Phytochrome action on the timing of cell division in adiantum gametophytes

When filamentous protonemata of Adiantum capillus-veneris L. precultured under continuous red light were transferred to the dark, the apical cell divided about 24 to 36 hours thereafter. The time of the cell division was delayed for several hours by a brief exposure to far red light given before the dark incubation. The effect of far red light was reversed by a small dose of red light given immediately after the preceding far red light. The effects of red and far red light were repeatedly reversible, indicating that the timing of cell division was regulated by a phytochrome system. When a brief irradiation with blue light was given before the dark incubation, the cell division occurred after 17 to 26 hours in darkness. A similar red far red reversible effect was also observed in the timing of the blue light-induced cell division. Thus, the timing of cell division appeared to be controlled by phytochrome and a blue light-absorbing pigment.     

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.     

Light Mediated Changes in the Chloroplasts of the Liverwort, Sphaerocarpos donnellii Aust

Dark-grown plants of Sphaerocarpos, incubated in a liquid medium containing sucrose and mineral salts, have a much lower chlorophyll and nitrogen content than do light-grown plants. Two minutes of red light per 12 hours is about two-thirds as effective in increasing chlorophyll and nitrogen content as is continuous white light. These red light-induced increases are mediated by phytochrome, as they are reversible by alternating exposures to red and far-red light. They appear to be related to differences in the ultrastructure of the chloroplasts. Plastids from dark-grown plants are full of starch and develop few lamellae, while light-grown plastids contain little starch and have many lamellae. The ultrastructural studies are supported by starch determinations which revealed a phytochrome-mediated decrease in starch content. The effect of white light in increasing the chlorophyll and nitrogen content above the level attained in red light-treated plants is not mediated by photosynthetic activity. These results are related to similar responses in other archegoniates and angiosperm seedlings.     

Evidence of phytochrome involvement in the entrainment of the circadian rhythm of carbon dioxide metabolism in Bryophyllum

The rhythm of carbon dioxide output in Bryophyllum leaves was entrained on exposure to 0.25 h of white light every 24 h. Entrainment also occurred on similar exposure to monochromatic radiation in spectral bands centred at 660 nm and, to a lesser extent, at 730 nm, but a band centred at 450 nm was without effect. A skeleton irradiation programme comprising two 0.25-h exposures to white light per 24 h also entrained the rhythm when the intervening dark periods were either 7.5 h and 16 h, or 10.5 h and 13 h. The rhythm disappeared when the two exposures were separated by 11.5-h and 12-h dark periods. Regular 0.25-h exposures to red light separated by 11.75-h periods of darkness also resulted in loss of the rhythm. Red/far-red reversibility was observed in irradiation schedules having either one or two exposures to red light daily. In the latter case, far-red reversal of the effects of one of the exposures to red light resulted in entrainment of the rhythm by the other, instead of abolition of the rhythm. The occurrence of distinct red/far-red reversibility suggests strongly that phytochrome is the pigment involved in entrainment of this rhythm by cycles of light and darkness.Abbreviation LD light-dark rhythm     

Phytochrome and potassium uptake by mung bean hypocotyl sections

Uptake of potassium (K) and ⁸⁶rubidiumlabelled potassium (⁸⁶Rb) by sub-hypocotyl hook sections of Phaseolus aureus L. was inhibited by red light. The effect was reversible with far red light. Using short exposures of high irradiance the effect on ⁸⁶Rb-labelled K uptake was observed after 5 min. The response showed no specificity for a particular anion. Uptake of ⁸⁶Rb-labelled K by sections cut immediately below the cotyledons was enhanced by red light after 10 min incubation and was also far red reversible. These results are interpreted as a rapid phytochrome-induced change in membrane properties resulting in modified K uptake.Abbreviations P Phytochrome - Pr red absorbing form of P - Pfr far red absorbing form of P - R red light - F far red light     

Blue light delays commitment to cell division in Chlamydomonas reinhardtii

In this study, we describe the effect of red and blue light on the timing of commitment to cell division in Chlamydomonas reinhardtii. The time point and cell size after which cells can complete their cell cycle with one division round were determined for cultures that were exposed to various red and blue light periods. We show that the commitment point of cells grown in blue light is shifted to a later time point and a larger cell size, when compared with cells grown in red light. This shift was reduced when cultures were exposed to shorter blue light periods. Furthermore, this shift occurred only when exposure to blue light started before the cells attained a particular size. We conclude that the critical cell size for cell division, which is the cell size at which commitment to cell division is attained, is dependent on spectral composition.     

Light-induced synchrony of cell division in the protonema of the fern,Pteris vittata

Summary In protonemata of Pteris vittata grown for 6 days under red light, which brings about a marked depression of mitotic activity, the first division of the cells was synchronously induced by irradiation with blue light, and subsequent cell divisions were also promoted. The peak of the mitotic index reached a maximum of about 70% at 11.5 hrs, and 90% of all protonemata divided between the 11th and 13th hour after exposure to blue light. When the protonemata were continuously irradiated with blue light, synchronism of the next cell division in the apical cells decreased to a mitotic index of about 30%, and further divisions occurred randomly.The synchronization of cell division was found to be a combined effect of red and blue light. Red light maintained the cells in the early G₁ phase of the cell cycle; blue light caused the cells to progress synchronously through the cell cycle, with an average duration of 12 hr. By using ³H-thymidine, the average duration of the G₁, S, G₂ and M phases was determined to be about 3.5, 5, 2.5 and 1 hr, respectively.Synchronous cell division could be induced in older protonemata grown for 6 to 12 days in red light and even in protonemata having two cells. It could be repeated in the same protonema by reexposure to red light for 24 hrs or more before another irradiation with blue light.     

Photocontrol of Hook Opening in Cuscuta gronovii Willd

Hook opening in seedlings of Cuscuta gronovii Willd. occurred only after prolonged exposures to blue, red, or far red light. Prolonged far red exposure was less effective than prolonged exposure to red or blue light. Brief far red irradiation inhibited the inductive effect of red light. The far red inhibition was in turn reversed by brief red irradiation. These effects suggest the involvement of two photosystems in the control of hook opening in Cuscuta gronovii Willd.: a phytochrome-mediated system and a separate high energy requirement.     

The Role of Light in Leaf and Flower Bud Break of the Peach (Prunus persica)

The effect of light on peach leaf and flower bud break was examined. It was found that leafless dormant shoots were light-perceptive organs. Darkness, after light preconditioning during dormancy, reduced leaf bud opening; however, light was obligatory when the shoots were preconditioned in the dark. Relatively short exposures to light were sufficient to stimulate leaf bud break. Terminal buds were less inhibited by darkness than were laterals. Flower bud break was inhibited in light after dark preconditioning. The red region of the spectrum was found to be active; the phytochorome system seems to be involved in the light reactions, as the red light effect was reversible with subsequent far-red illumination. Supplementary light, producing long-day conditions, could partly compensate for insufficient chilling. A possible sequence of reactions in the plant is suggested.     

Effect of red and blue light on the timing of cyclin-dependent kinase activity and the timing of cell division in Chlamydomonas reinhardtii.

In this study, we describe the effect of red and blue light on the timing of cell division, DNA synthesis, and activity and presence of cyclin-dependent kinases (CDKs), in synchronous cultures of the unicellular green alga Chlamydomonas reinhardtii. Cell division and DNA synthesis were found to occur later in cells grown in blue or white light, than in red light. CDK-like activity, measured using a histone H1 kinase assay, correspondingly occurred later in cultures that were grown in blue light compared to cultures grown in red light. The amount of CDK-like proteins, as detected using an antibody against the PSTAIRE motif, showed a maximum during the division phase. We conclude that the mechanism that causes the delay in the timing of cell division in blue light has its action before DNA replication takes place and also precedes the increase in CDK-like activity.     

PHOTOCONTROL OF THE ORIENTATION OF CELL DIVISION IN ADIANTUM. I. EFFECTS OF THE DARK AND RED PERIODS IN THE APICAL CELL OF GAMETOPHYTES

When protonemata of Adiantum capillus-veneris L. which had been grown filamentously under continuous red light were transferred to continuous white light, the apical cell divided transversely twice, but the 3rd division was longitudinal. An intervening period of darkness lasting from 0 to 90 hr either between the 1st and the 2nd cell division or between the 2nd and the 3rd one did not affect the number of protonemata in which the 3rd cell division was longitudinal. The insertion of red light instead of darkness greatly decreased the percentage of 1st longitudinal divisions occurring at the 3rd division, and increased the number of transverse divisions. Fifty percent reduction of induction of 1st longitudinal division was caused by ca. 50 hr exposure to red light between 1st and 2nd division and by ca. 20 hr between 2nd and 3rd division, and total loss was induced by an exposure of ca. 100 hr or longer to red light in the former and by ca. 40 hr longer in the latter. Thus, by using an appropriate intervening dark period or exposure to red light, the orientation and timing of cell division could be controlled in apical cell of the fern protonemata.     

Phytochrome-mediated growth responses in green and etiolated Lemna minor

Summary The growth of Lemna minor in darkness is log-linear, at a much reduced rate compared to growth in white or red light. This rate of frond production in darkness is stimulated by kinetin, yeast extract, and thiamine either in green plants transferred directly from the light or in plants which had been grown in the dark for 54 days. (Fig. 1).The magnitude of the stimulation of frond production by interruption of darkgrowth with red light (Fig. 2) is smaller in green than in etiolated plants, and is shown to depend upon the length of time that initially green plants were held in darkness (Fig. 4, Table 2). The stimulation of frond production in either green or etiolated plants does, however, obey the reciprocity law (Fig. 3).The stimulation by red light can be fully and repeatedly nullified by far red light only in etiolated plants, but the efficiency of nullification of the red effect by far red seems to increase in green plants with increasing sets of red + far red exposures (Fig. 5).As the dark-interval between red and far red exposures is lengthened, the efficiency of nullification is lessened significantly for etiolated plants only after 30 min (Fig. 6).     

EFFECTS OF PHYTOCHROME AND BLUE-NEAR ULTRAVIOLET LIGHT-ABSORBING PIGMENT ON DURATION OF COMPONENT PHASES OF THE CELL CYCLE IN ADIANTUM GAMETOPHYTES

Single-celled protonemata of the fern Adiantum capillus-veneris, kept under continuous red light, grew with a very low rate of cell division, and the cell cycle was arrested in the early G₁ phase. Cell division was induced by transferring the protonemata to the dark after various light treatments, and the duration of component phases in the cell cycle was determined by a continuous-labelling technique with 3H-thymidine. Blue light irradiation greatly reduced the duration of the G₁ phase but did not affect that of other phases. The greater the fluence of blue light, the shorter was the duration of G₁ phase was observed. In contrast, a brief exposure of red-light-grown protonemata to far-red light given immediately before the dark incubation showed no effect on the duration of G₁ S and M phases but significantly extended that of the G₂ phase. The effect of far-red light on the G₂ phase was reversed by red light, and the effects of red and far-red light were repeatedly reversible. The progression in the M phase was shown by means of a time-lapse video system to be not at all influenced by any pre-irradiation described above.     

STIMULATION OF LIGHT-SATURATED PHOTOSYNTHESIS IN LAMINARIA (PHAEOPHYTA) BY BLUE LIGHT1

The light-saturated rate of photosynthesis in blue light was 50-100% higher than that in red light for young sporophytes of Laminaria digitata (Huds.) Lamour., although photosynthetic rates were slightly higher in red than in blue light at low irradiances. Short exposures to low irradiances (e.g. 2 min at 20 μmol · m^?2· s^?1) of blue light also stimulated the subsequent photosynthesis of Laminaria sporophytes in saturating irradiances of red light but had little effect on photosynthesis in low irradiances of red light. The full stimulatory effect of short exposures to blue light was observed within 5 min of the blue treatment and persisted for at least 15 min in red light or in darkness. Thereafter, the effect began to decline, but some stimulation was still detectable 45 min after the blue treatment. The degree of stimulation was proportional to the logarithm of the photon exposure to blue light over the range 0.15-2.4 mmol · m^?2, and the effectiveness of an exposure to 0.6 mmol · m^?2at different wavelengths was high at 402-475 nm (with a peak at 460-475 nm) but declined sharply at 475-497 nm and was minimal at 544-701 nm. Blue light appears, therefore, to exert a direct effect on the dark reaction of photosynthesis in brown algae, possibly by activating carbon-fixing enzymes or by stimulating the uptake or transport of inorganic carbon in the plants.     

Phytochrome-mediated Electric Potential Changes in Oat Seedlings

Brief exposures to red light induce far red-reversible changes of 5 to 10 millivolts magnitude in the upper 1 centimeter of etiolated Avena coleoptiles. The changes begin within 15 seconds of the start of illumination and they continue for at least 12 minutes. The changes were measured using a flowing solution of 10 mm KCl to contact the surface of the coleoptile. A dark-grown coleoptile shows no change in response to far red light unless it first receives red light treatment. The second of two red light exposures is ineffective without an intervening far red treatment. Some characterization of these electric responses to light is presented.     

Effect of Low-intensity Red and Far-red Light and High-intensity White Light on the Flowering Response of the Long-day Plant Lemna gibba G3

The long-day plant Lemna gibba L., strain G3 exhibits a relatively low sensitivity to short, white-light interruptions given during the dark period of a short-day cycle. However, the plants are fairly sensitive to low-intensity red light treatments given during a 15-hour dark period on the third day of a 2LD-(9L:15D)-2LD-7SD schedule. Far-red light is almost as effective as red light, and attempts to reverse the red light response with subsequent far-red light treatments have not been successful. Blue light proved to be without effect. When plants were grown on a 48-hour cycle with 15 minutes of red light every 4 hours during the dark period, the critical daylength was reduced from about 32 hours to slightly less than 12 hours.

Continuous red light induced a fairly good flowering response. However, as little as 1 hour of white light each day gave a significant improvement in the flowering response over that of the continuous red light control. White light of 600 to 700 ft-c was more effective than white light of 60 to 70 ft-c. The white light was much more effective when divided into 2 equal exposures given 8 to 12 hours apart. These results suggest an increase in light sensitivity with regard to flower induction about 8 to 10 hours after the start of the light period.

     

Light-mediated bioregulation of tillering and photosynthate partitioning in wheat

The influence of plant population density on spectral distribution of light received by wheat ( Triticum aestivum L. cv. Coker 797) seedlings was measured under field conditions, and effects of red and far-red light on tillering and photosynthate partitioning were studied in controlled environments. Spectral distribution of light was measured in sunflecks at soil level in close-, intermediate-, and wide-spaced field populations during the tillering stage. Close-spaced seedlings received higher far-red/red light ratios than wide-spaced plants because of the larger amount of far-red reflected from green leaves of the more numerous nearby plants. The far-red/red light ratios in all population densities were higher in late afternoon than at noon. Close-spaced plants developed fewer tillers, less roots and longer leaves than wide-spaced seedlings under field conditions. In controlled environments, a higher far-red/red ratio during photosynthetic periods resulted in fewer tillers and longer leaves; whereas, brief red or far-red exposures at the end of each day had a more pronounced effect. Wheat seedlings that received 5-min exposures to far-red light at the end of the photosynthetic period each day for 20 consecutive days developed fewer tillers, longer leaves, less roots, and a higher shoot/root biomass ratio. The effects of far-red light were reversed by red light. The light spectral shifts associated with field plant population densities and the responses to red and far-red treatments under controlled environments suggest that phytochrome serves as a sensing mechanism that detects the amount of competition from other plants, and regulates the development of tillers and the partitioning of photosynthate between shoots and roots.     

Model for variable light sensitivity in imbibed dark-dormant seeds

The level of light-induced germination of the seed of common purslane (Portulaca oleracea L.) and curly dock (Rumex crispus L.) changes with dark incubation time prior to brief, low energy, red light treatment. The rate at which phytochrome—far red-absorbing form (Pfr) acts in the light-induced population of seeds was measured by quantitating per cent reversals of the red light effect with saturating far red light exposures at successive times after the red light exposure. A linear positive correlation was found between this rate and the final germination level. These results are compatible with a model involving changing levels, during dark incubation, of a component with which Pfr interacts. In this model, germination is initiated after attainment of a certain level of interaction between Pfr and this component. These findings also support the view that the Pfr to Pr decay rate constant and total phytochrome level are stable during dark incubation.     

Capacity of cytochrome and alternative path in coupled and uncoupled root respiration of Pisum and Plantago

A critical duration of darkness must be exceeded for the photoperiodic induction of flowering in short-day plants. This requires detection of the light/dark transition at dusk and the coupling of this information to a time-measuring system.
Lowering the P_fr/P_tot, ratio photochemically at the end of the day did not accelerate the onset of dark timing in Pharbitis nil Choisy cv. Violet. Time-measurement was initiated when, with no change in spectral quality, the irradiance fell below a threshold value. Thus, if the light/dark transition at dusk is sensed by a reduction in P_fr, this reduction can be achieved as rapidly through thermal reactions as through photochemical ones. When given at hourly intervals during a 6-h extension of a 24-h main light period in white light, pulses of red light were as effective as continuous red light in delaying the onset of timing; pulses every 2 or 3 h were less effective. The effectiveness of intermittent red light indicates that phytochrome is the photoreceptor and the requirement for frequent exposures suggests that P_fr is lost rapidly in the dark. However, the red light pulses could not be reversed by far-red light, which argues against this hypothesis. An alternative explanation is that the perception of light as being continuous occurs only when "new" P_fr is regenerated sufficiently frequently.
The nature of the coupling of the dusk signal to the time-measuring system is discussed and it is suggested that the effect of each red light pulse is to delay the phase of the photoperiodic rhythm by 1–3 h.