the photoreceptors, no protein kinase activity has been shown to be light-regulated in vivo. Using an in-gel protein kinase assay technique with histone H III SS as an exogenous substrate, we identified a light-regulated protein kinase activity with an apparent molecular weight ca 50 kDa. The kinase activity increased transiently after irradiation of dark-grown seedlings with continuous far red light (FR) and blue light (B) and decreased after irradiation with red light (R). The maximal activation was achieved after 30 min to 1 h with FR or B. After irradiation times longer than 2 h, the kinase activity decreased to below the sensitivity level of the assay. In Arabidopsis mutants lacking either the photoreceptors phytochrome A, phytochrome B or the blue-light receptor cryptochrome 1, kinase activity was undetectable, whereas in the photomorphogenic mutants cop1 and det1 the kinase activity was also observed in the absence of light signals, though still stimulated by B and FR. Interestingly, the R inhibition of the kinase activity was lost in the mutant hy5. Pretreatment with cycloheximide blocked the kinase activity.

Conditional circadian regulation of PHYTOCHROME A gene expression

The phytochrome photoreceptors and the circadian clock control many of the same developmental processes, in all organs and throughout the growth of Arabidopsis plants. Phytochrome A (phyA) provides light input signals to entrain the circadian clock. The clock is known to rhythmically regulate its light input pathway, so we tested rhythmic regulation of phyA, using transgenic plants carrying a PHYA promoter fusion to the luciferase reporter (PHYA:LUC). We provide the first images of LUC activity with subcellular resolution in intact tissue. PHYA transcription and the accumulation of all three PHYA mRNAs were indeed clock controlled. PHYA is expressed throughout the seedling, so we tested whether circadian rhythms were observed in all PHYA-expressing organs and whether the rhythms were autonomously controlled by each organ. In contrast to our previous results using other clock controlled genes, the rhythmic pattern of PHYA expression varied markedly among isolated organs and between isolated organs and intact plants. High-amplitude rhythms were maintained for many days in isolated leaves in darkness, whereas the leaves of intact plants rapidly lost rhythmicity. Wounding the leaves of intact plants had no effect. The rhythmic pattern of PHYA expression is not organ autonomous but depends upon the physical continuity or isolation of the rhythmic tissues, consistent with the presence of a transmitted signal that controls the overt expression of circadian rhythms without necessarily affecting the underlying clock. A circadian system might be present in most, if not all, plant cells, but its effect on intracellular rhythms can be controlled by supracellular signaling.

Carbon monoxide and nitric oxide: interacting messengers in muscarinic signaling to the brain's circadian clock

Within the central nervous system, acetylcholine (ACh) functions as a state-dependent modulator at a range of sites, but its signaling mechanisms are yet unclear. Cholinergic projections from the brain stem and basal forebrain innervate the suprachiasmatic nucleus (SCN), the master circadian clock in mammals, and cholinergic stimuli adjust clock timing. Cholinergic effects on clock state require muscarinic receptor-mediated activation of guanylyl cyclase and cGMP synthesis, although the effect is indirect. Here we evaluate the roles of carbon monoxide (CO) and nitric oxide (NO), major activators of cGMP synthesis. Both heme oxygenase 2 (HO-2) and neuronal nitric oxide synthase (nNOS), enzymes that synthesize CO and NO, respectively, are expressed in rat SCN, with HO-2 localized to the central core of the SCN, whereas nNOS is a punctate plexus. Hemin, an activator of HO-2, but not the NO donor, SNAP, mimicked cholinergic effects on circadian timing. Selective inhibitors of HO fully blocked cholinergic clock resetting, whereas NOS inhibition partially attenuated this effect. Hemoglobin, an extracellular scavenger of both NO and CO, blocked cholinergic stimulation of cGMP synthesis, whereas l-NAME, a specific inhibitor of NOS, had no effect on cholinergic stimulation of cGMP, but decreased the cGMP basal level. We conclude that basal NO production generates cGMP tone that primes the clock for cholinergic signaling, whereas HO/CO transmit muscarinic receptor activation to the cGMP-signaling pathway that modulates clock state. In light of the recently reported inhibitory interaction between HO-2/CO and amyloid-beta, a marker of Alzheimer's disease (AD), we speculate that HO-2/CO signaling may be a defective component of cholinergic neurotransmission in the pathophysiology of AD, whose manifestations include disintegration of circadian timing.

EARLY FLOWERING3 encodes a novel protein that regulates circadian clock function and flowering in Arabidopsis

Higher plants use photoperiodic cues to regulate many aspects of development, including the transition from vegetative to floral development. The EARLY FLOWERING3 (ELF3) gene is required for photoperiodic flowering and normal circadian regulation in Arabidopsis. We have cloned ELF3 by positional methods and found that it encodes a novel 695-amino acid protein that may function as a transcriptional regulator. ELF3 transcript level is regulated in a circadian manner, as is expected of a zeitnehmer input pathway component. Overexpression of the LATE ELONGATED HYPOCOTYL gene, which has been proposed to function as a clock component, did not abolish circadian regulation of ELF3 transcription, providing further evidence that ELF3 is a circadian clock input pathway component.

CIRCADIAN RHYTHMS IN PLANTS

Circadian rhythms, endogenous rhythms with periods of approximately 24 h, are widespread in nature. Although plants have provided many examples of rhythmic outputs and our understanding of photoreceptors of circadian input pathways is well advanced, studies with plants have lagged in the identification of components of the central circadian oscillator. Nonetheless, genetic and molecular biological studies, primarily in Arabidopsis, have begun to identify the components of plant circadian systems at an accelerating pace. There also is accumulating evidence that plants and other organisms house multiple circadian clocks both in different tissues and, quite probably, within individual cells, providing unanticipated complexity in circadian systems.

A clock- and light-regulated gene that links the circadian oscillator to LHCB gene expression

We have identified and characterized a novel tobacco gene, called ZGT (from the Chinese phrase zhong guang tiaokong, or clock and light controlled), that is regulated by the circadian clock and light. ZGT transcripts have alternate forms that are differentially expressed in different tissues. ZGT is expressed rhythmically in light/dark cycles and in constant light. Constitutive expression of ZGT sustains the expression of the clock-controlled LHCB1*1 gene in constant darkness, when it would normally dampen, but does not affect LHCB1*1 expression in constant light. ZGT expression is induced rapidly by light, and overexpression of ZGT increases the sensitivity of the circadian oscillator to brief light pulses. The ZGT promoter includes a G-box motif that is found in many light-regulated promoters in plants and is the same as the E box described for rhythmically regulated promoters of animal circadian clock genes. The ZGT promoter also includes "evening element" motifs that are correlated with circadian control of plant genes. We postulate that light- and clock-regulated expression of ZGT acts as a coupling agent between the central circadian oscillator and rhythmic LHCB1*1 expression and that it may function as a component in plant phototransduction pathways.

Phytochrome A mediates blue light and UV-A-dependent chloroplast gene transcription in green leaves

We characterized the photobiology of light-activated chloroplast transcription and transcript abundance in mature primary leaves by using the following two systems: transplastomic promoter-reporter gene fusions in tobacco (Nicotiana tabacum), and phytochrome (phyA, phyB, and hy2) and cryptochrome (cry1) mutants of Arabidopsis. In both dicots, blue light and UV-A radiation were the major signals that activated total chloroplast and psbA, rbcL, and 16S rrn transcription. In contrast, transcription activities in plants exposed to red and far-red light were 30% to 85% less than in blue light/UV-A, depending on the gene and plant species. Total chloroplast, psbA, and 16S rrn transcription were 60% to 80% less in the Arabidopsis phyA mutant exposed to blue light/UV-A relative to wild type, thus definitively linking phyA signaling to these photoresponses. To our knowledge, the major role of phyA in mediating the blue light/UV-A photoresponses is a new function for phyA in chloroplast biogenesis at this stage of leaf development. Although rbcL expression in plants exposed to UV-A was 50% less in the phyA mutant relative to wild type, blue light-induced rbcL expression was not significantly affected in the phyA, phyB, and cry1 mutants. However, rbcL expression in blue light was 60% less in the phytochrome chromophore mutant, hy2, relative to wild type, indicating that another phytochrome species (phyC, D, or E) was involved in blue light-induced rbcL transcription. Therefore, at least two different phytochromes, as well as phytochrome-independent photosensory pathways, mediated blue light/UV-A-induced transcription of chloroplast genes in mature leaves.

Interactions within a network of phytochrome, cryptochrome and UV-B phototransduction pathways regulate chalcone synthase gene expression in Arabidopsis leaf tissue

The Arabidopsis gene encoding the key flavonoid biosynthesis enzyme chalcone synthase (CHS) is regulated by several environmental and endogenous stimuli. Here we dissect the network of light signalling pathways that control CHS expression in mature leaves using cryptochrome (cry) and phytochrome (phy) deficient mutants. The UV-A/blue light induction of CHS is mediated principally by cry1, but neither cry1 nor cry2 is involved in UV-B induction or in the UV-A and blue light signalling pathways that interact synergistically with the UV-B pathway to enhance CHS expression. Moreover, these synergistic responses do not require phyA or phyB. Phytochrome is a positive regulator of the cry1 inductive pathway, mediating distinct potentiation and coaction effects. A red light pretreatment enhances subsequent cry1-mediated CHS induction. This potentiation is unaltered in phyA and phyB mutants but much reduced in a phyA phyB double mutant, indicating that it requires principally phyA or phyB. In contrast, the cry1-mediated induction of CHS, without pretreatment, is much reduced in phyB but not phyA mutants, indicating coaction between cry1 and phyB. Further experiments with phy-deficient mutants demonstrate that phyB is a negative regulator of the UV-B inductive pathway. We further show that phyB acts upstream of the points of interaction of the UV-A and blue synergism pathways with the UV-B pathway. We propose that phyB functions to balance flux through the cry1 and UV-B signalling pathways.

LKP1 (LOV kelch protein 1): a factor involved in the regulation of flowering time in arabidopsis

In plants, light is not only an energy source but also a very important signal that modulates development and differentiation. Here, we report a putative photo-regulatory factor sequence in LKP1 (LOV kelch protein 1). LKP1 cDNA encodes a protein of 610 amino acids and with a molecular weight of 65 905 with an LOV domain and kelch repeats. LOV domains are present in a number of sensor proteins involved in the detection of light, oxygen or voltage. The LKP1 LOV is very similar to the LOV domains in NPH1, a plasma membrane-associated blue light receptor kinase that regulates phototropism (Huala, E., Oeller, P.W., Liscum, E., Han, I-S., Larsen, E. & Briggs, W.R. (1997) Science, 278, 2120-2123). LKP1 mRNA accumulates in roots, stems, flowers and siliques. It is most abundant in leaves, and least abundant in seeds. Transgenic plants with a beta-glucuronidase (GUS) reporter gene driven by a 1.5 kb LKP1 promoter display strong GUS activity in leaves. Transgenic plants with a 35S:LKP1 cDNA gene overexpress LKP1 mRNA. These plants have elongated hypocotyls and petioles with elongated cells, and exhibit distinct cotyledon movement during the day. Expression of 35S:LKP1 in transgenic Arabidopsis promotes late flowering in plants grown under long-day, but not under short-day conditions. Vernalization does not affect the late flowering phenotype of the 35S:LKP1 plants. Transgenic plants possessing the 35S:GFP-LKP1 construct also have long hypocotyles and petioles, and a late flowering phenotype, suggesting that the GFP-LKP1 fusion protein is active. The GFP-associated fluorescence in 35S:GFP-LKP1 plants is observed in nuclei and cytosol, indicating that LKP1 is a new nucleo-cytoplasmic factor that influences flowering time in the long day pathway of Arabidopsis.

Plant blue-light receptors

Plants have several blue-light receptors, which regulate different aspects of growth and development. Recent studies have identified three such receptors: cryptochrome 1, cryptochrome 2 and phototropin. Cryptochromes 1 and 2 are photolyase-like receptors that regulate hypocotyl growth and flowering time; phototropin mediates phototropism in response to blue light. In addition, phytochrome A has also been found to mediate various blue-light responses. Although the signal-transduction mechanisms of blue-light receptors remain largely unclear, phototropin is probably a protein kinase that regulates cytoplasmic calcium concentrations, whereas the cryptochromes might regulate anion-channel activity and changes in gene expression.

Modulation by phytochrome of the blue light-induced extracellular acidification by leaf epidermal cells of pea (Pisum sativum l.): a kinetic analysis

Blue light induces extracellular acidification, a prerequisite of cell expansion, in epidermis cells of young pea leaves, by stimulation of the proton pumping-ATPase activity in the plasma membrane. A transient acidification, reaching a maximum 2.5-5 min after the start of the pulse, could be induced by pulses as short as 30 msec. A pulse of more than 3000 micromol m-2 saturated this response. Responsiveness to a second light pulse was recovered with a time constant of about 7 min. The fluence rate-dependent lag time and sigmoidal increase of the acidification suggested the involvement of several reactions between light perception and activation of the ATPase. In wild-type pea plants, the fluence response relation for short light pulses was biphasic, with a component that saturates at low fluence and one that saturates at high fluence. The phytochrome-deficient mutant pcd2 showed a selective loss of the high-fluence component, suggesting that the high-fluence component is phytochrome-dependent and the low-fluence component is phytochrome-independent. Treatment with the calmodulin inhibitor W7 also led to the elimination of the phytochrome-dependent high-fluence component. Simple models adapted from the one used to simulate blue light-induced guard cell opening failed to explain one or more elements of the experimental data. The hypothesis that phytochrome and a blue light receptor interact in a short-term photoresponse is endorsed by model calculations based upon a three-step signal transduction cascade, of which one component can be modulated by phytochrome.

Integrated temporal regulation of the photorespiratory pathway. Circadian regulation of two Arabidopsis genes encoding serine hydroxymethyltransferase

The photorespiratory pathway is comprised of enzymes localized within three distinct cellular compartments: chloroplasts, peroxisomes, and mitochondria. Photorespiratory enzymes are encoded by nuclear genes, translated in the cytosol, and targeted into these distinct subcellular compartments. One likely means by which to regulate the expression of the genes encoding photorespiratory enzymes is coordinated temporal control. We have previously shown in Arabidopsis that a circadian clock regulates the expression of the nuclear genes encoding both chloroplastic (Rubisco small subunit and Rubisco activase) and peroxisomal (catalase) components of the photorespiratory pathway. To determine whether a circadian clock also regulates the expression of genes encoding mitochondrial components of the photorespiratory pathway, we characterized a family of Arabidopsis serine hydroxymethyltransferase (SHM) genes. We examined mRNA accumulation for two of these family members, including one probable photorespiratory gene (SHM1) and a second gene expressed maximally in roots (SHM4), and show that both exhibit circadian oscillations in mRNA abundance that are in phase with those described for other photorespiratory genes. In addition, we show that SHM1 mRNA accumulates in light-grown seedlings, although this response is probably an indirect consequence of the induction of photosynthesis and photorespiration by illumination.