Effect of light quality on the organization of photosynthetic electron transport chain of pea seedlings

The activity of NADP and O₂ photoreduction by water is essentially higher in chloroplasts isolated from pea seedlings (Pisum sativum L.) grown under blue light as compared with that from plants grown under red light. In contrast, the photoreduction of NADP and O₂ with photosystem I only is practically the same or even lower in chloroplasts isolated from plants grown under blue light. The addition of plastocyanin does not affect the rate or the extent of NADP photoreduction by water in the chloroplasts isolated from plants grown under blue light, whereas it sharply activates NADP reduction in the chloroplasts isolated from plants grown under red light. The extent of the light-induced oxidation of cytochrome f is appreciably higher in chloroplasts isolated from plants grown under blue light. Cytochrome b₅₅₉ plays the predominant role in the oxidoreductive reactions of these chloroplasts. Furthermore, the fluorescence measurements indicate more effective transfer of excitation energy from chlorophyll to the photosystem II reaction center in chloroplasts isolated from plants grown under blue light.     

Enhancement of the Stomatal Response to Blue Light by Red Light, Reduced Intercellular Concentrations of CO(2), and Low Vapor Pressure Differences

The effects of environmental parameters on the blue light response of stomata were studied by quantifying transient increases in stomatal conductance in Commelina communis following 15 seconds by 0.100 millimole per square meter per second pulses of blue light. Because conductance increases were not observed following red light pulses of the same or greater (30 seconds by 0.200 millimole per square meter per second) fluences, the responses observed could be reliably attributed to the specific blue light response of the guard cells, rather than to guard cell chlorophyll. In both Paphiopedilum harrisianum, which lacks guard cell chloroplasts, and Commelina, the blue light response was enhanced by 0.263 millimole per square meter per second continuous background red light. Thus, the blue light response and its enhancement do not require energy derived from red-light-driven photophosphorylation by the guard cell chloroplasts. In Commelina, reduction of the intercellular concentration of CO₂ by manipulation of ambient CO₂ concentrations resulted in an enhanced blue light response. In both Commelina and Paphiopedilum, the blue light response was decreased by an increased vapor pressure difference. The magnitude of blue-light-specific stomatal opening thus appears to be sensitive to environmental conditions that affect the carbon and water status of the plant.     

Blue light-modulation of chlorophyll a fluorescence transients in guard cell chloroplasts

Chlorophyll a fluorescence transients from mesophyll and single guard cell pairs of Vicia faba were measured by microspectrofluorometry. In both chloroplast types, fluorescence induction (O to P) was similar under actinic blue and green light. In slow transients from mesophyll cell chloroplasts, blue and green light induced identical, typical rapid quenching from P to S, and the M peak. In contrast, the P to S transient from guard cell (GC) chloroplasts irradiated with blue light showed a much slower quenching rate, and the P to T transition showed no M peak. Actinic green light induced mesophyll-like transients in GC chloroplasts, including rapid quenching from P to S and the M peak. Detection of these transients in single pairs of GC and isolated protoplasts ruled out mesophyll contamination as a signal source. Green light induced a rapid quenching and the M peak in GC chloroplasts from several species. The effect of CO₂ concentration on the fluorescence transients was investigated in the presence of HCO₃⁻ at pH 6.8 and 10.0. In transients induced by green light in both chloroplast types, a pH increase concomitant with a reduction in CO₂ concentration caused an increase in the initial rate of quenching and the elimination of the M peak. Actinic blue light induced mesophyll-like transients from GC chloroplasts in the presence of 10 micromolar KCN, a concentration at which the blue light-induced stomatal opening is inhibited. Addition of 100 to 200 micromolar phosphate also caused large increases in fluorescence quenching rates and a M peak. These results indicate that blue light modulates photosynthetic activity in GC chloroplasts. This blue light effect is not observed in the absence of transduction events connected with the blue light response and in the presence of high phosphate concentrations.     

Fast Fluorescence Quenching from Isolated Guard Cell Chloroplasts of Vicia faba Is Induced by Blue Light and Not by Red Light

Chlorophyll a fluorescence transients from isolated Vicia faba guard cell chloroplasts were used to probe the response of these organelles to light quality. Guard cell chloroplasts were isolated from protoplasts by passing them through a 10-μm nylon net. Intact chloroplasts were purified on a Percoll gradient. Chlorophyll a fluorescence transients induced by actinic red or blue light were measured with a fluorometer equipped with a measuring beam. Actinic red light induced a monophasic quenching, and transients induced by blue light showed biphasic kinetics having a slow and a fast component. The difference between the red and blue light-induced transients could be observed over a range of fluence rates tested (200-800 μmol m⁻² s⁻¹). The threshold fluence rate of blue light for the induction of the fast component of quenching was 200 μmol m⁻² s⁻¹, but in the presence of saturating red light, fluence rates as low as 25 μmol m⁻² s⁻¹ induced the fast quenching. These results indicate that guard cell chloroplasts have a specific response to blue light.     

Phototropin Mediated Relocation of Myosins in Arabidopsis thaliana

Background

The mechanism of the light-dependent movements of chloroplasts is based on actin and myosin but its details are largely unknown. The movements are activated by blue light in terrestrial angiosperms. The aim of the present study was to determine the role of myosin associated with the chloroplast surface in the light-induced chloroplast responses in Arabidopsis thaliana. The localization of myosins was investigated under blue light intensities generating avoidance and accumulation responses of chloroplasts. The localization was compared in wild type plants and in phot2 mutant lacking the avoidance response.

Results

Wild type and phot2 mutant plants were irradiated with strong (36 µEm⁻²s⁻¹) and/or weak (0.8 µEm⁻²s⁻¹) blue light. The leaf tissue was immunolabeled with antimyosin antibodies. Different arrangements of myosins were observed in the mesophyll depending on the fluence rate in wild type plants. In tissue irradiated with weak blue light myosins were associated with chloroplast envelopes. In contrast, in tissue irradiated with strong blue light chloroplasts were almost myosin-free. The effect did not occur in red light and in the phot2 mutant.

Conclusions

Myosin displacement is blue light specific, i.e., it is associated with the activation of a specific blue-light photoreceptor. We suggest that the reorganization of myosins is essential for chloroplast movement. Myosins appear to be the final step of the signal transduction pathway starting with phototropin2 and leading to chloroplast movements.Key Words: Arabidopsis, blue light, chloroplast movements, myosins, phototropins     

Actin-based mechanisms for light-dependent intracellular positioning of nuclei and chloroplasts in Arabidopsis

The plant organelles, chloroplast and nucleus, change their position in response to light. In Arabidopsis thaliana leaf cells, chloroplasts and nuclei are distributed along the inner periclinal wall in darkness. In strong blue light, they become positioned along the anticlinal wall, while in weak blue light, only chloroplasts are accumulated along the inner and outer periclinal walls. Blue-light dependent positioning of both organelles is mediated by the blue-light receptor phototropin and controlled by the actin cytoskeleton. Interestingly, however, it seems that chloroplast movement requires short, fine actin filaments organized at the chloroplast edge, whereas nuclear movement does cytoplasmic, thick actin bundles intimately associated with the nucleus. Although there are many similarities between photo-relocation movements of chloroplasts and nuclei, plant cells appear to have evolved distinct mechanisms to regulate actin organization required for driving the movements of these organelles.Key words: actin, Arabidopsis, blue light, chloroplast positioning, phototropin, nuclear positioning     

Structural and functional properties of the coleoptile chloroplast: Photosynthesis and photosensory transduction

Recent studies have shown that guard cell and coleoptile chloroplasts appear to be involved in blue light photoreception during blue light-dependent stomatal opening and phototropic bending. The guard cell chloroplast has been studied in detail but the coleoptile chloroplast is poorly understood. The present study was aimed at the characterization of the corn coleoptile chloroplast, and its comparison with mesophyll and guard cell chloroplasts. Coleoptile chloroplasts operated the xanthophyll cycle, and their zeaxanthin content tracked incident rates of solar radiation throughout the day. Zeaxanthin formation was very sensitive to low incident fluence rates, and saturated at around 800–1000 mol m^–2 s^–1. Zeaxanthin formation in corn mesophyll chloroplasts was insensitive to low fluence rates and saturated at around 1800 mol m^–2 s^–1. Quenching rates of chlorophyll a fluorescence transients from coleoptile chloroplasts induced by saturating fluence rates of actinic red light increased as a function of zeaxanthin content. This implies that zeaxanthin plays a photoprotective role in the coleoptile chloroplast. Addition of low fluence rates of blue light to saturating red light also increased quenching rates in a zeaxanthin-dependent fashion. This blue light response of the coleoptile chloroplast is analogous to that of the guard cell chloroplast, and implicates these organelles in the sensory transduction of blue light. On a chlorophyll basis, coleoptile chloroplasts had high rates of photosynthetic oxygen evolution and low rates of photosynthetic carbon fixation, as compared with mesophyll chloroplasts. In contrast with the uniform chloroplast distribution in the leaf, coleoptile chloroplasts were predominately found in the outer cell layers of the coleoptile cortex, and had large starch grains and a moderate amount of stacked grana and stroma lamellae. Several key properties of the coleoptile chloroplast were different from those of mesophyll chloroplasts and resembled those of guard cell chloroplasts. We propose that the common properties of guard cell and coleoptile chloroplasts define a functional pattern characteristic of chloroplasts specialized in photosensory transduction.Abbreviations Ant or A antheraxanthin - dv/dt fluorescence quenching rate - Fm maximum yield of fluorescence with all PS II reaction centers closed - Fo yield of instantaneous fluorescence with all PS II reaction centers open - Vio or V violaxanthin - Zea or Z zeaxanthin     

Chloroplasts continuously monitor photoreceptor signals during accumulation movement

Under low light conditions, chloroplasts gather at a cell surface to maximize light absorption for efficient photosynthesis, which is called the accumulation response. Phototropin1 (phot1) and phototropin2 (phot2) were identified as blue light photoreceptors in the accumulation response that occurs in Arabidopsis thaliana and Adiantum capillus-veneris with neochrome1 (neo1) as a red light photoreceptor in A. capillus-veneris. However, the signal molecule that is emitted from the photoreceptors and transmitted to the chloroplasts is not known. To investigate this topic, the accumulation response was induced by partial cell irradiation with a microbeam of red, blue and far-red light in A. capillus-veneris gametophyte cells. Chloroplasts moved towards the irradiated region and were able to sense the signal as long as its signal flowed. The signal from neo1 had a longer life than the signal that came from phototropins. When two microbeams with the same wavelength and the same fluence rate were placed 20 μm apart from each other and were applied to a dark-adapted cell, chloroplasts at an equidistant position always moved towards the center (midpoint) of the two microbeams, but not towards either one. This result indicates that chloroplasts are detecting the concentration of the signal but not the direction of signal flow. Chloroplasts repeatedly move and stop at roughly 10 s intervals during the accumulation response, suggesting that they monitor the intermittent signal waves from photoreceptors.     

Absence of Variable Fluorescence from Guard Cell Chloroplasts of Stenotaphrum secundatum

The lack of detectable variable fluorescence from guard cell chloroplasts in both the albino and green portions of variegated leaves of St. Augustine grass (Stenotaphrum secundatum var variegatum A.S. Hitchc.) is reported. Fluorescence was measured either with a highly sensitive, modified fluorescence microscope which was capable of recording fluorescence induction curves from single chloroplasts, or with a spectrofluorometer. Both fast and slow fluorescence transients from S. secundatum guard cells showed a rapid rise and then remained at a steady level. Neither variable fluorescence increase (induction) nor decrease (quenching), properties normally associated with photosystem II, was observed from these chloroplasts. These fluorescence kinetics did not change either with alterations of the specimen preparation procedure or with alterations of the excitation light intensities and wavelengths. These results indicate that guard cell chloroplasts in this variety of S. secundatum do not conduct normal photosystem II electron transport. Light regulation of stomatal conductance in intact leaves of this plant did occur, however, and was similar to light regulation observed in other species. The conductance of the green portion of the leaves was much greater in the light than in the dark, and was much greater than the conductance of the albino portion of the leaves. Stomata in the green portion of the leaves also showed greater opening in blue light than in red light. These results provide evidence that stomatal regulation in this variety of S. secundatum does not rely on photosystem II electron transport in guard cell chloroplasts.     

Photoinduction of formation of circular structures by microfilaments on chloroplasts during intracellular orientation in protonemal cells of the fernAdiantum capillus-veneris

Summary Changes in the organization of cortical actin microfilaments during phytochrome-mediated and blue light-induced photoorientation of chloroplasts were investigated by rhodamine-phalloidin staining in protonemal cells of the fernAdiantum capillusveneris. Low- and high-fluence rate responses were induced by partial irradiation of individual cells with a microbeam of 20 m in width. In the low-fluence rate responses to red and blue light, a circular structure composed of microfilaments was induced on the chloroplast concentrated in the irradiated region, on the side facing the plasma membrane, as already reported in the case of the low-fluence rate response induced by polarized red or blue light. Such a structure was not observed on the chloroplasts located far from the microbeam. Time-course studies revealed that the structure was induced after the chloroplasts gathered in the illuminated region and that the structure disappeared before chloroplasts moved out of this region when the microbeam was turned off. In the high-fluence rate response to blue light, chloroplasts avoided the irradiated site but accumulated in the shaded area adjacent the edges of microbeam. The circular structure made of microfilaments was also observed on the chloroplasts gathered in the area and it showed the same behavior with respect to its appearance and disappearance during a light/dark regime as in the case of the low-fluence rate response. However, no such circular structure was observed in the high-fluence rate response to red light, in which case the chloroplasts also avoided the illuminated region but no accumulation in the adjacent areas was induced. These results indicate that the circular structure composed of microfilaments may play a role in the anchorage of the chloroplast during intracellular photo-orientation.     

Blue light-dependent nuclear positioning in Arabidopsis thaliana leaf cells

The plant nucleus changes its intracellular position not only upon cell division and cell growth but also in response to environmental stimuli such as light. We found that the nucleus takes different intracellular positions depending on blue light in Arabidopsis thaliana leaf cells. Under dark conditions, nuclei in mesophyll cells were positioned at the center of the bottom of cells (dark position). Under blue light at 100 mumol m(-2) s(-1), in contrast, nuclei were located along the anticlinal walls (light position). The nuclear positioning from the dark position to the light position was fully induced within a few hours of blue light illumination, and it was a reversible response. The response was also observed in epidermal cells, which have no chloroplasts, suggesting that the nucleus has the potential actively to change its position without chloroplasts. Light-dependent nuclear positioning was induced specifically by blue light at >50 mumol m(-2) s(-1). Furthermore, the response to blue light was induced in phot1 but not in phot2 and phot1phot2 mutants. Unexpectedly, we also found that nuclei as well as chloroplasts in phot2 and phot1phot2 mutants took unusual intracellular positions under both dark and light conditions. The lack of the response and the unusual positioning of nuclei and chloroplasts in the phot2 mutant were recovered by externally introducing the PHOT2 gene into the mutant. These results indicate that phot2 mediates the blue light-dependent nuclear positioning and the proper positioning of nuclei and chloroplasts. This is the first characterization of light-dependent nuclear positioning in spermatophytes.     

A simple low-cost microcontroller-based photometric instrument for monitoring chloroplast movement

A new microcontroller-based photometric instrument for monitoring blue light dependent changes in leaf transmission (chloroplast movement) was developed based on a modification of the double-beam technique developed by Walzcak and Gabrys [(1980) Photosynthetica 14: 65–72]. A blue and red bicolor light emitting diode (LED) provided both a variable intensity blue actinic light and a low intensity red measuring beam. A phototransistor detected the intensity of the transmitted measuring light. An inexpensive microcontroller independently and precisely controlled the light emission of the bicolor LED. A typical measurement event involved turning off the blue actinic light for 100 μs to create a narrow temporal window for turning on and measuring the transmittance of the red light. The microcontroller was programmed using LogoChip Logo (http://www.wellesley.edu/Physics/Rberg/logochip/) to record fluence rate response curves. Laser scanning confocal microscopy was utilized to correlate the changes in leaf transmission with intercellular chloroplast position. In the dark, the chloroplasts in the spongy mesophyll exhibited no evident asymmetries in their distribution, however, in the palisade layer the cell surface in contact with the overlying epidermis was devoid of chloroplasts. The low light dependent decrease in leaf transmittance in dark acclimated leaves was correlated with the movement of chloroplasts within the palisade layer into the regions previously devoid of chloroplasts. Changes in leaf transmittance were evident within one minute following the onset of illumination. Minimal leaf transmittance was correlated with chloroplasts having retreated from cell surfaces perpendicular to the incident light (avoidance reaction) in both spongy and palisade layers.Electronic Supplementary Material Supplementary material is available for this article at     

Evidence for a light dependent increase of phosphoglucomutase activity in isolated, intact spinach chloroplasts

Phosphoglucomutase (PGM) activity was measured in spinach (Spinacia oleracea L.) chloroplasts. Initial enzyme activity in a chloroplast lysate was 5 to 10% of total activity measured with 1 micromolar glucose 1,6-bisphosphate (Glc 1,6-P₂) in the assay. Initial PGM activity increased 2- to 3-fold when chloroplasts were illuminated for 10 minutes prior to enzyme measurement and then decreased slowly in the dark. Measurements of total enzyme activity were unchanged by prior light treatment. Initial PGM activity from light treated chloroplasts was sufficient to account for in vivo rates of starch synthesis. Changes in PGM activity were affected by stromal pH and orthophosphate concentration. Photosynthetic inhibitors, dl-glyceraldehyde, glycolaldehyde, and glyoxylate, decreased and 3-phosphoglyceric acid increased light induced changes of PGM activity. Dark preincubation of chloroplasts with 10 millimolar dithiothreitol had no effect upon initial PGM activity, suggesting that light effects did not involve a sulfhydryl mechanism. Hexose monophosphate levels increased in illuminated chloroplasts. Activation of PGM in a chloroplast lysate by Glc 1,6-P₂ was maximal between pH 7.5 and 8.5. Stromal concentrations of Glc 1,6-P₂ were between 20 and 30 micromolar for both light and dark incubated chloroplasts and these levels should saturate PGM activity. Light dependent alterations of enzyme activity may be due to changes of phosphorylated PGM levels in the stroma or are the result of changes in residual activity by the dephosphorylated form of the enzyme. The above results indicate that PGM activity in spinach chloroplasts may be regulated by light, stromal pH, and Glc 1,6-P₂ concentration.     

Phototropin‐ and photosynthesis‐dependent mitochondrial positioning in Arabidopsis thaliana mesophyll cells

Mitochondria are frequently observed in the vicinity of chloroplasts in photosynthesizing cells, and this association is considered necessary for their metabolic interactions. We previously reported that, in leaf palisade cells of Arabidopsis thaliana, mitochondria exhibit blue‐light‐dependent redistribution together with chloroplasts, which conduct accumulation and avoidance responses under the control of blue‐light receptor phototropins. In this study, precise motility analyses by fluorescent microscopy revealed that the individual mitochondria in palisade cells, labeled with green fluorescent protein, exhibit typical stop‐and‐go movement. When exposed to blue light, the velocity of moving mitochondria increased in 30 min, whereas after 4 h, the frequency of stoppage of mitochondrial movement markedly increased. Using different mutant plants, we concluded that the presence of both phototropin1 and phototropin2 is necessary for the early acceleration of mitochondrial movement. On the contrary, the late enhancement of stoppage of mitochondrial movement occurs only in the presence of phototropin2 and only when intact photosynthesis takes place. A plasma‐membrane ghost assay suggested that the stopped mitochondria are firmly adhered to chloroplasts. These results indicate that the physical interaction between mitochondria and chloroplasts is cooperatively mediated by phototropin2‐ and photosynthesis‐dependent signals. The present study might add novel regulatory mechanism for light‐dependent plant organelle interactions.     

The avoidance and aggregative movements of mesophyll chloroplasts in C(4) monocots in response to blue light and abscisic acid

In C(4) plants, mesophyll (M) chloroplasts are randomly distributed along the cell walls, whereas bundle sheath chloroplasts are located in either a centripetal or centrifugal position. It was reported previously that only M chloroplasts aggregatively redistribute to the bundle sheath side in response to extremely strong light or environmental stresses. The aggregative movement of M chloroplasts is also induced in a light-dependent fashion upon incubation with abscisic acid (ABA). The involvement of reactive oxygen species (ROS) and red/blue light in the aggregative movement of M chloroplasts are examined here in two distinct subtypes of C(4) plants, finger millet and maize. Exogenously applied hydrogen peroxide or ROS scavengers could not change the response patterns of M chloroplast movement to light and ABA. Blue light irradiation essentially induced the rearrangement of M chloroplasts along the sides of anticlinal walls, parallel to the direction of the incident light, which is analogous to the avoidance movement of C(3) chloroplasts. In the presence of ABA, most of the M chloroplasts showed the aggregative movement in response to blue light but not red light. Together these results suggest that ROS are not involved in signal transduction for the aggregative movement, and ABA can shift the blue light-induced avoidance movement of C(4)-M chloroplasts to the aggregative movement.     

Tetrazolium Reduction by Guard Cells in Abaxial Epidermis of Vicia faba: Blue Light Stimulation of a Plasmalemma Redox System

The stomata in the abaxial epidermis of Vicia faba were examined for the location of redox systems using tetrazolium salts. Three distinct redox systems could be demonstrated: chloroplast, mitochondrial, and plasmalemma. The chloroplast activity required light and NADP. Mitochondrial activity required added NADH and was suppressed by preincubation with KCN. The plasmalemma redox system in guard cells also required NADH, but was insensitive to KCN and was stimulated by blue light. The involvement of an NADH dehydrogenase in the blue light stimulated redox system in guard cells was suggested by the sensitivity to plantanetin, an inhibitor of NADH dehydrogenase. The redox system of mitochondria was the most active followed by that of plasmalemma. The activity of chloroplasts was the least among the three redox systems. The plasmalemma mediated tetrazolium reduction was stimulated by exogenous flavins and suppressed by Kl or phenylacetate, inhibitors of flavin excitation. We therefore conclude that an NADH-dependent, flavin mediated electron transport system, sensitive to blue light, operates in the plasmalemma of guard cells.     

Blue-light-induced reorganization of the actin cytoskeleton and the avoidance response of chloroplasts in epidermal cells of<Emphasis Type="Italic"> Vallisneria gigantea</Emphasis>

In leaf epidermal cells of the aquatic angiosperm Vallisneria gigantea Graebner, high-intensity blue light induces the actin-dependent avoidance response of chloroplasts. By semi-quantitative motion analysis and phalloidin staining, time courses of the blue-light-induced changes in the mode of movement of individual chloroplasts and in the configuration of actin filaments were examined in the presence and absence of a flavoprotein inhibitor, diphenylene iodonium. In dark-adapted cells, short, thick actin bundles seemed to surround each chloroplast, which was kept motionless in the outer periclinal cytoplasm of the cells. After 10 min of irradiation with high-intensity blue light, a rapid, unidirectional movement of chloroplasts was induced, concomitant with the appearance of aggregated, straight actin bundles stretched over the outer periclinal cytoplasm. Diphenylene iodonium inhibited the avoidance response of chloroplasts, apparently by delaying a change in the mode of chloroplast movement from random sway to unidirectional migration, by suppressing the appearance of aggregated, straight actin bundles. In partially irradiated individual cells, redistribution of chloroplasts and reorganization of actin filaments occurred only in the areas exposed to blue light. From the results, we propose that the short, thick actin bundles in the vicinity of chloroplasts function to anchor the chloroplasts in dark-adapted cells, and that the aggregated, straight actin bundles organized under blue-light irradiation provide tracks for unidirectional movement of chloroplasts.Preliminary results of part of the local irradiation study have already been reported in abstract form [N. Sakurai et al. (2002) J Photosci 9:326–328].     

Phytochrome-Mediated Photoorientation of Chloroplasts in Protonemal Cells of the Fern Adiantum Can be Induced by Brief Irradiation with Red Light

Measuring the ratio of the number of photooriented chloroplaststo the total number of chloroplasts, we found that photoorientationof chloroplasts in protonemata of the fern Adiantum capillus-veneriscould be induced by brief irradiation with polarized red light.After irradiation with red light (R) of 3 or 10 min, orientationalmovement was detected as early as 10 min after the irradiation;it continued during the subsequent dark period for 30–60min, after which chloroplasts gradually dispersed again. WhenR-treated protonemata were irradiated briefly with a second10-min pulse of R, 60 min after the onset of the first irradiation,the orientational response of chloroplasts was again observed.Typical red/far-red photoreversibility was apparent in the response,indicating the involvement of phytochrome. By contrast, irradiationwith polarized blue light for 10 min was ineffective, whileirradiation with blue light (B) at the same fluence for a longerperiod of time clearly induced the photoorientation of chloroplasts.It is likely that longterm irradiation is necessary for theresponse mediated by a blue-light receptor. When protonemata were irradiated with far-red light (FR) immediatelyafter R or after a subsequent dark period of 10 min, the magnitudeof the orientational response was smaller and chloroplasts dispersedmore quickly than those exposed to R alone. When FR was appliedat 50 min, when the response to R had reached the maximum level,chloroplasts again dispersed rapidly to their dark positions.These results indicate that P_FR not only induces the photoorientationmovement of chloroplasts but also fixes the chloroplasts atthe sites to which they have moved as a result of photoorientation. (Received June 2, 1993; Accepted January 11, 1994)     

Analyzing photosynthetic activity and growth of Solanum lycopersicum seedlings exposed to different light qualities

To investigate how light quality influences tomato (Solanum lycopersicum L) seedlings, we examined changes in plant growth, chloroplast ultrastructure, photosynthetic parameters and some photosynthesis-related genes expression levels. For this, tomato plants were grown under different light qualities with the same photosynthetic photon flux density: red (R), blue (B), yellow (Y), green (G) and white (W) lights. Our results revealed that, compared with plants grown under W light, the growth of plants grown under monochromatic lights was inhibited with the growth reduction being more significant in the plants grown under Y and G lights. However, the monochromatic lights had their own effects on the growth and photosynthetic function of tomato seedlings. The plant height was reduced under blue light, but expression of rbcS, rbcL, psbA, psbB genes was up-regulated, and the ΦPSII and electron transport rate (ETR) values were enhanced. More starch grains were accumulated in chloroplasts. The root elongation, net photosynthetic rate (Pn), NPQ and rbcS and psbA genes expression were promoted under red light. Yellow light- and green light-illuminated plants grew badly with their lower Rubisco content and Pn value observed, and less starch grains accumulated in chloroplast. However, less influence was noted of light quality on chloroplast structure. Compared with yellow light, the values of ΦPSII, ETR, qP and NPQ of plants exposed to green light were significantly increased, suggesting that green light was beneficial to both the development of photosynthetic apparatus to some extent.     

The hypocotyl chloroplast plays a role in phototropic bending of Arabidopsis seedlings: developmental and genetic evidence

Chloroplasts of guard cells and coleoptiles have been implicated in the sensory transduction of blue light. The present study was aimed at establishing whether the chloroplast of the hypocotyl from Arabidopsis, another blue light-responding organ, has similar characteristics to that of sensory-transducing guard cell and coleoptile chloroplasts. Results showed that the phototropic curvature and arch length induced by blue light in Arabidopsis seedlings matched the distribution of mature chloroplasts in the bending hypocotyl. The bending arch consistently included the region of the hypocotyl containing mature chloroplasts, and never extended beyond that region. Manipulation of the extent of greening of dark-grown hypocotyls by varying red light pretreatments elicited blue light-stimulated curvatures and arch lengths that depended on the duration of the red light pretreatment and on the distribution of mature chloroplasts in the hypocotyl. Albino psd2 mutants of Arabidopsis, which lack mature chloroplasts, are devoid of phototropic sensitivity under conditions in which wild-type seedlings show large curvatures. The star mutant of Arabidopsis has a delayed greening and a delayed phototropic response as compared with wild type. Measurements of photosynthetic oxygen evolution and carbon fixation, dark respiration, and light-dependent zeaxanthin formation in the hypocotyl showed features similar to those of guard cells and coleoptiles, and distinctly different from those of mesophyll tissue. These results indicate that the hypocotyl chloroplast has characteristics similar to those associated with guard cell and coleoptile chloroplasts, and that phototropic bending of Arabidopsis hypocotyls appears to require mature chloroplasts.