Photocontrol of stem elongation in light-grown plants of Fuchsia hybrida

Stems of the caulescent long-day plant, Fuchsia hybrida cv Lord Byron, showed 2 types of response to light. In one, internode length was increased by far-red irradiation given at the end of an 8 h photoperiod: the response was no greater with prolonged exposure and was less when the start of far-red was delayed. The effect of far-red was reversible by a subsequent exposure to red light. Internode length was inversely proportional to the P_fr/P ratio established before entry to darkness and there was no evidence for loss of P_fr during a 16 h dark period. The inhibitory effect of P_fr acted at a relatively late stage of internode growth. With the development of successive internodes a second response appeared in which stems lengthened following prolonged daily exposures to red or far-red light, or mixtures of the two, or to brief breaks with red or white light. In these later internodes, a short exposure to far-red near the middle of the night was not reversible by red because red alone promoted elongation at this time. Internode length increased with increase in the daily duration of light and, when light was given throughout an otherwise dark period of 16 h, with increase in illuminance to a saturation value of 200 lx from tungsten lamps. Elongation increased as a linear function of decrease in photostationary state of phytochrome down to P_fr/P0.3; however, internodes were shorter in far-red light than in 25% red/red+far-red. It was concluded that stem length is a net response to two modes of phytochrome action. An inductive effect of P_fr inhibits a late stage in internode expansion, and a phytochrome reaction which operates only in light (and may involve pigment cycling) promotes an early stage of internode development. Stem elongation is thus a function both of the daily duration of light and its red/red+far-red content. The outgrowth of axillary buds was controlled by the first type of phytochrome action only.Abbreviations and symbols FR far red light - R red light - P phytochrome - P_fr phytochrome in the far-red light absorbing form - SD 8 h short days - LDP long-day plant - SDP short-day plant     

Far red end-of-day treatment restores wild type-like plant length in hybrid aspen overexpressing phytochrome A

Shoot elongation in woody plants is modulated by a multitude of light signals, including irradiance, photoperiod and spectral composition, for which the phytochrome system is the probable photoreceptor. In hybrid aspen ( Populus tremula  ×  tremuloides ) overexpression of the oat phytochrome A ( PHYA ) prevents growth cessation in response to short photoperiod, and plants exhibit dwarf growth that is related to reduced cell numbers and reduced gibberellin contents. End-of-day far-red treatment significantly enhances internode elongation in PHYA overexpressors as well as in the wild type, and this was found here to be caused by stimulation of cell division and cell extension. In PHYA overexpressors the effects were substantially larger than in the wild type, and resulted in complete restoration of wild type-like plant length as well as cell numbers, and gibberellin content was greatly increased. No clear effect of far-red end-of-day treatment on gibberellin levels could be detected in the wild type. It thus appears that the far-red end-of-day treatment might modify the responsiveness of the tissue to GA rather than the GA levels. The observed effects were completely reversed by a subsequent irradiation with red light. The present data show that dwarfism due to PHYA overexpression can be completely overcome by far red end-of-day treatment, and the observations indicate that effects of far red end-of-day treatments appear to be mediated by phytochrome(s) other than phytochrome A.     

Ethylene and the Responses to Light of Rice Seedlings

The growth of rice seedlings (Oryza satira L.) in the presence of ethylene caused a change in the response to light of coleoptile elongation. In plants grown in air without added ethylene coleoptile elongation was promoted by red, far-red and yellow-green light only in very young seedlings; in older plants irradiation inhibited the growth of the coleoptile. The effect of growing plants in the presence of ethylene was to prolong the period during which light promoted coleoptile growth. Elongation of the first internode was inhibited by light whether or not the seedlings were grown in the presence of ethylene. A correlation existed between the growth effect of an irradiation and the initial decay rate of phytochrome which was established by the treatment. Regardless of wave length, light sources whose intensities were adjusted to produce a decay rate of about 10% per hour or less induced a moderate rate of coleoptile elongation which persisted for a relatively long period. Irradiation with red or yellow-green light of higher intensity which produced a higher rate of phytochrome decay induced a higher rate of coleoptile elongation, but growth stopped after several hours. Other observations, however, showed that one cannot establish a general simple correlation between the rate of elongation of rice coleoptiles under light and the status of measurable phytochrome in the plant.     

Action Spectra for Light-induced Elongation in Fern Protonemata

The action spectrum for promotion of elongation of protonemata of Onoclea sensibilis has peaks at 400–420, 580–600 and 640–660 nm. The largest growth increments at saturating light doses are produced by yellow and far-red light. Elongation induced by yellow and far-red irradiation persists in old as well as young filaments, while red-light promotion is found only in young filaments. The growth promotion caused by yellow light is partially reversed by red light down to the level of growth produced by red irradiation alone. Elongation of rhizoids is under reversible red, far-red control, while yellow light is inactive. A model is proposed and discussed in which the light-sensitive elongation of filaments is accounted for by the presence of three distinct photoreceptors: phytochrome; a pigment absorbing yellow light. P₅₈₀; and a pigment absorbing blue light, P₄₂₀.     

Photophysiology of the Elongated Internode (ein) Mutant of Brassica rapa: ein Mutant Lacks a Detectable Phytochrome B-Like Polypeptide

Several phytochrome-controlled processes have been examined in etiolated and light-grown seedlings of a normal genotype and the elongated internode (ein/ein) mutant of rapid-cycling Brassica rapa. Although etiolated ein seedlings displayed normal sensitivity to prolonged far-red light with respect to inhibition of hypocotyl elongation, expansion of cotyledons, and synthesis of anthocyanin, they displayed reduced sensitivity to prolonged red light for all three of these deetiolation responses. In contrast to normal seedlings, light-grown ein seedlings did not show a growth promotion in response to end-of-day far-red irradiation. Additionally, whereas the first internode of light-grown normal seedlings showed a marked increase in elongation in response to reduced ratio of red to far-red light, ein seedlings showed only a small elongation response. When blots of protein extracts from etiolated and light-treated ein and normal seedlings were probed with monoclonal antibody to phytochrome A, an immunostaining band at about 120 kD was observed for both extracts. The immunostaining intensity of this band was substantially reduced for extracts of light-treated normal and ein seedlings. A mixture of three monoclonal antibodies directed against phytochrome B from Arabidopsis thaliana immunostained a band at about 120 kD for extracts of etiolated and light-treated normal seedlings. This band was undetectable in extracts of ein seedlings. We propose that ein is a photoreceptor mutant that is deficient in a light-stable phytochrome B-like species.     

Phytochrome control mechanisms in leaf expansion of Phaseolus vulgaris cv. Limburg.

Abstract. The effectiveness of a red-light pulse acting through phytochrome in inducing primary leaf expansion in 9-d-old etiolated bean ( Phaseolus vulgaris L. ev. Limburg) seedlings is strongly increased by a continuous far-red light (CFR) pretreatment. This increase in effectiveness of a red pulse is positively correlated with the time and the fluence rate of the CFR pretreatment. Escape from photoreversibility of this red pulse after the CFR pretreatment is extremely slow (more than 3 d). When a dark period is interposed between the end of the CFR pretreatment and the inductive red pulse the photoreversible part of the response to this pulse is highly dependent upon the photostationary state of phytochrome at the onset of the dark period.
The results give strong evidence for the synergistic activity of two components of phytochrome action during leaf growth induction, one of them acting via a very stable Pfr fraction.     

Persistent effects of changes in phytochrome status on internode growth in light-grown mustard: Occurrence,kinetics and locus of perception

Extension growth of the first internode in fully de-etiolated mustard (Sinapis alba L.) seedlings (11–12.5 d old) is under the control of both the current phytochrome photoequilibrium (Pfr/P, ratio of the far-red-absorbing form of phytochrome to total phytochrome) and that established by short (<12 h) pretreatments. Plants were pretreated with either light pulses providing different calculated Pfr/P followed by dark incubations of different durations (a), or with a 12-h period of white light establishing different Pfr/P (b). After the pretreatments, the plants received either light pulses providing different Pfr/P, followed by dark incubations (c), or continuous white light with or without addtional far-red light (d). Thus, four experimental approaches were followed: (a)(c); (a)(d); (b)(c) and (b)(d). Extension growth during the second period (c or d) was not only affected by the current phytochrome status, but also by that established during the pretreatment period (a or b). The results show the existence of a long-term promotion of stem growth which persists after the end of the low Pfr/P pretreatment. This effect is different from the previously reported rapid effect of far-red light added to background white light as follows: (i) the duration of low Pfr/P required to effect a full response is longer (2.5 h); (ii) the duration of the promotion after returning to high Pfr/P is longer (approx. 24 h) and (iii) the locus of perception is mainly in the leaves, rather than the growing internode.Abbreviations FR far-red light - PAR photosynthetically active radiation - Pfr/P ratio between the FR-absorbing form and total phytochrome - R red light - WL white light     

Conditional Synergism between Cryptochrome 1 and Phytochrome B Is Shown by the Analysis of phyA, phyB, and hy4 Simple, Double, and Triple Mutants in Arabidopsis

Wild-type or phyA, phyB, or hy4 mutant Arabidopsis seedlings lacking phytochrome A (phyA), phytochrome B (phyB), or cryptochrome 1 (cry1), respectively, and the double and triple mutants were used in combination with blue-light treatments given simultaneously with red or far-red light. We investigated the interaction between phytochromes and cry1 in the control of hypocotyl growth and cotyledon unfolding. Under conditions deficient for cry1 (short exposures to blue light) or phyB (far-red background), these photoreceptors acted synergistically: Under short exposures to blue light (3 h/d) added to a red-light background, cry1 activity required phyB (e.g. the hy4 mutant was taller than the wild type but the phyBhy4 mutant was not taller than the phyB mutant). Under prolonged exposures to blue light (24 h/d) added to a far-red light background, phyB activity required cry1 (e.g. the phyAphyB mutant was taller than the phyA mutant but the phyAphyBhy4 mutant was not taller than the phyAhy4 mutant). Under more favorable light inputs, i.e. prolonged exposures to blue light added to a red-light background, the effects of cry1 and phyB were independent. Thus, the synergism between phyB and cry1 is conditional. The effect of cry1 was not reduced by the phyA mutation under any tested light condition. Under continuous blue light the triple mutant phyAphyBhy4 showed reduced hypocotyl growth inhibition and cotyledon unfolding compared with the phyAphyB mutant. The action of cry1 in the phyAphyB double mutant was higher under the red-light than the far-red-light background, indicating a synergistic interaction between cry1 and phytochromes C, D, or E; however, a residual action of cry1 independent of any phytochrome is likely to occur.     

Red-light and blue-light photoreceptors controlling chlorophyll a synthesis in the red alga Porphyra umbilicalis and in the green alga Ulva rigida

Chlorophyll synthesis is stimulated by red light in the green alga Ulva rigida C. Ag. and in the red alga Porphyra umbilicalis (L.) Kützing. Because the effect of red light showed some far-red reversibility in successive red and far-red light treatments, the involvement of phytochrome or a phytochrome-like photoreceptor is suggested. The extent of the response is dependent on exposure and photon fluence rate of red-light pulses. In addition to the effect of red light, a strong stimulation of chlorophyll synthesis by blue light was only observed in Ulva rigida. The effect of blue light shows also some far-red reversibility. In the green alga the accumulated chlorophyll is higher after blue light pulses than after red light pulses. In Porphyra umbilicalis , however, the contrary is observed. In Ulva rigida the involvement of a blue light photoreceptor in addition to phytochrome or a phytochrome-like photoreceptor is proposed. The different responses to red and blue light in both algae are explained in terms of their adaptation to the natural light environment.     

Temperature-dependent internode elongation in vegetative plants of Arabidopsis thaliana lacking phytochrome B and cryptochrome 1

 Vegetative plants of Arabidopsis thaliana (L.) Heynh. form a compact rosette of leaves in which internode growth is virtually arrested. Rapid extension of the internodes occurs after flower buds are present in the reproductive apex. Under natural radiation, continuous light from fluorescent lamps, or short photoperiods of light from fluorescent lamps, plants of the phyB cry1 double mutant (lacking both phytochrome B and cryptochrome 1) did not form normal rosettes because all the internodes showed some degree of elongation. Internode elongation was weak in the phyB single mutant and absent in the cry1 mutant, indicating redundancy between phytochrome B and cryptochrome 1. The absence of phytochrome A caused no effects. The failure to form normal rosettes was conditional because internode elongation was arrested at low temperatures in all the mutant combinations. In contrast, the temperature dependence of phytochrome B and cryptochrome 1 effects on hypocotyl growth was weak. The elongation of the internodes in phyB cry1 was not accompanied by early flowering as showed by the lack of effects on the final number of leaves. Apex dissection indicated that in phyB cry1 double mutants internode elongation anticipated the transition from the vegetative to the reproductive stage. Thus, stem growth in Arabidopsis thaliana is not fully dependent on the program of reproductive development. Received: 2 June 1999 / Accepted: 13 August 1999     

Photo-inhibition of internode elongation rate in light-grown Vigna sinensis L. Control by light quality

Abstract. In white light(W) stem elongation of green plants is stimulated when some far-red light (F) is added to W (Holmes & Smith, 1975, 1977b). The effect of adding F to a background W was quantified using Vigna sinensis L. plants and relatively high fluence rate W. F was given selectively to the 1st internode and the internode elongation rate (E.R) was recorded using linear voltage displacement transducers. Results confirm that adding F to background W stimulates internode E. R. The stimulation is very quickly measurable after switching F on and falls off rapidly after switching F off. The stimulation is not dependent on F fluence rate per per se but is strictly dependent on the quantum flux ratio F/W. These data are strongly in favour of phytochrome as the receptor, whose activity is controlled by the P_FR content or the [P_FR]/[P] ratio. A common mode of action for phytochrome in W and in darkness is suggested. The results indicate independence and additivity of the E. R.modulation by W light quantity and of the modulation by light quality.     

Phytochrome-mediated branch formation in protonemata of the mossCeratodon purpureus

We have analyzed light induction of side-branch formation and chloroplast re-arrangement in protonemata of the mossCeratodon purpureus. After 12 hr of dark adaptation, the rate of branch formation was as low as 5%. A red light treatment induced formation of side branches up to 75% of the dark-adapted protonema. The frequency of light induced branch formation differed between cells of different ages, the highest frequency being found in the 5th cell, the most distal cell studied from the apex. We examined the effect of polarized light given parallel to the direction of filament growth. The position of branching within the cell depended on the vibration plane of polarized red light. Branch formation was highest when the electric vector of polarized light vibrates parallel to the cell surface and is fluence rate dependent. The positional effect of polarized red light could be nullified to some extent by simultaneous irradiation with polarized far-red light. An aphototropic mutant,ptr116, shows characteristics of deficiency in biosynthesis of the phytochrome chromophore and exhibits no red-light induced branch formation. Biliverdin, a precursor of the phytochrome chromophore, rescued the red-light induced branching when added to the medium, supporting the conclusion that phytochrome acts as photoreceptor for red light induced branch formation. The light effect on chloroplast re-arrangement was also analyzed in this study. We found that polarized blue light induced chloroplast re-arrangement in wild-type cells, whereas polarized red light was inactive. This result suggests that chloroplast re-arrangement is only controlled by a blue light photoreceptor, not by phytochrome inCeratodon.     

Isolation and Initial Characterization of Arabidopsis Mutants That Are Deficient in Phytochrome A

Phytochrome, a red/far-red-light photoreceptor protein of plants, is encoded by a small gene family. Phytochrome A (PHYA), the product of the PHYA gene, is the predominant molecular species of phytochrome in etiolated tissue and has been best characterized biochemically. To define a role for PHYA, we isolated new mutants, designated fre1 (far-red elongated), in Arabidopsis thaliana that were specifically deficient in PHYA spectral activity and protein accumulation. These mutants were identified on the basis of their long hypocotyl phenotype under continuous far-red light. Although the fre1 mutants lacked the hypocotyl response to continuous far-red light, their responses to continuous white light and to end-of-day far-red-light treatments were normal. Thus, PHYA appears to play only a minor role in the regulation of hypocotyl elongation under natural conditions. In contrast, the fre1 mutation affected greening a fre1 mutant was less able than the wild type to deetiolate after growth in the dark. However, the potentiation effect of a red-light pulse on accumulation of chlorophyll was not changed significantly in the fre1 mutants. Thus, the function of PHYA might be highly specialized and restricted to certain phases of Arabidopsis development.     

The Aspergillus nidulans phytochrome FphA represses sexual development in red light

Phytochrome photoreceptors sense red and far-red light through photointerconversion between two stable conformations, a process mediated by a linear tetrapyrrole chromophore. Originally, phytochromes were thought to be confined to photosynthetic organisms including cyanobacteria, but they have been recently discovered in heterotrophic bacteria and fungi, where little is known about their functions. It was shown previously in the ascomycetous fungus Aspergillus nidulans that asexual sporulation is stimulated and sexual development repressed by red light. The effect was reminiscent of a phytochrome response, and indeed phytochrome-like proteins were detected in several fungal genomes. All fungal homologs are more similar to bacterial than plant phytochromes and have multifunctional domains where the phytochrome region and histidine kinase domain are combined in a single protein with a C-terminal response-regulator domain. Here, we show that the A. nidulans phytochrome FphA binds a biliverdin chromophore, acts as a red-light sensor, and represses sexual development under red-light conditions. FphA-GFP is cytoplasmic and excluded from the nuclei, suggesting that red-light photoperception occurs in the cytoplasm. This is the first phytochrome experimentally characterized outside the plant and bacterial kingdoms and the second type of fungal protein identified that functions in photoperception.     

Phytochrome control of short-day-induced bud set in black cottonwood

In trees and other woody perennial plants, short days (SDs) typically induce growth cessation, the initiation of cold acclimation, the formation of a terminal bud and bud dormancy. Phytochrome control of SD-induced bud set was investigated in two northern clones of black cottonwood (Populus trichocarpa Torr. & Gray) by using night breaks with red light (R) and far-red light (FR). For both clones (BC-1 and BC-2), SD-induced bud set was prevented when R night breaks as short as 2 min were given in the middle of the night. When night breaks with 2 min of R were immediately followed by 2 min of FR, substantial reversibility of bud set was observed for BC-1 but not for BC-2. By comparing the effects of the R night breaks on bud set and the length of specific internodes, we determined that the R night breaks influenced internode elongation in two opposing ways. First, the addition of a R night break to the SD treatment prevented the cessation of internode elongation that is associated with bud set. Those internodes that would not have elongated under SDs (and would have been found within the terminal bud) elongated in the R treatment. Second, the R night breaks decreased internode length relative to the long-day (LD) control. In contrast to the clonal differences in reversibility that we observed for bud set, the decrease in internode length (i.e. the second effect of R) was R/FR reversible in both clones. Based on these results, we conclude that internode elongation is influenced by two distinct types of phytochrome-mediated response. The first response is a typical response to photpperiod, whereas the second response is a typical “end-of-day” response to light quality. Our results demonstrate that SD-induced bud set in black cottonwood is controlled by phytochrome but that clonal differences have an important influence on the R/FR reversibility of this response. The availability of an experimental system in which SD-induced bud set is R/FR reversible will be valuable for studying the physiological genetics of photoperiodism in trees.     

Regulation of pea epicotyl elongation by blue light : fluence-response relationships and growth distribution

Irradiation with blue light causes a rapid decrease in stem elongation in Pisum sativum. Growing plants under continuous red light allowed us to study the fluence dependence and spatial distribution of blue-induced growth effects without interference from large changes in the ratio of the far-red absorbing form of phytochrome to total phytochrome. The magnitude of the inhibition generated by a 30-second pulse of blue light was linearly related to the log of the fluence applied over two orders of magnitude. Reciprocity held for irradiations with a pulse length shorter than the lag time for the response. The spatial distribution of inhibition was studied by marking the growing zone and photographing the stem at 10-minute intervals before, during, and after a 1-hour exposure to blue light. The region just below the hook does not undergo any perceptible change in growth rate while growth is nearly 100% inhibited in the base of the third internode.     

The function of phytochrome A

Knowledge of the photoperceptive function of phytochrome A has improved substantially thanks to the availability of mutants lacking phytochrome A and transgenic plants transformed with the PHYA gene in sense or anti-sense orientation. In imbibed seeds, phytochrome A mediates very-low-fluence responses. In etiolated seedlings, phytochrome A mediates very-low-fluence responses, high-irradiance responses under continuous far-red light, responsivity amplification to phytochrome B and red-light enhancement of the phototropic response to blue light. In light-grown seedings, phytochrome A modulates the extent of response to reductions in red/far-red ratio perceived by phytochrome B, perceives daylength extensions and night interruptions affecting flowering, and perceives light treatments resetting endogenous rhythms. Under natural radiation these abilities are manifested during seed germination and seedling de-etiolation under dense canopies or extremely low light fluences, and during early neighbour detection, but other processes await experimental evaluation. Phytochrome A affects growth and development throughout the whole life cycle of angiosperms.     

Photoresponses of transgenic Arabidopsis overexpressing the fern Adiantum capillus-veneris PHY1.

The phytochrome gene (PHY1) cDNA from the fern Adiantum capillus-veneris encodes an amino acid sequence that shows equal similarity (50-60%) to all five Arabidopsis phytochromes (PHYA-E). The A. capillus-veneris PHY1 cDNA was transformed into Arabidopsis ecotype Landsberg erecta to investigate its activity in angiosperms. Three of the resulting lines contained at least 8 times more spectrally active phytochrome than the wild type, indicating that A. capillus-veneris phytochrome can incorporate the chromophore of the host plants. Hypocotyl growth inhibition of these transgenic lines was investigated under red and far-red light. The results indicated dominant negative activity of A. capillus-veneris phy1 on the phytochrome A response in the host plants under continuous far-red light. However, the fern phytochrome did not interfere with the red-light repression of hypocotyl growth mediated by endogenous phytochrome B, and it failed to complement a phyB mutant phenotype. These observations suggest that the phy1 phytochrome molecule is too diverged from those of Arabidopsis to be fully functional.     

Phytochrome and internode elongation in Chenopodium polyspermum L. the light fluence rate during the day and the end-of-day effect

The elongation of the fourth internode of fully green Chenopodium polyspermum L. is strongly stimulated by far-red light (FR) given at the end of the day. The end-of-day effect is more important when the plants had been cultivated for several days with a main light period of 140 Wm^-2 than with a main light period of 85 Wm^-2. There exists a quantitative relationship between the FR end-of-day effect mediated by phytochrome and the value of the light fluence during the day.Abbreviations D darkness - FR far-red light - HWL white light at 140 Wm^-2 - LWL white light at 85 Wm^-2 - PAR photosynthetically active radiation - R red light - WL white light