Interaction of gravi- and phototropic stimulation in the response of maize (Zea mays L.) coleoptiles

The influence of gravitropic stimulation upon blue-light-induced first positive phototropism for stimulations in the same (light source and center of gravity opposite to each other) and in opposing directions was investigated in maize cole-optiles by measuring fluence-response patterns. As a result of gravitropic counterstimulation, phototropic bending was transient with maximum curvature occurring 100 min after stimulation. On a horizontal clinostat, however, the seedlings curved for 20 h. Gravistimulation in the opposite direction acted additively upon blue-light curvature. Gravistimulation in the same direction as phototropic stimulation produced a complex behaviour deviating from simple additivity. This pattern can be explained by a gravitropically mediated sensitization of the phototropic reaction, an optimal dependence of differential growth on the sum of photo-and gravistimulation, and blue-light-induced inhibition of gravitropic curvature at high fluences. These findings indicate that several steps of photo-and gravitransduction are separate. Preirradiation with red light desensitized the system independently of applied gravity-treatment, indicating that the site of red-light interaction is common to both transduction chains.Abbreviations BL blue light - G⁺ stimulation by light and gravity in the same direction (i.e. light source and center of gravity opposite to each other) - G^- stimulation by light and gravity in opposing directions     

Blue-Light-Induced Shrinking of Protoplasts from Maize Coleoptiles and Its Relationship to Coleoptile Growth

Protoplasts isolated from red-light-grown maize (Zea mays L.) coleoptiles shrank transiently upon brief exposure (e.g. 30 s) to blue light under background irradiation with red light. The maximal volume reduction (about 4% at a saturating fluence) occurred about 5 min after blue-light stimulation. The response was prevented by the anion-channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoic acid. Red light and far-red light did not induce any comparable response. Protoplasts of different sizes and those isolated from different coleoptile positions showed similar responses. After treatment with a saturating blue-light pulse, the protoplasts became responsive to a second pulse and gained full responsiveness within 5 min, suggesting that the photoreceptor system involves a dark-reversible component. The response to continuous blue light was also found to be transient. The protoplast volume was reduced during about 6 to 9 min of irradiation and returned within the next 30 min to the control level. The response to continuous blue light was saturated at 30 [mu]mol m-2 s-1. However, when the fluence rate was enhanced 10-fold after a period of irradiation at 30 [mu]mol m-2 s-1, the protoplasts showed another shrinking response. These and other kinetic results indicate that the photoreceptor system undergoes a photosensory adaptation. Growth in different zones of the coleoptile was inhibited by blue light transiently after pulse stimulation, as well as during continuous stimulation. It was concluded that the observed protoplast shrinking is related to the blue-light-induced inhibition of coleoptile growth.     

KINETICS OF BLUE-LIGHT STIMULATION AND CIRCADIAN RHYTHMICITY OF LIGHT-SATURATED PHOTOSYNTHESIS IN BROWN ALGAE: A SPECIES COMPARISON1

The time courses of photosynthetic rates in red light, with and without additional blue light, were investigated and compared in 20 species of brown algae. Species could be separated into two groups on the basis of the rhythmicity of their photosynthesis in red light and the kinetics of their responses to blue-light pulses. One group, which consisted of members of the Ectocarpales, Chordariales, and Dictyosiphonales, was characterized by strong and persistent circadian rhythmicity in red light. The photosynthetic responses of these species to blue-light pulses started within 10–30 s of the beginning of blue-light treatment and mostly contained at least two distinct kinetic components. An early component, which reached a maximum about 5–10 min after the blue-light pulse, was always detectable. Later components were seen as separate peaks or shoulders after an additional 10–20 min. The decay of the response in this group of species was mostly slow, with half-lives of between 0.5 and 1.5 h. In the second group of species, consisting of members of the Dictyotales, Laminariales, and Fucales, photosynthesis in red light was usually non-rhythmic, although circadian rhythms with a weak amplitude or of transient occurrence were observed in some plants of some species. The increase in photosynthesis in response to a blue-light pulse was not detectable until 70–330 s after the start of blue-light treatment, and the response itself had only a single component, with a maximum after about 10 min and half-life of 10–20 min. The lengths of the lag-phases were positively correlated with the times taken to reach the peak in this group, although the lag-phases and the half lives sometimes varied with time in individual plants. Two members of the Sphacelariales (Sphacelaria, Cladostephus) did not fit into either of the two groups because their photosynthesis was rhythmic, but their responses had long lag-phases, a single component, and moderately long half-lives. The differences in the kinetics of the photosynthetic response to blue-light pulses, which have been described for the two main groups of species, are thought to indicate that there are two distinct mechanisms by which light-saturated photosynthesis responds to blue light in brown algae. Since in some species the maximal photosynthesis after a blue-light pulse and the rate of photosynthesis in continuous blue light also varied in a circadian pattern, the response to blue light itself may be under circadian control.     

Evidence for the involvement of plasma-membrane-bound nitrate reductase in signal transduction during blue-light stimulation of nitrate uptake in Chlorella saccharophila

Nitrate uptake in Chlorella saccharophila (Krüger) Nadson was found to be stimulated by blue light, leading to a doubling of the rate. In the presence of background red light (300 mol photons · m^-2 · s^-1), only 15–20 mol photons · m^-2 · s^-1 of blue light was sufficient to saturate this increased uptake rate. Incubation of Chlorella cells with anti-nitrate-reductase immunoglobulin-G fragments inhibited blue-light stimulation. However, ferricyanide (10 M) doubled and dithiothreitol (100 M) inhibited the stimulatory effect of blue light. Among the protein-kinase inhibitors used, only staurosporine (10 M) prevented the blue-light stimulation. Phosphatase inhibitors were without effect and sodium vanadate totally inhibited nitrate uptake, pointing to an involvement of the plasma-membrane ATPase. Preincubation of the cells with calmodulin antagonists or calcium ionophores did not significantly reduce blue-light stimulation of nitrate uptake. The data are discussed with regard to transduction of the signal for blue-light stimulation of nitrate uptake and the possibility that the plasma-membrane-bound nitrate reductase is the blue-light receptor.Abbreviations Chl chlorophyll - DMSO dimethylsulfoxide - 1,2-DHG 1,2-dihexanoylglycerol - ML-9 1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine - NR nitrate reductase - H-7 1-(5-isoquinolinyl-sulfonyl)-2-methylpiperazine - IgG immunoglobulin G - PFD photon flux density - PM plasma membrane - W-7 N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide This work was supported by a grant from the Deutsche Forschungs-gemeinschaft to R.T.     

Blue-light-dependent osmoregulation in protoplasts of Phaseolus vulgaris Pulvini.

Blue light was found to induce shrinkage of the protoplasts isolated from first-leaf lamina pulvini of 18-day-old Phaseolus vulgaris. The response was transient following pulse stimulation, while it was sustainable during continuous stimulation. No apparent difference was found between flexor and extensor protoplasts. Protoplasts of the petiolar segment located close to the pulvinus showed no detectable response. In the plants used, the pulvinus was fully matured and the petiole was ceasing its elongation growth. When younger, 12-day-old, plants were used, however, the petiolar protoplasts did respond to blue light. The pulse-induced response was similar to that in pulvinar protoplasts, although the response to continuous stimulation was transient and differed from that in pulvinar protoplasts. No shrinkage was induced in pulvinar protoplasts when the far-red-light-absorbing form of phytochrome was absent for a period before blue-light stimulation, indicating that the blue-light responsiveness is strictly controlled by phytochrome. Inhibitors of anion channels and H(+)-ATPase abolished the shrinking response, supporting the view that protoplasts shrink by extruding ions. The response of pulvinar protoplasts is probably involved in the blue-light-induced, turgor-based movement of pulvini. The blue-light responding system in pulvini is suggested to have evolved from that functioning in other growing organs.     

Two distinct blue-light responses regulate epicotyl elongation in pea

Blue light induces a long-term suppression of epicotyl elongation in red-light-grown pea (Pisum sativum L.) seedlings. The fluence-response characteristics are bell-shaped, indicating the possibility of two different blue-light responses: a lower fluence response causing suppression and a higher fluence response alleviating the suppression. To determine if two responses are in effect, we have grown pea seedlings under dark conditions hoping to eliminate one or the other response. Under these growth conditions, only the lower fluence portion of the response (suppression of elongation) is apparent. The kinetics of suppression are similar to those observed for the lower fluence response of red-light-grown seedlings. The response to blue light in the dark-grown seedlings is not due to the excitation of phytochrome because a pulse of far-red light large enough to negate phytochrome-induced suppression has no effect on the blue-light-induced suppression. Furthermore, treatment of the dark-grown seedlings with red light immediately prior to treatment with high fluence blue light does not elicit the higher fluence response, indicating that the role of red light in the blue high fluence response is to allow the plant to achieve a specific developmental state in which it is competent to respond to the higher fluences of blue light.     

Inhibition of stem elongation in cucumis seedlings by blue light requires calcium

The effects of blue light and calcium on elongation of hypocotyl segments of Cucumber (Cucumis sativa L. cv Burpee's Pickler) were studied. Cucumber seedlings grown in dim red light showed a rapid decline in the rate of hypocotyl elongation when irradiated with high intensity (100 micromoles per square meter per second) blue light. In intact, 4-day-old seedlings the inhibition began within 2 minutes after the onset of blue-light irradiation and reached a maximum of approximately 55% within 4 minutes. Hypocotyl segments cut from 4-day-old seedlings also showed an inhibition of elongation in response to blue light when segments were floated on aqueous buffer and exposed to blue light for 3 hours. In the presence of 2 micromolar indole-3-acetic acid, blue light caused a 50% inhibition of elongation. Buffering free calcium in the incubation medium with 0.1 millimolar ethylene glycol bis(-aminoethyl ether)- N,N,N′,N′-tetraacetic acid eliminated the blue-light inhibition of segment elongation. Several experiments confirmed a specific requirement for calcium for the blue-light-induced inhibition of segment elongation. Treating segments with 0.2 micromolar fusicoccin abolished the inhibition of elongation by blue light as did buffering the medium at pH 4. Adding 1 millimolar ascorbate to incubation medium also eliminated the inhibition of segment elongation caused by blue light. Several compounds implicated in cell-wall redox reactions alter the magnitude of the blue-light-induced inhibition. The activity of peroxidase isolated from the cell-wall free space of cucumber hypocotyls was inhibited by ascorbate and low pH. The results are consistent with the hypothesis that blue light inhibits elongation by inducing an increase in cell-wall peroxidase activity and implicate calcium ions in the response to blue light.     

Light induction of genes preceding chloroplast differentiation in cultured plant cells

Our aim was to identify, using their complementary DNAs (cDNAs), genes which are rapidly and transiently expressed during the initial phase of blue-light-induced chloroplast differentiation in cultured plant cells (Chenopodium rubrum L.), and to evaluate the role of their prospective products with regard to light perception, signal transduction and response, as well as coordination in the expression of other blue-light-induced genes. A cDNA library (λ gt10) was established using polyadenylated RNAs from cells exposed to blue light for 6, 12, and 24 h. Selection of the relevant species from the combined preparations was achieved by applying hybridization techniques and hydroxylapatite chromatography, thus eliminating most of the mRNAs representative of dark-grown and fully greened cells. By differential screening, several clones corresponding to genes rapidly induced by blue light were identified. For most of these a temporary accumulation of the specific mRNA between 30 min and 72 h of blue-light irradiation was observed. With regard to the nucleotide sequence and the respective deduced amino-acid sequence, a glycine-rich protein, a β-tubulin-like protein and one species resembling an acidic ribosomal protein (RLAO) were among the products of the early light-induced genes. Dedicated to Professor Hans Mohr on the occasion of his 60th birthday Sequence data reported will appear in the EMBL Genbank and DDBJ Nucleotide Sequence Databases under the following accession numbers: X14067 for clone CRHC1 (glycine-rich protein), X15456 for clone CRHG1 (tubulin-like protein) and X15206 for clone CRE2 (RLAO-like protein)     

Intracellular chloroplast photorelocation in the moss Physcomitrella patens is mediated by phytochrome as well as by a blue-light receptor

 The light-induced intracellular relocation of chloroplasts was examined in red-light-grown protonemal cells of the moss Physcomitrella patens. When irradiated with polarized red or blue light, chloroplast distribution in the cell depended upon the direction of the electrical vector (E-vector) in both light qualities. When the E-vector was parallel to the cross-wall (i.e. perpendicular to the protonemal axis), chloroplasts accumulated along the cross-wall; however, no accumulation along the cross-wall was observed when the E-vector was perpendicular to it (i.e. parallel to the protonemal axis). When a part of the cell was irradiated with a microbeam of red or blue light, chloroplasts accumulated at or avoided the illumination point depending on the fluence rate used. Red light of 0.1–18 W m⁻² and blue light of 0.01–85.5 W m⁻² induced an accumulation response (low-fluence-rate response; LFR), while an avoidance response (high-fluence-rate response; HFR) was induced by red light of 60 W m⁻² or higher and by blue light of 285 W m⁻². The red-light-induced LFR and HFR were nullified by a simultaneous background irradiation of far-red light, whereas the blue-light-induced LFR and HFR were not affected at all by this treatment. These results show, for the first time, that dichroic phytochrome, as well as the dichroic blue-light receptor, is involved in the chloroplast relocation movement in these bryophyte cells. Further, the phytochrome-mediated responses but not the blue-light responses were revealed to be lost when red-light-grown cells were cultured under white light for 2 d. Received: 7 September 1999 / Accepted: 15 October 1999     

Regulation of blue-light-induced proton pumping by Vicia faba L. guard-cell protoplasts: Energetic contributions by chloroplastic and mitochondrial activities

An initial response during signal transduction in guard cells, following absorption of blue light, is the extrusion of protons. Translocation of protons across the guard-cell plasmalemma is an energy-requiring activity. The present study has investigated the energetic contribution from guard-cell chloroplasts and mitochondria to blue-light-induced proton pumping by Vicia faba guard-cell protoplasts. The addition of 3(3,4-dichlorophenyl)-1,1-dimethylurea to the protoplast suspension had a minimal effect on rates of acidification when oxygen concentrations of the medium were maintained close to near-saturating levels. Under the same conditions, oligomycin reduced both the rates of blue-light-induced acidification and total proton efflux. Lowering the oxygen concentration of the suspending medium to approximately 20 M resulted in complete inhibition of blue-light-induced acidification activity. Swelling of protoplasts induced by blue light was also inhibited by low oxygen levels. Levels of ATP from whole-protoplast extracts were reduced by about 64% when exposed to low levels of oxygen. Increasing oxygen levels to near-saturating levels restored both blue-light-induced acidification rates and swelling of the protoplasts within a 60-min recovery period. Levels of ATP also increased during the recovery period. Addition of 3(3,4-dichlorophenyl)-1,1-dimethylurea or oligomycin to the suspending medium prior to increasing the oxygen concentration caused a reduction in acidification rates after the recovery period by 40 and 80%, respectively. Levels of ATP in guard-cell protoplasts were also reduced by both inhibitors after a 60-min recovery period. The results demonstrate that both guard-cell chloroplasts and mitochondria contribute energetically to blue-light-induced proton pumping by guard-cell protoplasts. Furthermore, both energy sources are inhibited by low oxygen concentrations, suggesting coordinated metabolic regulation between photo- and oxidative phosphorylation in guard cells.Abbreviations BL blue light - Chl chlorophyll - DCMU 3(3,4-dichlorophenyl)-1,1-dimethylurea - GCPs guard-cell protoplasts This research was supported by an operating grant from the Natural Sciences and Engineering Research Council of Canada and a University Research Grant from The University of Calgary. Dr. L. Gedamu (University of Calgary) is thanked for providing access to the bioluminometer. Technical assistance by C. Chmielewski, C. Turnnir, S. Ham and K. Meyer is gratefully acknowledged.     

Rapid,Blue-Light-Induced Acidifications at the Surface of Ectocarpus and Other Marine Macroalgae

In most brown algae, photosynthesis saturated with red light can be stimulated by continuous blue light. Pulses of blue light lead to transient increases in photosynthetic rate. When a CO2-sensitive electrode was used, occasionally blue light was observed to cause an apparent increase of CO2 instead of the expected decrease. This was changed by buffering the seawater medium and, under these conditions, blue light caused stimulation of CO2 consumption. These results led to investigations of blue-light-dependent pH changes at the outer surface of the plants. Shifts of the pH were recorded in the presence of the photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea. In all brown algae tested and in the green algae Ulva and Enteromorpha, blue-light pulses caused transient acidification of 0.03 to 0.18 pH units, depending on the species. The kinetics showed lag phases of a few seconds and the minimum was reached after 5 to 9 min. Fluence response relationships indicated that the sensitivity (threshold) to blue light was very similar in all species. The responses in Ectocarpus changed with time, and about 5 h after the beginning of red light or darkness, a second component became evident, which peaked 20 min after the blue-light pulse. The refractory period of the whole system was about 3 h in Ectocarpus. The blue-light-dependent pH changes show striking similarities to those of higher plant guard cells, and it is possible that similar responses may occur in other tissues of higher plants. In red algae, however, no blue-light-dependent acidifications could be detected. The possible role of the observed pH shifts in a mechanism of CO2 acquisition is discussed.     

Absence of long-wavelength photic potentiation of murine intrinsically photosensitive retinal ganglion cell firing in vitro

Melanopsin is an opsin-family photopigment required for photosensitivity of the intrinsically photosensitive retinal ganglion cells (ipRGCs), which subserve photic entrainment of circadian rhythms in mammals. The melanopsin photocycle is presently unknown but is independent of the enzymatic photocycle employed by rhodopsin and cone opsins. Recent experiments have demonstrated that red-light exposure potentiates circadian phase-shifting responses to blue-light stimuli, consistent with the hypothesis that melanopsin functions as a bistable photopigment. To further test this hypothesis, we analyzed ipRGC firing activity in response to 480-nm blue light with or without intervening long-wavelength 620-nm red-light stimulation, using in vitro multielectrode array recording of postnatal day 8 to 10 murine retina. Cell-firing responses to 480-nm light were highly reproducible. No significant potentiating or bleaching effect of intervening subthreshold 620-nm light on ipRGC firing to 480-nm light could be discerned. Further physiologic and biochemical analysis of the ipRGC photoreception is required to reconcile the presence of long-wavelength potentiation at the level of the SCN with its absence in light-induced ipRGC firing.     

Second positive phototropism results from coordinated co-action of the phototropins and cryptochromes

Phototropism and hypocotyl growth inhibition are modulated by the coaction of different blue-light photoreceptors and their signaling pathways. How seedlings integrate the activities of the different blue-light photoreceptors to coordinate these hypocotyl growth responses is still unclear. We have used time-lapse imaging and a nontraditional mathematical approach to conduct a detailed examination of phototropism in wild-type Arabidopsis and various blue-light photoreceptor mutants. Our results indicate that high fluence rates of blue light (100 micro mol m(-)(2) s(-)(1)) attenuate phototropism through the coaction of the phototropin and cryptochrome blue-light photoreceptors. In contrast, we also demonstrate that phototropins and cryptochromes function together to enhance phototropism under low fluence rates (<1.0 micro mol m(-)(2) s(-)(1)) of blue light. Based on our results, we hypothesize that phototropins and cryptochromes regulate phototropism by coordinating the balance between stimulation and inhibition of growth of the hypocotyl depending on the fluence rate of blue light.     

BLUE LIGHT IN THE DEVELOPMENT OF FERN GAMETOPHYTES AND ITS INTERACTION WITH FAR-RED AND RED LIGHT

Two effects of blue light on the development of Onoclea sensibilis spores are demonstrated. Brief irradiation promotes the protonemal or filamentous type of growth, and the rate of filament elongation is greater than in darkness. Longer periods of irradiation induce the formation of 2-dimensional prothallia. Blue-light treatments which promote filament elongation interact with elongation-promoting far-red light. Far-red irradiation alone promotes filament elongation to a greater extent than blue light, but a blue-light irradiation, either following or preceding far-red treatment, strongly inhibits the far-red promotion. In darkness, a slow recovery from the blue-light-induced loss of sensitivity to far-red takes place. The recovery may be greatly accelerated by interposing a red-light treatment between blue and far-red irradiation.     

Circadian rhythm and fast responses to blue light of photosynthesis in Ectocarpus (Phaeophyta,Ectocarpales)

Photosynthesis of Ectocarpus siliculosus (Dillwyn) Lyngb. under continuous saturating red irradiation follows a circadian rhythm. Blue-light pulses rapidly stimulate photosynthesis with high effectiveness in the troughs of this rhythm but the effectiveness of such pulses is much lower at its peaks. In an attempt to understand how blue light and the rhythm affected photosynthesis, the effects of inorganic carbon on photosynthetic light saturation curves were studied under different irradiation conditions. The circadian rhythm of photosynthesis was apparent only at irradiances which were not limiting for photosynthesis. The same was found for blue-light-stimulated photosynthesis, although stimulation was observed also under very low red-light irradiances after a period of adaptation, provided that the inorganic-carbon concentration was not in excess. Double-reciprocal plots of light-saturated photosynthetic rates versus the concentration of total inorganic carbon (up to 10 mM total inorganic carbon) were linear and had a common constant for half-saturation (3.6 mM at pH 8) at both the troughs and the peaks of the rhythm and before and after blue-light pulses. Only at very low carbon concentrations was a clear deviation found from these lines for photosynthesis at the rhythm maxima (red and blue light), which indicated that the strong carbon limitation specifically affected photosynthesis at the peak phases of the rhythm. Very high inorganic carbon concentrations (20 mM) in the medium diminished the responses to blue light, although they did not fully abolish them. The kinetics of the stimulation indicate that the rate of photosynthesis is affected by two blue-light-dependent components with different time courses of induction and decay. The faster component seemed to be at least partially suppressed at red-light irradiances which were not saturating for photosynthesis. Lowering the pH of the medium had the same effects as an increase of the carbon concentration to levels of approx. 10 mM. This indicates that Ectocarpus takes up free CO₂ only and not bicarbonate, although additional physiological mechanisms may enhance the availability of CO₂.Abbreviation TIC total inorganic carbon     

Acclimation of Arabidopsis thaliana to the light environment: regulation of chloroplast composition

The regulation by light of the composition of the photosynthetic apparatus was investigated in Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta. When grown in high- and low-irradiance white light, wild-type plants and photomorphogenic mutants showed large differences in their maximum photosynthetic rate and chlorophyll a/b ratios; such changes were abolished by growth in red light. Photosystem I (PSI) and PSII levels were measured in wild-type plants grown under a range of light environments; the results indicate that regulation of photosystem stoichiometry involves the specific detection of blue light. Supplementing red growth lights with low levels of blue light led to large increases in PSII content, while further increases in blue irradiance had the opposite effect; this latter response was abolished by the hy4 mutation, which affects certain events controlled by a blue-light receptor. Mutants defective in the phytochrome photoreceptors retained regulation of photosystem stoichiometry. We discuss the results in terms of two separate responses controlled by blue-light receptors: a blue-high-fluence response which controls photosystem stoichiometry; and a blue-low-fluence response necessary for activation of such control. Variation in the irradiance of the red growth light revealed that the blue-high-fluence response is attenuated by red light; this may be evidence that photosystem stoichiometry is controlled not only by photoreceptors, but also by photosynthetic metabolism.Abbreviations BHF blue-high-fluence - BLF blue-low-fluence - Chl chlorophyll - FR far-red light - LHCII light-harvesting complex of PSII - P_max maximum photosynthetic rate - R red light - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase This work was supported by Natural Environment Research Council Grant No. GR3/7571A. We would like to thank H. Smith (Botany Department, University of Leicester) and E. Murchie (INRA, Versailles) for helpful discussions.     

Color-sensitive motility and methanol release responses in Rhodobacter sphaeroides

Blue-light-induced repellent and demethylation responses, characteristic of behavioral adaptation, were observed in Rhodobacter sphaeroides. They were analyzed by computer-assisted motion analysis and through the release of volatile tritiated compounds from [methyl-(3)H]methionine-labeled cells, respectively. Increases in the stop frequency and the rate of methanol release were induced by exposure of cells to repellent light signals, such as an increase in blue- and a decrease in infrared-light intensity. At a lambda of >500 nm the amplitude of the methanol release response followed the absorbance spectrum of the photosynthetic pigments, suggesting that they function as photosensors for this response. In contrast to the previously reported motility response to a decrease in infrared light, the blue-light response reported here does not depend on the number of photosynthetic pigments per cell, suggesting that it is mediated by a separate sensor. Therefore, color discrimination in taxis responses in R. sphaeroides involves two photosensing systems: the photosynthetic pigments and an additional photosensor, responding to blue light. The signal generated by the former system could result in the migration of cells to a light climate beneficial for photosynthesis, while the blue-light system could allow cells to avoid too-high intensities of (harmful) blue light.     

Light perception and the role of the xanthophyll cycle in blue-light-dependent chloroplast movements in lemna trisulca L

In most higher plants, chloroplasts move towards the periclinal cell walls in weak blue light (WBL) to increase light harvesting for photosynthesis, and towards the anticlinal walls as an escape reaction, thus avoiding photo-damage in strong blue light (SBL). The photo- receptor(s) triggering these responses have not yet been identified. In this study, the role of zeaxanthin as a blue-light photoreceptor in chloroplast movements was investigated. Time-lapse 3D confocal imaging in Lemna trisulca showed that individual chloroplasts responded to local illumination when one half of the cell was treated with light of different intensity or spectral quality to that received by the other half, or was maintained in darkness. Thus the complete signal perception, transduction and effector system has a high degree of spatial resolution and is consistent with localization of part of the transduction chain in the chloroplasts. Turnover of xanthophylls was determined using HPLC, and a parallel increase was observed between zeaxanthin and chloroplast movements in SBL. Ascorbate stimulated both a transient increase in zeaxanthin levels and chloroplast movement to profile in physiological darkness. Conversely, dithiothreitol blocked zeaxanthin production and responses to SBL and, to a lesser extent, WBL. Norflurazon preferentially inhibited SBL-dependent chloroplast movements. Increases in zeaxanthin were also observed in strong red light (SRL) when no directional chloroplast movements occurred. Thus it appears that a combination of zeaxanthin and blue light is required to trigger responses. Blue light can cause cis-trans isomerization of xanthophylls, thus photo-isomerization may be a critical link in the signal transduction pathway.