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First published online January 17, 2003; 10.1105/tpc.007237 American Society of Plant Biologists Regulation of Actin-Dependent Cytoplasmic Motility by Type II Phytochrome Occurs within Seconds in Vallisneria gigantea Epidermal Cells
a Department of Biology, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan 1 To whom correspondence should be addressed. E-mail shingot{at}bio.sci.osaka-u.ac.jp; fax 81-6-6850-5817
The effects of light on actin-dependent cytoplasmic motility in epidermal cells of green leaves of the aquatic angiosperm Vallisneria gigantea were investigated quantitatively using a custom-made dynamic image analyzer. Cytoplasmic motility was measured by monitoring changes in the brightness of individual pixels on digitized images taken sequentially under infrared light. Acceleration and deceleration of cytoplasmic motility were regulated photoreversibly by type II phytochrome(s). This phytochrome-dependent induction of cytoplasmic motility did not occur uniformly in cytoplasm but took place as scattered patches in which no particular organelles, including nucleus, existed. The induction became detectable at 2.5 s after the start of irradiation with pulsed red light. In cells exposed to microbeam irradiation, cytoplasmic motility was induced only in sites in the cytoplasm that were irradiated directly, whereas nonirradiated neighboring areas were unaffected. The effect was short-lived, disappearing within a few minutes, and no signal was transmitted from an irradiated cell to its neighbors. Anti-phytochrome antibody–responsive protein(s) was detectable in the leaf extract by immunoblot and zinc blot analyses and in cryosections of the epidermis by immunocytochemistry. Although the phytochrome-dependent cytoplasmic motility was blocked by exogenously applied latrunculin B or cytochalasins, treatment of the dark-adapted cells with Ca2+-chelating reagents induced the cytoplasmic motility. We have proposed a model for the phytochrome regulation of cytoplasmic motility as one of the earliest responses to a light stimulus.
Cytoplasmic movement is a phenomenon that is ubiquitous throughout the plant kingdom, and different types of movement have been described in detail by Britz (1979)  , Gunning (1982), and Nagai (1993). In many cases, prominent cell organelles such as nucleus and plastids move either constitutively or in response to external stimuli together with a mobile cytoplasmic matrix. For active cytoplasmic movements to occur, the cytoplasm must have a high motility. A change in the mode of cytoplasmic motility was induced rapidly by light in a green alga (Schönbohm, 1972) and higher plants (Takagi and Nagai, 1985; Kagawa and Wada, 2000). Although the light-dependent regulation of cytoplasmic motility in plant cells has long been of interest, no quantitative analysis of this phenomenon has been reported. This is attributable mainly to the fact that movements occur so rapidly that they are unable to be dissected by conventional methods of light microscopy.
To address this problem, a digital image-processing technique was developed based on the temporal analysis of changes in the brightness of individual pixels on optical images (Mineyuki et al., 1983
The recent progress of molecular approaches to the study of phytochromes has resulted in an enormous increase in our knowledge of the structure-function relationship of phytochromes (Quail et al., 1995
Phytochrome-regulated responses in plants have been classified into three categories based on fluence and/or wavelength dependence. These are the very-low-fluence responses, low-fluence red/far-red reversible responses (Shinomura et al., 1996
Quantitative Visualization of Cytoplasmic Motility The newly developed image-processing method was applied to a quantitative analysis of cytoplasmic motility in epidermal cells of Vallisneria. After irradiation with white light of different intensities, samples determined visually to show low (Figure 1A) , intermediate (Figure 1D), and high (Figure 1G) cytoplasmic motility were analyzed by the dynamic image-processing method. The equipment is illustrated schematically in Figure 1J. Each result was demonstrated in terms of the processed image in false color (Figures 1B, 1E, and 1H) and the frequency distribution of calculated indices for individual pixels (Figures 1C, 1F, and 1I). The peak values of the calculated indices of cytoplasmic motility for the low-, intermediate-, and high-motility samples were 13.5 (Figure 1C), 21.6 (Figure 1F), and 39.2 (Figure 1I), respectively. We concluded that this index can be used to provide a quantitative scale of cytoplasmic motility.
Using this method, the effects of dark adaptation and light irradiation on cytoplasmic motility were investigated. Sample leaves were exposed to white light for 12 h under the daily regime and then kept in complete darkness for various periods of time. During a 12-h dark incubation, the cytoplasmic motility index decreased gradually from >20 to 14. Upon white light illumination of the dark-adapted samples, numerous clusters of bright pixels appeared as scattered patches in the cytoplasm facing the outer periclinal wall of the epidermal cells (Figure 2) . The number and size of bright clusters depended on the fluence of the incident light. The site of these areas of cytoplasmic movement did not correspond to any particular visible organelles, such as nuclei and plastids, but appeared to occur in a patchy distribution throughout the cytoplasm. With longer periods of observation, new clusters of bright pixels were seen to emerge spontaneously, and some of the clusters expanded gradually, whereas others shrank. On average, the index of cytoplasmic motility of such samples increased rapidly with time.
Regulation of Cytoplasmic Motility by Type II Phytochrome Dark-adapted samples were irradiated for 1.0 s with monochromatic blue (460 nm), green (520 nm), red (660 nm), and far-red (760 nm) light of 100 µmol·m-2·s-1. Immediately after these light treatments, optical images of each sample were recorded under IR light (850 nm) and analyzed by the image-processing method. The resulting index of cytoplasmic motility indicated that red light was most effective at increasing the index, whereas blue light exhibited a much smaller effect and green and far-red light showed almost no effect (Table 1). The effect of red light (100 µmol·m-2·s-1 for 1.0 s) was canceled by a subsequent irradiation with far-red light of the same fluence as the red light. The effects of red and far-red light were repeatedly reversible upon the alternation of irradiation with red and far-red light (Table 1).
After samples were irradiated with red light of 10 to 103 µmol·m-2·s-1 for different time periods (0.1 to 5.0 s), we determined the magnitude of the response and plotted the resulting values against the fluence of applied light (Figure 3) . The response was inducible at a fluence of 30 µmol/m2 and reached saturation at  1 mmol/m2. The photoreversible effect (Table 1) and the effective range of fluences (Figure 3) strongly suggested that a type II phytochrome (Furuya, 1993) regulated the cytoplasmic motility in the epidermal cells of Vallisneria. However, there was no indication of the involvement of type I phytochrome in this effect because far-red light alone failed to induce detectable cytoplasmic motility (Table 1).
Time Requirement for the Photoinduction of Cytoplasmic Motility Dark-adapted samples were irradiated with red or far-red light for 0.25, 0.5, or 1.0 s at a fixed fluence of 2 mmol/m2. To determine when the effect of light on cytoplasmic motility was first detectable, we recorded optical images of the sample cells under the IR microscope sequentially at intervals of 0.2 s immediately after the termination of the pulse irradiation, as illustrated in Figure 4B . From a series of sets of nine IR light images, processed images were obtained sequentially with intervals of 0.2 s. The interval of 0.2 s was chosen because it was the shortest period of time for which it was possible to determine the index of cytoplasmic motility with an acceptable signal-to-noise ratio.
In the case of red light irradiation for 0.25 s, the first processed image (Figure 4A, R0.25, top) was obtained after calculation of the index of cytoplasmic motility using the first set of nine serial IR light images that were recorded at 0.25, 0.45, 0.65, 0.85, 1.05, 1.25, 1.45, 1.65, and 1.85 s after the start of red light irradiation (Figure 4B). The next image (Figure 4A, R0.25, second from top) was obtained using the next set of nine serial IR light images recorded at 0.45 to 2.05 s. Such image-processing procedures were performed successively at intervals of 0.2 s with the sets of nine serial IR light images (Figure 4A, R0.25). No prominent change in the index of cytoplasmic motility was detected in the top four processed images (shown in false color), indicating that the magnitude of cytoplasmic motility did not change substantially from the dark-adapted level in the first 2.45 s after the start of the red light irradiation. By contrast, a cluster of bright pixels became evident in the fifth processed image and thereafter (Figure 4A, R0.25), indicating that the first significant changes in pixel brightness occurred in the last IR light image, captured 2.65 s after the start of pulse irradiation with red light.
In the case of red light irradiation for 0.5 s (Figure 4A, R0.5), a cluster of bright pixels first became evident in the fourth processed image, whereas in the case of a 1.0-s red light pulse (Figure 4A, R1.0), cytoplasmic movement was first seen in the second processed image. These results represent the induction of significant cytoplasmic movement at 2.7 and 2.8 s after the start of the 0.5- and 1.0-s red light pulse, respectively. In repeated experiments, we found that the earliest change in the cytoplasmic motility detectable in processed images occurred at
When far-red light was applied for 0.5 s immediately after irradiation with red light for 0.25 s, the induction of cytoplasmic motility was not observed (Figure 4A, R0.25/FR0.5). Irradiation with far-red pulse light and the observing IR light did not produce any changes in the processed images for the period of these experiments (Figure 4A, FR0.5 and IR). This finding confirmed our previous conclusion that the photoregulation of cytoplasmic motility is controlled by type II phytochrome but not by type I phytochrome.
Effects of Microbeam Light on Cytoplasmic Motility
Indices of cytoplasmic motility were determined for both microbeam-irradiated and unirradiated cells. Although microbeam irradiation did not always produce a significant increase in the index in the irradiated cell, the results clearly showed that when the response was detected, it was limited exclusively to irradiated cells and there was no indication of any long-distance signaling through the leaf tissue (Table 3, experiments 1 and 2). Moreover, even in a single cell, the red light–induced signal did not spread from the irradiated region to neighboring regions (Table 3, experiment 3). The effect of a microbeam of red light was antagonized almost completely by subsequent irradiation with far-red light (data not shown). In these experiments, changes in the cytoplasmic motility index were exaggerated compared with those in the whole irradiation experiments (Table 1). This finding is attributable to the fact that measurements were made only in the samples that responded to microbeam irradiation and were based on a much smaller number of pixels. In the whole irradiation experiments, the index was determined for a large area that included several cells, of which the whole outer periclinal layer had been irradiated. Because the light-induced cytoplasmic motility occurred in a patchy manner (Figure 2), the average indices became much smaller compared with those in the microbeam irradiation experiments.
Immunochemical Detection of Phytochromes in Vallisneria To demonstrate that the material used in the present study does contain phytochrome(s), we first prepared extracts of soluble proteins from dark-adapted whole leaves of Vallisneria and performed immunoprecipitation with eight monoclonal anti-phytochrome antibodies produced previously against different phytochromes from various plants (Table 4). Immunoblot analysis revealed that polypeptides of 120 kD (as determined by SDS-PAGE) were precipitated with those specific antibodies, and the precipitated polypeptides exhibited different immunoaffinity to the individual antibodies (Figure 6A)
. Furthermore, Zn2+-induced luminescence signal was evident in all 120-kD bands precipitated with the anti-phytochrome antibodies (Figure 6B), suggesting that the anti-phytochrome antibody–responsive polypeptides incorporated a bilin chromophore. Using the anti-PHYB antibodies mBA02 and mBP01, we further confirmed that the antibody-responsive polypeptides of 120 kD also were precipitated from the protein extract prepared from dark-adapted epidermal tissues (Figure 6C). The molecular mass of these polypeptides was consistent with that of the previously reported type I and type II phytochromes in other plant species, which ranged from 118 to 125 kD (Furuya, 1993), lending further support to the conclusion that Vallisneria leaf epidermal cells contain phytochrome(s).
Because the anti-PHYB antibodies seemed to have higher affinity for Vallisneria phytochrome(s) than the anti-PHYA antibodies (Figure 6), we used those antibodies as the primary antibody to perform immunocytochemical assays on cryosections prepared from the dark-adapted Vallisneria leaves. Strong immunosignals were observed in the symplastic region of the epidermal cells with monoclonal antibodies mBA01 (Figure 7A) , mBA02 (Figure 7B), and mBP01 (Figure 7C), all of which positively detected the 120-kD polypeptide in immunoprecipitation (Figure 6). By contrast, only very weak immunosignals were detectable with mBT04 (Figure 7D). With nonimmune serum, no signal was detected (Figure 7E).
Involvement of the Actin Cytoskeleton and Calcium in Cytoplasmic Motility Finally, we attempted to characterize a motile apparatus that drives cytoplasmic motility in Vallisneria. Processed images were obtained sequentially from dark-adapted samples under IR light, in which numerous tiny clusters of pixels with very small indices of cytoplasmic motility emerged and disappeared spontaneously. On average, the index of such samples remained at a constant level. When such samples were treated with latrunculin B or cytochalasin B in darkness, the index declined dramatically (Table 5, experiment 1). In the processed images of those treated samples, we observed no spontaneous appearance and disappearance of tiny clusters of pixels with very small indices of cytoplasmic motility. Moreover, in the presence of latrunculin B or cytochalasin B, light irradiation did not induce any increase in the index of cytoplasmic motility (Table 5, experiment 1). By staining with fluorescently labeled phalloidin, we confirmed that the actin filaments along the outer periclinal walls of epidermal cells were destroyed almost completely after treatment with latrunculin B (Figure 8) . In such samples, a pulse irradiation with red light did not induce a rapid increase in cytoplasmic motility (Figure 4A, +LB).
When latrunculin B and cytochalasin B were removed by washing, the effects disappeared and the index returned to the level of the dark-adapted samples (Table 5, experiment 1). Furthermore, in the washed samples, the responsiveness to light irradiation was recovered fully. Another inhibitor, cytochalasin D, exhibited quite similar effects at 0.1 mM (data not shown). By contrast, the microtubule-disrupting reagent colchicine affected neither the dark-adapted index of cytoplasmic motility nor the responsiveness to light irradiation (Table 5, experiment 1). These results strongly suggest that the observed cytoplasmic motility is an active process driven by an actin-dependent motile apparatus and that the movement of the cytoplasm with very small amplitude can occur in a patchy manner even in complete darkness. When the dark-adapted samples were treated with the Ca2+-chelating reagent 1,2-bis(o-aminophenoxy)ethane-N,N,N,N-tetraacetic acid or EGTA, the induction of cytoplasmic motility was observed even in darkness (Figure 9) . The effects of Ca2+-chelating reagents were canceled when those reagents were applied with a high concentration of Ca2+. Although these results suggested the possible involvement of Ca2+ in the regulation of cytoplasmic motility in the material studied here, the photoinduction of cytoplasmic motility did not seem to be affected by an inhibitor of plasma membrane ATPase (vanadate) or by a plasma membrane Ca2+ channel blocker (La3+) (Table 5, experiment 2).
Model for the Rapid Regulation of Cytoplasmic Motility by Phytochrome In this study, we have established a new method to quantify and visualize cytoplasmic motility using an IR microscope combined with a dynamic image-processing procedure. Furthermore, the microirradiation equipment enabled a precise spatiotemporal analysis of the photoregulation of the cytoplasmic motility. From the results obtained, we propose the following hypothesis for the phytochrome regulation of cytoplasmic motility in Vallisneria epidermal cells. Phototransformation of the red light–absorbing form of type II phytochrome, which is located in the cytoplasm, to the far-red light–absorbing form produces an altered interaction with a cytoplasmic partner, such as the PKS1 protein characterized previously in Arabidopsis (Fankhauser et al., 1999 ). This change evokes a modulation in the cytoplasmic level of Ca2+. Ca2+ interacts with Ca2+-sensitive cytoskeletal factors, which might include the motor protein myosin (Yokota et al., 1999), which is known to function in the generation of motive force through an interaction with actin filaments and the Ca2+-sensitive actin binding protein (Yokota et al., 2000) necessary for the reorganization of actin filaments. Transduction of the type II phytochrome-dependent signals onto the actin-based motile apparatus is completed within 2.5 s after the start of light irradiation only in the light-exposed cytoplasmic region.
Cytoplasmic Motility under the Control of Type II Phytochrome
Rapidly Photoinducible Responses in Plant Cells
Localized Responses and Long-Distance Signaling in Phytochrome-Mediated Regulation
By contrast, phytochrome in the cotyledons of Sinapis (Nick et al., 1993
Actin-Dependent Cytoplasmic Motility and Its Regulation by Ca2+
Because the exogenous application of Ca2+-chelating reagents has been demonstrated to effectively modulate cytoplasmic Ca2+ in plant cells using a fluorescent probe (Gilroy et al., 1991
Plant Material and Preparation of Specimens Young plants of Vallisneria gigantea were cultured in buckets filled with tap water, with soil on the bottom, under a daily regime of 12 h of light (0.5 W/m2) and 12 h of darkness at 20 to 25°C. The light source was a bank of 20-W fluorescent lamps (FL20S-PG; National, Kadoma, Japan). The procedures for the preparation of specimens were described in detail previously (Dong et al., 1995 ). In brief, adaxial pieces of leaf 5 to 10 mm in width and 2 to 3 mm in length were removed at the end of a light period. Each piece was floated in artificial pond water, which contained 0.5 mM KCl, 0.2 mM NaCl, 0.1 mM Ca(NO3)2, 0.1 mM Mg(NO3)2, and 2 mM Pipes, pH 7.0, for one cycle of darkness and light, and then it was mounted on a glass slide. The glass slide was immersed in fresh artificial pond water in a Petri dish and kept in complete darkness for another 12 to 18 h. Stock solutions of latrunculin B and cytochalasin B were prepared in DMSO and diluted with artificial pond water before use.
Light Treatments
Dynamic Processing of Digital Images The brightness of each pixel on each optical image was assigned one of 256 levels. From an appropriate number of such images, the standard deviation for all levels of brightness was calculated for individual pixels at the same position. If a pixel exhibited frequent changes in brightness, the resulting value became larger, whereas when a pixel exhibited only occasional changes in brightness, a smaller value was obtained. The calculated standard deviations were taken as an index of cytoplasmic motility after multiplication by an appropriate constant. For the visual demonstration of cytoplasmic motility, the index of cytoplasmic motility for individual pixels was displayed as a processed image in 15-step false color. Unless noted otherwise, optical images were recorded at intervals of 1.0 s, and the index of cytoplasmic motility for individual pixels was calculated from nine serial images at the multiplying constant of 32. Thus, one processed image represented the cytoplasmic motility detected during 8.0 s of observation. When the time course of the photoinduction of cytoplasmic motility was examined (Figure 4, Table 2), optical images were recorded at intervals of 0.2 s (Figure 4B). The dimensions of each pixel were 0.8 x 0.8 µm2 under the experimental conditions of the present study. Except for the microbeam irradiation study (Figure 5, Table 3), the index of cytoplasmic motility was determined on 200,000 to 250,000 pixels, which covered several epidermal cells for each sample. Autofluorescence from chlorophylls was eliminated efficiently by sheets of blue cellophane (transmissible from 350 to 530 nm and >700 nm; Hino-Asahido Corp., Tokyo, Japan) placed in the light path in front of the IR light–sensitive camera. All of the experiments were performed under dim green light at 18 to 22°C.
Protein Extraction
Immunoblot and Zinc Blot Analysis
Immunostaining of Cryosections
Staining of Actin Filaments Upon request, all novel materials described in this article will be made available in a timely manner for noncommercial research purposes.
This paper is dedicated to the late Prof. emeritus Noburo Kamiya of Osaka University, who had made a great contribution to plant cell biology. The authors are deeply indebted to Akira Nagatani (Kyoto University) for permission to use purified monoclonal antibodies, to James L. Weller (University of Tasmania) for critical reading of the manuscript, and to Shigeru Uchiyama (Hamamatsu Photonics) for kind cooperation in improving the software. Thanks are due as well to Nami Sakurai (Osaka University) for excellent staining of actin filaments and to Michinari Kouzuma (Hitachi Ltd., Instrument Division) for collaboration in the early stages of this study. This work was supported in part by Grant B2023 from the Hitachi Advanced Research Laboratory and by a grant from the Program for the Promotion of Basic Research Activities for Innovative Biosciences to M.F.
Article, publication date, and citation information can be found at www.plantcell.org/cgi/doi/10.1105/tpc.007237.
2 Current address: Department of Botany, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan. Received August 18, 2002; accepted November 6, 2002.
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Abstract
INTRODUCTION
