Photomorphogenic Responses Research Articles

FAR-RED INSENSITIVE219 Modulates CONSTITUTIVE PHOTOMORPHOGENIC1 Activity via Physical Interaction to Regulate Hypocotyl Elongation in Arabidopsis

FAR-RED INSENSITIVE219 (FIN219) in Arabidopsis (Arabidopsis thaliana) is involved in phytochrome A-mediated far-red (FR) light signaling. Previous genetic studies revealed that FIN219 acts as an extragenic suppressor of CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1). However, the molecular mechanism underlying the suppression of COP1 remains unknown. Here, we used a transgenic approach to study the regulation of COP1 by FIN219. Transgenic seedlings containing ectopic expression of the FIN219 amino (N)-terminal domain in wild-type Columbia (named NCox for the expression of the N-terminal coiled-coil domain and NTox for the N-terminal 300-amino acid region) exhibited a dominant-negative long-hypocotyl phenotype under FR light, reflected as reduced photomorphogenic responses and altered levels of COP1 and ELONGATED HYPOCOTYL5 (HY5). Yeast two-hybrid, pull-down, and bimolecular fluorescence complementation assays revealed that FIN219 could interact with the WD-40 domain of COP1 and with its N-terminal coiled-coil domain through its carboxyl-terminal domain. Further in vivo coimmunoprecipitation study confirms that FIN219 interacts with COP1 under continuous FR light. Studies of the double mutant fin219-2/cop1-6 indicated that HY5 stability requires FIN219 under darkness and FR light. Moreover, FIN219 levels positively regulated by phytochrome A can modulate the subcellular location of COP1 and are differentially regulated by various fluence rates of FR light. We conclude that the dominant-negative long-hypocotyl phenotype conferred by NCox and NTox in a wild-type background was caused by the misregulation of COP1 binding with the carboxyl terminus of FIN219. Our data provide a critical mechanism controlling the key repressor COP1 in response to FR light.

Negative feedback regulation of UV-B–induced photomorphogenesis and stress acclimation in Arabidopsis

Plants respond to low levels of UV-B radiation with a coordinated photomorphogenic response that allows acclimation to this environmental stress factor. The key players in this UV-B response are COP1 (an E3 ubiquitin ligase), UVR8 (a β-propeller protein), and HY5 (a bZIP transcription factor). We have shown previously that an elevated UV-B-specific response is associated with dwarf growth, indicating the importance of balancing UV-B-specific signaling. Negative regulators of this pathway are not known, however. Here, we describe two highly related WD40-repeat proteins, REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1 (RUP1) and RUP2, that interact directly with UVR8 as potent repressors of UV-B signaling. Both genes were transcriptionally activated by UV-B in a COP1-, UVR8-, and HY5-dependent manner. rup1 rup2 double mutants showed an enhanced response to UV-B and elevated UV-B tolerance after acclimation. Overexpression of RUP2 resulted in reduced UV-B-induced photomorphogenesis and impaired acclimation, leading to hypersensitivity to UV-B stress. These results are consistent with an important regulatory role for RUP1 and RUP2, which act downstream of UVR8-COP1 in a negative feedback loop impinging on UVR8 function, balancing UV-B defense measures and plant growth.

Evidence of physiological phototropin1 (phot1) action in response to UV-C illumination

Stem growth kinetics were measured in cucumber (Cucumis sativus) and Arabidopsis thaliana using highly-sensitive monitoring with 5-minute resolution, in darkness and in response to a short, single pulse of UV-C illumination. The results show that UV-C, like blue light, induces a rapid decrease in seedling growth rate. The fluence-response kinetics and time course were similar to the phototropin1 mediated response observed following a blue pulse. Arabidopsis seedlings were used to assess the genetic mechanism of this response. The phot1 mutant exhibited defects in stem growth rate inhibition, with sustained growth inhibition completely absent following specific treatments. The cryptochrome and phytochrome mutants exhibited responses comparable to wild type, suggesting that these receptor classes do not contribute to this response. The work demonstrates in two species that UV-C has an effect on a rapid plant photomorphogenic response and that the response is partially mediated by the phot1 photoreceptor.

Signaling Events Leading to Red-Light-Induced Suppression of Photomorphogenesis in Wheat (Triticum aestivum)

Perception of red light (400 μmol photon m²/s) by the shoot bottom turned off the greening process in wheat. To understand the signaling cascade leading to this photomorphogenic response, certain signaling components were probed in seedlings grown in different light regimes. Upon analysis the gene expression of heterotrimeric Gα and Gβ were severely down-regulated in seedlings grown without vermiculite and having their shoot bottom exposed to red light (R/V-) and was similar to that of dark-grown seedlings. Supplementing the red-light-grown V- seedlings with blue light resulted in up-regulation of both Gα and Gβ expression, suggesting that blue light is able to modulate G protein expression. Treatment of cytokinin analog benzyladenine to cytokinin-deficient red-light-grown R/V- seedlings resulted in up-regulation of gene expression of both Gα and Gβ. To probe further, modulators of signal transduction pathway--AlF₃ (G protein activator), LaCl₃ (Ca(2+) channel blocker), NaF (nonspecific phosphatase inhibitor), or calmodulin (CaM) antagonists trifluoperazine (TFP) and N-(6-aminohexyl)-5-chloro-1-nafthalene-sulfonamide (W-7)--were added along with Hoagland solution to the roots of 4-day-old etiolated seedlings, grown on germination paper and transferred to red light. AlF₃, LaCl₃, NaF failed to elicit any photomorphogenic response. However, CaM antagonists TFP and W-7 significantly reversed the red-light-induced suppression of photomorphogenesis. Phosphorylation of proteins assayed in the absence or presence of CaM antagonist TFP revealed respective up-regulation or down-regulation of phosphorylation of several plastidic proteins in R/V- seedlings. These suggest that signal transduction of red light perceived by the shoot bottom to suppress photomorphogenesis is mediated by CaM-dependent protein kinases.

Subcellular Sites of the Signal Transduction and Degradation of Phytochrome A

Phytochrome regulates various physiological and developmental processes throughout the life cycle of plants. Among the members of the phytochrome family, phytochrome A (phyA) exclusively mediates the far-red light high irradiance response (FR-HIR), which is elicited by continuous far-red light. In FR-HIR, nuclear accumulation of phyA, which precedes physiological responses, is proposed to be required for the response. In contrast to FR, red light induces rapid degradation of phyA to suppress undesirable long-term photomorphogenic responses of phyA. In the present study, we compared biological activities between phyA derivatives to which either a nuclear localization (NLS) or export (NES) signal sequence was attached. Those derivatives were expressed under the control of the PHYA promoter in the Arabidopsis phyA mutant. Detailed microscopic observation revealed that the phyA-green fluorescent protein (GFP) without a signal sequence is localized exclusively in the cytoplasm in darkness. Rapid nuclear entry was observed after exposure to both red and far-red light. Interestingly, both phyA-GFP-NLS and phyA-GFP-NES were rapidly degraded under continuous red light. Furthermore, a proteasome inhibitor delayed degradation equally under these two conditions. Therefore, similar mechanisms for phyA degradation may exist in the cytoplasm and nucleus. As expected from previous reports, phyA-GFP-NLS, but not phyA-GFP-NES, mediated different aspects of FR-HIR, such as inhibition of hypocotyl elongation and rapid induction of gene expression, confirming that phyA nuclear localization is required for FR-HIR. In addition, a detailed time course analysis of phyA-GFP and phyA-GFP-NLS responses revealed that they were almost indistinguishable, raising the question of the physiological relevance of phyA cytoplasmic retention in darkness.

EIN3/EIL1 cooperate with PIF1 to prevent photo-oxidation and to promote greening of Arabidopsis seedlings

The ability to switch from skotomorphogenesis to photomorphogenesis is essential for seedling development and plant survival. Recent studies revealed that COP1 and phytochrome-interacting factors (PIFs) are key regulators of this transition by repressing the photomorphogenic responses and/or maintaining the skotomorphogenic state of etiolated seedlings. Here we report that the plant hormone ethylene plays a crucial role in the transition from skotomorphogenesis to photomorphogenesis by facilitating greening of etiolated seedlings upon light irradiation. Activation of EIN3/EIL1 is both necessary and sufficient for ethylene-induced enhancement of seedling greening, as well as repression of the accumulation of protochlorophyllide, a phototoxic intermediate of chlorophyll synthesis. EIN3/EIL1 were found to induce gene expression of two key enzymes in the chlorophyll synthesis pathway, protochlorophyllide oxidoreductase A and B (PORA/B). ChIP and EMSA assays demonstrated that EIN3 directly binds to the specific elements present in the PORA and PORB promoters. Genetic studies revealed that EIN3/EIL1 function in cooperation with PIF1 in preventing photo-oxidative damage and promoting cotyledon greening. Moreover, activation of EIN3 reverses the blockage of greening triggered by cop1 mutation or far-red light irradiation. Consistently, EIN3 acts downstream of COP1 and its protein accumulation is enhanced by COP1 but decreased by light. Taken together, EIN3/EIL1 represent a new class of transcriptional regulators along with PIF1 to optimize de-etiolation of Arabidopsis seedlings. Our study highlights the essential role of ethylene in enhancing seedling development and survival through protecting etiolated seedlings against photo-oxidative damage.

UV‐B Action Spectrum for UVR8‐Mediated HY5 Transcript Accumulation in Arabidopsis

Arabidopsis thaliana UV RESISTANCE LOCUS8 (UVR8) is a UV-B-specific signaling component that mediates low fluence, photomorphogenic responses to UV-B. It is required for UV-B-induced expression of the gene encoding the ELONGATED HYPOCOTYL5 (HY5) transcription factor. HY5 is a key effector of responses mediated by UVR8. In mature leaf tissue, HY5 transcript accumulation occurred rapidly in response to a brief UV-B treatment and no induction was observed in a uvr8 mutant over a broad range of UV wavelengths. In response to monochromatic light, maximal transcript accumulation occurred in wild-type plants at wavelengths 280-300 nm. HY5 transcript accumulation showed reciprocity between the fluence rate and duration of UV-B exposure, and on this basis conditions were chosen to generate an action spectrum for the UVR8 signaling pathway. Dose-response curves were produced for a range of UV wavelengths using 20 min exposure to UV and harvesting tissue 2 h after the start of illumination. Experiments using mutants defective in sinapate ester and flavonoid biosynthesis indicated that the presence of UV-absorbing compounds did not affect the construction of an action spectrum under the conditions employed. The action spectrum for the induction of HY5 by the UVR8 pathway showed a main peak at 280 nm with a smaller peak at 300 nm. The data are discussed in relation to the proposed mechanisms of UV-B photoreception.

Signal Transduction in Responses to UV-B Radiation

UV-B radiation is a key environmental signal that initiates diverse responses in plants that affect metabolism, development, and viability. Many effects of UV-B involve the differential regulation of gene expression. The response to UV-B depends on the nature of the UV-B treatment, the extent of adaptation and acclimation to UV-B, and interaction with other environmental factors. Responses to UV-B are mediated by both nonspecific signaling pathways, involving DNA damage, reactive oxygen species, and wound/defense signaling molecules, and UV-B-specific pathways that mediate photomorphogenic responses to low levels of UV-B. Importantly, photomorphogenic signaling stimulates the expression of genes involved in UV-protection and hence promotes plant survival in UV-B. Photomorphogenic UV-B signaling is mediated by the UV-B-specific component UV RESISTANCE LOCUS8 (UVR8). Both UVR8 and CONSTITUTIVE PHOTOMORPHOGENESIS1 (COP1) are required for UV-B-induced expression of the ELONGATED HYPOCOTYL5 (HY5) transcription factor, which plays a central role in the regulation of genes involved in photomorphogenic UV-B responses.

Improved elongation of Scots pine seedlings under blue light depletion is not dependent on resource acquisition.

Removal of blue light (400-500 nm) induced shoot elongation of 2-year-old Scots pine (Pinus sylvestris L.) seedlings, which was not related to resource acquisition (carbohydrates, C/N ratio and soluble proteins) and frost hardening. The seedlings were grown in northern Finland (64°N) in plexiglass chambers, either orange in colour or transparent, during elongation and cold hardening periods in 2001. The orange chamber removed the blue wavelengths. The results suggest that the growth inhibiting effect of blue light on Scots pine elongation is probably a photomorphogenic regulation response; the removal of blue light did not affect the gas exchange and accumulation of growth resources. In addition, the removal of blue light also did not affect the physiological parameters (pigment composition, chlorophyll fluorescence and lipid peroxidation) measured during the preparation for winter.

Modelling leaf phototropism in a cucumber canopy.

Leaf phototropism might have significant effects on the light interception, dry matter production and yield of cucumber (Cucumis sativus L.). The objective of the present study was to model the phototropism of leaves in a greenhouse cucumber canopy. The dynamic structural model of cucumber using a parametric L-system was extended to mimic the leaf movement induced by gradients in the local light environment of each leaf. The red to far-red (R : FR) ratio is known to be a driving force in shade avoidance reactions of plants. In the model, R : FR ratios on the left and right leaf half of each individual leaf lamina were calculated and the change in tropism angle per phyllochron was based on the R : FR gradient across the leaf halves. The tropism angle of a leaf describes the deviation of the present leaf azimuth from the initial leaf orientation, which is predefined by the phyllotaxis of the plant. Even in its simple form, the model simulated photo-morphogenic canopy responses.

Arabidopsis thaliana Circadian Clock Is Regulated by the Small GTPase LIP1

At the core of the eukaryotic circadian network, clock genes/proteins form multiple transcriptional/translational negative-feedback loops and generate a basic approximately 24 hr oscillation, which provides daily regulation for a wide range of processes. This temporal organization enhances the fitness of the organism only if it corresponds to the natural day/night cycles. Light is the most effective signal in synchronizing the oscillator to environmental cycles. The lip1-1 (light insensitive period 1) mutant isolated from the model plant Arabidopsis thaliana displays novel circadian phenotypes arising from specific defects in the light input pathway to the oscillator. In wild-type plants, period length shortens with increasing light fluence rates and the phase of rhythms can be shifted by light pulses administered to dark-adapted plants. In contrast, in lip1-1, period length is nearly insensitive to light intensity and significantly larger phase shifts (delays) can be induced during the subjective night. The mutant also displays elevated photomorphogenic responses to red and blue light, which cannot be explained by the circadian defect, suggesting distinct functions for LIP1 in the circadian light input and photomorphogenesis. The LIP1 gene encodes a functional, plant-specific atypical small GTPase, and therefore we postulate that it acts similarly to ZEITLUPE at postranscriptional level. LIP1 represents the first small GTPase implicated in the circadian system of plants. LIP1 plays a unique negative role in controlling circadian light input and is required for precise entrainment of the plant clock.

A study of gibberellin homeostasis and cryptochrome-mediated blue light inhibition of hypocotyl elongation.

Cryptochromes mediate blue light-dependent photomorphogenic responses, such as inhibition of hypocotyl elongation. To investigate the underlying mechanism, we analyzed a genetic suppressor, scc7-D (suppressors of cry1cry2), which suppressed the long-hypocotyl phenotype of the cry1cry2 (cryptochrome1/cryptochrome2) mutant in a light-dependent but wavelength-independent manner. scc7-D is a gain-of-expression allele of the GA2ox8 gene encoding a gibberellin (GA)-inactivating enzyme, GA 2-oxidase. Although scc7-D is hypersensitive to light, transgenic seedlings expressing GA2ox at a level higher than scc7-D showed a constitutive photomorphogenic phenotype, confirming a general role of GA2ox and GA in the suppression of hypocotyl elongation. Prompted by this result, we investigated blue light regulation of mRNA expression of the GA metabolic and catabolic genes. We demonstrated that cryptochromes are required for the blue light regulation of GA2ox1, GA20ox1, and GA3ox1 expression in transient induction, continuous illumination, and photoperiodic conditions. The kinetics of cryptochrome induction of GA2ox1 expression and cryptochrome suppression of GA20ox1 or GA3ox1 expression correlate with the cryptochrome-dependent transient reduction of GA(4) in etiolated wild-type seedlings exposed to blue light. Therefore we propose that in deetiolating seedlings, cryptochromes mediate blue light regulation of GA catabolic/metabolic genes, which affect GA levels and hypocotyl elongation. Surprisingly, no significant change in the GA(4) content was detected in the whole shoot samples of the wild-type or cry1cry2 seedlings grown in the dark or continuous blue light, suggesting that cryptochromes may also regulate GA responsiveness and/or trigger cell- or tissue-specific changes of the level of bioactive GAs.

Phytochrome-mediated photomorphogenesis in plants

Photomorphogenesis is the process by which plants grow and develop in response to light signals. This process is mediated by a sophisticated network of photoreceptors among which phytochromes play a key role. Phytochrome-mediated photomorphogenic responses are characterized by the complex variety of relationships between light input and physiological outputs, including germination, de-etiolation, shade avoidance, circadian rhythm, and flowering. Recent studies have resulted in several important advances, and have revealed the major consequences of phytochrome activity in terms of controlling protein subcellular localization, transcription, protein stability, and protein phosphorylation. In addition, many downstream components in the phytochrome signaling have now been identified, and a complex, highly regulated signaling network is envisaged. Here, we review the current knowledge about red/far-red photoreceptor phytochromes and provide a comprehensive summary of the phytochrome-mediated photomorphogenesis signaling network.

How do UV Photomorphogenic Responses Confer Water Stress Tolerance?¶†

How do UV Photomorphogenic Responses Confer Water Stress Tolerance?¶†

Derepression of the NC80 motif is critical for the photoactivation of Arabidopsis CRY2

Cryptochromes are blue light receptors that regulate photomorphogenesis in plants and the circadian clock in animals and plants. Arabidopsis cryptochrome 2 (CRY2) mediates blue light inhibition of hypocotyl elongation and photoperiodic control of floral initiation. CRY2 undergoes blue light-induced phosphorylation, which was hypothesized to be associated with CRY2 photoactivation. To further investigate how light activates CRY2, we analyzed the physiological activities and phosphorylation of various CRY2 fusion proteins in transgenic plants. Our results showed that an 80-residue motif, referred to as NC80, was sufficient to confer the physiological function of CRY2. The GUS-NC80 fusion protein expressed in transgenic plants is constitutively active but unphosphorylated, suggesting that the blue light-induced CRY2 phosphorylation causes a conformational change to derepress the NC80 motif. Consistent with this hypothesis, the CRY2 C-terminal tail was found to be required for the blue light-induced CRY2 phosphorylation but not for the CRY2 activity. We propose that the PHR domain and the C-terminal tail of the unphosphorylated CRY2 form a "closed" conformation to suppress the NC80 motif in the absence of light. In response to blue light, the C-terminal tail of CRY2 is phosphorylated and electrostatically repelled from the surface of the PHR domain to form an "open" conformation, resulting in derepression of the NC80 motif and signal transduction to trigger photomorphogenic responses.

Positive Role for COP1 in UV-B Responses

Under dark conditions, the E3 ubiquitin ligase COP1 is enriched in the nucleus, where it contributes to the degradation of various transcriptional regulators involved in the photomorphogenic response, including HY5 (ELONGATED HYPOCOTYL 5). Thus, in Arabidopsis , COP1 is a negative regulator of photomorphogenesis. Oravecz et al . report that COP1 has a positive role in plant responses to low-level ultraviolet B (UV-B) irradiation through a mechanism that includes stimulation of expression of the HY5 gene. Microarray analysis and phenotypic analysis of wild-type and hy5-1 mutant plants indicated that HY5 was necessary for the stimulation of the expression of genes involved in tolerance to UV-B. The nonlethal COP1 mutant cop1-4 also exhibited decreased UV-B-mediated stimulation of several genes, including HY5 , and about half of the genes that required COP1 also required HY5. Plants respond to UV-B by stimulation of the phenylpropanoid biosynthetic pathway, which produces "sunscreen" flavonoids. Both hy5-1 and cop1-4 plants were deficient in this response to UV-B; however, plants with another COP1 mutant allele, cop1 eid6 , did not show this deficiency, which suggests that the structural requirements and functions of COP1 in visible light (photomorphogenesis) and UV-B signaling (flavonoid biosynthesis) are different. Exposure of light-grown plants to supplemental UV-B irradiation promoted the nuclear accumulation of COP1 (in just light-grown plants, COP1 is excluded from the nucleus). HY5 is also abundant in the nucleus of light-grown plants exposed to supplemental UV-B irradiation, which is unexpected given that COP1 promotes its degradation under dark conditions when both proteins are present in the nucleus. How COP1 can mediate degradation of HY5 in the dark, yet be required for HY5 to accumulate in response to UV-B irradiation, remains to be determined. A. Oravecz, A. Baumann, Z. Máté, A. Brzezinska, J. Molinier, E. J. Oakeley, E. Ádám, E. Schäfer, F. Nagy, R. Ulm, CONSTITUTIVELY PHOTOMORPHOGENIC1 is required for the UV-B response in Arabidopsis . Plant Cell 18 , 1975-1990 (2006). [Abstract] [Full Text]

CONSTITUTIVELY PHOTOMORPHOGENIC1 is required for the UV-B response in Arabidopsis.

CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) is a negative regulator of photomorphogenesis in Arabidopsis thaliana. COP1 functions as an E3 ubiquitin ligase, targeting select proteins for proteasomal degradation in plants as well as in mammals. Among its substrates is the basic domain/leucine zipper (bZIP) transcription factor ELONGATED HYPOCOTYL5 (HY5), one of the key regulators of photomorphogenesis under all light qualities, including UV-B responses required for tolerance to this environmental threat. Here, we report that, in contrast with the situation in visible light, COP1 is a critical positive regulator of responses to low levels of UV-B. We show that in the cop1-4 mutant, flavonoid accumulation and genome-wide expression changes in response to UV-B are blocked to a large extent. COP1 is required for HY5 gene activation, and both COP1 and HY5 proteins accumulate in the nucleus under supplementary UV-B. SUPPRESSOR OF PHYTOCHROME A-105 family proteins (SPA1 to SPA4) that are required for COP1 function in dark and visible light are not essential in the response to UV-B. We conclude that COP1 performs a specific and novel role in the plants' photomorphogenic response to UV-B, coordinating HY5-dependent and -independent pathways, which eventually results in UV-B tolerance.

Systems biology flowering in the plant clock field

Mol Syst Biol. 2: 60 In the 18th Century, the Swedish botanist Karl von Linne designed a ‘Flower‐Clock’ by arranging a series of various plant species according to the respective time their flowers open or close every day. Watching this ‘Flower‐Clock’, one can then estimate the time of the day by noting the pattern of flower opening and closing. It has been a well‐known fact since Linne's early times that plants can open or close their flowers at a precise time of the day. However, we still do not fully understand the design principles of the molecular network that underlies the cellular circadian clock, which achieves to predict, often with exquisite precision, the cyclic changes in the environment due to the rotation of earth. In two articles currently published in Molecular Systems Biology , Millar and co‐workers (Locke et al , 2006) and Doyle and co‐workers (Zeilinger et al , 2006) propose a plausible design for the plant circadian clock. In previous work, Millar and co‐workers extended an initial ‘one‐loop model’ of the plant circadian clock into a ‘two‐loop model’ (Figure 1) (Locke et al , 2005). In the simple ‘one‐loop model’ (Figure 1 …

Phenotypic responses to early signals of neighbour proximity in Picea omorika, a pioneer conifer tree

Developmental “shade-avoidance” plasticity induced by the low ratio of red to far-red radiation reflected from neighbouring plants is considered adaptive in crowded habitats, because it allows plants to detect the presence of potential competitors and adopt a morphology that enhances light interception. We examined the pattern and adaptive effects of phenotypic responses to early signals of neighbour proximity in Picea omorika by exposing the seedlings from a synthetic population comprising 21 families (offspring groups) from six natural populations to the alternative density conditions in a controlled-environment growth-room. A set of measurements was taken at each of a series of predetermined seedling ages, for a number of shoot traits that participate in the formation of the shade-avoidance phenotype. A repeated-measures MANOVA (plus profile analysis) revealed that, apart from bud traits, the overall shape of the ontogenetic trajectories for all epicotyl and branch traits differed significantly between high and low density. However, individual ANOVAs applied on each of the two successive time-intervals (starting at week 12 after epicotyl appearance), identified a significant difference in the slope of the response curves only in the lather. Because the onset of photomorphogenic responses to neighbour proximity in P. omorika coincided with the changes in the spectral composition of the incident radiation to which the growing seedlings were exposed, the observed differences in the slopes of the shade-avoidance response curves could be causally related to a variation in the amount of the far-red photons reflected from the foliage of the surrounding plants. A multivariate analysis of phenotypic selection revealed that the association between epicotyl lenght and relative fitness was significantly greater at high than at low density, indicating that taller seedlings were more efficient in light aquisition than suppressed ones, which could be causally related to the effect of direct selection on epicotyl lenght in that environment. Die Entwicklung einer „Schattenvermeidungs”-Plastizität, die durch den geringen Anteil der roten und infraroten Strahlung induziert wird, die von benachbarten Pflanzen reflektiert wird, wird in überfüllten Habitaten als adaptiv gewertet, da es den Pflanzen erlaubt, die Anwesenheit potenzieller Konkurrenten zu bemerken und eine Morphologie anzunehmen, welche die Lichtausbeute verbessert. Wir untersuchten die Muster und adaptiven Effekte der phänotypischen Reaktionen auf frühe Signale der Nähe von Nachbarn bei Picea omorika indem wir Keimlinge einer synthetischen Population, die 21 Familien (Nachkommengruppen) aus sechs natürlichen Populationen enthielt, den alternativen Dichtebedingungen in einem Wachstumsraum unter kontrollierten Umweltbedingungen aussetzten. Eine Anzahl von Messungen wurde bei einer Serie von verschiedenen bestimmten Keimlingsaltern gemacht, und zwar für eine Anzahl von Sprossmerkmalen, die zur Ausformung des schattenvermeidenden Phänotyps beitragen. Eine MANOVA für wiederholte Messungen enthüllte, dass sich abgesehen von den Knospenmerkmalen die allgemeine Form der ontogenetischen Trajektorien für alle Epikotyl- und Astmerkmale signifikant zwischen der geringen und hohen Dichte unterschieden. Individuelle ANOVAs, die auf jedes der zwei aufeinanderfolgenden Zeitintervalle (beginnend mit der 12 Woche nach dem Erscheinen des Epikotyls) angewendet wurden, identifizierten nur bei den letzteren einen signifikanten Unterschied in der Steigung der Reaktionskurven. Weil der Beginn der photomorphogenetischen Reaktion auf die Nähe von Nachbarn bei P. omorika zusammen mit den Veränderungen in der spektralen Zusammensetzung der einfallenden Strahlung einsetzte, der die Keimlinge ausgesetzt waren, könnten die beobachteten Unterschiede in den Steigungen der Schattenvermeidungs-Reaktionskurven kausal mit einer Varibilität des Anteils der infraroten Photonen verbunden sein, die von den Blättern der benachbarten Pflanzen reflektiert werden. Eine multivariate Analyse der phänotypischen Selektion enthüllte, dass die Assoziation zwischen Epikotyllänge und der relativen Fitness bei hohen Dichten signifikant größer als bei niedrigen Dichten war, und wies darauf hin, dass größere Keimlinge effektiver bei der Lichtausnutzung waren als unterdrückte Keimlinge. Dies könnte kausal mit dem Effekt einer direkten Selektion der Epikotyllänge in diesem Umfeld in Beziehung stehen.

PSEUDO-RESPONSE REGULATORS, PRR9, PRR7 and PRR5, Together Play Essential Roles Close to the Circadian Clock of Arabidopsis thaliana

In Arabidopsis thaliana, a number of clock-associated protein components have been identified. Among them, CCA1 (CIRCADIAN CLOCK-ASSOCIATED 1)/LHY (LATE ELONGATED HYPOCOTYL) and TOC1 (TIMING OF CAB EXPRESSION 1) are believed to be the essential components of the central oscillator. CCA1 and LHY are homologous and partially redundant Myb-related DNA-binding proteins, whereas TOC1 is a member of a small family of proteins, designated as PSEUDO-RESPONSE REGULATOR. It is also believed that these two different types of clock components form an autoregulatory positive/negative feedback loop at the levels of transcription/translation that generates intrinsic rhythms. Nonetheless, it was not yet certain whether or not other PRR family members (PRR9, PRR7, PRR5 and PRR3) are implicated in clock function per se. Employing a set of prr9, prr7 and prr5 mutant alleles, here we established all possible single, double and triple prr mutants. They were examined extensively by comparing them with each other with regard to their phenotypes of circadian rhythms, photoperiodicity-dependent control of flowering time and photomorphogenic responses to red light during de-etiolation. Notably, the prr9 prr7 prr5 triple lesions in plants resulted in severe phenotypes: (i) arrhythmia in the continuous light conditions, and an anomalous phasing of diurnal oscillation of certain circadian-controlled genes even in the entrained light/dark cycle conditions; (ii) late flowering that was no longer sensitive to the photoperiodicity; and (iii) hyposensitivity (or blind) to red light in the photomorphogenic responses. The phenotypes of the single and double mutants were also characterized extensively, showing that they exhibited circadian-associated phenotypes characteristic for each. These results are discussed from the viewpoint that PRR9/PRR7/PRR5 together act as period-controlling factors, and they play overlapping and distinctive roles close to (or within) the central oscillator in which the relative, PRR1/TOC1, plays an essential role.