INTERACTION BETWEEN CRYPTOCHROME AND PHYTOCHROME IN HIGHER PLANT PHOTOMORPHOGENESIS

At least three photoreceptors are involved in the mediation of light action in higher plant photomorphogenesis: cryptochrome (UV-A/blue light photoreceptor), UV-B photoreceptor, and phytochrome. The degree of photoreceptor interaction in photomorphogenesis can apparently vary depending on the response, the species, and the stage of development of the biological system. In most cases of interaction studied so far, Pfr, the physiologically active form of phytochrome, is apparently required for the final expression of the response. In some systems, the cryptochrome and/or UV-B photoreceptor mediated action of UV/blue radiation is required to establish/enhance/maintain responsiveness toward Pfr. There is no evidence for photoreceptor interaction in some response-system combinations. It is not known for sure if this apparent lack of photoreceptor interaction represents a real situation or just a failure to detect it because of experimental limitations. Practically nothing is known about the mechanism of photoreceptor interaction at the molecular level.     

Positive and negative tropic curvature induced by microbeam irradiation of protonemal tip cells of the moss Ceratodon purpureus

The photoreceptor phytochrome mediates tropic responses in protonemata of the moss Ceratodon purpureus. Under standard conditions the tip cells grow towards unilateral red light, or perpendicular to the electrical vector of polarized light. In this study the response of tip cells to partial irradiation of the apical region was analysed using a microbeam apparatus. The fluence response curve gave an unexpected pattern: whereas a 15-min microbeam with light intensities around 3 micro mol m (-2) s (-1) induced a growth curvature towards the irradiated side, higher light intensities around 100 micro mol m (-2) s (-1) caused a negative response, the cells grew away from the irradiated side. This avoidance response is explained by two effects: the light intensity is high enough to induce photoconversion into the active Pfr form of phytochrome, not only on the irradiated but also on the non-irradiated side by stray light. At the same time, the strong light on the irradiated side acts antagonistically to Pfr. As a result of this inhibition, the growth direction is moved to the light-avoiding side. Such a Pfr-independent mechanism might be important for the phototropic response to distinguish between the light-directed and light-avoiding side under unilateral light.     

Rhodopsin Receptors of Phototaxis in Green Flagellate Algae

Green flagellate algae are capable of the active adjustment of their swimming path according to the light direction (phototaxis). This direction is detected by a special photoreceptor apparatus consisting of the photoreceptor membrane and eyespot. Receptor photoexcitation in green flagellates triggers a cascade of rapid electrical events in the cell membrane which plays a crucial role in the signal transduction chain of phototaxis and the photophobic response. The photoreceptor current is the earliest so far detectable process in this cascade. Measurement of the photoreceptor current is at present the most suitable approach to investigation of the photoreceptor pigment in green flagellate algae, since a low receptor concentration in the cell makes application of optical and biochemical methods so far impossible. A set of physiological evidences shows that the phototaxis receptor in green flagellate algae is a unique rhodopsin-type protein. It shares common chromophore properties with retinal proteins from archaea. However, the involvement of photoelectric processes in the signal transduction chain relates it to animal visual rhodopsins. The presence of some enzymatic components of the animal visual cascade in isolated eyespot preparations might also point to this relation. A retinal-binding protein has been identified in such preparations, the amino acid sequence of which shows a certain homology to sequences of animal visual rhodopsins. However, potential function of this protein as the phototaxis receptor has been questioned in recent time.     

Genetic and molecular analysis of light-regulated plant development

Light regulates many physiological and developmental events in plants through the action of multiple sensory pigment systems. Although our understanding of the regulatory photoreceptors, including phytochromes (that principally absorb red and far-red energy) and blue light receptors, has advanced considerably in the recent past, the mechanisms of light signal transduction in higher plants are poorly understood. To unravel the molecular events associated with light-regulated plant development, a large number of photomorphogenic mutants have been isolated in several different plant species, including Arabidopsis, cucumber, tomato, pea, Brassica and Sorghum, which are either impaired in normal perception of light signal (photoreceptor mutants) or are affected in some specific or a sub-set of phenotypic traits (signal transduction mutants). Their physiological and molecular analysis is proving to be valuable in (1) assigning specific function to discrete phytochrome species, (2) elucidation of elements that constitute the transduction pathway downstream of signal perception, and (3) determining how different photosensory systems regulate many diverse responses. The progress made in the analysis of photomorphogenic mutants, as reviewed in this article, clearly indicates that multiple photoreceptors, either of the same or different class, interact through an intricate network of signal transduction pathways to finally determine the light-dependent phenotype of both monocots and dicots.     

Blue-light control of sporangiophore initiation in Phycomyces

Summary Many fungi produce spores or spore-bearing structures under the control of blue light. Sporangiophores of Phycomyces blakesleeanus are produced continuously along racing tube cultures grown in constant darkness or constant light. However, if a dark-grown culture is exposed to light for a short time on one day a narrow, dense band of sporangiophores is observed the next day at that point of the tube occupied by the mycelial tips during the light pulse. A periodic program with short days (e.g., 4 h light; 20 h dark), leads to periodic bands of sporangiophores spaced at intervals corresponding to one period-length (in this case 24 h) of mycelial growth. Sporangiophore initiation is inhibited by a light to dark transition and is stimulated by a dark to light transition. A partial action spectrum of the initiation response, covering the critical 480–540 nm region, strongly suggests that the same photoreceptor pigment is involved as in the phototropic response and light growth response of sporangiophores. Mutants with altered light control of sporangiophore initiation have been found among those selected for altered phototropism. This joint elimination of these two responses to blue light by a single mutation is evidence for a common early transduction system.     

The signaling pathway in photoresponses that may be mediated by visual pigments in erythrophores of Nile tilapia

The ability to increase the synthesis or vary the distribution of pigment in response to light is an important feature of many pigment cells. Unlike other light-sensitive pigment cells, erythrophores of Nile tilapia change the direction of pigment migration depending on the peak wavelength of incident light: light near 365, 400 or 600 nm induces pigment aggregation, while dispersion occurs in response to light at 500 nm. How these phenomena are achieved is currently unknown. In the present study, the phototransduction involved in the pigment dispersion caused by light at 500 nm or the aggregation by light at 600 nm was examined, using pertussis toxin, cholera toxin, blockers of ion channels, various chemicals affecting serial steps of signaling pathways and membrane-permeable cAMP analog. The results show that light-induced bidirectional movements in tilapia erythrophores may be controlled by cytosolic cAMP levels via Gi- or Gs-type G proteins. In addition, RT-PCR demonstrated for the first time the expression of mRNAs encoding red and green opsins in tilapia fins, only where erythrophores exist. Here, we suggest that multiple cone-type visual pigments may be present in the erythrophores, and that unique cascades in which such opsins couple to Gi or Gs-type G proteins are involved in the photoresponses in these pigment cells. Thus, tilapia erythrophore system seems to be a nice model for understanding the photoresponses of cells other than visual cells.     

Photoregulation systems for light-oriented chloroplast movement

Movements of the chloroplasts induced by a directional stimulus of light are found to occur in various plant materials ranging from algae to terrestrial angiosperms. Depending on the fluence rate of light, chloroplasts move toward the area of the maximum light absorption and escape from it, which are named as low- and high-fluence-rate responses, respectively. In most materials the effective wavelengths are exclusively found in the blue-UV region of spectrum, (a) flavin pigment(s) being considered as the photoreceptor, while in a few species of plants phytochrome is involved. The arrangement of chloroplasts as a result of the movement depends on the orientation of the electrical vector of light, which reflects the dichroic orientation of the photoreceptor for perception of light direction. Photosystems involved in these responses, however, are not only for perception of light direction, but also for realization of the movement and for holding of the chloroplasts in the reached site. Possible interactions and dual roles of photosystems in regulation of chloroplast movement are discussed.     

Photoinduced electric currents in carotenoid-deficient Chlamydomonas mutants reconstituted with retinal and its analogs.

Reconstitution of the photoelectric responses involved in photosensory transduction in "blind" cells of Chlamydomonas reinhardtii carotenoid-deficient mutants was studied by means of a recently developed population method. Both the photoreceptor current and the regenerative response can be restored by addition of all-trans-retinal, 9-demethyl-retinal, or dimethyl-octatrienal, while the retinal analogs prevented from 13-cis/trans isomerization, 13-demethyl-retinal and citral, are not effective. Fluence dependence, spectral sensitivity, and effect of hydroxylamine treatment on retinal-induced photoelectric responses are similar to those found earlier in green strains of Chlamydomonas, although an alternative mechanism of antenna directivity in white cells of reconstituted "blind" mutants (likely based on the focusing effect of the transparent cell bodies) leads to the reversed sign of phototaxis in mutant cells under the same conditions. The results obtained indicate that both photoreceptor current and regenerative response are initiated by the same or similar rhodopsins with arhaebacterial-like chromophore(s) and prove directly the earlier suggested identity of the photoreceptor pigment(s) involved in photomotile and photoelectric responses in flagellated algae.     

Are two photoreceptors involved in the flowering of a long-day plant?

The effect of daylength extension with narrow spectral bands on the flowering of a long-day plant, Brassica campestris L. cv. Ceres, was investigated to obtain clues to the identity of the photoreceptor involved. Extension of a 9 h photoperiod with 5 h of light pulses at various wavelengths resulted in maximal flowering occurring after irradiation at 710 nm, less at 730 nm, and none at 550, 660 and 750 nm. Flowering at 710 and 730 nm was negated by simultaneous exposures at 550 nm, but not at 660 nm. A short preirradiation at 660 nm enabled a following irradiation at 750 nm to induce flowering. This latter induction was prevented by 550 nm irradiation.
Short flashes of light at 710 nm induced flowering that was negated by a following flash at 550 nm but not at 660 nm. The negation by 550 nm radiation was prevented by subsequent flashes at 710 nm, indicating photoreversibility. A flash at 660 nm enabled subsequent light flashes at 750 nm to initiate flowering that was reversed by a following 550 nm flash.
From the results showing the necessity of red and far-red lights, it is proposed that flowering in this long-day plant is due to two photoreceptors - one is phytochrome and the other an unknown pigment with far-red, green photoreversible properties. By using fluence response data, it is deduced that the unidentified photoreceptor has weak absorption bands in the far-red, but has a strong absorption band in the green. Flowering is induced when effects of red light absorbed by phytochrome interact with effects of far-red light absorbed by the unidentified photoreceptor.     

Lipid Metabolism in Photoreceptor Membranes: Regulation and Mechanisms

Lipid metabolism in photoreceptor rod outer segments has attracted considerable attention because of its importance in providing the appropriate environment for supporting an efficient phototransduction mechanism. Recent studies suggest that lipid metabolism in these membranes is involved in the generation of second messengers and in signal transduction mechanisms. Phospholipid turnover is tightly regulated by phosphorylation-dephosphorylation reactions and light, and provides, in turn, with molecules capable of activating protein kinases and cellular processes such as membrane fusion or light-adaptation. These findings suggest that photoreceptor membrane lipids are more than just important structural components of the visual cell rod outer segment.     

The pH dependence of photosensory responses in Stentor coeruleus and model system

1. Live Stentor coeruleus exhibits a substantially red-shifted fluorescence maximum, corresponding to the anionic species of the photoreceptor chromophore. No change was observed in either the absorption or fluorescence excitation spectrum, indicating an efficient deprotonation of the photoreceptor pigment upon excitation by light. 2 Changes in external pH exhibit a dramatic effect on the pulmonary response of Stentor. Phototaxis is specifically inhibited at pH less than 6, with loss of photosensory perception which is restored when the pH is returned to pH greater than 6. 3. Fluorescence changes of 9-aminoacridine in suspensions of live Stentor indicate the generation of a pH gradient upon irradiation with light. Both pH gradient and phototaxis were inhibited by the addition of nigericin and p-tri-fluoromethoxy carbonyl cyanide phenylhydrazone (FCCP). 4. Incorporation of the Stentor photoreceptor protein in to artificial liposomes demonstrates the ability of the system to generate pH gradients across model membranes as monitored by the quenching of 9-aminoacridine fluorescence. The effect of external pH on net proton movement in the model system is strikingly similar to the pH dependent of the liver Stentor, thus lending support for transient proton flux being an important mode of light signal processing for photosensory transduction.     

Mutational analysis of blue-light sensing in Arabidopsis

Blue light regulates many physiological and developmental processes in higher plants through the action of multiple photosensory systems. The analysis of photomorphogenic mutants is leading to a better understanding of how the different photosensory systems mediate the wide range of responses observed in blue light. A review of the current literature on photomorphogenic mutants makes it apparent that redundancies exist in photoreceptor function. For example, many blue-light responses that have been shown to be regulated by a blue-light photosensory system are also under phytochrome control. The study of various light-response mutants suggests that a complex sensory network regulates light-mediated responses. This article attempts to piece together information regarding the sensory systems responsible for blue-light-regulated responses.     

Phototaxis of the green algae: the new class of rhodopsin receptors

Photomotility behavior in green flagellate algae is mediated by rhodopsin-like receptors, which was initially suggested on the basis of physiological evidence. The cascade of rapid Ca(2+)-dependent electrical responses in the plasma membrane plays a key role in the signal transduction chain during both phototaxis and the photophobic response. The photoreceptor current through the plasma membrane is the earliest detectable event upon photoexcitation of the photoreceptors. Analysis of this current revealed that it consists of at least two components with different characteristics. Genes encoding two archaeal-type rhodopsins (type I rhodopsins) were recently identified in the genome of Chlamydomonas reinhardtii and named (Chlamydomonas Sensory Rhodopsins A and B CSRA and CSRB). The measurements of photoelectric and motor responses in genetic transformants of C. reinhardtii enriched in each of these receptor proteins showed that the two components of the photoreceptor current are mediated by the two rhodopsins, and that both CSRA and CSRB are involved in phototaxis and the photophobic response. The CSRA-mediated current dominates at high light intensities and contributes primarily to the photophobic response. The CSRB-initiated transduction involves an efficient amplification cascade and mediates the highly sensitive phototaxis at low light intensities. CSRA and CSRB expressed heterologously in oocytes of Xenopus laevis act as light-gated proton channels, although it is unclear whether this channel activity plays a functional role in the initiation of motor responses and/or occurs in the native system.     

Light perception in guard cells

Abstract. Guard cells perceive light via two photoreceptor systems: a blue-light-dependent photosystem and the guard cell chloroplast. Chloroplasts stimulate stomatal opening by transducing photosynthetic active radiation into proton pumping at the guard cell plasma membrane. In addition, guard cell chloroplasts fix CO₂ photosynthetically. Sugar from guard cell photosynthesis can contribute to the osmotic build-up required for opening. The blue-light-dependent photosystem activates proton pumping at the guard cell plasma membrane and stimulates starch hydrolysis. Available information on the photobiological properties of guard cells makes it possible to describe stomatal function in terms of the cellular components regulating stomatal movements. The blue light response is involved in stomatal opening in the early morning and stomatal responses to sunflecks. The guard cell chloroplast is likely to be involved in stomatal adaptations to sun, shade and to temperature. Interactions between these photosystems, a third photoreceptor in guard cells, phytochrome, and other mechanisms transducing stomatal responses such as VPD and carbon dioxide, provide the cellular basis for stomatal regulation.     

Photophobic stop-response in a dinoflagellate: Modulation by preirradiation

Gyrodinium dorsum Kofoid responds photophobically to flashes of blue light. The photophobic response consists of a cessation of movement (stop-response). Without background light and after a flash fluence above 10 J m⁻², 75–85% of the cells show a stop-response, while only 50% of the cells show this response at 5 J m⁻². With a flash fluence of 5 J m⁻², background light of different wavelengths either increases (614 nm. 5.5–18.2 μmol m⁻² s⁻¹) or decreases (700 nm, 18.4–36.0 μmol m⁻² s⁻¹) the stop-response. Two hypotheses for the mechanism of the modulation by background light of the photophobic response are discussed: an effect of light on the balance of the photosynthetic system (PS I/PS II) or an effect on a phytochrome-like pigment (P_r/P_fr). This study supports the idea that a phytochrome-like pigment works in combination with a blue light-absorbing pigment. It was also found that cells of Gyrodinium dorsum cultured in red light (39.8 μmol m⁻²) had a higher absorption in the red region of the absorption spectra than those cultured in white light (92.7 μmol m⁻²).     

Wirkungsspektren der Anthocyansynthese in Gewebekulturen und Keimlingen von Haplopappus gracilis

Summary The biosynthesis of anthocyanin in tissue cultures and intact seedlings of Haplopappus gracilis is a light-dependent reaction which can be induced by blue light only. Anthocyanin appeared in all organs of the seedling.Wounding of the plant led to an increase in the content of anthocyanin due to increased anthocyanin synthesis in the cotyledons.The action spectra of anthocyanin formation in tissue cultures and intact seedlings have two peaks, one at 438 nm and the other at 372 nm. The limit of activity in the direction of longer wavelengths lies between 474 and 493 nm. Red light of short and long wavelength is ineffective in the induction of pigment synthesis. The photoreceptor of the light reaction is supposed to be a yellow pigment (flavoprotein or carotinoid). In contrast to the intact plants, isolated cotyledons and wounded seedlings are able to form anthocyanin not only in the blue region but also during irradiation with red light of high intensity. The action spectrum of anthocyanin synthesis in the isolated cotyledons has a marked maximum at about 440 nm and a second one at about 660 nm. A little activity can be observed throughout the visible spectrum. The pigment synthesis induced by red light can be completely suppressed by DCMU, an inhibitor of photosynthesis. This indicates that in the case of the activity in the red light caused by wounding chlorophyll serves as photoreceptor.The anthocyanin synthesis in tissue cultures and seedlings could not be influenced by low energy radiation in the red or in the far red region, even after induction of anthocyanin synthesis by blue light of high intensity. Therefore it seems that the phytochrome system is not involved in anthocyanin synthesis in Haplopappus gracilis.     

Phytochrome phosphorylation in plant light signaling.

Reversible protein phosphorylation is a switching mechanism used in eukaryotes to regulate various cellular signalings. In plant light signaling, sophisticated photosensory receptor systems operate to modulate growth and development. The photoreceptors include phytochromes, cryptochromes and phototropins. Despite considerable progresses in defining the photosensory roles of these photoreceptors, the primary biochemical mechanisms by which the photoreceptor molecules transduce the perceived light signals into cellular responses remain to be elucidated. The signal-transducing photoreceptors in plants are all phosphoproteins and/or protein kinases, suggesting that light-dependent protein phosphorylation and dephosphorylation play important roles in the function of the photoreceptors. This review focuses on the role of phytochromes' reversible phosphorylation involved in the light signal transduction in plants.     

植物的双组分信号系统

双组分系统由感受信号输入的组氨酸(His) 蛋白激酶和调节信号输出的反应调控因子组成,涉及许多原核生物、真菌、黏菌和植物的各种信号转导途径。在植物中,还存在更复杂的包括杂合的His激酶、磷酸传递中间体和反应调控因子的信号系统,称为多步骤双组分系统。最近的研究表明,双组分系统在对环境刺激和生长调节剂(如乙烯、细胞分裂素、光和渗透胁迫)的反应中起重要作用。     

Direct reception of light by chromatophores of lower vertebrates

Rapid color changes of lower vertebrates are caused by the motile activities of pigment cells (chromatophores) present in the skin tissue. Chromatophore motility is generally regulated by neural and/or by endocrine systems. However, in some cases, light also induces pigment aggregation or dispersion directly, which suggests the existence of visual pigments in chromatophores. In fact, some opsins, including melanopsin, have been identified. This article reviews light-sensitive chromatophores of lower vertebrates. Photoreceptive molecules (visual pigments) and signal transduction of light via a GTP-binding protein (G protein) are also discussed.     

植物光信号途径重要新调控因子TZP的研究进展

TZP(TANDEM ZINC-FINGER/PLUS3)是近年来鉴定到的一个光信号转导途径新组分,在光介导的植物生长发育过程中发挥重要调控作用。TZP不仅负调控蓝光信号途径,参与光敏色素B(phyB)介导的开花调控过程,还参与调控phyA在体内的蛋白质磷酸化。对TZP生化活性和作用机制的深入研究,不仅有助于进一步完善光信号调控网络,也可为设计和培育具有耐密理想株型及高光效作物新品种提供理论依据。该文系统总结了TZP在植物光信号途径中发挥的重要调控作用,并提出未来TZP功能研究的重要问题。