Photocontrol of the hypocotyl hook opening of Sinapis alba seedlings. Involvement of phytochrome and a high irradiance response

Baumgartner, N. and Fondeville, J. C. 1989. Photocontrol of the hypocotyl hook opening of Sinapis alba seedlings. Involvement of phytochrome and a high irradiance response.
A statistical evaluation of the hypocotyl hook opening (hook opening index) was used for measurement of the hook angle in lots of etiolated Sinapis alba L. cv. Albatros seedlings. Studies of the kinetics for hook opening were carried out in continuous fluorescent white, blue and red light (6, 15 and 40 μmol m^-2s^-1) with 2-day-old dark-grown seedlings. At the beginning of the irradiation period the photoresponse in red light was the opposite to that in blue (low photon fluences). Blue rapidly induced the hook opening (in less than 20 min), while red produced hook tightening (photon fluences up to 70 mmol m^-2), which precedes the normal progressive hook opening. For low fluences, the data were consistent with the involvement of phytochrome and a specific blue light photoreceptor. A phytochrome effect was observed in the hook opening, dependent upon a high irradiance response (HIR). This HIR (like that for the inhibition of the hypocotyl elongation) was characterized by a wavelength response curve with maxima in the blue and far-red regions of the spectrum.     

Action spectra for the inhibition of growth in radish hypocotyls

In etiolated seedlings of Raphanus sativus L. the inhibition of hypocotyl elongation by continuous light showed a major bimodal peak of action in the red and far-red, and two minor peaks in the blue regions of the spectrum. It is argued that, under conditions of prolonged irradiation, phytochrome is the pigment controlling the inhibition of hypocotyl elongation by red and far-red light, but that its mode of action in far-red is different from that in red. A distinct pigment is postulated for blue light.Abbreviations B blue - FR far red - G green - R red - HIR high irradiance reaction - P_r and P_fr red and far red absorbing forms of phytochrome - R red     

The effect of red and far-red light in the high irradiance reaction of phytochrome (hypocotyl growth in dark-grown Sinapis alba L.)

Abstract. Fluence-rate response curves were determined for the inhibition of hypocotyl growth in 54-h-old dark-grown Sinapis alba L. seedlings by continuous or hourly 5 min far-red light irradiation (24 h). Just as in red light ( Heim & Schäfer, 1982 ), a fluence-rate dependence was observed for both kinds of irradiations, even if only 35% of the continuous light effect could be substituted for by hourly far-red pulses. The same total fluence was used for the two different light regimes. Measurements of Pfr and Pfr/Ptot showed a strong fluence-rate dependence under continuous light which only partially paralleled the fluence-rate response curves for the inhibition of the hypocotyl growth. It was concluded that neither spectrophotometrically determined levels of Pfr nor Pfr/Ptot can be the only light-dependent factor controlling hypocotyl lengthening under continuous irradiation.     

Photoregulation of hypocotyl growth: geotropic evidence for the operation of two photosystems

Abstract. The rate of curvature of etiolated cress ( Lepi-dium sativum L. ) hypocotyls in response to gravity (negative geotropism) was retarded by red or blue light; far-red irradiation was without effect. The timing of the irradiation period in relation to the presentation for geostimulus markedly affected the response. When seedlings were irradiated during the 1–2 h period of geostimulus, blue light was more effective than red at retarding curvature; when seedlings were irradiated prior to geostimulus, only red light affected geocurvature. These results are interpreted as a further example of the kinetically distinct effects of red and blue light on hypocotyl development. Blue light elicited a rapid, immediate response effective only during the period of irradiation; red light induced a response characterized by a lag period and persistence in subsequent darkness. Etiolated mustard seedlings showed similar responses to light and gravity. The results are discussed in relation to the possibility that two photosystems operate in hypocotyl growth.     

Shoot Auxin and Light in the Mechanism Maintaining the Posture of the Cotyledons of Linum usitatissimum

The mean cotyledon angle (liminal day posture) of flax is linearlyrelated to light intensity, the cotyledons being raised withincreasing intensity; adjustments occur when plants are transferredto new conditions. Short days, low light intensity, and prolongeddarkness result in epinasty. Epinastic and hyponastic responsesare enhanced by excision of the epicotyl shoot. Applicationof indoleacetic acid to the cut shoot is an effective substitutefor the excised portion. Ringing the intact epicotyl with 2,3,5-tri-iodobenzoicacid has an effect comparable to excision. Given continuously or in a light/dark cycle, blue light induceshyponasty, and red light maintains the liminal posture for about48 h after which time epinasty occurs. Far-red light given continuouslyinduces epinasty to an extent comparable to the effect of darkness.Irradiation with far-red light even for brief periods priorto placing the plants in darkness enhances the epinastic response:the effect of far-red light is reversed by a subsequent shortexposure to red light. Interruption of a prolonged period of darkness by blue lightconsiderably lessens the subsequent epinastic reaction whetheror not any epinasty has occurred before the light-break. Redor white light given under these circumstances has less effect.     

A light-regulated pool of phytochrome and rudimentary high-irradiance responses under far-red light in Pinus elliottii and Pseudotsuga menziesii

The presence of a phytochrome pool down-regulated by light and the occurrence of high-irradiance responses to far-red light are well documented in angiosperms but not in gymnosperms. A pool of phytochrome was identified in Pinus elliottii and Pseudotsuga menziesii seedlings grown in darkness with a monoclonal antibody developed against oat phytochrome A. This pool was barely detectable in light-grown tissues. Dark-grown conifer seedlings transferred to continuous red light showed a gradual decrease of the levels of immunodetectable phytochrome. This decrease was significantly slower in gymnosperms than in angiosperms. Dark-grown seedlings of P. elliottii and P. menziesii showed enhanced growth of the cotyledonary whorl and increased anthocyanin pigmentation of the hypocotyl, but no hypocotyl-growth inhibition in response to continuous far-red light. Hourly pulses were significantly less effective than continuos far-red light. The response to far-red light was not observed in seedlings pretreated with red light to reduce the levels of immunodetectable phytochrome. Rudiments of phytochrome A-like function and kinetics are present in P. elliottii and P. menziesii.     

Green light stimulates early stem elongation, antagonizing light-mediated growth inhibition

During the transition from darkness to light, the rate of hypocotyl elongation is determined from the integration of light signals sensed through the phototropin, cryptochrome, and phytochrome signaling pathways. In all light conditions studied, from UV to far-red, early hypocotyl growth is rapidly and robustly suppressed within minutes of illumination in a manner dependent upon light quality and quantity. In this study, it is shown that green light (GL) irradiation leads to a rapid increase in the growth rate of etiolated Arabidopsis seedlings. GL-mediated growth promotion was detected in response to constant irradiation or a short, single pulse of light with a similar time course. The response has a threshold between 10(-1) and 10(0) micromol m(-2), is saturated before 10(2) micromol m(-2) and obeys reciprocity. Genetic analyses indicate that the cryptochrome or phototropin photoreceptors do not participate in the response. The major phytochrome receptors influence the normal amplitude and timing of the GL response, yet the GL response is normal in seedlings grown for hours under constant dim-red light. Therefore, phytochrome activation enhances, but is not required for, the GL response. Seedlings grown under green, red, and blue light together are longer than those grown under red and blue alone. These data indicate that a novel GL-activated light sensor promotes early stem elongation that antagonizes growth inhibition.     

Photosensory mechanisms in the lettuce seedling hypocotyl

Summary A number of differences in the responses of Great Lakes lettuce seedlings to blue and far-red light indicate that more than one photo-sensitive pigment is involved in the photo-inhibition of hypocotyl elongation under highenergy conditions. In far-red light the inhibitory effect is restricted to young seedlings and is of limited duration; after 24 hours in far-red a rapid growth rate similar to that of plants maintained in darkness is resumed, despite continued irradiation. The onset of inhibition is relatively slow. Blue light, in contrast, exerts a strongly inhibitory effect on elongation at any age, and a slow rate of growth persists throughout the entire irradiation period. The onset of inhibition is very rapid. Furthermore, even when the inhibition in far-red had already been exhausted after prolonged exposure, transfer to blue light resulted in a prompt reduction in growth rate. Also the effect of far-red is almost completely lost after a pre-irradiation with red light which does not affect the response to blue. It is concluded that the responses to blue and far-red light in Great Lakes lettuce are not mediated by a single pigment system and that a distinct blue-sensitive pigment is present in addition to phytochrome. Red light has a number of different effects depending on conditions: (1) a pretreatment with red light almost completely prevents the inhibitory effect of a subsequent far-red irradiation, (2) a brief terminal treatment with red increases the inhibitory effect of either far-red or blue light; this is reversed by far-red, and (3) prolonged exposure to red light given alone increases the growth rate relative to darkness, because the more rapid elongation rate characteristic of young seedlings continues for longer with red light than in plants grown in darkness throughout.     

Obtention d'épinastie des cotylédons de Sinapis alba en éclairement rouge Iointain continu.

Obtention of epinasty in the cotyledons of Sinapis alba under continuous far-red illumination. Relation to the development of hypocotyl and cotyledons. Treatment by far red light induces epinasty of the cotyledonary leaf. The effects of light treatments of different intensities were recorded for hypocotyl length, percentage of epinasty and the cotyledon leaf area. The percentage of epinasty increases as a function of treatment duration and light intensity. As from the second day of treatment a correlation is obtained between the percentage of epinasty and hypocotyl length when the seedlings are given far-red light treatment of different durations.     

Light quantity and quality interactions in the control of elongation growth in light-grown Chenopodium rubrum L. seedlings

The spectral control of hypocotyl elongation in light-grown Chenopodium rubrum L. seedlings has been studied. The results showed that although the seedlings responded to changes in the quantity of combined red and far-red radiation, they were also very sensitive to changes in the quantity of blue radiation reaching the plant. Altering the proportion of red: far-red radiation in broad waveband white light caused marked differences in hypocotyl extension. Comparison of the responses of green and chlorophyll-free seedlings indicated no qualitative difference in the response to any of the light sources used, although photosynthetically incompetent plants were more sensitive to all wavelengths. Blue light was found to act primarily of a photoreceptor which is different from phytochrome. It is concluded that hypocotyl extension rate in vegetation shade is photoregulated by the quantity of blue light and the proportion of red: far-red radiation. In neutral shade, such as that caused by stones or overlying soil, hypocotyl extension appears to be regulated primarily by the quantity of light in the blue waveband and secondarily by the quantity of light in the red and far-red wavebands.Abbreviations B blue - FR far-red - k ₁, k ₂ rate constants for photoconverison of Pr to Pfr and Pfr to Pr, respective - k ₁/k ₁ +k ₂= phytochrome photoequilibrium - k ₁ +k ₂= phytochrome cycling rate - Pr=R absorbing form of phytochrome - Pfr=FR absorbing form of phytochrome - P_tot Pr+Pfr - PAR photosynthetically active radiation = 400–700 nm - R red - WL white light     

Role of the Cotyledons in the Phototropic Response of Lavatera cretica Seedlings

Young seedlings of Lavatera cretica L. exhibit positive phototropism. The hypocotyl perceives unilateral illumination with blue light and curves towards the light source by unequal growth. In addition, the cotyledonary laminas perceive the vectorial component of unilateral illumination with blue light and reorient normal to the beam by creating a turgor differential in their pulvini. Excision of one cotyledon resulted in negative organotropic curvature of the hypocotyl, away from the remaining cotyledon. Illumination of the cotyledonary lamina did not participate in the phototropic curvature of the hypocotyl, so long as the lamina was free to reorient to face the beam. When the lamina was continuously exposed to vectorial photoexcitation, elongation of the hypocotyl on the side carrying the cotyledon could be enhanced, or inhibited, depending on the direction of the beam striking its lamina.     

In vitro shoot development of Prunus GF 655–2: interaction between light and benzyladenine

Experiments designed to study the effect of light quality and quantity and their possible interaction with benzyladenine (BA) in the control of in vitro proliferation of Prunus insititia Schneider GF 655–2 microcuttings are reported. The action of BA as a promoting factor of shoot formation was expressed only in the presence of light. The concentration response curves for BA-induced proliferation were very similar under the different light sources, irrespective of proliferation rate values.
Shoot formation under blue, far-red and white light was enhanced by the highest photon fluence rates, while the efficiency of red seemed to be independent of this factor. The results suggest the action of a low-energy response in the red waveband and under the low photon fluence rates of blue and white. The inhibition of shoot elongation induced by BA in the dark as well as under all light treatments indicates that, while the BA-induced release from the apical dominance is light dependent. BA inhibition of shoot elongation is entirely light independent.     

Genetic analysis of the roles of phytochromes A and B1 in the reversed gravitropic response of the lz-2 tomato mutant

The lz-2 mutation in tomato ( Lycopersicon esculentum ) causes conditional reversal of shoot gravitropism by light. This response is mediated by phytochrome. To further elicit the mechanism by which phytochrome regulates the lz-2 phenotype, phytochrome-deficient lz-2 plants were generated. Introduction of au alleles, which severely block chromophore biosynthesis, eliminated the reversal of hypocotyl gravitropism in continuous red and far-red light. The fri ¹ and tri ¹ alleles were introduced to specifically deplete phytochromes A and B1, respectively. In dark-grown seedlings, phytochrome A was necessary for response to high-irradiance far-red light, a complete response to low fluence red light, and also mediated the effects of blue light in a far-red reversible manner. Loss of phytochrome B1 alone did not significantly affect the behaviour of lz-2 plants under any light treatment tested. However, dark-grown lz-2 plants lacking both phytochrome A and B1 exhibited reduced responses to continuous red and were less responsive to low fluence red light and high fluence blue light than plants that were deficient for phytochrome A alone. In high light, full spectrum greenhouse conditions, lz-2 plants grew downward regardless of the phytochrome deficiency. These results indicate that phytochromes A and B1 play significant roles in mediating the lz-2 phenotype and that at least one additional phytochrome is involved in reversing shoot gravitropism in this mutant.     

The Influence of Chlorophyll on the Spectral Control of Elongation Growth in Chenopodium rubrum L. Hypocotyls

The inhibitory effectiveness of various monochromatic wavelengthsbetween 399 and 802 nm on hypocotyl elongation growth in light-grownChenopodium rubrum L. seedlings has been studied. The responsesof normal light-grown seedlings and chlorophyll-free light-grownseedlings were compared. Both types of seedling responded moststrongly to the blue and red waveband although a distinct peakof red light effectiveness was not observed in normal greenseedlings. The presence of chlorophyll also correlates witha lower inhibitory effectiveness of most wavelengths in the400–700 nm waveband. Photon fluence-rate response curves were not parallel; whereasthe plants were very sensitive to changes in fluence-rate inthe blue waveband, a much less marked fluence-rate dependencywas observed in the red and far-red wavebands. (Received September 10, 1981; Accepted April 26, 1982)     

Photocontrol of hypocotyl elongation in light-grown Cucumis sativus L.

An interaction is demonstrated between the effects of phytochrome and cryptochrome (the specific blue-light photoreceptor) in the inhibition of hypocotyl elongation of light-grown cucumber (Cucumis sativus L.) cv. Ridge Greenline seedlings. At certain fluence rates of blue light the total inhibition response is greater than the sum of the separate responses to each photoreceptor. The threshold for response to blue light is reduced at least 30-fold by additional red-light irradiation. The synergistic effect is demonstrated for two different fluence rates of red light. Synergism is mediated by phytochrome in both the cotyledons and the hypocotyl.Abbreviations and symbols BL blue light - FR far-red light - Pfr far-red-absorbing form of phytochrome - R red light - photostationary state of phytochrome - _c calculated      

Photoregulation of asymmetric cell division followed by rhizoid development in the fern Ceratopteris prothalli

Strap-shaped prothalli of CERATOPTERIS: richardii grown in the dark have an apical meristem, a subapical elongation zone and a basal growth cessation zone [Murata et al. (1997) Plant Cell Physiol. 38: 201]. When the dark-grown prothalli were irradiated with continuous white light, marginal cells of the elongation zone divided asymmetrically, and the resulting smaller cells developed into rhizoids. The asymmetric division was also induced by brief irradiation of red light. The effect of red light was cancelled by subsequent irradiation of far-red light, indicating that the asymmetric division was regulated by phytochrome. Since the response to red light was not observed at 10(1) J m(-2) and saturated at 10(2) J m(-2) and the response is photoreversible by far-red light, the photoresponse was classified as a low-fluence response of phytochrome. Although the asymmetric division was induced by brief irradiation of red light, continuous irradiation of white, blue or red light was necessary to induce rhizoid growth. These results indicate that asymmetric division and subsequent cell growth are independently regulated by light in CERATOPTERIS: prothalli.     

Control of hypocotyl elongation in Arabidopsis thaliana by photoreceptor interaction

In order to test the interaction of different phytochromes and blue-light receptors, etiolated seedlings of wild-type Arabidopsis thaliana (L.) Heynh., a phytochrome (phy) B-overexpressor line (ABO), and the photoreceptor mutants phyA-201, phyB-5, hy4-2.23n, fha-1, phyA-201/phyB-5, and phyA-201/hy4-2.23n were exposed to red and far-red light pulses after various preirradiations. The responsiveness to the inductive red pulses is primarily mediated by phyB which is rather stable in its far-red-absorbing form as demonstrated by a very slow loss of reversibility. Without preirradiation the red pulses had an impact on hypocotyl elongation only in PHYA mutants but not in the wild type. This indicates a suppression of phyB function by the presence of phyA. Preirradiation with either far-red or blue light resulted in an inhibition of hypocotyl elongation by red pulses in the wild type. Responsiveness amplification by far-red light is mediated by phyA and disappears slowly in the dark. The extent of responsiveness amplification by blue light was identical in the wild type and in the absence of phyA, or the cryptochromes cryl (hy4-2.23n) or cry2 (fha-1). Therefore, we conclude that stimulation of phyB by blue light preirradiation is either mediated by an additional still-unidentified blue-light-absorbing pigment or that phyA, cry1 and cry2 substitute for each other completely. Both blue and red preirradiation established responsiveness to red pulses in phyA-201/phyB-5 double mutants. These results demonstrate that inhibition of hypocotyl elongation by red pulses is not only mediated by phyB but also by a phytochrome(s) other than phyA and phyB. Received: 21 July 1998 / Accepted: 7 December 1998     

Photomorphogenetic responses to UV radiation and short-term red light in lettuce seedlings

Effects of red light (R), far-red light (FR) and UV radiation on growth and greening of lettuce seedlings (Latuca sativa L., cv. Berlinskii) have been investigated. UV-B and UV-C inhibited hypocotyl elongation and stimulated cotyledonary growth. R in combination with UV-B and UV-C partly eliminated these effects, but FR increased those and reversed the R effect. Chlorophyll accumulation was inhibited by UV-B and UV-C. In comparison with cotyledonary growth, R strengthened the UV inhibitory effect, and FR reversed this effect of R. Thus, UV and phytochrome system modify the effects of each other on hypocotyl and leaf growth in lettuce seedlings depending on the level of active phytochrome formed.