Photomorphogenesis in Sinningia speciosa, cv. Queen Victoria I. Characterization of Phytochrome Control

The morphological development of Sinningia speciosa plants that were exposed to supplementary far red light was very different from that of plants receiving dark nights. After several nights of such irradiation, stems and petioles were elongated, petioles were angulated, leaf blade expansion was inhibited, plants were chlorotic and the accumulation of shoot dry weight was retarded.

Red reversibility of the morphological changes potentiated by far red light indicated control by the phytochrome system. A high P_FR level during the last half of the night inhibited stem elongation and promoted leaf blade expansion, but both of these processes were hardly affected by the P_FR level during the first half of the night. Thus sensitivity to P_FR was cyclic.

The interpretation of our experiments was complicated by quantitative morphological differences resulting from long, as compared to short, far red irradiations.

     

Photocontrol of Anthocyanin Synthesis: VI. Spectral Sensitivity, Irradiance Dependence, and Reciprocity Relationships

The spectral sensitivity and the irradiance dependence of anthocyanin synthesis, a “high irradiance response,” in cabbage (Brassica oleracea, cv. Red Acre) and tomato (Lycopersicon esculentum, cv. Beefsteak) seedlings exposed to continuous irradiation depend upon the length of the exposure. In cabbage, blue and red are more effective than far red when the irradiations are shorter than 12 hours and less effective than far red when the irradiations are longer than 12 hours. The irradiance dependence is negligible under red and becomes evident under blue and far red red only for exposures longer than 12 hours. Anthocyanin synthesis under intermittent light treatments, of efficiency comparable to that of continuous treatments, obeys the Bunsen-Roscoe reciprocity law and is a function of the dose (irradiance × time), rather than of the irradiance alone. The validity of the reciprocity relationships suggests that only one photoreceptor is responsible for the photocontrol of the response in the blue, red, and far red spectral regions. The characteristics of the response suggest that the photoreceptor is phytochrome, at least in cabbage.     

Light-dependent anthocyanin synthesis: A model system for the study of plant photomorphogenesis

The biosynthesis of anthocyanins in plant tissues either requires light or is enhanced by it. Light-dependent anthocyanin synthesis has been extensively used as a model system for studies of the mechanism of photoregulation of plant development. Two components can be distinguished in the action of light on anthocyanin production. The first component is the red-far red reversible, phytochrome-mediated response induced by short irradiations; the amount of anthocyanin formed in response to a single, short irradiation is small. The second component is the response to prolonged exposures; the formation of large amounts of anthocyanin requires prolonged exposures to high fluence rates of visible and near-visible radiation (290 to 750 nm) and shows the typical properties of the “High Irradiance Reaction” (HIR) of plant photomorphogenesis. Phytochrome is involved in the photoregulation of the HIR response and is the only photoreceptor mediating the action of prolonged red and far red irradiations. The response to prolonged ultraviolet and blue radiation is probably mediated, at least in some systems, by two photoreceptors: phytochrome and cryptochrome, the latter being a specific ultraviolet-blue-light photoreceptor. The nature of the interaction between phytochrome and cryptochrome in the regulation of plant photomorphogenic responses is still unclear.     

Phytochrome-mediated responses in light-grown corn showing rapid,reverse reciprocity failure*

Abstract. Three responses (mesocotyl and coleoptile elongation and anthocyanin accumulation in the coleoptile) to end- of-day far-red irradiation in light-grown corn show rapid failure of the reciprocity law such that short, high fluence rate irradiations are much more effective than long, low fluence rate ones of the same fluence (reverse reciprocity failure). The reciprocity failure cannot be explained by escape from photoreversibility, a change in sensitivity to Pfr, reciprocity failure for photoconversion, or a high irradiance response taking over for long irradiation times. Fluence–response curves measured by varying irradiation time at a low fluence rate show the threshold fluence shifted to higher energy in comparison with fluence–response curves obtained at a high fluence rate. Red reversal of these responses also shows rapid reciprocity failure in the same direction, a process which can be only partially explained by escape. These responses to end-of-day far-red and red illumination are distinguished from high irradiance reactions by their low fluence requirements and ready reversibility. These same characteristics are similar to those of classical phytochrome- mediated, induction-reversion responses in etiolated tissue, but it is difficult to explain the rapid, reverse reciprocity failure in terms of standard phytochrome dogma.     

Involvement of phytochrome and a blue light photoreceptor in UV-B induced flavonoid synthesis in parsley (Petroselinum hortense Hoffm.) cell suspension cultures

Flavonoid synthesis in cell suspension cultures of parsley (Petroselinum hortense Hoffm.) occurs only after irradiation with ultraviolet light (UV), mainly from the UV-B (280–320 nm) spectral range. However, it is also controlled by phytochrome. A P_fr/P_tot ratio of approximately 20% is sufficient for a maximum phytochrome response as induced by pulse irradiation. Continuous red and far red light, as well as blue light, given after UV, are more effective than pulse irradiations. The response to blue light is considerably greater than that to red and far red light. Continuous red and blue light treatments can be substituted for by multiple pulses and can thus probably be ascribed to a multible induction effect. Continuous irradiations with red, far red and blue light also increase the UV-induced flavonoid synthesis if given before UV. The data indicate that besides phytochrome a separate blue light photoreceptor is involved in the regulation of the UV-induced flavonoid synthesis. This blue light receptor seems to require the presence of P_fr in order to be fully effective.Abbreviations HIR high irradiance response - P_fr far red absorhing form of phytochrome - P_tet total phytochrome - UV ultraviolet light     

Reorientation of microtubules at the outer epidermal wall of maize coleoptiles by phytochrome,blue-light photoreceptor,and auxin

Summary The effects of red and blue light on the orientation of cortical microtubules (MTs) underneath the outer epidermal wall of maize (Zea mays L.) coleoptiles were investigated with immunofluorescent techniques. The epidermal cells of dark-grown coleoptiles demonstrated an irregular pattern of regions of parallel MTs with a random distribution of orientations. This pattern could be changed into a uniformly transverse MT alignment with respect to the long cell axis by 1 h of irradiation with red light. This response was transient as the MTs spontaneously shifted into a longitudinal orientation after 1–2 h of continued irradiation. Induction/reversion experiments with short red and far-red light pulses demonstrated the involvement of phytochrome in this response. In contrast to red light, irradiation with blue light induced a stable longitudinal MT alignment which was established within 10 min. The blue-light response could not be affected by subsequent irradiations with red or far-red light indicating the involvement of a separate blue-light photoreceptor which antagonizes the effect of phytochrome. In mixed light treatments with red and blue light, the blue-light photoreceptor always dominated over phytochrome which exhibited an apparently less stable influence on MT orientation. Long-term irradiations with red or blue light up to 6 h did not reveal any rhythmic changes of MT orientation that could be related to the rhythmicity of helicoidal cell-wall structure. Subapical segments isolated from dark-grown coleoptiles maintained a longitudinal MT arrangement even in red light indicating that the responsiveness to phytochrome was lost upon isolation. Conversely auxin induced a transverse MT arrangement in isolated segments even in blue light, indicating that the responsiveness to blue-light photoreceptor was eliminated by the hormone. These complex interactions are discussed in the context of current hypotheses on the functional significance of MT reorientations for cell development.Abbreviations MT cortical microtubule - P_r, P_fr red and far-red absorbing form of phytochrome     

Light Rquirements for Photoperiodic Sensitivity in Cotyledons of Dark-grown Pharbitis nil

The light requirements for induction of flowering by a long dark period were investigated in dark-grown seedlings of Pharbitis nil Chois, cv. Violet. The cotyledons bcame photoperiodically sensitive to a 24 h dark period by two 1 min red irradiations (6.3 μmol m⁻² S⁻¹) separated by a 24 h dark period. The reversibility of the effect of brief red irradiations, and the effectiveness of low energies of red irradiation suggest the involvement of phytochrome in the induction of photoperiodic sensitivity. Partial de-etiolation occurred after these brief periods of red irradiation but the seedlings were not capable of net CO₂ uptakeeven 7 h after the start of the main light period that followed the critical dark period. A changing response to the duration of the priod of darkness given between the two short red irradiations showed the the correct phasing of an endogenous photoperiodic rhythm is needed for the attainment of photoperiodic snsitivity.     

Photocontrol of Anthocyanin Synthesis: IV. Dose Dependence and Reciprocity Relationships in Anthocyanin Synthesis

Under continuous far red light, anthocyanin synthesis in young, dark-grown cabbage seedlings (Brassica oleracea cv. Red Acre) is irradiance-dependent and fails to follow the reciprocity (irradiance × time = constant) relationships. Under intermittent far red treatments extended over a prolonged period of time, anthocyanin synthesis becomes dose dependent, and reciprocity relationships are valid. Intermittent far red treatments with short dark intervals between successive irradiations are as effective as continuous treatments, if the total radiation doses applied with the two types of treatments are equal and are applied over equally long periods of time. The high effectiveness of inter-mittent treatments, the dose dependence, and the validity of the reciprocity relationships suggest that cycling between red-absorbing form of phytochrome and far red-absorbing form of phytochrome and the formation of electronically excited far red-absorbing form of phytochrome, or the involvement of a second photoreactive system, besides phytochrome, may play only a minor role in high irradiance reaction anthocyanin synthesis brought about by prolonged exposures to far red irradiation.     

Action of light and streptomycin on protein synthesis in cabbage seedlings

The action of light on protein synthesis was examined in the cabbage seedlings, a system extensively used in the studies of anthocyanin synthesis. Continuous red and far red light have no effect on total protein content while they cause a marked decrease in the level of free amino acids in cabbage seedlings. The rate of protein synthesis, measured as incorporation of radioaetively-labelled amino acids into proteins, is clearly stimulated by light. Phytochrome involvement in the light stimulation of the incorporation is also demonstrated by the red-far red reversibility of the response. The relative effectiveness of continuous red and far red light upon the incorporation of amino acids into proteins is affected by the nature of the system used to study the incorporation process. When excised cotyledons and short period of incorporation were used, continuous far red was more effective than red. However, when whole seedlings and long period of incorporation were used, red and far red were equally effective. Streptomycin causes a 10– 15% decrease in the rate of incorporation of amino acids into proteins of all cellular fractions, except the plastid fraction where a much higher inhibition (30%) was observed.     

Red Light-inhibited Mesocotyl Elongation in Maize Seedlings: II. Kinetic and Spectral Studies

Red light induces two distinct inhibition responses in mesocotyls of etiolated corn seedlings. A light dose of 10 nanoeinsteins per square centimeter is saturating for the more sensitive response, whereas doses above 1,000 nanoeinsteins per square centimeter are required to exceed the threshold sensitivity of the less sensitive one. The sensitive response can be detected within 20 minutes of the onset of illumination whereas the other response does not become apparent until more than 4 hours after the beginning of irradiation. The reciprocity law is valid for the first response, but probably not for the second. An action spectrum for the first response shows two maxima, one at 640 nanometers and the other between 660 and 670 nanometers, with a pronounced minimum near 650 nanometers. The effects both of 640 and 665 nanometers of light were reversible by far red light, but doses of far red required for full reversibility were almost three orders of magnitude greater than the doses of red required either to saturate the initial inhibition or to reverse the effect of far red light. The results suggest that corn may contain a red-absorbing pigment other than phytochrome which in some way interacts with phytochrome in the inhibition of mesocotyl elongation by red light.     

The characteristics of light in floral induction in vitro of Cichorium intybus. The possible role of phytochrome

The action of light in the initiation of floral buds in vitro has been studied using monochromatic light qualities on root explants of a long day plant, Cichorium intybus L. cv. Witloof. Red light (660 nm, 0.30 W m^-2) promotes flowering, while far-red (730 nm, 0.31 W m^-2) and irradiation with combined red + far-red (0.20 + 0.41 W m^-2) have no effect. In short day conditions floral response can be obtained in two ways: 1) by interrupting the dark period with 5 brief irradiations of red light (0.45 W m^-2, 12 min) at regular intervals, although these are counteracted by far-red irradiations of equal intensity and duration; 2) by interrupting the long night with 5 h red light applied during the second third of the night, while at the beginning or at the end it is ineffective. Red light efficiency appears to depend on the photosynthetic activity of the tissues, so that flowering increases with increasing intensity of white light and is suppressed if no white light is supplied. The reproductive development is determined by the coordination of proper irradiation conditions with sufficient sensitivity of the perceiving meristematic cells. The period of highest sensitivity to environmental light conditions in the life cycle of a Cichorium root explant occurs between the 8th and the 16th day after the start of the culture. The data strongly suggest that phytochrome is involved in flower induction of Cichorium in vitro.     

Phytochrome and blue light-mediated stomatal opening in the orchid,paphiopedilum

Guard cells of the orchid genus, Paphiopedilum have been reported to lack developed chloroplasts and detectable chlorophyll a autofluorescence. Paphiopedilum stomata lack a photosynthesis-dependent opening response but have a blue light-specific opening. The present study found that low fluence rate green and red light elicited stomatal opening in Paphiopedilum and this opening was reversed by far red light, indicating the presence of a phytochrome-mediated opening response. Phytochrome-dependent, red light-stimulated opening was largest under low fluence rates and decreased to near zero as fluence rate increased. A recently discovered green light reversibility of blue light-specific stomatal opening was used to probe the properties of the blue light response in Paphiopedilum stomata. Blue light-stimulated opening was completely reversed by green light in the presence of far red light. Red light enhanced the blue light response of Paphiopedilum guard cells when given as a pretreatment or together with blue light. Analysis of guard cell pigments showed that guard cells have small amounts of chlorophyll a and b, zeaxanthin, violaxanthin, antheraxanthin and lutein. Zeaxanthin content increased in response to blue light or ascorbate and declined in the dark or under illumination in the presence of dithiothreitol, indicating the presence of an active xanthophyll cycle. Thus Paphiopedilum stomata possess both a blue light-mediated opening response with characteristics similar to species with normal chloroplast development and a novel phytochrome-mediated opening response.     

Photocontrol of spore germination in the fern Thelypteris kunthii

The light requirement for germination in spores of the fern Thelypteris kunthii (Desv.) Morton was fully satisfied by a long period of continuous red light or partially by intermittent, short periods of red light. Red light-potentiated spore germination was inhibited by brief far-red light irradiation, indicating phytochrome involvement. Repeated exposure of spores to prolonged red and short far-red irradiations, or exposure of red-potentiated spores to far-red light after an extended period in darkness, led to their escape from inhibition of germination by far-red light. Prolonged irradiation of spores with blue light before or after red light treatment partially antagonized the effect of red light.     

The Effect of Temperature and Light on the Germination of Lettuce Seeds

‘Grand Rapids’ lettuce. seed will not germinate in darkness at 30°. An exposure in temperatures between 5°C and 25°C for a short period after the initiation of germination can effectively overcome the high temperature imposed dormancy. If the exposure to low temperature is from the beginning of germination it is less effective. The low temperature induced germination is not reversed by far red light of 725 nm and seed not responding to the low temperature do respond in a classical fashion to red and far red irradiations. It is considered that the results justify acceptance of the hypothesis that an inhibitor of germination is produced during the initial stages of germination and that this formation is strongly temperature dependent so that there is much accumulation. At low temperatures an alternative metabolic pathway predominates leading to the production of an essential metabolite. At high temperatures this metabolite is produced from the inhibitor (or inert compound) by a mechanism under the control of the phytochrome system.     

Phytochrome control of two low-irradiance responses in etiolated oat seedlings

Light-induced coleoptile stimulation and mesocotyl suppression in etiolated Avena sativa (cv. Lodi) has been quantitated. Etiolated seedlings showed the greatest response to light when they were illuminated 48 to 56 hours after imbibition. Two low-irradiance photoresponses for each tissue have been described. Red light was 10 times more effective than green and 1,000 times more effective than far red light in evoking these responses. The first response, which resulted in a 45% mesocotyl suppression and 30% coleoptile stimulation, had a threshold at 10⁻¹⁴ einsteins per square centimeter and was saturated at 3.0 × 10⁻¹² einsteins per square centimeter of red light. This very low-irradiance response could be induced by red, green, or far red light and was not photoreversible. Reciprocity failed if the duration of the red illumination exceeded 10 minutes. The low-irradiance response which resulted in 80% mesocotyl suppression and 60% coleoptile stimulation, had a threshold at 10⁻¹⁰ einsteins per square centimeter and was saturated at 3.0 × 10⁻⁸ einsteins per square centimeter of red light. A complete low-irradiance response could be induced by either red or green light but not by far red light. This response could be reversed by a far red dose 30 times greater than that of the initial red dose for both coleoptiles and mesocotyls. Reciprocity failed if the duration of the red illumination exceeded 170 minutes. Both of these responses can be explained by the action of phytochrome.     

Phytochrome-Mediated Control of Diamine Oxidase Level in the Epicotyl of Etiolated Lentil (Lens culinaris Medicus) Seedlings

Diamine oxidase (DAO; EC 1.4.3.6) levels are strongly reduced in epicotyls of 3-day-old etiolated lentil (Lens culinaris Medicus) seedlings upon exposure to continuous red and blue light, as compared to etiolated controls. Far-red light inhibits DAO activity to a lesser extent. A less marked effect can also be obtained by short (5-10 min) daily exposures. Phytochrome involvement in this light-mediated response has been demonstrated by red/far-red reversibility experiments. These findings provide the first evidence that mechanisms underlying the photoregulation of DAO level in the Leguminosae are related to photomorphogenesis and are essentially unrelated to the photosynthetic capacity of the seedling.     

Studies of the Mechanism of Enhancement of Phytochrome-dependent Lettuce Seed Germination by Prechilling

Temperature and kinetic studies were performed to examine the mechanism by which prechilling stimulates phytochrome-dependent seed germination in lettuce, Lactuca sativa, L. cv. Grand Rapids. Imbibed seeds were given a short far red irradiation and one day of dark incubation at 20 C to establish very low levels of the far red-absorbing form of phytochrome—(Pfr). Germination was greatly stimulated by subsequent prechilling treatments when they were followed by a second short far red irradiation. Prechilling therefore increased germination sensitivity to the low, normally inhibitory Pfr levels established by far red irradiation. This sensitivity increased with lowered prechilling temperature to a maximum near 4 C. It was linearly dependent upon duration of prechilling at 4 C up to a near maximal response at 10 hours, and it decayed in a converse manner when seeds were returned to 20 C after 10 hours at 4 C. Prechilling also increased germination responses to subsequent periods of high levels of Pfr which were initiated by red and terminated by far red irradiations. High Pfr periods adequate to promote the germination of unchilled seeds produced sharp inflections at 18 C in the dependence of germination on prechilling temperature. Rates of phytochrome potentiation of germination were not affected by prechilling. The response to prechilling fit a mechanism involving homeoviscous adaptation of membrane lipids to temperature.     

The influence of de-etiolation on the sensitivity of Avena coleoptiles to the far-red absorbing form of phytochrome

In photoresponses regulated by phytochrome the effect of a red irradiation is not always reversed by far-red. This applies for instance to the influence of red light on the geotropic reactions of Avena coleoptiles. We could induce red/far-red reversibility by a short de-etiolating exposure to red light about 20 h prior to the experimental irradiations. This, was due to a decrease of the sensitivity to the low level of the far-red absorbing form of phytochrome that is established by far-red. Since etiolated plants react also to a wavelength of 520 nm (green light), it is advisable to expose the coleoptiles to a de-etiolating irradiation prior to manipulations in green safelight in order to prevent the plants from reacting to the green light.     

Synergism between gibberellic acid and low Pfr levels inducing germination of Kalanchoë seeds

Sown on water, seeds of Kalanchoëbiossfetdiana Poelln. cv. Feuerblute are absolutely light-requiring and show full red/far-red reversibility. In seeds, sown on 2 ×10^-3 M gibberellic acid, red/far-red reversibility disappears and both short red and far-red irradiations induce germination. Gibberellic acid alone does not induce germination, but it increases the physiological activity of P_fr to the extent, that the low P_fr level obtained by far-red irradiation becomes very effective. The synergism between gibberellic acid and far-red light appears after a two-day incubation; period. The nature of this lag phase was examined by measuring both germination and uptake of labelled gibberellic acid in intact seeds and seeds with a punctured seed coat. The lag phase was shown to be independent of the uptake kinetics of gibberellic acid and allows development to a specific stage, necessary for germination after phytochrome-phototransformation. The kinetics of the uptake of gibberellic acid by intact seeds and embryos of intact seeds are different. In intact seeds most of the gibberellic acid is retained in the seed coat; only a small fraction actually penetrates to the embryo where it can exert its physiological activity.     

Effects of Red and Far Red Light on the Initiation of Cold Acclimation in Cornus stolonifera Michx

Red and far red light distinctly influence the initial phytochrome-mediated phase of cold acclimation in red-osier dogwood (Cornus stolonifera). Under controlled conditions, short days and end-of-day far red light exposure after long days promote growth cessation, cold acclimation, and subsequent cold hardening of dogwood stems in response to low temperature. Nuclear magnetic resonance absorption spectra of the water in internode stem sections imply that the short day-induced phase of cold acclimation involves a change in tissue hydration, at least in part, due to a substantial reduction in bulk phase water as a result of senescence and loss of water from the pith. Seasonal responses to light and an attempt to induce early acclimation under natural conditions with end-of-day far red light are discussed.