Seasonal Changes In Day Length Research Articles

S.09.03 Serotonergic parameters in the treatment of FTD

Humans retain neurobiological responses to circadian day-night cycles and seasonal changes in daylength in spite of a life-style usually independent of dawn-dusk signals. Seasonality has been documented in many functions, from mood to hormones to gene expression. Research on seasonal affective disorder initiated the first use of timed bright light as therapy, a treatment since extended to non-seasonal major depression and sleep-wake cycle disturbances in many psychiatric and medical illnesses. The growing recognition that sufficient light is important for psychological and somatic well-being is leading to the development of novel lighting solutions in architecture as well as focus on a more conscious exposure to natural daylight.

The Soybean-Specific Maturity Gene E1 Family of Floral Repressors Controls Night-Break Responses through Down-Regulation of FLOWERING LOCUS T Orthologs.

Photoperiodism is a rhythmic change of sensitivity to light, which helps plants to adjust flowering time according to seasonal changes in daylength and to adapt to growing conditions at various latitudes. To reveal the molecular basis of photoperiodism in soybean (Glycine max), a facultative short-day plant, we analyzed the transcriptional profiles of the maturity gene E1 family and two FLOWERING LOCUS T (FT) orthologs (FT2a and FT5a). E1, a repressor for FT2a and FT5a, and its two homologs, E1-like-a (E1La) and E1Lb, exhibited two peaks of expression in long days. Using two different approaches (experiments with transition between light and dark phases and night-break experiments), we revealed that the E1 family genes were expressed only during light periods and that their induction after dawn in long days required a period of light before dusk the previous day. In the cultivar Toyomusume, which lacks the E1 gene, virus-induced silencing of E1La and E1Lb up-regulated the expression of FT2a and FT5a and led to early flowering. Therefore, E1, E1La, and E1Lb function similarly in flowering. Regulation of E1 and E1L expression by light was under the control of E3 and E4, which encode phytochrome A proteins. Our data suggest that phytochrome A-mediated transcriptional induction of E1 and its homologs by light plays a critical role in photoperiodic induction of flowering in soybean.

Editorial: "Thyroid Hormone in Brain and Brain Cells".

Editorial: "Thyroid Hormone in Brain and Brain Cells".

Existence of a photoinducible phase for ovarian development and photoperiod-related alteration of clock gene expression in a damselfish

The sapphire devil, Chrysiptera cyanea, is a reef-associated damselfish and their ovarian development can be induced by a long photoperiod. In this study, we demonstrated the existence of a photoinducible phase for the photoperiodic ovarian development in the sapphire devil. Induction of ovarian development under night-interruption light schedules and Nanda–Hamner cycles revealed that the photoinducible phase appeared in a circadian manner between ZT12 and ZT13. To characterize the effect of photoperiod on clock gene expression in the brain of this species, we determined the expression levels of the sdPer1, sdPer2, sdCry1, and sdCry2 clock genes under constant light and dark conditions (LL and DD) and photoperiodic (short and long photoperiods). The expression of sdPer1 exhibited clear circadian oscillation under both LL and DD conditions, while sdPer2 and sdCry1 expression levels were lower under DD than under LL conditions and sdCry2 expression was lower under LL than under DD conditions. These results suggest a key role for sdPer1 in circadian clock cycling and that sdPer2, sdCry1, and sdCry2 are light-responsive clock genes in the sapphire devil. After 1week under a long photoperiod, we observed photoperiod-related changes in sdPer1, sdPer2, and sdCry2 expression, but not in sdCry1 expression. These results suggest that the expression patterns of some clock genes exhibit seasonal variation according to seasonal changes in day length and that such seasonal alteration of clock gene expression may contribute to seasonal recognition by the sapphire devil.

Evolution of CONSTANS Regulation and Function after Gene Duplication Produced a Photoperiodic Flowering Switch in the Brassicaceae.

Environmental control of flowering allows plant reproduction to occur under optimal conditions and facilitates adaptation to different locations. At high latitude, flowering of many plants is controlled by seasonal changes in day length. The photoperiodic flowering pathway confers this response in the Brassicaceae, which colonized temperate latitudes after divergence from the Cleomaceae, their subtropical sister family. The CONSTANS (CO) transcription factor of Arabidopsis thaliana, a member of the Brassicaceae, is central to the photoperiodic flowering response and shows characteristic patterns of transcription required for day-length sensing. CO is believed to be widely conserved among flowering plants; however, we show that it arose after gene duplication at the root of the Brassicaceae followed by divergence of transcriptional regulation and protein function. CO has two close homologs, CONSTANS-LIKE1 (COL1) and COL2, which are related to CO by tandem duplication and whole-genome duplication, respectively. The single CO homolog present in the Cleomaceae shows transcriptional and functional features similar to those of COL1 and COL2, suggesting that these were ancestral. We detect cis-regulatory and codon changes characteristic of CO and use transgenic assays to demonstrate their significance in the day-length-dependent activation of the CO target gene FLOWERING LOCUS T. Thus, the function of CO as a potent photoperiodic flowering switch evolved in the Brassicaceae after gene duplication. The origin of CO may have contributed to the range expansion of the Brassicaceae and suggests that in other families CO genes involved in photoperiodic flowering arose by convergent evolution.

Enhanced Phase Resetting in the Synchronized Suprachiasmatic Nucleus Network

The suprachiasmatic nucleus (SCN) adapts to both the external light-dark (LD) cycle and seasonal changes in day length. In short photoperiods, single-cell activity patterns are tightly synchronized (i.e., in phase); in long photoperiods, these patterns are relatively dispersed, causing lower amplitude rhythms. The limit cycle oscillator has been used to describe the SCN's circadian rhythmicity and predicts that following a given perturbation, high-amplitude SCN rhythms will shift less than low-amplitude rhythms. Some studies reported, however, that phase delays are larger when animals are entrained to a short photoperiod. Because phase advances and delays are mediated by partially distinct (i.e., nonoverlapping) biochemical pathways, we investigated the effect of a 4-h phase advance of the LD cycle in mice housed in either short (LD 8:16) or long (LD 16:8) photoperiods. In vitro recordings revealed a significantly larger phase advance in the SCN of mice entrained to short as compared to long photoperiods (4.2 ± 0.3 h v. 1.4 ± 0.9 h, respectively). Surprisingly, in mice with long photoperiods, the behavioral phase shift was larger than the phase shift of the SCN (3.7 ± 0.4 h v. 1.4 ± 0.9 h, respectively). To exclude a confounding influence of running-wheel activity on the magnitude of the shifts of the SCN, we repeated the experiments in the absence of running wheels and found similar shifts in the SCN in vitro in short and long days (3.0 ± 0.5 h v. 0.4 ± 0.9 h, respectively). Interestingly, removal of the running wheel reduced the phase-shifting capacity of mice in long days, leading to similar behavioral shifts in short and long photoperiods (1.0 ± 0.1 h v. 1.0 ± 0.4 h). As the behavioral shifts in the presence of wheels were larger than the shift of the SCN, it is suggested that additional, non-SCN neuronal networks in the brain are involved in regulating the timing of behavioral activity. On the basis of the phase shifts observed in vitro, we conclude that highly synchronized SCN networks with high-amplitude rhythms show a larger phase-shifting capacity than desynchronized networks of low amplitude.

The effects of the photoperiod-insensitive alleles, se13, hd1 and ghd7, on yield components in rice

Flowering time is closely associated with grain yield in rice (Oryza sativa L.). In temperate regions, seasonal changes in day length (known as the photoperiod) are an important environmental cue for floral initiation. The timing of flowering is important not only for successful reproduction, but also for determining the ideal balance between vegetative growth and reproductive growth duration. Recent molecular genetics studies have revealed key flowering time genes responsible for photoperiod sensitivity. In this study, we investigated the effect of three recessive photoperiod-insensitive alleles, se13, hd1 and ghd7, on yield components in rice under Ehd1-deficient genetic background conditions to ensure vegetative growth of each line. We found that se13-bearing plants had fewer panicles, hd1-bearing plants showed decreased grain-filling percentage, and ghd7-bearing plants appeared to have fewer grains per panicle and fewer secondary branches. Our results indicate that the pleiotropic effects of photoperiod-insensitive genes on yield components are independent of short vegetative growth. This will provide critical information which can be used to create photoperiod-insensitive varieties that can be adapted to a wide range of latitudes.

Endocrine rhythms in the brown bear (Ursus arctos): Evidence supporting selection for decreased pineal gland size.

Many temperate zone animals adapt to seasonal changes by altering their physiology. This is mediated in large part by endocrine signals that encode day length and regulate energy balance and metabolism. The objectives of this study were to determine if the daily patterns of two important hormones, melatonin and cortisol, varied with day length in captive brown bears (Ursus arctos) under anesthetized and nonanesthetized conditions during the active (March–October) and hibernation periods. Melatonin concentrations varied with time of day and season in nonanesthetized female bears despite exceedingly low nocturnal concentrations (1–4 pg/mL) in the active season. In contrast, melatonin concentrations during hibernation were 7.5-fold greater than those during the summer in anesthetized male bears. Functional assessment of the pineal gland revealed a slight but significant reduction in melatonin following nocturnal light application during hibernation, but no response to beta-adrenergic stimulation was detected in either season. Examination of pineal size in two bear species bears combined with a phylogenetically corrected analysis of pineal glands in 47 other species revealed a strong relationship to brain size. However, pineal gland size of both bear species deviated significantly from the expected pattern. Robust daily plasma cortisol rhythms were observed during the active season but not during hibernation. Cortisol was potently suppressed following injection with a synthetic glucocorticoid. The results suggest that melatonin and cortisol both retain their ability to reflect seasonal changes in day length in brown bears. The exceptionally small pineal gland in bears may be the result of direct or indirect selection.

The Antarctic Krill Euphausia superba Shows Diurnal Cycles of Transcription under Natural Conditions

BackgroundPolar environments are characterized by extreme seasonal changes in day length, light intensity and spectrum, the extent of sea ice during the winter, and food availability. A key species of the Southern Ocean ecosystem, the Antarctic krill (Euphausia superba) has evolved rhythmic physiological and behavioral mechanisms to adapt to daily and seasonal changes. The molecular organization of the clockwork underlying these biological rhythms is, nevertheless, still only partially understood.Methodology/Principal FindingsThe genome sequence of the Antarctic krill is not yet available. A normalized cDNA library was produced and pyrosequenced in the attempt to identify large numbers of transcripts. All available E. superba sequences were then assembled to create the most complete existing oligonucleotide microarray platform with a total of 32,217 probes. Gene expression signatures of specimens collected in the Ross Sea at five different time points over a 24-hour cycle were defined, and 1,308 genes differentially expressed were identified. Of the corresponding transcripts, 609 showed a significant sinusoidal expression pattern; about 40% of these exibithed a 24-hour periodicity while the other 60% was characterized by a shorter (about 12-hour) rhythm. We assigned the differentially expressed genes to functional categories and noticed that those concerning translation, proteolysis, energy and metabolic process, redox regulation, visual transduction and stress response, which are most likely related to daily environmental changes, were significantly enriched. Two transcripts of peroxiredoxin, thought to represent the ancestral timekeeping system that evolved about 2.5 billion years ago, were also identified as were two isoforms of the EsRh1 opsin and two novel arrestin1 sequences involved in the visual transduction cascade.ConclusionsOur work represents the first characterization of the krill diurnal transcriptome under natural conditions and provides a first insight into the genetic regulation of physiological changes, which occur around the clock during an Antarctic summer day.

The saccus vasculosus of fish is a sensor of seasonal changes in day length

The pars tuberalis of the pituitary gland is the regulatory hub for seasonal reproduction in birds and mammals. Although fish also exhibit robust seasonal responses, they do not possess an anatomically distinct pars tuberalis. Here we report that the saccus vasculosus of fish is a seasonal sensor. We observe expression of key genes regulating seasonal reproduction and rhodopsin family genes in the saccus vasculosus of masu salmon. Immunohistochemical studies demonstrate that all of these genes are expressed in the coronet cells of the saccus vasculosus, suggesting the existence of a photoperiodic signalling pathway from light input to neuroendocrine output. In addition, isolated saccus vasculosus has the capacity to respond to photoperiodic signals, and its removal abolishes photoperiodic response of the gonad. Although the physiological role of the saccus vasculosus has been a mystery for several centuries, our findings indicate that the saccus vasculosus acts as a sensor of seasonal changes in day length in fish.

LNK genes integrate light and clock signaling networks at the core of the Arabidopsis oscillator

Light signaling pathways and the circadian clock interact to help organisms synchronize physiological and developmental processes with periodic environmental cycles. The plant photoreceptors responsible for clock resetting have been characterized, but signaling components that link the photoreceptors to the clock remain to be identified. Here we describe a family of night light-inducible and clock-regulated genes (LNK) that play a key role linking light regulation of gene expression to the control of daily and seasonal rhythms in Arabidopsis thaliana. A genomewide transcriptome analysis revealed that most light-induced genes respond more strongly to light during the subjective day, which is consistent with the diurnal nature of most physiological processes in plants. However, a handful of genes, including the homologous genes LNK1 and LNK2, are more strongly induced by light in the middle of the night, when the clock is most responsive to this signal. Further analysis revealed that the morning phased LNK1 and LNK2 genes control circadian rhythms, photomorphogenic responses, and photoperiodic dependent flowering, most likely by regulating a subset of clock and flowering time genes in the afternoon. LNK1 and LNK2 themselves are directly repressed by members of the TIMING OF CAB1 EXPRESSION/PSEUDO RESPONSE REGULATOR family of core-clock genes in the afternoon and early night. Thus, LNK1 and LNK2 integrate early light signals with temporal information provided by core oscillator components to control the expression of afternoon genes, allowing plants to keep track of seasonal changes in day length.

Reproductive costs for everyone: How female loads impact human mobility strategies

While mobility strategies are considered important in understanding selection pressures on individuals, testing hypotheses of such strategies requires high resolution datasets, particularly at intersections between morphology, ecology and energetics. Here we present data on interactions between morphology and energetics in regards to the cost of walking for reproductive women and place these data into a specific ecological context of time and heat load. Frontal loads (up to 16% of body mass), as during pregnancy and child-carrying, significantly slow the optimal and preferred walking speed of women, significantly increase cost at the optimal speed, and make it significantly more costly for women to walk with other people. We further show for the first time significant changes in the curvature in the Cost of Transport curve for human walking, as driven by frontal loads. The impact of these frontal loads on females, and the populations to which they belong, would have been magnified by time constraints due to seasonal changes in day length at high latitudes and thermoregulatory limitations at low latitudes. However, wider pelves increase both stride length and speed flexibility, providing a morphological offset for load-related costs. Longer lower limbs also increase stride length. Observed differences between preferred and energetically optimal speeds with frontal loading suggest that speed choices of women carrying reproductive loads might be particularly sensitive to changes in heat load. Our findings show that female reproductive costs, particularly those related to locomotion, would have meaningfully shaped the mobility strategies of the hominin lineage, as well as modern foraging populations.

Daily variation in markers of nutritional condition in wintering Black‐capped Chickadees Poecile atricapillus

The plasma metabolites triglycerides (TRIG) and β‐hydroxybutyrate (BUTY) are used as indices of nutritional condition in migrating birds during refuelling and can provide a measure of relative fattening rates in individual birds. Because non‐migratory birds wintering at northern latitudes also fatten on a daily basis to support their overnight fast, blood metabolites could provide a useful tool to measure individual performance in energy acquisition. However, daily patterns of metabolite change may differ between species and could be affected by thermoregulatory requirements. We studied daily variation in TRIG and BUTY over a complete winter in Black‐capped Chickadees to determine the pattern of daily and seasonal change in these markers. We also assessed how short‐term variation (up to 7 days) in weather parameters that influence heat exchange may affect TRIG and BUTY levels. In contrast to a linear gain of body mass, TRIG increased non‐linearly during the day, with a rapid increase in the morning that levelled off in the afternoon, whereas BUTY did not change significantly. Metabolites varied with sampling time and the seasonal change in day length, suggesting higher fat catabolism and fattening rates in mid‐winter. TRIG and BUTY also differed between capture sites, possibly due to differences in shelter quality. Weather variation did not affect TRIG levels and had a significant but marginal effect on BUTY, explaining at best 3% of the variation. Our results suggest that these markers can be used as indicators of energy turnover in resident wintering passerines.

Flowering and Fruiting Patterns in a Subtropical Rain Forest, Taiwan

AbstractReproductive patterns of tropical and temperate plants are usually associated with climatic seasonality, such as rainfall or temperature, and with their phylogeny. It is still unclear, however, whether plant reproductive phenology is influenced by climatic factors and/or phylogeny in aseasonal subtropical regions. The plant reproductive phenology of a subtropical rain forest in northern Taiwan (24°45′ N, 121°35′ E) was monitored at weekly intervals during a 7‐yr period (2002–2009). The flowering patterns of 46 taxa and fruiting patterns of 26 taxa were examined and evaluated in relation to climatic seasonality, phylogenetic constraints, and different phenophases. Our results indicated that most of the studied species reproduced annually. Additionally, both community‐wide flowering and fruiting patterns exhibited distinct annual rhythms and varied little among years. The community flowering peak matched seasonal changes in day length, temperature, and irradiance; while the community fruiting peak coincided with an increase in bird richness and the diet‐switching of resident omnivorous birds. In addition, phylogenetically closely related species tended to reproduce during the same periods of a year. Neither the mean flowering dates nor seasonal variation in solar radiation, however, was related to the fruit development times. Our results indicate the importance of abiotic, biotic, and evolutionary factors in determining the reproductive phenology in this subtropical forest.

A model of the gene network for flowering time regulation in winter wheat and barley

Transition from the vegetative to the reproductive stage of plant development depends on photoperiod and vernalization. A simple logical model of a gene network that regulates flowering time of winter wheat is suggested. In winter varieties of cereals, vernalization sensitivity is controlled by the VRN1, VRN2 and VRN3 genes. After vernalization, the VRN1 gene product suppresses the VRN2 gene, which encodes a repressor of flowering. As a result, the level of expression of VRN3 increases and stimulates a further increase in the level of transcription of the VRN1 gene. The genes form a positive-feedback loop that enhances the transcription of VRN1 to the level that is required for the initiation of flowering. Under long-day conditions, the expression of VRN3 is enhanced by the PPD1 and CO2 gene products, which determine sensitivity to photoperiod. Seasonal changes in day length are transmitted through photoreceptors to a circadian clock that modulates flowering time. Data on barley and wheat genes that control sensitivity to vernalization and photoperiod are integrated into the gene network. Using a synchronous Boolean model, the dynamics of the gene network were reproduced. An extended model of the gene network can be used to check the consistency of the experimental data with proposals of new hypotheses on the interactions of genes.

Celestial Moderation of Tropical Seabird Behavior

Most animals, including birds, have cyclic life histories and numerous studies generally conducted on captive animals have shown that photoperiod is the main factor influencing this periodicity. Moon cycles can also affect periodic behavior of birds. Few studies have investigated the influence of these environmental cues in natural settings, and particularly in tropical areas where the change in photoperiod is slight and some bird species keep cyclic behaviors. Using miniaturized light sensors, we simultaneously investigated under natural conditions the influence of photoperiod and moon phases on the migration dates and at-sea activity of a tropical seabird species, the Barau's petrel, throughout its annual cycle. Firstly, we found that birds consistently started their pre- and post-breeding migrations at precise dates corresponding in both cases to a day-duration of 12.5 hours, suggesting a strong influence of the photoperiod in the regulation of migration behavior. We also found that mean population arrival dates to the colony changed from year to year and they were influenced by moon phases. Returns at their colonies occurred around the last full moon of the austral winter, suggesting that moon cycle is used by birds to synchronize their arrival. Secondly, variations of day-time activity were sinusoidal and correlated to seasonal changes of daylength. We thus hypothesize that the photoperiod could directly affect the behavior of the birds at sea. Night-time at-sea activity exhibited a clear cycle of 29.2 days, suggesting that nocturnal foraging was highly regulated by moon phase, particularly during the non-breeding season. To our knowledge, this is the first study to document a mixed regulation of the behavior of a wild bird by photoperiod and moon phases throughout its annual cycle.

Acute induction of Eya3 by late-night light stimulation triggers TSHβ expression in photoperiodism

Living organisms detect seasonal changes in day length (photoperiod) [1-3] and alter their physiological functions accordingly to fit seasonal environmental changes. TSHβ, induced in the pars tuberalis (PT), plays a key role in the pathway that regulates vertebrate photoperiodism [4, 5]. However, the upstream inducers of TSHβ expression remain unknown. Here we performed genome-wide expression analysis of the PT under chronic and conditions in melatonin-proficient CBA/N mice, in which the photoperiodic TSHβ expression response is preserved [6]. This analysis identified short-day and long-day genes, including TSHβ, and further predicted the acute induction of genes by late-night light stimulation. We verified this by advancing and extending the light period by 8 hr, which induced TSHβ expression within one day. In the following genome-wide expression analysis under this acute condition, we searched for candidate upstream genes by looking for expression that preceded TSHβ's, and we identified the Eya3 gene. We demonstrated that Eya3 and its partner Six1 synergistically activate TSHβ expression and that this activation is further enhanced by Tef and Hlf. These results elucidate the comprehensive transcriptional photoperiodic response in the PT, revealing the complex regulation of TSHβ expression and unexpectedly rapid response to light changes in the mammalian photoperiodic system.

Effect of Birth Season on Circadian Typology Appearing in Japanese Young Children Aged 2 to 12 Years Disappears in Older Students Aged 18 to 25 Years

Several studies suggest that season of birth differentially affects the physiological characteristics of humans. Those living at relatively high latitude, such as Canada, Spain, and Italy (44°N–45°N), and born in the fall tended to be “morning-type” persons in comparison to those born in other seasons. There are relatively little data on the affect of season of birth on people residing at low latitude. Here the authors show that at low latitude, Kochi, Japan (33°N), the effect of season of birth on the morningness chronotype is confined to young children aged 1–12 yrs, disappearing in elderly persons. Only female participants aged 2–12 yrs born in the fall, especially in November, were significantly morning-typed (p < .001) in comparison to those born in the other seasons, whereas there were no such significant season-of-birth differences in morningness-eveningness preference among male participants. Moreover, both female and male participants aged 13–25 yrs showed no significant seasonal differences in morningness-eveningness preference. The small effects detected in this study might be due to smaller seasonal change in day length at the relatively lower latitude of Kochi. (Author correspondence: haratets@kochi-u.ac.jp)

Can Photoperiod Predict Mortality in the 1918-1920 Influenza Pandemic?

Amplitude of the seasonal change in day length increases with distance from the equator, and changes in day length markedly alter immune function in diverse nonhuman animal models of infection. Historical records of mortality data, ambient temperature, population density, geography, and economic indicators from 42 countries during 1918-1920 were analyzed to determine relative contributions toward human mortality during the "Spanish" influenza pandemic of 1918-1920. The data identify a strong negative relation between distance from the equator and mortality during the 1918-1920 influenza pandemic, which, in a multiple regression model, manifested independent of major economic, demographic, and temperature variables. Enhanced survival was evident in populations that experienced a winter nadir day length ≤10 h light/day, relative to those that experienced lower amplitude changes in photoperiod. Numerous reports indicate that exposure to short day lengths, typical of those occurring outside the tropics during winter, yields robust and enduring reductions in the magnitude of cytokine, febrile, and behavioral responses to infection. The present results are preliminary but prompt the conjecture that, if similar mechanisms are operant in humans, then they would be predicted to mitigate symptoms of infection in proportion to an individual's distance from the equator. Although limitations and uncertainties accompany regression-based analyses of historical epidemiological data, latitude, per se, may be an underrecognized factor in mortality during the 1918-1920 influenza pandemic. The author proposes that some proportion of the global variance in morbidity and mortality from infectious diseases may be explained by effects of day length on the innate immune response to infection.

COP1-Mediated Degradation of BBX22/LZF1 Optimizes Seedling Development in Arabidopsis

Light regulates multiple aspects of growth and development in plants. Transcriptomic changes govern the expression of signaling molecules with the perception of light. Also, the 26S proteasome regulates the accumulation of positive and negative regulators for optimal growth of Arabidopsis (Arabidopsis thaliana) in the dark, light, or light/dark cycles. BBX22, whose induction is both light regulated and HY5 dependent, is a positive regulator of deetiolation in Arabidopsis. We found that during skotomorphogenesis, the expression of BBX22 needs to be tightly regulated at both transcriptional and posttranslational levels. During photomorphogenesis, the expression of BBX22 transiently accumulates to execute its roles as a positive regulator. BBX22 protein accumulates to a higher level under short-day conditions and functions to inhibit hypocotyl elongation. The proteasome-dependent degradation of BBX22 protein is tightly controlled even in plants overexpressing BBX22. An analysis of BBX22 degradation kinetics shows that the protein has a short half-life under both dark and light conditions. COP1 mediates the degradation of BBX22 in the dark. Although dispensable in the dark, HY5 contributes to the degradation of BBX22 in the light. The constitutive photomorphogenic development of the cop1 mutant is enhanced in cop1BBX22ox plants, which show a short hypocotyl, high anthocyanin accumulation, and expression of light-responsive genes. Exaggerated light responsiveness is also observed in cop1BBX22ox seedlings grown under short-day conditions. Therefore, the proper accumulation of BBX22 is crucial for plants to maintain optimal growth when grown in the dark as well as to respond to seasonal changes in daylength.