Recruitment of myosin VIII towards plastid surfaces is root-cap specific and provides the evidence for actomyosin involvement in root osmosensing

An Update on the Role of the Actin Cytoskeleton in Plasmodesmata: A Focus on Formins

Cell-to-cell communication in plants is mediated by plasmodesmata (PD) whose permeability is tightly regulated during plant growth and development. The actin cytoskeleton has been implicated in regulating the permeability of PD, but the underlying mechanism remains largely unknown. Recent characterization of PD-localized formin proteins has shed light on the role and mechanism of action of actin in regulating PD-mediated intercellular trafficking. In this mini-review article, we will describe the progress in this area.

Collagenase as a useful tool for the analysis of plant cellular peripheries

A technique for the selective loosening of the cell wall structure and the isolation of proteins permanently knotted in the cell walls was elaborated. Following treatment with collagenase, some proteins, such as calreticulin (CRT) and auxin binding protein 1 (ABP1) were released from purified cell walls, most probably through destruction of respective interacting proteins. The results were confirmed by the immunolocalization of the ABP1 and CRT with confocal and electron microscopy. On the other hand, potential substrates of collagenase, among them annexin 1 have been recognized. Mass spectra of annexin 1 obtained after collagenase digestion and results from analysis of potential cleavage sites suggested that the mechanism of enzyme cleavage might not depend on the amino acid sequence. Summarizing, collagenase was found to be a very useful tool for exploring molecules involved in the functioning of cellular peripheries.

Role of plant myosins in motile organelles: is a direct interaction required?

Plant organelles are highly motile, with speed values of 3-7 µm/s in cells of land plants and about 20-60 µm/s in characean algal cells. This movement is believed to be important for rapid distribution of materials around the cell, for the plant's ability to respond to environmental biotic and abiotic signals and for proper growth. The main machinery that propels motility of organelles within plant cells is based on the actin cytoskeleton and its motor proteins the myosins. Most plants express multiple members of two main classes: myosin VIII and myosin XI. While myosin VIII has been characterized as a slow motor protein, myosins from class XI were found to be the fastest motor proteins known in all kingdoms. Paradoxically, while it was found that myosins from class XI regulate most organelle movement, it is not quite clear how or even if these motor proteins attach to the organelles whose movement they regulate.

Molecular characterization and subcellular localization of Arabidopsis class VIII myosin, ATM1

Land plants possess myosin classes VIII and XI. Although some information is available on the molecular properties of class XI myosins, class VIII myosins are not characterized. Here, we report the first analysis of the enzymatic properties of class VIII myosin. The motor domain of Arabidopsis class VIII myosin, ATM1 (ATM1-MD), and the motor domain plus one IQ motif (ATM1-1IQ) were expressed in a baculovirus system and characterized. ATM1-MD and ATM1-1IQ had low actin-activated Mg(2+)-ATPase activity (Vmax = 4 s(-1)), although their affinities for actin were high (Kactin = 4 μM). The actin-sliding velocities of ATM1-MD and ATM1-1IQ were 0.02 and 0.089 μm/s, respectively, from which the value for full-length ATM1 is calculated to be ∼0.2 μm/s. The results of actin co-sedimentation assay showed that the duty ratio of ATM1 was ∼90%. ADP dissociation from the actin·ATM1 complex (acto-ATM1) was extremely slow, which accounts for the low actin-sliding velocity, low actin-activated ATPase activity, and high duty ratio. The rate of ADP dissociation from acto-ATM1 was markedly biphasic with fast and slow phase rates (5.1 and 0.41 s(-1), respectively). Physiological concentrations of free Mg(2+) modulated actin-sliding velocity and actin-activated ATPase activity by changing the rate of ADP dissociation from acto-ATM1. GFP-fused full-length ATM1 expressed in Arabidopsis was localized to plasmodesmata, plastids, newly formed cell walls, and actin filaments at the cell cortex. Our results suggest that ATM1 functions as a tension sensor/generator at the cell cortex and other structures in Arabidopsis.

Immunofluorescent localization of MAPKs in Steedman's wax sections

Signals of different nature are transduced in cells through signal transduction pathways, where mitogen-activated protein kinases (MAPKs) play an important role as signaling molecules. Views into intracellular localization of MAPKs are critical for the understanding of their spatial and temporal functions, like activation-based relocation, compartmentation, or interactions with local substrates. Localization of MAPKs in cells is thus very useful cell biological approach, extending complex mode of cell signaling characterization in plants. Here, we present a method for subcellular immunofluorescence localization of MAPKs using protein- or phospho-specific antibodies, performed on sectioned fixed plant samples. It is based on embedding of samples in the Steedman's wax, a low-melting point polyester wax embedding medium, which maintains high antigenicity of studied proteins. In addition, exposure of dewaxed sections to antibodies allows for their efficient penetration. Altogether, it makes this simple method a good tool in the efficient subcellular localization of diverse proteins, including plant MAPKs.

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.

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.

In vivo reorganization of the actin cytoskeleton in leaves of Nicotiana tabacum L. transformed with plastin-GFP. Correlation with light-activated chloroplast responses

BACKGROUND

The actin cytoskeleton is involved in the responses of plants to environmental signals. Actin bundles play the role of tracks in chloroplast movements activated by light. Chloroplasts redistribute in response to blue light in the mesophyll cells of Nicotiana tabacum. The aim of this work was to study the relationship between chloroplast responses and the organization of actin cytoskeleton in living tobacco cells. Chloroplast movements were measured photometrically as changes in light transmission through the leaves. The actin cytoskeleton, labeled with plastin-GFP, was visualised by confocal microscopy.

RESULTS

The actin cytoskeleton was affected by strong blue and red light. No blue light specific actin reorganization was detected. EGTA and trifluoperazine strongly inhibited chloroplast responses and disrupted the integrity of the cytoskeleton. This disruption was reversible by Ca(2+) or Mg(2+). Additionally, the effect of trifluoperazine was reversible by light. Wortmannin, an inhibitor of phosphoinositide kinases, potently inhibited chloroplast responses but did not influence the actin cytoskeleton at the same concentration. Also this inhibition was reversed by Ca(2+) and Mg(2+). Magnesium ions were equally or more effective than Ca(2+) in restoring chloroplast motility after treatment with EGTA, trifluoperazine or wortmannin.

CONCLUSION

The architecture of the actin cytoskeleton in the mesophyll of tobacco is significantly modulated by strong light. This modulation does not affect the direction of chloroplast redistribution in the cell. Calcium ions have multiple functions in the mechanism of the movements. Our results suggest also that Mg(2+) is a regulatory molecule cooperating with Ca(2+) in the signaling pathway of blue light-induced tobacco chloroplast movements.

Nitric oxide modulates dynamic actin cytoskeleton and vesicle trafficking in a cell type-specific manner in root apices

NO is an important regulatory molecule in eukaryotes. Much of its effect is ascribed to the action of NO as a signalling molecule. However, NO can also directly modify proteins thus affecting their activities. Although the signalling functions of NO are relatively well recognized in plants, very little is known about its potential influence on the structural integrity of plant cells. In this study, the reorganization of the actin cytoskeleton, and the recycling of wall polysaccharides in plants via the endocytic pathway in the presence of NO or NO-modulating substances were analysed. The actin cytoskeleton and endocytosis in maize (Zea mays) root apices were visualized with fluorescence immunocytochemistry. The organization of the actin cytoskeleton is modulated via NO levels and the extent of such modulation is cell-type specific. In endodermis cells, actin cables change their orientation from longitudinal to oblique and cellular cross-wall domains become actin-depleted/depolymerized. The reaction is reversible and depends on the type of NO donor. Actin-dependent vesicle trafficking is also affected. This was demonstrated through the analysis of recycled wall material transported to newly-formed cell plates and BFA compartments. Therefore, it is concluded that, in plant cells, NO affects the functioning of the actin cytoskeleton and actin-dependent processes. Mechanisms for the reorganization of the actin cytoskeleton are cell-type specific, and such rearrangements might selectively impinge on the functioning of various cellular domains. Thus, the dynamic actin cytoskeleton could be considered as a downstream effector of NO signalling in cells of root apices.

The Arabidopsis class VIII myosin ATM2 is involved in endocytosis

Members of the class XI of the myosin superfamily comprising higher plant, actin-based molecular motors have been shown to be involved in peroxisome and Golgi vesicle trafficking comparable to yeast and animal class V myosins. The tasks of the second class of myosins of higher plants, class VIII, are unclear. In this study the class VIII myosin ATM2 from the model plant Arabidopsis thaliana was selected for the examination of cargo specificity in vivo. Fluorescent protein-fusion plasmid constructs with fragments of the ATM2 cDNA were generated and used for Agrobacterium tumefaciens-based transient transformation of Nicotiana benthamiana leaves. The resulting subcellular localization patterns were recorded by live imaging with confocal laser scanning microscopy (CLSM) in epidermal leaf cells. Expression of a nearly full-length construct displayed labeling of filaments and vesicles, a head + neck fragment led to decoration of filaments only. However, expression of fluorescent protein-tagged C-terminal tail domain constructs labeled vesicular structures of different appearance. Most importantly, coexpression of different RFP/YFP-ATM2 tail fusion proteins showed colocalization and, hence, binding to the same type of vesicular target. Further coexpression experiments of RFP/YFP-ATM2 tail fusion proteins with the endosomal marker FYVE and the endosomal tracer FM4-64 demonstrated colocalization with endosomes. Colocalization was also detected by expression of the CFP-tagged membrane receptor BRI1 as marker, which is constantly recycled via endosomes. Occasionally the ATM2 tail targeted to sites at the plasma membrane closely resembling the pattern obtained upon expression of the YFP-ATM1 C-terminal tail. ATM1 is known for its localization at the plasma membrane at sites of plasmodesmata.

Different subcellular localizations and functions of Arabidopsis myosin VIII

BACKGROUND

Myosins are actin-activated ATPases that use energy to generate force and move along actin filaments, dragging with their tails different cargos. Plant myosins belong to the group of unconventional myosins and Arabidopsis myosin VIII gene family contains four members: ATM1, ATM2, myosin VIIIA and myosin VIIIB.

RESULTS

In transgenic plants expressing GFP fusions with ATM1 (IQ-tail truncation, lacking the head domain), fluorescence was differentially distributed: while in epidermis cells at the root cap GFP-ATM1 equally distributed all over the cell, in epidermal cells right above this region it accumulated in dots. Further up, in cells of the elongation zone, GFP-ATM1 was preferentially positioned at the sides of transversal cell walls. Interestingly, the punctate pattern was insensitive to brefeldin A (BFA) while in some cells closer to the root cap, ATM1 was found in BFA bodies. With the use of different markers and transient expression in Nicotiana benthamiana leaves, it was found that myosin VIII co-localized to the plasmodesmata and ER, colocalized with internalized FM4-64, and partially overlapped with the endosomal markers ARA6, and rarely with ARA7 and FYVE. Motility of ARA6 labeled organelles was inhibited whenever associated with truncated ATM1 but motility of FYVE labeled organelles was inhibited only when associated with large excess of ATM1. Furthermore, GFP-ATM1 and RFP-ATM2 (IQ-tail domain) co-localized to the same spots on the plasma membrane, indicating a specific composition at these sites for myosin binding.

CONCLUSION

Taken together, our data suggest that myosin VIII functions differently in different root cells and can be involved in different steps of endocytosis, BFA-sensitive and insensitive pathways, ER tethering and plasmodesmatal activity.

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 filaments modulate hypoosmotic-responsive K+ efflux channels in specialised cells of developing bean seed coats

In developing bean (Phaseolus vulgaris L.) seeds, nutrients move in the symplasm from sieve elements to ground-parenchyma cells where they are transported across the plasma membrane into the seed apoplasm. Release of nutrients to the seed apoplasm is related to the osmotic conditions of the apoplasm. A hypoosmotic solution, resulting from enhanced uptake of nutrients by cotyledons, stimulates nutrient release from seed coat to the apoplasm. We investigated hypoosmotic nutrient release by examining the ionic membrane currents that respond to hypoosmotic treatment in protoplasts derived from three important cell types that occur at the seed coat-cotyledonary boundary. A non-selective but predominantly K⁺ efflux current that displayed a distinct time-dependent inactivation was elicited by membrane depolarisation under hypoosmotic conditions only in ground-parenchyma protoplasts. Hypoosmotic treatment had little effect on whole-cell ionic currents in protoplasts derived from coat chlorenchyma cells and cotyledon dermal cells. The inactivating K⁺ efflux current was elicited under isosmotic conditions by treatment with cytochalasin D, which disrupts actin filaments. Hypoosmotic treatment and cytochalasin D failed to induce the K⁺ current in ground-parenchyma protoplasts in the presence of the actin stabiliser, phalloidin. The net efflux of K⁺ from intact seed coats was enhanced by hypoosmotic treatment and cytochalasin D, and the stimulation of K⁺ efflux induced by the hypoosmotic treatment and cytochalasin D was abolished by phalloidin. A bursting Cl^- channel previously described showed a similar pattern of responses. These results suggest that hypoosmotic-dependent KCl efflux from seed coats is mediated by the inactivating K⁺ outward current and bursting Cl^- channel, and that actin filaments act as components of the transduction process that is a function of cell volume.

Phototropin Mediated Relocation of Myosins in Arabidopsis thaliana

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Domain-specific mechanosensory transmission of osmotic and enzymatic cell wall disturbances to the actin cytoskeleton

Plant protoplasts are embedded within surrounding cell walls and the cell wall-plasma membrane-cytoskeleton (WMC) structural continuum seems to be crucial for the proper functioning of plant cells. We have utilised the protoplast preparation methodology to study the organisation and the putative components of the WMC continuum. Application of an osmotic agent evoked plasmolysis of the Zea mays root apex cells which appeared to be cell type- and growth stage-specific. Simultaneous use of wall polysaccharide-digesting enzymes selectively severed linkages between the components of the WMC continuum which changed the plasmolytic patterns in various cell types. This was followed by a reorganisation of filamentous actin aimed to reinforce protoplast boundaries and maintain the functioning of intercellular contact sites, especially at the cross walls. Particularly strong effects were evoked by pectin-degrading enzymes. Such treatments demonstrated directly the differentiated composition of various wall domains surrounding individual cells with the pectin-enriched cross walls (synapses), and the cellulose-hemicellulose network dominating the side walls. The same wall-degrading enzymes were used for in vitro digestion of isolated Lupinus albus cell walls followed by the extraction of wall proteins. Selective release of proteins suggested the importance of wall polysaccharide-protein interactions in the maintenance of the functioning and mechanical stability of root cell walls.

Gravity: one of the driving forces for evolution

Mechanical load is 10(3) larger for land-living than for water-living organisms. As a consequence, antigravitational material in form of compound materials like lignified cell walls in plants and mineralised bones in animals occurs in land-living organisms preferentially. Besides cellulose, pectic substances of plant cell walls seem to function as antigravitational material in early phases of plant evolution and development. A testable hypothesis including vesicular recycling processes into the tensegrity concept is proposed for both sensing of gravitational force and responding by production of antigravitational material at the cellular level.

Macrotubule-dependent protoplast volume regulation in plasmolysed root-tip cells of Triticum turgidum: involvement of phospholipase D

The probable involvement of phospholipase D (PLD)/phosphatidic acid (PA) signalling in the hyperosmotic stress response of Triticum turgidum root cells was investigated by examining the effects of butanol-1, butanol-2, phosphatidylbutanol (PtdBut), N-acylethanolamine (NAE) and PA on the hyperosmotic response, the organization of the tubulin cytoskeleton and the accumulation of a phosphorylated p38-like mitogen-activated protein (MAP) kinase (phospho-p46) in plasmolysed root cells. The effects of all the treatments were assessed by differential interference contrast (DIC) microscopy of living cells, tubulin immunofluorescence, conventional transmission electron microscopy (TEM), tubulin immunogold localization, protoplast volume measurements and western blot analysis. Butanol-1 and NAE compromised the viability of plasmolysed cells, induced a marked reduction in the plasmolysed protoplast volume, and inhibited hyperosmotically induced tubulin macrotubule formation and the accumulation of phospho-p46. Exogenous PA reinforced the hyperosmotic response of T. turgidum root cells and positively affected tubulin macrotubule formation. Additionally, PA reduced the effects of butanol-1 in plasmolysed cells. Taken together, the data suggest that PLD-mediated PA synthesis occurs upstream of the accumulation of phospho-p46 to regulate hyperosmotically induced macrotubule formation in plasmolysed T. turgidum root cells.