THE RELATIONSHIP BETWEEN THE PROMOTION OF ELONGATION OF FERN PROTONEMATA BY LIGHT AND GROWTH SUBSTANCES

Germinating spores of the fern Onoclea sensibilis L. were grown in darkness, so that they developed as filaments (protonemata). Brief daily exposure of the filaments to red, far-red or blue light increased the rate of filament elongation. Filament elongation was also promoted by indoleacetic acid. When filament elongation was promoted with both indoleacetic acid and exposure to light, the growth promotions caused by red and far-red light were additive to auxin-induced growth. Blue light promoted elongation only at sub-optimal concentrations of auxin. Elongation induced by guanine was additive to red- and far-red-induced elongation. Gibberellic acid had no effect on elongation under any condition. Blue-light-induced elongation resembled auxin-induced elongation in its requirement for exogenous sucrose and sensitivity to inhibition by parachlorophenoxyisobutyric acid. Red and far-red light were active regardless of the presence or absence of sucrose and promoted elongation at a concentration of parachlorophenoxyisobutyric acid which completely inhibited blue-light-induced elongation.     

Cytokinin, acting through ethylene, restores gravitropism to Arabidopsis seedlings grown under red light.

Cytokinin replaces light in several aspects of the photomorphogenesis of dicot seedlings. Arabidopsis thaliana seedlings grown under red light have been shown to become disoriented, losing the negative hypocotyl gravitropism that has been observed in seedlings grown in darkness or white light. We report here that cytokinin at micromolar concentrations restores gravitropism to seedlings grown under red light. Cytokinin cancels the effect of red light on the gravity-sensing system and at the same time replaces light in the inhibition of hypocotyl elongation. Furthermore, application of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid acts similarly to cytokinin. Cytokinin cannot restore gravitropism under red light to an ethylene-insensitive mutant that is defective at the EIN2 locus. Stimulation of ethylene production, therefore, can explain the action of cytokinin in restoring negative gravitropism to the hypocotyls of Arabidopsis seedlings grown under continuous red light.     

A phytochrome-mediated alteration from stem to rosette growth on chicory root explants in vitro

Chicory root explants (Cichorium intybus L. var. foliosum) of two cultivars, taken before and after hydroponic forcing, were cultured in vitro in complete darkness supplemented with red and far-red light treatments. Using 5 min red light per day, the strong stem elongation occurring in complete darkness was converted to rosette formation. This reaction was reversed to stem elongation (accompanied by leaf formation) adding 15 min far-red light after the red light. Fifteen min far-red light per day alone caused the same reaction as 5 min red/15 min far-red light. Far-red light followed by red light caused rosette formation. In stems, formed under complete darkness in vitro, the presence of phytochrome was shown. No phytochrome was detected in the root fragment itself.Abbreviations R red light - FR far-red light - GA gibberellinic acid - A absorbance - FW fresh weight     

Changes in photosensitive stem growth in intact peas following irradiation

Etiolated pea seedlings given a short red-light pretreatment followed by 30 hr of darkness no longer showed a typical red-light inhibition of internode elongation. The induction of phytochrome-insensitive growth was itself mediated by phytochrome, since far-red light reversed the effect of the short red-light pretreatment. Peas grown in white light showed a similar insensitivity to red light. However, in this instance the phytochrome system exerted some control over internode elongation since far-red light promoted growth slightly, and this effect was red-reversible.     

Cell Elongation and Microtubule Behavior in the Arabidopsis Hypocotyl: Responses to Ethylene and Auxin

During elongation of the Arabidopsis hypocotyl, each cell reacts to light and hormones in a time- and position-dependent manner. Growth in darkness results in the maximal length a wild-type cell can reach. Elongation starts at the base and proceeds in the acropetal direction. Cells in the upper half of the hypocotyl can become the longest of the whole organ. Light strongly inhibits cell elongation all along the hypocotyl, but proportionally more in the upper half. The ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) is known to stimulate hypocotyl elongation in the light. Here we show that this stimulation only occurs in cells of the apical half of the hypocotyl. Moreover, ACC application can partially overcome light inhibition, whereas indole-3-acetic acid (IAA) cannot. On low-nutrient medium (LNM) in the light, elongation is severely reduced as compared to growth on rich medium, and both ACC and IAA can stimulate elongation to the levels reached on a nutrient-rich medium. Furthermore, microtubule orientation was studied in vivo. During elongation in darkness, transverse and longitudinal patterns are clearly related with rates of elongation. In other conditions, except for the association of longitudinally orientated microtubules with growth arrest, microtubule orientation is merely an indicator of developmental age, not of elongation activity. A hypothesis on the relation between microtubules and elongation rate is discussed.     

Oak Seedlings Grown in Different Light Qualities

Seedlings of oak (Quercus robur) were germinated in darkness for 3 weeks and then given continuous light or short pulses of light (5–8 min every day). The morphological development was followed during 25 days. In continuous white, blue, and red light the stem growth terminated after about 10 days by formation of a resting bud. At that time the seedlings were about 100 mm high. In con tinuous long wavelength farred light (wavelength longer than 700 nm) the stem growth including leaf formation was continuous without the formation of resting buds, and the stem length was about 270 mm after 25 days. The number of nodes developed became twice that of the seedlings grown in while light. The leaves became well developed in all light colours, but leaf areas were largest in plants cultivated in white light. Compared to dark grown seedlings the mean area per leaf was increased about five times in continuous long wavelength far red light. A supplement with short (5 min) pulses of red light each day increased the leaf area up to 20 times. The stem elongation showed a high energy reaction response, i.e. the stem length increased only in continuous long wavelength far-red light but was not influenced by short pulses of red light or far-red light. The leaf expansion, however, was increased by short pulses of red light with a partial reversion of the effect by a subsequent pulse of far-red light. The fraction of the plant covered with periderm was higher in plants given continuous light. In respect to periderm inhibition continuous long wavelength far red light was the most effective. The transfer of seedlings from darkness to continuous white light gave anthocyanin formation in the stem 10–20 mm below the apex. This formation took place in the cortex and was evident in plants grown in darkness or under short pulses of light. Plants grown in continuous red, blue or long wavelength Far red light showed only traces of anthocyanin.     

Light inhibition of internode elongation in green plants

The clongation of the first internode of fully greenVigna sinensis L. is inhibited by white light (W). This inhibition is fluence-rate dependent between 0 and 70 Wm^–2. The kinetics of elongation rate in the light after darkness were investigated with linear displacement transducers. The internode elongation rate does not exhibit any endogenous rhythm. A rapid inhibition occurs during the first 2 or 3 h after the onset of light, and a second type of inhibition (slow reaction) increases from the beginning to the 8th hour of light. The rapid inhibition is not fluence-rate dependent between 20 and 70 Wm^–2, but the slow reaction is. There is no rapid inhibition in a low fluence rate white light to high fluence rate white light transition, only the slow reaction is observed. The responses to different wavebands, i.e., blue light (B), yellow and green light (YG), and red light (R), are the same for the two inhibition reactions. Each waveband used separately does not reproduce the full effect observed in W. Results show a stimulation with B, a greater inhibition activity with YG than with R, and a synergistic action of B and R which when given together lead to an inhibition similar to that obtained in W. Plants returned from the light to darkness progressively recover a high elongation rate without any latent period. The W light regulating internode elongation rate is mainly perceived by the growing internode itself.Abbreviations B blue light - D darkness - F far-red light - HW high fluence rate white light - LW low fluence rate white light - R red light - W white light - YG yellow and green light     

EFFECT OF LIGHT ON AUXIN TRANSPORT AND ELONGATION OF AVENA MESOCOTYL

The present work was undertaken to find if there are relations between light and auxin action on elongation of coleoptilar node and mesocotyl with Avena seedlings. Red light inhibited the elongation of mesocotyl and simultaneously decreased the rate of transport of diffusible auxin through the node. Red light also inhibited the transport of exogenously given IAA through the nodal region. The light inhibition of IAA transport was closely related to the increase of IAA immobilization. As the age proceeds, the ability of IAA immobilization increased with the decrease in the rate of mesocotyl elongation, even if the seedling was grown in complete darkness. The nature of radioactive substances found in the IAA-C¹⁴ treated tissue was examined by paper chromatography. The above results strongly suggested that the increase of IAA immobilization might result in the inhibition of mesocotyl elongation.     

Ornithine cycle inNostoc PCC 73102: Stimulation of in vitro ornithine carbamoyl transferase activity by addition of arginine

Cells ofNostoc PCC 73102, a free-living cyanobacterium originally isolated from the cycadMacrozamia, were cultured under different conditions and examined for the presence ofin vitro active ornithine carbamoyl transferase (OCT). Cells grown in darkness showed a significant increase ofin vitro OCT activity compared with the activity when grown in light. Addition of external arginine in the growth medium increasedin vitro OCT activity both in light and in darkness. Moreover, the highestin vitro OCT activity was observed in cells grown in darkness and with the addition of external arginine, a sevenfold increase compared with cells grown in light. Native-PAGE in combination with on gel OCT activity stain demonstrated that external arginine induced the presence of twoin vitro active OCT. In addition to the previously described 80 kDa OCT [Physiol Plant 84:275–282, 1992], a secondin vitro active enzyme with a molecular weight of approximately 118 kDa appeared. Western immunoblots, with native cell-free extracts and antibodies directed either against native or denatured OCT purified fromPisum sativum, confirmed that both enzymes were OCT. Moreover, with a denatured cell-free extract only one polypeptide, with a molecular weight of about 40 kDa, was recognized, indicating that the secondin vitro active OCT might be a trimer with three identical subunits.     

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.     

Effects of Colchicine and Light on Cell Form in Fern Gametophytes. Implications for a Mechanism of Light-induced Cell Elongation

Spores of the fern, Onoclea sensihilis L., suffer a disruption of normal development when they are cultured on media containing colchicine. Cell division is inhibited, and the spores develop into giant spherical cells under continuous white fluorescent light. In darkness only slight cell expansion occurs. Spherical cell expansion in the light requires continuous irradiation. Photosynthesis does not seem to be involved, since variations in light intensity do not affect the final cell diameter; the addition of sucrose to the medium does not permit cell expansion in darkness; and the inhibitor DCMU does not block the light-induced cell expansion. Continuous irradiation of colchicine-treated spores with blue, red or far-red light produces different patterns of cell expansion. Blue light permits spherical growth, similar to that found under white light, whereas red and far-red light promote the reestablishment of polarized filamentous growth. Although ethylene is unable to induce polarized cell expansion in colchicine-treated spores in darkness or white and blue light, it enhances filamentous growth which already is established by red or far-red irradiation. Both red and far-red light increase the elongation of normal filaments (untreated with colchicine) above that of dark-grown plants, but under all 3 conditions the rates of volume growth are identical. Light, however, does cause a decrease in the cell diameters of irradiated filaments. These data are used to construct an hypothesis to explain the promotion of cell elongation in fern protonemata by red and far-red light. The model proposes light-mediated changes in microtubular orientation and cell wall structure which lead to restriction of lateral cell expansion and enhanced elongation growth.     

The Geotropic Curvature in Roots of Norway Spruce (Picea abies) Containing Anthocyanins

Seedlings of Norway spruce (Picea abies L.) have been found to synthesize anthocyanins in the root tips as well as in the hypocotyls upon irradiation with white light when kept at 4°C for 6–8 days. In addition, it has also been found that the elongation and the geotropic curvature of spruce roots are dependent on the light conditions. The course of the geotropic curvature in spruce roots containing anthocyanins has been followed during a period of 5 h, in which the seedlings were geotropically stimulated continuously in the horizontal position. When the stimulation was performed in white light and in darkness at 21°C, significantly larger curvatures were observed in the roots pretreated at 4°C in darkness than in the roots containing anthocyanins. The specific curvature (curvature in degrees per mm elongation), however, was approximately the same in both types of roots stimulated in white light. This was due to a retarded elongation of the roots pretreated with light at 4°C and containing anthocyanins. A smaller difference in elongation rate between roots with and without anthocyanins was observed in the dark than in the light, but even in the dark the anthocyanin-containing roots grew more slowly than roots without anthocyanins. In order to find out if it is the anthocyanin content or the illumination which affects the elongation and geotropic curvature in the roots, a series of similar experiments was performed using cress seedlings grown at 4°C in light or darkness. Roots of cress seedlings cultivated under conditions which would induce anthocyanin formation in spruce roots exhibited the highest geotropic responses both in light and darkness as compared to cress seedlings grown at 4°C in darkness. As in the case of spruce roots an increase in elongation was observed in cress roots illuminated during the geotropic stimulation. These similarities in the behaviour made it relevant to compare the development of the geotropic curvature in cress and spruce roots.     

The Relationship of Endogenous Gibberellins to Light-regulated Stem Elongation Rates in Dwarf and Normal Cultivars of Pisum sativum L.

Dwarf cultivar Progress No. 9 and normal cultivar Alaska ofPisum sativum L. were grown under conditions of darkness orred light. Red light decreased the stem elongation rate of bothcultivars. Gibberellins present during the linear phase of stemelongation were isolated and the two main components were tentativelyidentified by gas chromato-chromatography and mass spectrometryas Gibberellin A₁ and A₅. Both gibberellins varied quantitativelywithin and between cultivar and treatment groups, and the amountspresent were inversely related to stem elongation rate. Althoughthe stem elongation rate of plants grown under red light wasrepressed, endogenous gibberellin was not limiting and was asmuch as two-fold higher in red light-grown plants than in dark-grownplants. The levels of endogenous gibberellin in dawrf plantsindicated that the genetic growth limitation was not due toa gibberellin deficiency. (Received May 26, 1981; Accepted January 28, 1982)     

Responses of Heterotrophic Cultures of Chlorella vulgaris Beyerinck to Darkness and Light. I. Pigment and pH Changes

Glucose cultures of Chlorella vulgaris were grown in white light, in monochromatic light, and in darkness. Difference spectra showed that all wavelengths resulted in increased pigmentation over the dark controls.

Cells irradiated with the 600 mμ beam showed a much higher absorption in the blue end of the spectrum with respect to the red end than is normally found in absorption spectra of white-light grown Chlorella cells.

Dry weight comparisons between monochromatic light and dark controls showed the controls to be somewhat higher. This demonstrated that the monochromatic irradiation produced pigment synthesis but no increase in growth. Dark growth experiments suggested that cultures incubated in darkness on glucose excreted an acidic product.

     

The response of Cuscuta planiflora seedlings to red and far-red, blue light and end-of-day irradiations

Irradiation of excised stem segments from de-etiolated seedlings of Cuscuta planiflora for 24 h with mixtures of red and far-red light with red to far-red ratios between 0.02 to 1.0 enhanced coiling and formation of prehaustoria. Maximum number of prehaustoria were recorded when red:far-red was near 0.1. Coiling and prehaustoria were observed whenever estimated in vivo Pfr/Ptotal at photoequilibrium was between 0.06 and 0.67. Irradiation of excised stem segments from white light grown seedlings with 12 h blue light also promoted coiling and prehaustoria formation after another 38 h in darkness. Coiling and prehaustoria were not observed in segments pulsed with 10 min red light at the end of 12 h in blue light. Coiling and prehaustoria were observed after photoreversible end-of-day far-red/red/far-red pulses but not after red/far-red/red pulses. A far-red pulse may not reverse inhibition by end-of-day red pulse when far-red is given more than 12 h after the red pulse.     

Effects of red and blue light on the composition and morphology of maize kernels grown in vitro

Growth and development of plants are known to be affected by exposure to red and blue light. Mechanisms by which light quality influences gene expression in maize (Zea mays L.) embryos have not been explored. Maize kernels can be cultured in vitro allowing experimental manipulation of environmental factors during seed development. We used the in vitro kernel culture system to investigate the response of developing maize seeds, which normally develop without exposure to light, to controlled light quality. Kernels grown under red light accumulated more dry weight than those grown in darkness, whereas kernels grown under blue light accumulated less. Reciprocal color shift experiments showed that light quality during the first week in culture had more influence on kernel weight than during the subsequent three weeks in culture. Soluble sugars were higher in both light treatments than in darkness. Blue-grown kernels had higher amino acid and lower lipid levels than red-or dark-grown kernels. Embryo morphology was markedly affected by red light, under which the upper shoot axis was longer than under blue light or in darkness. Embryo morphology was influenced by light quality during the later stages of development rather than the first week. We suggest, based on these results, that gene expression in the embryo and endosperm of developing maize seeds is sensitive to light quality, and the mechanism and time dependence of this effect warrant further study. In vitro maize kernel culture affords a convenient system for such light quality experiments.     

A role of G-proteins and calcium in light-regulated primary leaf formation in Sorghum bicolor

Primary leaf development of Sorghum bicolor is a phytochrome-mediated response. Primary leaves are not produced in Sorghum seedlings even after 10 d of germination if grown in darkness. However, 5 min irradiation with white light or red light given to 5 d etiolated seedlings resulted in the formation of etiolated leaves. This effect of red light was reversed by far-red light. When calcium (3-5 mM) was added exogenously, complete leaf formation was obtained in darkness; however, the kinetics of the response was slower than that seen with light irradiation. This effect was also obtained with potassium ions but magnesium ions had no effect. Light- and calcium-mediated leaf development could be arrested at the stage of leaf emergence or leaf expansion by the addition of inhibitors of G-proteins or by calcium channel blockers suggesting a role of G-proteins and calcium in phytochrome signal transduction during primary leaf development.Key words: Leaf formation, G-proteins, calcium, potassium, Sorghum bicolor.      

ULTRASTRUCTURE OF CHLORELLA PYRENOIDOSA AS AFFECTED BY ENVIRONMENTAL CHANGES

Chlorella pyrenoidosa (van Niel's strain) was cultured in four environmental conditions: light-glucose, light-galactose, dark-glucose, and dark-galactose. The gradual change in structure was observed when the cultures were transferred from light to darkness (dark adaptation). Cells grown in light contained well-developed chloroplasts and the normal cytoplasmic sub-units. During the transition to the complete heterotrophic mode of nutrition, the chloroplast regressed to the proplastid stage with a gradual reduction in thylakoid number. A prolamellar body was noted in the most regressed proplastid condition and plastid storage material was observed in all stages. The cell wall thickened during the transition. Cells adapted to darkness on galactose exhibited abundant cytoplasmic storage material with osmiophilic properties.     

Evidence against the involvement of phytochrome in UVB-induced inhibition of stem growth in green tomato plants

The effects of UVB on the kinetics of stem elongation of wild type (WT) and photomorphogenic mutants of tomato were studied by using linear voltage transducers connected to a computer. Twenty-one or twenty-six-day-old plants, grown in 12 h white light (150 μmol m⁻² s⁻¹ PAR)/12 h dark cycles, were first transferred to 200 μmol m⁻² s⁻¹ monochromatic yellow light for 12 h, then irradiated with 0.1 or 4.5 μmol m⁻² s⁻¹ UVB for 12 h and finally kept in darkness for another 24 h. The measurements of the kinetics of stem elongation started after 4 h under yellow light. Significant differences in stem growth during the irradiation with yellow light, as well as during the dark period, were found between the genotypes. In darkness, the magnitude of stem growth followed the order: tri > AC = fri > MMau > hp1. Two factors determined the large differences of growth in darkness: 1) the different stem elongation rate (SER) and 2) the different duration of the growing phase among the genotypes. In darkness the stem growth of au and hp1 mutants lasted for about 18 h, whereas it continued for the whole experimental period (36 h) in the other genotypes. UVB irradiation substantially reduced elongation growth of all genotypes (4.5 μmol m⁻² s⁻¹ being more effective than 0.1 μmol m⁻² s⁻¹). Both fluence rates of UVB induced a detectable reduction of SER already after 15 min of irradiation. Red light inhibited, while far red light promoted stem growth of all the genotypes tested. fri (phyA null), tri (phyB1 null), hp1 (exhibiting exaggerated phytochrome responses) mutants and WT tomato showed similar levels of UVB–induced inhibition of growth, while the aurea mutant showed the largest growth inhibition during the 12 h of irradiation. These results indicate that phytochrome is not directly involved in UVB control of stem elongation. The results of dichromatic irradiations UVB + red or UVB + far red indicate the presence of distinct and additive action of UVB photoreceptor and of the phytochrome system in the photoregulation of stem growth. This revised version was published online in June 2006 with corrections to the Cover Date.     

Ornithine cycle in Nostoc PCC 73102. Arginase,OCT and arginine deiminase,and the effects of addition of external arginine,ornithine, or citrulline

Arginase, ornithine carbamoyl transferase (OCT) and arginine deiminase activities were found in cell-free extracts of Nostoc PCC 73102, a free-living cyanobacterium originally isolated from the cycad Macrozamia. Addition of either arginine, ornithine or citrulline to the growth medium induced significant changes in their in vitro activities. Moreover, growth in darkness, compared to in light, induced higher in vitro activities. The in vitro activities of arginase and arginine deiminase, two catabolic enzymes primarily involved in the breakdown of arginine, increased substantially by a combination of growth in darkness and addition of either arginine, or ornithine, to the growth medium. The most significant effects on the in vitro OCT activities where observed in cells grown with the addition of ornithine. Cells grown in darkness exhibited about 6% of the in vivo nitrogenase activity observed in cells grown in light. However, addition of external carbon (glucose and fructose) to cells grown in darkness resulted in in vivo nitrogenase activity levels similar to, or even higher than, cells grown in light. Growth with high in vivo nitrogenase activity or in darkness with the addition of external carbon, resulted in repressed levels of in vitro arginase and arginine deiminase activities. It is suggested that nitrogen starvation induces a mobilization of the stored nitrogen, internal release of the amino compound arginine, and an induction of two catabolic enzymes arginase and arginine deiminase. A similar and even more pronunced induction can be observed by addition of external arginine to the growth medium.