Circadian clock component,LHY, tells a plant when to respond photosynthetically to light in nature

The circadian clock is known to increase plant growth and fitness, and is thought to prepare plants for photosynthesis at dawn and dusk; whether this happens in nature was unknown. We transformed the native tobacco, Nicotiana attenuata to silence two core clock components, NaLHY (irLHY) and NaTOC1 (irTOC1). We characterized growth and light‐ and dark‐adapted photosynthetic rates (A_c) throughout a 24 h day in empty vector‐transformed (EV), irLHY, and irTOC1 plants in the field, and in NaPhyA‐ and NaPhyB1‐silenced plants in the glasshouse. The growth rates of irLHY plants were lower than those of EV plants in the field. While irLHY plants reduced A_c earlier at dusk, no differences between irLHY and EV plants were observed at dawn in the field. irLHY, but not EV plants, responded to light in the night by rapidly increasing A_c. Under controlled conditions, EV plants rapidly increased A_c in the day compared to dark‐adapted plants at night; irLHY plants lost these time‐dependent responses. The role of NaLHY in gating photosynthesis is independent of the light‐dependent reactions and red light perceived by NaPhyA, but not NaPhyB1. In summary, the circadian clock allows plants not to respond photosynthetically to light at night by anticipating and gating red light‐mediated in native tobacco.     

Dawn signal as a rhythmical timer for the seasonal adaptive variation of CAM: a model

Abstract Photoperiodism, known to control the level of CAM in different types of CAM plants (e.g. Kalanchoe blossfeldiana, Bryophyllum daigremontianum), would act as a reliable timer for seasonal coherent adaptation. Two different endogenous rhythms (malic enzyme activity, PEP carboxylase capacity) appear to be separately coupled to dawn and dusk, respectively, thus achieving time-compartmentation of CAM; this feature suggests involvement of an ‘internal coincidence’ type of clock mechanism. Persistence in continuous darkness of the rhythm of malic enzyme activity (measured by label incorporation into pyruvate or by CO₂ output) establishes its endogenous character and shows that light is not necessary for malate decarboxylation. The role of the dawn signal would be to entrain the CAM system, i.e. to phase the endogenous rhythm of malic enzyme activity correctly to local time. The possibility of an effect of the phase of the PEP carboxylase rhythm on the phase of the decarboxylation rhythm is ruled out by the presence of the intermediary malate storage step. Phase responses to red and to far-red show that phytochrome is involved in rephasing the rhythm of malic enzyme activity. The relative position of dusk affords a ‘measurement’ of the season by the CAM system entrained by dawn. According to the dusk-dawn interval, the level of PEP carboxylase capacity (amount of active enzyme) is modified, resulting in changes of CAM level (high activity under short days).     

Circadian regulation of teleost retinal cone movements in vitro

In the retinas of many species of lower vertebrates, retinal photoreceptors and pigment epithelium pigment granules undergo daily movements in response to both diurnal, and in the case of teleost cone photoreceptors, endogenous circadian signals. Typically, these cone movements take place at dawn and at dusk when teleosts are maintained on a cyclic light (LD) regime, and at expected dawn and expected dusk when animals are maintained in continuous darkness (DD). Because these movements are so strictly controlled, they provide an overt indicator of the stage of the underlying clock mechanism. In this study we report that both light-induced and circadian-driven cone myoid movements in the Midas cichlid (Cichlasoma citrinellum), occur normally in vitro. Many of the features of retinomotor movements found in vivo also occur in our culture conditions, including responses to light and circadian stimuli and dopamine. Circadian induced predawn contraction and maintenance of expected day position in response to circadian modulation, are also normal. Our studies suggest that circadian regulation of cone myoid movement in vitro is mediated locally by dopamine, acting via a D2 receptor. Cone myoid contraction can be induced at midnight and expected mid-day by dark culture with dopamine or the D2 receptor agonist LY171555. Further, circadian induced predawn contraction can be increased with either dopamine or LY171555, or may be reversed with the dopamine D2 antagonist, sulpiride. Sulpiride will also induce cone myoid elongation in retinal cultures at expected mid- day, but will not induce cone myoid elongation at dusk. In contrast, circadian cone myoid movements in vitro were unaffected by the D1 receptor agonist SCH23390, or the D1 receptor antagonist SKF38393. Our short-term culture experiments indicate that circadian regulation of immediate cone myoid movement does not require humoral control but is regulated locally within the retina. The inclusion of dopamine, or dopamine receptor agonists and antagonists in our cultures, has indicated that retinal circadian regulation may be mediated by endogenously produced dopamine, which acts via a D2 mechanism.     

Effects of light pulses on the locomotor activity rhythm of Orchestia montagui (Amphipoda,Talitridae)

Animals of the amphipod Orchestia montagui are kept in constant darkness with two short light pulses. One pulse is applied at the beginning of subjective night (around the dusk) and the other one at the end of subjective night (around the dawn). The pulse duration is estimated in the order of one or two hours around the dusk as well as the dawn. The locomotor activity rhythm was monitored in individual animals in summer under constant temperature. Results revealed that whatever the experimental conditions, under continuous or interrupted darkness by pulses, two endogenous components have been highlighted. In fact, Periodogram analysis showed the presence of ultradian and circadian periods around 12 and 24 h, respectively. The shortest circadian period and the most important inter-individual variability was observed under pulse of 2 h around the dusk with mean value equal to τ_DD+pulse = 24h38′ ± 4h34′. The activity profiles are in majority unimodal. Moreover, the most activity peak showed a slipping of its location from the middle of subjective night under constant darkness to the middle of subjective day under pulse. Globally, the locomotor activity rhythm of O. montagui was better defined under pulses and specimens were significantly more active under continuous darkness. Moreover, a great variability around the activity time was observed especially with pulse of 1 h.     

Night length measurements by the circadian clock controlling diapause induction in the mosquito Aedes atropalpus

Night interruption experiments were used to investigate the behavior of the clock controlling diapause induction in the mosquito, Aedes atropalpus. The data from these experiments indicated that the clock included a circadian oscillator which was phase set at dusk. Following this event the oscillator proceeded to drive a nightly rhythm of sensitivity to light. This rhythm included a photoinducible phase where light interruption inhibited diapause. The photoinducible phase was fixed, occurring 7 to 9 hr after dusk in all photoperiod regimens tested. The photoinducible phase was followed by a refractory phase, which continued until dawn. During the refractory period light did not inhibit diapause. These observations indicated that the circadian clock behaved like an interval timer which was set at dusk. The rhythm of sensitivity to light, an inherited time scale, limited the induction of diapause to seasonal periods when nights were longer than 9 hr. As a result, diapause was induced only when the daylength dropped below the critical photoperiod of L15:D9 (hours of light:hours of dark).A ‘T’ experimental design was used to confirm the importance of the length of the night in clock controlled induction of diapause in this mosquito.     

The effect of dusk and dawn on the locomotor activity rhythm of Talitrus saltator (Montagu) (Crustacea: Amphipoda)

Entrainment of the nocturnal, endogenous locomotor activity rhythm of Talitrus saltator (Montagu) by the natural light-dark cycle is non-parametric, the phase of the rhythm shifting only in response to changes in the time of an experimentally simulated ‘dawn’ transition. The difference in response to light at ‘dusk’ and ‘dawn’ illustrates a phase-dependent responsiveness common to endogenous rhythms. Activity begins after complete darkness, with cessation always occurring during the ‘dawn’ transition and never continuing past the onset of total experimental illumination. The point of activity cessation is taken to be the position of a synchronizing cue controlling entrainment. The ‘dawn’ cue appears to be an absolute irradiance level of approximately 1.5 × 10^?4 W/m² (1.5 lux). The implications of such a cue are discussed in relation to field behaviour.     

Temporal Links in Daily Activity Patterns between Coral Reef Predators and Their Prey

Few studies have documented the activity patterns of both predators and their common prey over 24 h diel cycles. This study documents the temporal periodicity of two common resident predators of juvenile reef fishes, Cephalopholis cyanostigma (rockcod) and Pseudochromis fuscus (dottyback) and compares these to the activity and foraging pattern of a common prey species, juvenile Pomacentrus moluccensis (lemon damselfish). Detailed observations of activity in the field and using 24 h infrared video in the laboratory revealed that the two predators had very different activity patterns. C. cyanostigma was active over the whole 24 h period, with a peak in feeding strikes at dusk and increased activity at both dawn and dusk, while P. fuscus was not active at night and had its highest strike rates at midday. The activity and foraging pattern of P. moluccensis directly opposes that of C. cyanostigma with individuals reducing strike rate and intraspecific aggression at both dawn and dusk, and reducing distance from shelter and boldness at dusk only. Juveniles examined were just outside the size-selection window of P. fuscus. We suggest that the relatively predictable diel behaviour of coral reef predators results from physiological factors such as visual sensory abilities, circadian rhythmicity, variation in hunting profitability, and predation risk at different times of the day. Our study suggests that the diel periodicity of P. moluccensis behaviour may represent a response to increased predation risk at times when both the ability to efficiently capture food and visually detect predators is reduced.     

GIGANTEA and EARLY FLOWERING 4 in Arabidopsis exhibit differential phase-specific genetic influences over a diurnal cycle

The endogenous circadian clock regulates many physiological processes related to plant survival and adaptability. GIGANTEA (GI), a clock-associated protein, contributes to the maintenance of circadian period length and amplitude, and also regulates flowering time and hypocotyl growth in response to day length. Similarly, EARLY FLOWERING 4 (ELF4), another clock regulator, also contributes to these processes. However, little is known about either the genetic or molecular interactions between GI and ELF4 in Arabidopsis. In this study, we investigated the genetic interactions between GI and ELF4 in the regulation of circadian clock-controlled outputs. Our mutant analysis shows that GI is epistatic to ELF4 in flowering time determination, while ELF4 is epistatic to GI in hypocotyl growth regulation. Moreover, GI and ELF4 have a synergistic or additive effect on endogenous clock regulation. Gene expression profiling of gi, elf4, and gi elf4 mutants further established that GI and ELF4 have differentially dominant influences on circadian physiological outputs at dusk and dawn, respectively. This phasing of GI and ELF4 influences provides a potential means to achieve diversity in the regulation of circadian physiological outputs, including flowering time and hypocotyl growth.     

PHASE ADVANCE AFTER ONE OR THREE SIMULATED DAWNS IN HUMANS

A specially designed apparatus that can simulate the waveform of the dawn or dusk signal at any latitude and any day of the year has been shown to phase shift the circadian pacemaker in rodents and primates at a fraction of the illuminance previously used. Until recently, it was considered that rather high illuminances or rather long exposure episodes to room light were necessary to phase shift human circadian rhythms. This experiment shows that, under controlled conditions of a modified constant routine protocol, a single dawn signal is sufficient to phase advance the timing of the onset of secretion of the pineal hormone melatonin. The significant phase advance of salivary melatonin of 20 minutes, which is enhanced to 34 minutes after three consecutive dawn signals, is small, but appears to be of sufficient magnitude to entrain the human circadian pacemaker, which has an endogenous period of about 24.2h. (Chronobiology International, 17(5), 659–668, 2000)     

Activation of MT(2) melatonin receptors in rat suprachiasmatic nucleus phase advances the circadian clock

The aim of this study was to identify the melatonin receptor type(s) (MT(1) or MT(2)) mediating circadian clock resetting by melatonin in the mammalian suprachiasmatic nucleus (SCN). Quantitative receptor autoradiography with 2-[(125)I]iodomelatonin and in situ hybridization histochemistry, with either (33)P- or digoxigenin-labeled antisense MT(1) and MT(2) melatonin receptor mRNA oligonucleotide probes, revealed specific expression of both melatonin receptor types in the SCN of inbred Long-Evans rats. The melatonin receptor type mediating phase advances of the circadian rhythm of neuronal firing rate in the SCN slice was assessed using competitive melatonin receptor antagonists, the MT(1)/MT(2) nonselective luzindole and the MT(2)-selective 4-phenyl-2-propionamidotetraline (4P-PDOT). Luzindole and 4P-PDOT (1 nM-1 microM) did not affect circadian phase on their own; however, they blocked both the phase advances (approximately 4 h) in the neuronal firing rate induced by melatonin (3 pM) at temporally distinct times of day [i.e., subjective dusk, circadian time (CT) 10; and dawn, CT 23], as well as the associated increases in protein kinase C activity. We conclude that melatonin mediates phase advances of the SCN circadian clock at both dusk and dawn via activation of MT(2) melatonin receptor signaling.     

Relationships Between Behavioral Rhythms,Plasma Corticosterone and Hypothalamic Circadian Rhythms

Circadian rhythms in physiological processes and behaviors were compared with hypothalamic circadian rhythms in norepinephrine (NE) metabolites, adrenergic transmitter receptors, cAMP, cGMP and suprachiasmatic nucleus (SCN) arginine vasopressin (AVP) in a single population of rats under D: D conditions. Eating, drinking and locomotor activity were high during the subjective night (the time when lights were out in L: D) and low during the subjective day (the time when lights were on in L: D). Plasma corticosterone concentration rose at subjective dusk and remained high until subjective dawn. Binding to hypothalamic α₁- and β-adrenergic receptors also peaked during the subjective night. Cyclic cGMP concentration was elevated throughout the 24-hr period except for a trough at dusk, whereas DHPG concentration peaked at dawn. Arginine vasopressin levels in the suprachiasmatic nucleus peaked in the middle of the day. No rhythm was found either in binding to the α₂-adrenergic receptor, or in MHPG or cAMP concentration. Behavioral and corticosterone rhythms, therefore, are parallel to rhythms in hypothalamic α₁-and β-receptor binding and NE-release. Cyclic GMP falls only at dusk, suggesting the possibility that cGMP inhibits activity much of the day and that at dusk the inhibition of nocturnal activity is removed. SCN AVP, on the other hand, peaking at 1400 hr, may play a role in the pacemaking function of the SCN that drives these other rhythms.     

Age affects photoentrainment in a nocturnal primate

The endogenous circadian clock is entrained by external cues, mainly the light-dark cycle received by photopigments located in the retina. The authors investigated (1) the effect of aging on the synchronization of the rest-activity rhythm and (2) the physiological basis of light photoreception in the gray mouse lemur, a nocturnal Malagasy primate. Old individuals were tested at different irradiance levels under 3 different light wavelengths previously shown to trigger maximal response in young adults. Investigators analyzed animal activity and temperature waveforms and used 2 reference treatments, strong white light and continuous darkness. The investigation revealed manifold effects of aging picturing a general loss of responsiveness to light and age-related changes in activity and temperature. In addition, the investigation shows that photoentrainment is a continuous process. Short wavelengths (430 nm) are more efficient than longer wavelengths (470-540 nm) at low light levels at dawn and across all light levels at dusk. This suggests an implication of S-cones, differential at dawn and dusk. This results, surprising for several reasons, calls for further investigation. The study brings an interesting contribution to the understanding of the physiological processes underlying synchronization to light.     

The correlation between crassulacean acid metabolism and water uptake in Senecio medley-woodii

The combination of a chamber for CO₂ gas exchange with a potometric measuring arrangement allowed concomitant investigations into CO₂ gas exchange, transpiration and water uptake by the roots of whole plants of Senecio medley-woodii, a species which exhibits Crassulacean acid metabolism. The water-uptake rate showed the same daily pattern as malate concentration and osmotic potential. The accumulation of organic acids resulting from nocturnal CO₂ fixation enhanced the water-uptake rate from dusk to dawn. During the day the water-uptake rates decreased with decreasing organic-acid concentration. With gradually increasing water stress, CO₂ dark fixation of S. medley-woodii was increased as long as water could be taken up by the roots. It was also shown that a reestablished water supply after drought caused a similar increase which in both cases ameliorated the water uptake in order to conserve a positive water balance for as long as possible. This water-uptake pattern shows that Crassulacean acid metabolism is not only a water-saving adaptation but also enhances water uptake and is directly correlated with the amelioration of the plant water status.Abbreviation CAM Crassulacean acid metabolism     

Evidence for ‘dawn’ and ‘dusk’ oscillators in the Nasonia photoperiodic clock

Females of Nasonia vitripennis were maintained in light cycles from 12 to 72 hr in length, with 4 to 28 hr photoperiods, and their offspring examined for larval diapause. This ‘resonance’ technique revealed periodic maxima of diapause induction, about 24 hr apart. The ‘ascending slopes’ of these maxima appeared to obtain their principal time cue from dusk and the ‘descending slopes’ from dawn. This suggests that two independent—dawn and dusk—oscillators are involved in the Nasonia photoperiodic clock. The results are interpreted in terms of ‘internal coincidence’.N. vitripennis was shown to be able to distinguish between 12 and 18 hr of red light (>600 nm) in the photoperiodic sense. A ‘positive’ resonance experiment using such a red light was also performed. This shows that the spectral sensitivity of the pigment coupling the circadian system to the environmental light cycle extends into the red end of the spectrum.