Blue- and red-light action in photoorientation of chloroplasts in Adiantum protonemata

A PSTAIRE-type cyclin-dependent kinase controls light responses in land plants

Light is a critical signal perceived by plants to adapt their growth rate and direction. Although many signaling components have been studied, how plants respond to constantly fluctuating light remains underexplored. Here, we showed that in the moss Physcomitrium (Physcomitrella) patens, the PSTAIRE-type cyclin-dependent kinase PpCDKA is dispensable for growth. Instead, PpCDKA and its homolog in Arabidopsis thaliana control light-induced tropisms and chloroplast movements by probably influencing the cytoskeleton organization independently of the cell cycle. In addition, lower PpCDKA kinase activity was required to elicit light responses relative to cell cycle regulation. Thus, our study suggests that plant CDKAs may have been co-opted to control multiple light responses, and owing to the bistable switch properties of PSTAIRE-type CDKs, the noncanonical functions are widely conserved for eukaryotic environmental adaptation.

The cold-induced switch in direction of chloroplast relocation occurs independently of changes in endogenous phototropin levels

When exposed to fluctuating light intensity, chloroplasts move towards weak light (accumulation response), and away from strong light (avoidance response). In addition, cold treatment (5°C) induces the avoidance response even under weak-light conditions (cold-avoidance response). These three responses are mediated by the phototropin (phot), which is a blue-light photoreceptor and has also been reported to act as a thermosensory protein that perceives temperature variation. Our previous report indicated that cold-induced changes in phot biochemical activity initiate the cold-avoidance response. In this study, we further explored the induction mechanism of the cold-avoidance response in the liverwort Marchantia polymorpha and examined the relationship between changes in the amount of phot and the induction of the cold-avoidance response. The switch between the accumulation and avoidance responses occurs at a so-called 'transitional' light intensity. Our physiological experiments revealed that a cold-mediated decrease in the transitional light intensity leads to the induction of the cold-avoidance response. While artificial overexpression of phot decreased the transitional light intensity as much as cold treatment did, the amount of endogenous phot was not increased by cold treatment in wild-type M. polymorpha. Taken together, these findings show that the cold-avoidance response is initiated by a cold-mediated reduction of the transitional light intensity, independent of the amount of endogenous phot. This study provides a clue to understanding the mechanism underlying the switch in direction of chloroplast relocation in response to light and temperature.

Light-dependent spatiotemporal control of plant cell development and organelle movement in fern gametophytes

The haploid gametophyte generation of ferns is an excellent experimental material for cell biology studies because of its simple structure and high sensitivity to light. Each step of the developmental process, such as cell growth, cell cycle and the direction of cell division, is controlled, step by step, by light, unlike what happens in complex seed plant tissues. To perform analyses at the cell or organelle level, we have developed special tools, instruments and techniques, such as a cuvette suitable for repeated centrifugation in particular directions, microbeam irradiators for partial cell irradiation and single-cell ligation technique to create enucleated cells. Some of our main discoveries are as follows: (1) changes in the intracellular position of the nucleus in long protonemal cells by centrifugation revealed that the nuclear position or a factor(s) that is/are co-centrifuged with the nucleus is important for the decision regarding the place of the formation of preprophase bands and the timing of their disappearance, which determines the position where the new cell wall attaches to the mother cell wall; (2) even within a single cell, various phenomena could be induced by blue or red light, with the localization of the blue or red light receptors being different depending on the phenomenon; (3) de novo mRNA synthesis is not involved in the signal transduction pathways underlying light-induced chloroplast movements. In this review article, various microscopic techniques, in addition to the results of physiology studies in fern gametophytes, are described.

Actin-mediated movement of chloroplasts

Plants are sessile and require diverse strategies to adapt to fluctuations in the surrounding light conditions. Consequently, the photorelocation movement of chloroplasts is essential to prevent damages that are induced by intense light (avoidance response) and to ensure efficient photosynthetic activities under weak light conditions (accumulation response). The mechanisms that underlie chloroplast movements have been revealed through analysis of the behavior of individual chloroplasts and it has been found that these organelles can move in any direction without turning. This implies that any part of the chloroplast periphery can function as the leading or trailing edge during movement. This ability is mediated by a special structure, which consists of short actin filaments that are polymerized at the leading edge of moving chloroplasts and are specifically localized in the space between the chloroplast and the plasma membrane, and is called chloroplast-actin. In addition, several of the genes that encode proteins that are involved in chloroplast-actin polymerization or maintenance have been identified. In this Review, we discuss the mechanisms that regulate chloroplast movements through polymerization of the chloroplast-actin and propose a model for actin-driven chloroplast photorelocation movement.

Ferns, mosses and liverworts as model systems for light-mediated chloroplast movements

Light-induced chloroplast movement is found in most plant species, including algae and land plants. In land plants with multiple small chloroplasts, under weak light conditions, the chloroplasts move towards the light and accumulate on the periclinal cell walls to efficiently perceive light for photosynthesis (the accumulation response). Under strong light conditions, chloroplasts escape from light to avoid photodamage (the avoidance response). In most plant species, blue light induces chloroplast movement, and phototropin receptor kinases are the blue light receptors. Molecular mechanisms for photoreceptors, signal transduction and chloroplast motility systems are being studied using the model plant Arabidopsis thaliana. However, to further understand the molecular mechanisms and evolutionary history of chloroplast movement in green plants, analyses using other plant systems are required. Here, we review recent works on chloroplast movement in green algae, liverwort, mosses and ferns that provide new insights on chloroplast movement.

Temperature-dependent signal transmission in chloroplast accumulation response

Chloroplast photorelocation movement, well-characterized light-induced response found in various plant species from alga to higher plants, is an important phenomenon for plants to increase photosynthesis efficiency and avoid photodamage. The signal for chloroplast accumulation movement connecting the blue light receptor, phototropin, and chloroplasts remains to be identified, although the photoreceptors and the mechanism of movement via chloroplast actin filaments have now been revealed in land plants. The characteristics of the signal have been found; the speed of signal transfer is about 1 µm min^-1 and that the signal for the accumulation response has a longer life and is transferred a longer distance than that of the avoidance response. Here, to collect the clues of the unknown signal substances, we studied the effect of temperature on the speed of signal transmission using the fern Adiantum capillus-veneris and found the possibility that the mechanism of signal transfer was not dependent on the simple diffusion of a substance; thus, some chemical reaction must also be involved. We also found new insights of signaling substances, such that microtubules are not involved in the signal transmission, and that the signal could even be transmitted through the narrow space between chloroplasts and the plasma membrane.

Molecular basis of chloroplast photorelocation movement

Chloroplast photorelocation movement is an essential physiological response for sessile plant survival and the optimization of photosynthetic ability. Simple but effective experiments on the physiological, cell biological and molecular genetic aspects have been widely used to investigate the signaling components of chloroplast photorelocation movement in Arabidopsis for the past few decades. Although recent knowledge on chloroplast photorelocation movement has led us to a deeper understanding of its physiological and molecular basis, the biochemical roles of the downstream factors remain largely unknown. In this review, we briefly summarize recent advances regarding chloroplast photorelocation movement and propose that a new high-resolution approach is necessary to investigate the molecular mechanism underlying actin-based chloroplast photorelocation movement.

Chloroplast and nuclear photorelocation movements

Chloroplasts move toward weak light to increase photosynthetic efficiency, and migrate away from strong light to protect chloroplasts from photodamage and eventual cell death. These chloroplast behaviors were first observed more than 100 years ago, but the underlying mechanism has only recently been identified. Ideal plant materials, such as fern gametophytes for photobiological and cell biological approaches, and Arabidopsis thaliana for genetic analyses, have been used along with sophisticated methods, such as partial cell irradiation and time-lapse video recording under infrared light to study chloroplast movement. These studies have revealed precise chloroplast behavior, and identified photoreceptors, other relevant protein components, and novel actin filament structures required for chloroplast movement. In this review, our findings regarding chloroplast and nuclear movements are described.

Non-invasive, whole-plant imaging of chloroplast movement and chlorophyll fluorescence reveals photosynthetic phenotypes independent of chloroplast photorelocation defects in chloroplast division mutants

Leaf chloroplast movement is thought to optimize light capture and to minimize photodamage. To better understand the impact of chloroplast movement on photosynthesis, we developed a technique based on the imaging of reflectance from leaf surfaces that enables continuous, high-sensitivity, non-invasive measurements of chloroplast movement in multiple intact plants under white actinic light. We validated the method by measuring photorelocation responses in Arabidopsis chloroplast division mutants with drastically enlarged chloroplasts, and in phototropin mutants with impaired photorelocation but normal chloroplast morphology, under different light regimes. Additionally, we expanded our platform to permit simultaneous image-based measurements of chlorophyll fluorescence and chloroplast movement. We show that chloroplast division mutants with enlarged, less-mobile chloroplasts exhibit greater photosystem II photodamage than is observed in the wild type, particularly under fluctuating high levels of light. Comparison between division mutants and the severe photorelocation mutant phot1-5 phot2-1 showed that these effects are not entirely attributable to diminished photorelocation responses, as previously hypothesized, implying that altered chloroplast morphology affects other photosynthetic processes. Our dual-imaging platform also allowed us to develop a straightforward approach to correct non-photochemical quenching (NPQ) calculations for interference from chloroplast movement. This correction method should be generally useful when fluorescence and reflectance are measured in the same experiments. The corrected data indicate that the energy-dependent (qE) and photoinhibitory (qI) components of NPQ contribute differentially to the NPQ phenotypes of the chloroplast division and photorelocation mutants. This imaging technology thus provides a platform for analyzing the contributions of chloroplast movement, chloroplast morphology and other phenotypic attributes to the overall photosynthetic performance of higher plants.

Phototropin encoded by a single-copy gene mediates chloroplast photorelocation movements in the liverwort Marchantia polymorpha

Blue-light-induced chloroplast photorelocation movement is observed in most land plants. Chloroplasts move toward weak-light-irradiated areas to efficiently absorb light (the accumulation response) and escape from strong-light-irradiated areas to avoid photodamage (the avoidance response). The plant-specific kinase phototropin (phot) is the blue-light receptor for chloroplast movements. Although the molecular mechanisms for chloroplast photorelocation movement have been analyzed, the overall aspects of signal transduction common to land plants are still unknown. Here, we show that the liverwort Marchantia polymorpha exhibits the accumulation and avoidance responses exclusively induced by blue light as well as specific chloroplast positioning in the dark. Moreover, in silico and Southern-blot analyses revealed that the M. polymorpha genome encodes a single PHOT gene, MpPHOT, and its knockout line displayed none of the chloroplast photorelocation movements, indicating that the sole MpPHOT gene mediates all types of movement. Mpphot was localized on the plasma membrane and exhibited blue-light-dependent autophosphorylation both in vitro and in vivo. Heterologous expression of MpPHOT rescued the defects in chloroplast movement of phot mutants in the fern Adiantum capillus-veneris and the seed plant Arabidopsis (Arabidopsis thaliana). These results indicate that Mpphot possesses evolutionarily conserved regulatory activities for chloroplast photorelocation movement. M. polymorpha offers a simple and versatile platform for analyzing the fundamental processes of phototropin-mediated chloroplast photorelocation movement common to land plants.

Recent advances in understanding the molecular mechanism of chloroplast photorelocation movement

Plants are photosynthetic organisms that have evolved unique systems to adapt fluctuating environmental light conditions. In addition to well-known movement responses such as phototropism, stomatal opening, and nastic leaf movements, chloroplast photorelocation movement is one of the essential cellular responses to optimize photosynthetic ability and avoid photodamage. For these adaptations, chloroplasts accumulate at the areas of cells illuminated with low light (called accumulation response), while they scatter from the area illuminated with strong light (called avoidance response). Plant-specific photoreceptors (phototropin, phytochrome, and/or neochrome) mediate these dynamic directional movements in response to incident light position and intensity. Several factors involved in the mechanisms underlying the processes from light perception to actin-based movements have also been identified through molecular genetic approach. This review aims to discuss recent findings in the field relating to how chloroplasts move at molecular levels. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.

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.

Photosynthesis-dependent but neochrome1-independent light positioning of chloroplasts and nuclei in the fern Adiantum capillus-veneris

Chloroplasts change their positions in the cell depending on the light conditions. In the dark, chloroplasts in fern prothallia locate along the anticlinal wall (dark position). However, chloroplasts become relocated to the periclinal wall (light position) when the light shines perpendicularly to the prothallia. Red light is effective in inducing this relocation in Adiantum capillus-veneris, and neochrome1 (neo1) has been identified as the red light receptor regulating this movement. Nevertheless, we found here that chloroplasts in neo1 mutants still become relocated from the dark position to the light position under red light. We tested four neo1 mutant alleles (neo1-1, neo1-2, neo1-3, and neo1-4), and all of them showed the red-light-induced chloroplast relocation. Furthermore, chloroplast light positioning under red light occurred also in Pteris vittata, another fern species naturally lacking the neo1-dependent phenomenon. The light positioning of chloroplasts occurred independently of the direction of red light, a response different to that of the neo1-dependent movement. Photosynthesis inhibitors 3-(3,4 dichlorophenyl)-1,1-dimethylurea or 2,5-dibromo-3-isopropyl-6-methyl-p-benzoquinone blocked this movement. Addition of sucrose (Suc) or glucose to the culture medium induced migration of the chloroplasts to the periclinal wall in darkness. Furthermore, Suc could override the effects of 3-(3,4 dichlorophenyl)-1,1-dimethylurea. Interestingly, the same light positioning was evident for nuclei under red light in the neo1 mutant. The nuclear light positioning was also induced in darkness with the addition of Suc or glucose. These results indicate that photosynthesis-dependent nondirectional movement contributes to the light positioning of these organelles in addition to the neo1-dependent directional movement toward light.

Chloroplast outer envelope protein CHUP1 is essential for chloroplast anchorage to the plasma membrane and chloroplast movement

Chloroplasts change their intracellular distribution in response to light intensity. Previously, we isolated the chloroplast unusual positioning1 (chup1) mutant of Arabidopsis (Arabidopsis thaliana). This mutant is defective in normal chloroplast relocation movement and shows aggregation of chloroplasts at the bottom of palisade mesophyll cells. The isolated gene encodes a protein with an actin-binding motif. Here, we used biochemical analyses to determine the subcellular localization of full-length CHUP1 on the chloroplast outer envelope. A CHUP1-green fluorescent protein (GFP) fusion, which was detected at the outermost part of mesophyll cell chloroplasts, complemented the chup1 phenotype, but GFP-CHUP1, which was localized mainly in the cytosol, did not. Overexpression of the N-terminal hydrophobic region (NtHR) of CHUP1 fused with GFP (NtHR-GFP) induced a chup1-like phenotype, indicating a dominant-negative effect on chloroplast relocation movement. A similar pattern was found in chloroplast OUTER ENVELOPE PROTEIN7 (OEP7)-GFP transformants, and a protein containing OEP7 in place of NtHR complemented the mutant phenotype. Physiological analyses of transgenic Arabidopsis plants expressing truncated CHUP1 in a chup1 mutant background and cytoskeletal inhibitor experiments showed that the coiled-coil region of CHUP1 anchors chloroplasts firmly on the plasma membrane, consistent with the localization of coiled-coil GFP on the plasma membrane. Thus, CHUP1 localization on chloroplasts, with the N terminus inserted into the chloroplast outer envelope and the C terminus facing the cytosol, is essential for CHUP1 function, and the coiled-coil region of CHUP1 prevents chloroplast aggregation and participates in chloroplast relocation movement.

Chloroplast photorelocation movement mediated by phototropin family proteins in green plants

Chloroplasts gather in areas irradiated with weak light to maximize photosynthesis (the accumulation response). They move away from areas irradiated with strong light to minimize damage of the photosynthetic apparatus (the avoidance response). The processes underlying these chloroplast movements can be divided into three parts: photoperception, signal transduction, and chloroplast movement. Photoreceptors for chloroplast movement have been identified recently in various plant species. A blue light receptor phototropin (phot) mediates chloroplast photorelocation movement in the seed plant Arabidopsis thaliana, the fern Adiantum capillus-veneris, the moss Physcomitrella patens and possibly the green alga Mougeotia scalaris. A chimeric photoreceptor between phytochrome and phototropin, neochrome (neo), was found in some advanced ferns and in the green alga M. scalaris. While the mechanism of chloroplast movement is not well understood, it is known that actin filaments play an important role in this process. To understand the molecular mechanisms associated with chloroplast movement, several mutants were isolated in A. thaliana (jac1 and chup1) and the corresponding genes were cloned. In this review, recent progress in photoreceptor research into chloroplast movement in various plant species and the possible factors functioning in signal transduction or the regulation of actin filaments identified in A. thaliana is discussed.

Cryptochrome signaling in plants

Cryptochromes are blue light receptors that mediate various light-induced responses in plants and animals. They share sequence similarity to photolyases, flavoproteins that catalyze the repair of UV light-damaged DNA, but do not have photolyase activity. Arabidopsis cryptochromes work together with the red/far-red light receptor phytochromes to regulate various light responses, including the regulation of cell elongation and photoperiodic flowering, and are also found to act together with the blue light receptor phototropins to mediate blue light regulation of stomatal opening. The signaling mechanism of Arabidopsis cryptochromes is mediated through negative regulation of COP1 by direct CRY-COP1 interaction through CRY C-terminal domain. Arabidopsis CRY dimerized through its N-terminal domain and dimerization of CRY is required for light activation of the photoreceptor activity. Recently, significant progresses have been made in our understanding of cryptochrome functions in other dicots such as pea and tomato and lower plants including moss and fern. This review will focus on recent advances in functional and mechanism characterization of cryptochromes in plants. It is not intended to cover every aspect of the field; readers are referred to other review articles for historical perspectives and a more comprehensive understanding of this photoreceptor.

Phytochrome-dependent photomovement responses mediated by phototropin family proteins in cryptogam plants

In this review, we describe the regulation of photomovement responses by phototropin and phytochrome photoreceptors. The blue light receptor phototropin mediates various photomovement responses such as phototropism, chloroplast movement and stomatal opening. In cryptogamic plants including ferns, mosses and green alga, red as well as blue light mediates phototropism and chloroplast movement. The red/far-red light reversibility suggests the involvement of phytochrome in these responses. Thereby, plant growth is presumably promoted by coordinating these photomovements to capture efficiently light for photosynthesis.

The fern as a model system to study photomorphogenesis

The fern gametophyte is a good model system for studying cell biological, physiological, and photobiological aspects of the fundamental processes of plant development and physiological phenomena, because of its autotrophic characteristics and its simple structure. The cells, moreover, are not surrounded by tissue, so observation and manipulation of the cells are very easy. Here I summarize a part of my knowledge of fern systems, which I have studied for nearly 40 years.

The sliding theory of cytoplasmic streaming: fifty years of progress

Fifty years ago, an important paper appeared in Botanical Magazine Tokyo. Kamiya and Kuroda proposed a sliding theory for the mechanism of cytoplasmic streaming. This pioneering study laid the basis for elucidation of the molecular mechanism of cytoplasmic streaming--the motive force is generated by the sliding of myosin XI associated with organelles along actin filaments, using the hydrolysis energy of ATP. The role of the actin-myosin system in various plant cell functions is becoming evident. The present article reviews progress in studies on cytoplasmic streaming over the past 50 years.

Blue-light-induced reorganization of the actin cytoskeleton and the avoidance response of chloroplasts in epidermal cells of Vallisneria gigantea

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.

Chloroplast movement: dissection of events downstream of photo- and mechano-perception

The study of chloroplast photorelocation movement is progressing rapidly now that mutants for chloroplast movement have become available in Arabidopsis thaliana. However, mechanistic approaches in cell biology still stand to elucidate the mechanisms and regulations of such movement. The fern Adiantum capillus-veneris and the moss Physcomitrella patens are particularly suitable materials for analyzing the kinetics of intracellular chloroplast movement. In these plants, chloroplast movement is induced by red light as well as blue light, mediated by phytochrome and blue light receptor, respectively. In this paper, we review the unique force-generating system for chloroplast motility in P. patens. In addition to light-induced chloroplast movement, we also summarize mechanically induced chloroplast movement in these plants and the motility systems involved. Finally, the different dependency of mechano- and photo-relocation movement on external Ca(2+) is discussed.

The phototropin family as photoreceptors for blue light-induced chloroplast relocation

Blue light-induced chloroplast accumulation and avoidance relocation movements are controlled by the blue light photoreceptor phototropin. The Arabidopsis thaliana genome has two phototropin genes encoding phot1 and phot2. Each of these photoreceptors contains two LOV (light oxygen and voltage) domains and a kinase domain. The LOV domains absorb blue light though an associated flavin mononucleotide chromophore, while the kinase domain is thought to be associated with signal transduction. The phototropins control not only chloroplast relocation movement, but also blue light-induced phototropic responses, leaf expansion and stomatal opening. Here I review the role of phototropin as a photoreceptor for chloroplast photorelocation movement.

Responses of ferns to red light are mediated by an unconventional photoreceptor

Efficient photosynthesis is essential for plant survival. To optimize photosynthesis, plants have developed several photoresponses. Stems bend towards a light source (phototropism), chloroplasts move to a place of appropriate light intensity (chloroplast photorelocation) and stomata open to absorb carbon dioxide. These responses are mediated by the blue-light receptors phototropin 1 (phot1) and phototropin 2 (phot2) in Arabidopsis (refs 1-5). In some ferns, phototropism and chloroplast photorelocation are controlled by red light as well as blue light. However, until now, the photoreceptor mediating these red-light responses has not been identified. The fern Adiantum capillus-veneris has an unconventional photoreceptor, phytochrome 3 (phy3), which is a chimaera of the red/far-red light receptor phytochrome and phototropin. We identify here a function of phy3 for red-light-induced phototropism and for red-light-induced chloroplast photorelocation, by using mutational analysis and complementation. Because phy3 greatly enhances the sensitivity to white light in orienting leaves and chloroplasts, and PHY3 homologues exist among various fern species, this chimaeric photoreceptor may have had a central role in the divergence and proliferation of fern species under low-light canopy conditions.

Chloroplast movement

The study of chloroplast movement made a quantum leap at the beginning of the twenty-first century. Research based on reverse-genetic approaches using targeted mutants has brought new concepts to this field. One of the most exciting findings has been the discovery of photoreceptors for both accumulation and avoidance responses in Arabidopsis and in the fern Adiantum. Evidence for the adaptive advantage of chloroplast avoidance movements in plant survival has also been found. Additional discoveries include mechano-stress-induced chloroplast movement in ferns and mosses, and microtubule-mediated chloroplast movement in the moss Physcomitrella. The possible ecological significance of chloroplast movement is discussed in the final part of this review.

Blue light-induced chloroplast relocation

Chloroplast relocation movement is induced by blue light in most plants tested. Under weak light, chloroplasts move toward a brighter area in a cell (called low-fluence-rate response or accumulation movement), but they avoid strong light and move away from the light (called high-fluence-rate response or avoidance movement). Recently, mutants deficient in the chloroplast avoidance movement were isolated from Arabidopsis thaliana. The results of mutant analyses revealed that the phototropin photoreceptors phot1 and phot2 both control chloroplast accumulation while phot2 alone controls the avoidance movements.

External Ca(2+) is essential for chloroplast movement induced by mechanical stimulation but not by light stimulation

In the fern Adiantum capillus-veneris, chloroplast movement is induced by mechanical stimulation as well as by light stimulation. Directional movement of both types depends on an actin-based motile system. To investigate the physiological relationship between mechanical and light signaling in the regulation of chloroplast movement, we examined the mechano-response of chloroplasts whose motility had been already restricted after photo-relocation. Chloroplast mechano-avoidance movement was induced under all of the photo-relocation conditions tested, indicating that mechano-specific signals generated by mechanical stimulation dominate over the light signals and reactivate the motility of chloroplasts. When the effects of external Ca(2+) on the induction of mechano- and light responses were examined, strikingly different requirements of external Ca(2+) were found for each. In medium without Ca(2+), the mechano-response was suppressed but no effects were observed on photo-response. Mechano-relocation movement of chloroplasts was inhibited by 100 microM lanthanum (La(3+)), a plasma membrane calcium channel blocker, and by 10 microM gadolinium (Gd(3+)), a stretch-activated channel blocker. However, the same concentrations of these drugs did not affect the photo-relocation movement at all. These results suggest that the influx of external Ca(2+) is crucial for the early signaling step of chloroplast mechano-relocation but not for that of photo-relocation. This is the first report showing the separation of signaling pathways in mechano- and photo-relocation of chloroplasts.

Arabidopsis NPL1: a phototropin homolog controlling the chloroplast high-light avoidance response

Chloroplasts relocate their positions in a cell in response to the intensity of incident light, moving to the side wall of the cell to avoid strong light, but gathering at the front face under weak light to maximize light interception. Here, Arabidopsis thaliana mutants defective in the avoidance response were isolated, and the mutated gene was identified as NPL1 (NPH-like 1), a homolog of NPH1 (nonphototropic hypocotyl 1), a blue light receptor used in phototropism. Hence, NPL1 is likely a blue light receptor regulating the avoidance response under strong light.

Choice of tracks, microtubules and/or actin filaments for chloroplast photo-movement is differentially controlled by phytochrome and a blue light receptor

Light induced chloroplast movement has been studied as a model system for photoreception and actin microfilament (MF)-based intracellular motilities in plants. Chloroplast photo-accumulation and -avoidance movement is mediated by phytochrome as well as blue light (BL) receptor in the moss Physcomitrella patens. Here we report the discovery of an involvement of a microtubule (MT)-based system in addition to an MF-based system in photorelocation of chloroplasts in this moss. In the dark, MTs provided tracks for rapid movement of chloroplasts in a longitudinal direction and MFs contributed the tracks for slow movement in any direction. We found that phytochrome responses utilized only the MT-based system, while BL responses had an alternative way of moving, either along MTs or MFs. MT-based systems were mediated by both photoreceptors, but chloroplasts showed movements with different velocity and pattern between them. No apparent difference in the behavior of chloroplast movement between the accumulation and avoidance movement was detected in phytochrome responses or BL responses, except for the direction of the movement. The results presented here demonstrate that chloroplasts use both MTs and MFs for motility and that phytochrome and a BL receptor control directional photo-movement of chloroplasts through the differential regulation of these motile systems.

SIGNALING TO THE ACTIN CYTOSKELETON IN PLANTS

Plants have developed finely tuned, cellular mechanisms to respond to a variety of intrinsic and extrinsic stimuli. In several examples, these responses necessitate rearrangements of the cytoplasm that are coordinated by a network of actin microfilaments and microtubules, dynamic polymers collectively known as the cytoskeleton. This review focuses on five different cellular responses in which the actin cytoskeleton redistributes following extracellular stimulation: pollen tube tip growth and the self-incompatibility response; root hair responses to bacterial nodulation factors; light-mediated plastid positioning; nonhost resistance to fungal attack; and guard cell shape and turgor changes. For each of these systems, there is reasonable knowledge about what signals induce the plant response and the function(s) of the actin rearrangement. This review aims to build beyond a description of cytoskeletal changes and look at specific actin-binding proteins that have been implicated as effectors of each response, as sites of action for second messengers, and as fundamental coordinators of actin dynamics.

Chloroplast-avoidance response induced by high-fluence blue light in prothallial cells of the fern adiantum capillus-veneris as analyzed by microbeam irradiation

Chloroplast movement was induced by partial cell illumination using a high-fluence blue microbeam in light-grown and dark-adapted prothallial cells of the fern Adiantum capillus-veneris. Chloroplasts inside the illuminated area moved out (high-fluence response [HFR]), whereas those outside moved toward the irradiated area (low-fluence response [LFR]), although they stopped moving when they reached the border. These results indicate that both HFR and LFR signals are generated by high-fluence blue light of the same area, and that an LFR signal can be transferred long-distance from the beam spot, although an HFR signal cannot. The lifetime of the HFR signal was calculated from the traces of chloroplast movement induced by a brief pulse from a high-fluence blue microbeam to be about 6 min. This is very short compared with that of the LFR (30-40 min; T. Kagawa, M. Wada [1994] J Plant Res 107: 389-398). These data indicate that the signal transduction pathways of the HFR and the LFR must be distinct.