Possible association of actin filaments with chloroplasts of spinach mesophyll cells in vivo and in vitro

HSP90.2 promotes CO2 assimilation rate, grain weight and yield in wheat

Wheat fixes CO₂ by photosynthesis into kernels to nourish humankind. Improving the photosynthesis rate is a major driving force in assimilating atmospheric CO₂ and guaranteeing food supply for human beings. Strategies for achieving the above goal need to be improved. Here, we report the cloning and mechanism of CO₂ ASSIMILATION RATE AND KERNEL-ENHANCED 1 (CAKE1) from durum wheat (Triticum turgidum L. var. durum). The cake1 mutant displayed a lower photosynthesis rate with smaller grains. Genetic studies identified CAKE1 as HSP90.2-B, encoding cytosolic molecular chaperone folding nascent preproteins. The disturbance of HSP90.2 decreased leaf photosynthesis rate, kernel weight (KW) and yield. Nevertheless, HSP90.2 over-expression increased KW. HSP90.2 recruited and was essential for the chloroplast localization of nuclear-encoded photosynthesis units, for example PsbO. Actin microfilaments docked on the chloroplast surface interacted with HSP90.2 as a subcellular track towards chloroplasts. A natural variation in the hexaploid wheat HSP90.2-B promoter increased its transcription activity, enhanced photosynthesis rate and improved KW and yield. Our study illustrated an HSP90.2-Actin complex sorting client preproteins towards chloroplasts to promote CO₂ assimilation and crop production. The beneficial haplotype of Hsp90.2 is rare in modern varieties and could be an excellent molecular switch promoting photosynthesis rate to increase yield in future elite wheat varieties.

Comparative Proteomic and Physiological Analysis Reveals the Variation Mechanisms of Leaf Coloration and Carbon Fixation in a Xantha Mutant of Ginkgo biloba L

Yellow-green leaf mutants are common in higher plants, and these non-lethal chlorophyll-deficient mutants are ideal materials for research on photosynthesis and plant development. A novel xantha mutant of Ginkgo biloba displaying yellow-colour leaves (YL) and green-colour leaves (GL) was identified in this study. The chlorophyll content of YL was remarkably lower than that in GL. The chloroplast ultrastructure revealed that YL had less dense thylakoid lamellae, a looser structure and fewer starch grains than GL. Analysis of the photosynthetic characteristics revealed that YL had decreased photosynthetic activity with significantly high nonphotochemical quenching. To explain these phenomena, we analysed the proteomic differences in leaves and chloroplasts between YL and GL of ginkgo using two-dimensional gel electrophoresis (2-DE) coupled with MALDI-TOF/TOF MS. In total, 89 differential proteins were successfully identified, 82 of which were assigned functions in nine metabolic pathways and cellular processes. Among them, proteins involved in photosynthesis, carbon fixation in photosynthetic organisms, carbohydrate/energy metabolism, amino acid metabolism, and protein metabolism were greatly enriched, indicating a good correlation between differentially accumulated proteins and physiological changes in leaves. The identifications of these differentially accumulated proteins indicates the presence of a specific different metabolic network in YL and suggests that YL possess slower chloroplast development, weaker photosynthesis, and a less abundant energy supply than GL. These studies provide insights into the mechanism of molecular regulation of leaf colour variation in YL mutants.

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.

Evolution of the Cp-Actin-based Motility System of Chloroplasts in Green Plants

During the course of green plant evolution, numerous light responses have arisen that optimize their growth under fluctuating light conditions. The blue light receptor phototropin mediates several photomovement responses at the tissue, cellular and organelle levels. Chloroplast photorelocation movement is one such photomovement response, and is found not only in most green plants, but also in some red algae and photosynthetic stramenopiles. In general, chloroplasts move toward weak light to maximally capture photosynthetically active radiation (the chloroplast accumulation response), and they move away from strong light to avoid photodamage (the avoidance response). In land plants, chloroplast movement is dependent on specialized actin filaments, chloroplast-actin filaments (cp-actin filaments). Through molecular genetic analysis using Arabidopsis thaliana, many molecular factors that regulate chloroplast photorelocation were identified. In this Perspective, we discuss the evolutionary history of the molecular mechanism for chloroplast photorelocation movement in green plants in view of cp-actin filaments.

A protein phosphatase 2A catalytic subunit modulates blue light-induced chloroplast avoidance movements through regulating actin cytoskeleton in Arabidopsis

Chloroplast avoidance movements mediated by phototropin 2 (phot2) are one of most important physiological events in the response to high-fluence blue light (BL), which reduces damage to the photosynthetic machinery under excess light. Protein phosphatase 2A-2 (PP2A-2) is an isoform of the catalytic subunit of PP2A, which regulates a number of developmental processes. To investigate whether PP2A-2 was involved in high-fluence BL-induced chloroplast avoidance movements, we first analyzed chloroplast migration in the leaves of the pp2a-2 mutant in response to BL. The data showed that PP2A-2 might act as a positive regulator in phot2-mediated chloroplast avoidance movements, but not in phot1-mediated chloroplast accumulation movements. Then, the effect of okadaic acid (OA) and cantharidin (selective PP2A inhibitors) on high-fluence BL response was further investigated in Arabidopsis thaliana mesophyll cells. Within a certain concentration range, exogenously applied OA or cantharidin inhibited the high-fluence BL-induced chloroplast movements in a concentration-dependent manner. Actin depolymerizing factor (ADF)/cofilin phosphorylation assays demonstrated that PP2A-2 can activate/dephosphorylate ADF/cofilin, an actin-binding protein, in Arabidopsis mesophyll cells. Consistent with this observation, the experiments showed that OA could inhibit ADF1 binding to the actin and suppress the reorganization of the actin cytoskeleton after high-fluence BL irradiation. The adf1 and adf3 mutants also exhibited reduced high-fluence BL-induced chloroplast avoidance movements. In conclusion, we identified that PP2A-2 regulated the activation of ADF/cofilin, which, in turn, regulated actin cytoskeleton remodeling and was involved in phot2-mediated chloroplast avoidance movements.

Distribution pattern changes of actin filaments during chloroplast movement in Adiantum capillus-veneris

Chloroplasts change their positions in a cell in response to light intensities. The photoreceptors involved in chloroplast photo-relocation movements and the behavior of chloroplasts during their migration were identified in our previous studies, but the mechanism of movement has yet to be clarified. In this study, the behavior of actin filaments under various light conditions was observed in Adiantum capillus-veneris gametophytes. In chloroplasts staying in one place under a weak light condition and not moving, circular structures composed of actin filaments were observed around the chloroplast periphery. In contrast, short actin filaments were observed at the leading edge of moving chloroplasts induced by partial cell irradiation. In the dark, the circular structures found under the weak light condition disappeared and then reappeared around the moving chloroplasts. Mutant analyses revealed that the disappearance of the circular actin structure was mediated by the blue light photoreceptor, phototropin2.

Actin-dependent chloroplast anchoring is regulated by Ca(2+)-calmodulin in spinach mesophyll cells

Chloroplasts are actively anchored at the appropriate intracellular regions to maintain advantageous distribution patterns under specific environmental conditions. Redistribution of chloroplasts is accompanied by their de-anchoring and re-anchoring, respectively, from and to the cortical cytoplasm. In spinach mesophyll cells, high-intensity blue light and Ca(2+) treatment induced the disappearance of the meshwork-like array of actin filaments surrounding chloroplasts, which was suppressed by a calmodulin antagonist. Regulatory mechanisms of chloroplast anchoring were investigated using plasma membrane (PM) ghosts, on which the cortical cytoplasm underlying the PM was exposed. Addition of an actin-depolymerizing reagent or > 1 µM Ca(2+) induced detachment of a substantial number of chloroplasts from the PM ghosts concomitant with disordered actin organization. Calmodulin antagonists and anti-calmodulin antibodies negated the effects of Ca(2+). In addition, Ca(2+)-induced detachment of chloroplasts was no longer evident on the calmodulin-depleted PM ghosts. We propose that chloroplasts are anchored onto the cortical cytoplasm through interaction with the actin cytoskeleton, and that Ca(2+)-calmodulin-sensitized de-anchoring of chloroplasts is a critical early step in chloroplast redistribution induced by environmental stimuli.

New insights into dynamic actin-based chloroplast photorelocation movement

Chloroplast movement is essential for plants to survive under various environmental light conditions. Phototropins-plant-specific blue-light-activated receptor kinases-mediate the response by perceiving light intensity and direction. Recently, novel chloroplast actin (cp-actin) filaments have been identified as playing a pivotal role in the directional chloroplast photorelocation movement. Encouraging progress has recently been made in this field of research through molecular genetics and cell biological analyses. This review describes factors that have been identified as being involved in chloroplast movement and their roles in the regulation of cp-actin filaments, thus providing a basis for reflection on their biochemical activities and functions.

Stochastic dynamics of actin filaments in guard cells regulating chloroplast localization during stomatal movement

Actin filaments and chloroplasts in guard cells play roles in stomatal function. However, detailed actin dynamics vary, and the roles that they play in chloroplast localization during stomatal movement remain to be determined. We examined the dynamics of actin filaments and chloroplast localization in transgenic tobacco expressing green fluorescent protein (GFP)-mouse talin in guard cells by time-lapse imaging. Actin filaments showed sliding, bundling and branching dynamics in moving guard cells. During stomatal movement, long filaments can be severed into small fragments, which can form longer filaments by end-joining activities. With chloroplast movement, actin filaments near chloroplasts showed severing and elongation activity in guard cells during stomatal movement. Cytochalasin B treatment abolished elongation, bundling and branching activities of actin filaments in guard cells, and these changes of actin filaments, and as a result, more chloroplasts were localized at the centre of guard cells. However, chloroplast turning to avoid high light, and sliding of actin fragments near the chloroplast, was unaffected following cytochalasin B treatment in guard cells. We suggest that the sliding dynamics of actin may play roles in chloroplast turning in guard cells. Our results indicate that the stochastic dynamics of actin filaments in guard cells regulate chloroplast localization during stomatal movement.

Chloroplast actin filaments organize meshwork on the photorelocated chloroplasts in the moss Physcomitrella patens

Cytoskeleton dynamics during phototropin-dependent chloroplast photorelocation movement was analyzed in protonemal cells of actin- and microtubule-visualized lines of Physcomitrella patens expressing GFP- or tdTomato-talin and GFP-tubulin. Using newly developed epi- and trans-microbeam irradiation systems that permit fluorescence observation of the cell under blue microbeam irradiation inducing chloroplast relocation, it was revealed that meshwork of actin filaments formed at the chloroplast-accumulating area both in the avoidance and accumulation movements. The structure disappeared soon when blue microbeam was turned off, and it was not induced under red microbeam irradiation that did not evoke chloroplast relocation movement. In contrast, no apparent change in microtubule organization was detected during the movements. The actin meshwork was composed of short actin filaments distinct from the cytoplasmic long actin cables and was present between the chloroplasts and plasma membrane. The short actin filaments emerged from around the chloroplast periphery towards the center of chloroplast. Showing highly dynamic behavior, the chloroplast actin filaments (cp-actin filaments) were rapidly organized into meshwork on the chloroplast surface facing plasma membrane. The actin filament configuration on a chloroplast led to the formation of actin meshwork area in the cell as the chloroplasts arrived at and occupied the area. After establishment of the meshwork, cp-actin filaments were still highly dynamic, showing appearance, disappearance, severing and bundling of filaments. These results indicate that the cp-actin filaments have significant roles in the chloroplast movement and positioning in the cell.

Why have chloroplasts developed a unique motility system?

Organelle movement in plants is dependent on actin filaments with most of the organelles being transported along the actin cables by class XI myosins. Although chloroplast movement is also actin filament-dependent, a potential role of myosin motors in this process is poorly understood. Interestingly, chloroplasts can move in any direction, and change the direction within short time periods, suggesting that chloroplasts use the newly formed actin filaments rather than preexisting actin cables. Furthermore, the data on myosin gene knockouts and knockdowns in Arabidopsis and tobacco do not support myosins' XI role in chloroplast movement. Our recent studies revealed that chloroplast movement and positioning are mediated by the short actin filaments localized at chloroplast periphery (cp-actin filaments) rather than cytoplasmic actin cables. The accumulation of cp-actin filaments depends on kinesin-like proteins, KAC1 and KAC2, as well as on a chloroplast outer membrane protein CHUP1. We propose that plants evolved a myosin XI-independent mechanism of the actin-based chloroplast movement that is distinct from the mechanism used by other organelles.

Light, genotype, and abscisic acid affect chloroplast positioning in guard cells of Arabidopsis thaliana leaves in distinct ways

The goal of this study was to investigate the effects of light intensity, genotype, and various chemical treatments on chloroplast movement in guard cells of Arabidopsis thaliana leaves. After treatment at various light intensities (dark, low, and high light), leaf discs were fixed with glutaraldehyde, and imaged using confocal laser microscopy. Each chloroplast was assigned a horizontal (close to pore, center, or epidermal side) and vertical (outer, middle, inner) position. White light had a distinct effect on chloroplast positioning, most notably under high light (HL) when chloroplasts on the upper leaf surface of wild-type (WT) moved from epidermal and center positions toward the pore. This was not the case for phot1-5/phot2-1 or phot2-1 plants, thus phototropins are essential for chloroplast positioning in guard cells. In npq1-2 mutants, fewer chloroplasts moved to the pore position under HL than in WT plants, indicating that white light can affect chloroplast positioning also in a zeaxanthin-dependent way. Cytochalasin B inhibited the movement of chloroplasts to the pore under HL, while oryzalin did not, supporting the idea that actin plays a role in the movement. The movement along actin cables is dependent on CHUP1 since chloroplast positioning in chup1 was significantly altered. Abscisic acid (ABA) caused most chloroplasts in WT and phot1-5/phot2-1 to be localized in the center, middle part of the guard cells irrespective of light treatment. This indicates that not only light but also water stress influences chloroplast positioning.

AT_CHLORO, a comprehensive chloroplast proteome database with subplastidial localization and curated information on envelope proteins

Recent advances in the proteomics field have allowed a series of high throughput experiments to be conducted on chloroplast samples, and the data are available in several public databases. However, the accurate localization of many chloroplast proteins often remains hypothetical. This is especially true for envelope proteins. We went a step further into the knowledge of the chloroplast proteome by focusing, in the same set of experiments, on the localization of proteins in the stroma, the thylakoids, and envelope membranes. LC-MS/MS-based analyses first allowed building the AT_CHLORO database (http://www.grenoble.prabi.fr/protehome/grenoble-plant-proteomics/), a comprehensive repertoire of the 1323 proteins, identified by 10,654 unique peptide sequences, present in highly purified chloroplasts and their subfractions prepared from Arabidopsis thaliana leaves. This database also provides extensive proteomics information (peptide sequences and molecular weight, chromatographic retention times, MS/MS spectra, and spectral count) for a unique chloroplast protein accurate mass and time tag database gathering identified peptides with their respective and precise analytical coordinates, molecular weight, and retention time. We assessed the partitioning of each protein in the three chloroplast compartments by using a semiquantitative proteomics approach (spectral count). These data together with an in-depth investigation of the literature were compiled to provide accurate subplastidial localization of previously known and newly identified proteins. A unique knowledge base containing extensive information on the proteins identified in envelope fractions was thus obtained, allowing new insights into this membrane system to be revealed. Altogether, the data we obtained provide unexpected information about plastidial or subplastidial localization of some proteins that were not suspected to be associated to this membrane system. The spectral counting-based strategy was further validated as the compartmentation of well known pathways (for instance, photosynthesis and amino acid, fatty acid, or glycerolipid biosynthesis) within chloroplasts could be dissected. It also allowed revisiting the compartmentation of the chloroplast metabolism and functions.

A myosin XI tail domain homologous to the yeast myosin vacuole-binding domain interacts with plastids and stromules in Nicotiana benthamiana

The actin cytoskeleton plays a role in mobility of many different organelles in plant cells, including chloroplasts, mitochondria, Golgi, and peroxisomes. While progress has been made in identifying the myosin motors involved in trafficking of various plant organelles, not all of the cargoes mobilized by different members of the myosin XI family have yet been identified. The involvement of myosins in chloroplast positioning and mitochondrial movement was demonstrated by expression of a virus-induced gene silencing (VIGS) construct in tobacco. When VIGS with two different conserved sequences from a myosin XI motor was performed in plants with either GFP-labeled plastids or mitochondria, chloroplast positioning in the dark was abnormal, and mitochondrial movement ceased. Because these and prior observations have implicated a role for myosins and the actin cytoskeleton in plastid and stromule movement, we searched for myosin tail domains that could associate with plastids and stromules. While a yellow fluorescent protein (YFP) fusion with the entire tail region of myosin XI-F was usually found only in the cytoplasm, we observed that an Arabidopsis or Nicotiana benthamiana YFP::myosin XI-F tail domain homologous to the yeast myo2p vacuole-binding domain associated with plastids and stromules after transient expression in N. benthamiana. Taken together, these observations implicate myosin motor proteins in dynamics of plastids and stromules.

Is chloroplast import of photosynthesis proteins facilitated by an actin-TOC-TIC-VIPP1 complex?

Actin filaments are major components of the cytoskeleton that interact with chloroplast envelope membranes to allow chloroplast positioning and movement, stromule mobility and gravitropism perception. We recently reported that Toc159, a component of the TOC complex of the chloroplast protein import apparatus, interacts directly with actin. The interaction of Toc159 and actin was identified by co-immunoprecipitation and co-sedimentation experiments with detergent-solubilised pea chloroplast envelope membranes. In addition, many of the components of the TOC-TIC protein import apparatus and VIPP1 (vesicle-inducing protein in plastids 1) were identified by mass spectroscopy in the material co-immunoprecipitated with antibodies to actin. Toc159 is the receptor for the import of photosynthesis proteins and VIPP1 is involved in thylakoid membrane formation by inducing vesicle formation from the chloroplast inner envelope membrane, suggesting we may have identified an actin-TOC-TIC-VIPP1 complex that may provide a means of channeling cytosolic preproteins to the thylakoid membrane. The interaction of Toc159 with actin may facilitate exchange between the putative soluble and membrane forms of Toc159 and promote the interaction of cytosolic preproteins with the TOC complex.

Short actin-based mechanism for light-directed chloroplast movement in Arabidopsis

Organelle movement is essential for proper function of living cells. In plants, these movements generally depend on actin filaments, but the underlying mechanism is unknown. Here, in Arabidopsis, we identify associations of short actin filaments along the chloroplast periphery on the plasma membrane side associated with chloroplast photorelocation and anchoring to the plasma membrane. We have termed these chloroplast-actin filaments (cp-actin filaments). Cp-actin filaments emerge from the chloroplast edge and exhibit rapid turnover. The presence of cp-actin filaments depends on an actin-binding protein, chloroplast unusual positioning1 (CHUP1), localized on the chloroplast envelope. chup1 mutant lacked cp-actin filaments but showed normal cytoplasmic actin filaments. When irradiated with blue light to induce chloroplast movement, cp-actin filaments relocalize to the leading edge of chloroplasts before and during photorelocation and are regulated by 2 phototropins, phot1 and phot2. Our findings suggest that plants evolved a unique actin-based mechanism for organelle movement.

Interaction of actin and the chloroplast protein import apparatus

Actin filaments are major components of the cytoskeleton and play numerous essential roles, including chloroplast positioning and plastid stromule movement, in plant cells. Actin is present in pea chloroplast envelope membrane preparations and is localized at the surface of the chloroplasts, as shown by agglutination of intact isolated chloroplasts by antibodies to actin. To identify chloroplast envelope proteins involved in actin binding, we have carried out actin co-immunoprecipitation and co-sedimentation experiments on detergent-solubilized pea chloroplast envelope membranes. Proteins co-immunoprecipitated with actin were identified by mass spectrometry and by Western blotting and included the Toc159, Toc75, Toc34, and Tic110 components of the TOC-TIC protein import apparatus. A direct interaction of actin with Escherichia coli-expressed Toc159, but not Toc33, was shown by co-sedimentation experiments, suggesting that Toc159 is the component of the TOC complex that interacts with actin on the cytosolic side of the outer envelope membrane. The physiological significance of this interaction is unknown, but it may play a role in the import of nuclear-encoded photosynthesis proteins.

Chloroplast anchoring: its implications for the regulation of intracellular chloroplast distribution

The intracellular distribution of organelles plays a pivotal role in the maintenance and adaptation of a wide spectrum of cellular activities in plants. Chloroplasts are a special type of organelle able to photosynthesize, capturing light energy to fix atmospheric CO2. Consequently, the intracellular positioning of chloroplasts is crucial for plant growth and development. Knowledge of the photoreceptors and cellular apparatus responsible for chloroplast movement has gradually accumulated over time, yet recent advances have allowed improved understanding. In this article, several aspects of research progress into the mechanisms for maintaining the specific intracellular distribution patterns of chloroplasts, namely, chloroplast anchoring, are summarized, together with a brief consideration of the future prospects of this subject. Our discussion covers developmental, physiological, ecophysiological, and recent cell biological research areas.

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.

The chloroplast outer membrane protein CHUP1 interacts with actin and profilin

Chloroplasts accumulate in response to low light, whereas high light induces an actin-dependent avoidance movement. This is a long known process, but its molecular base is barely understood. Only recently first components of the blue light perceiving signal cascade initiating this process were described. Among these, a protein was identified by the analysis of a deletion mutant in the corresponding gene resulting in a chloroplast unusual positioning phenotype. The protein was termed CHUP1 and initial results suggested chloroplast localization. We demonstrate that the protein is indeed exclusively and directly targeted to the chloroplast surface. The analysis of the deletion mutant of CHUP1 using microarray analysis shows an influence on the expression of genes found to be up-regulated, but not on genes found to be down-regulated upon high light exposure in wild-type. Analyzing a putative role of CHUP1 as a linker between chloroplasts and the cytoskeleton, we demonstrate an interaction with actin, which is independent on the filamentation status of actin. Moreover, binding of CHUP1 to profilin -- an actin modifying protein -- could be shown and an enhancing effect of CHUP1 on the interaction of profilin to actin is demonstrated. Therefore, a role of CHUP1 in bridging chloroplasts to actin filaments and a regulatory function in actin polymerization can be discussed.

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.

Actin microfilament dynamics and actin side-binding proteins in plants

Actin microfilaments are highly organized and essential intracellular components of organelle movement and cell morphogenesis in plants. The organization of these microfilaments undergoes dynamic changes during cell division, elongation, and differentiation. Recent live-cell imaging of plant actin microfilaments has revealed their native organization and remarkable dynamics. In addition, characterization of plant actin side-binding proteins has progressed rapidly by genetic, biochemical, and bioinformatic approaches. The gathering and integration of microscopy-based information from actin microfilament dynamics and the molecular identification of actin side-binding proteins have provided considerable insights into actin microfilament-dependent events and actin microfilament organization in plants.

Membrane contact sites: physical attachment between chloroplasts and endoplasmic reticulum revealed by optical manipulation

Chloroplasts and their surrounding cell are highly interdependent. One example is lipid metabolism, where the cell depends on its chloroplasts to provide fatty acids for lipid synthesis in the endoplasmic reticulum (ER) and in turn, chloroplasts rely on import of lipid precursors from the ER. Despite its fundamental importance, the route for lipid trafficking into and out of chloroplasts remains unknown. Biochemical studies of plant membrane lipid metabolism have suggested the possibility of lipid transport at membrane contact sites (MCSs) between the ER and chloroplasts. With the aid of optical manipulation, we recently could present physical evidence for this association. Leaf protoplasts isolated from Arabidopsis thaliana expressing green fluorescent protein (GFP) in the ER lumen were observed by confocal microscopy. A laser scalpel was used to rupture the protoplasts. ER fragments associated with the released chloroplasts could be stretched out by optical tweezers but remained attached to the chloroplast surface, even when a stretching force of 400 pN was applied. We thus provided the first physical evidence for MCSs between two membranes and we propose for the ER-chloroplast pair, that such tight associations are involved in bidirectional lipid trafficking between the two compartments.