Actin organization during the cell cycle in meristematic plant cells. Actin is present in the cytokinetic phragmoplast

Actin isovariant ACT7 controls root meristem development in Arabidopsis through modulating auxin and ethylene responses

The meristem is the most functionally dynamic part in a plant. The shaping of the meristem requires constant cell division and elongation, which are influenced by hormones and the cytoskeletal component, actin. Although the roles of hormones in modulating meristem development have been extensively studied, the role of actin in this process is still elusive. Using the single and double mutants of the vegetative class actin, we demonstrate that actin isovariant ACT7 plays an important role in root meristem development. In the absence of ACT7, but not ACT8 and ACT2, depolymerization of actin was observed. Consistently, the act7 mutant showed reduced cell division, cell elongation, and meristem length. Intracellular distribution and trafficking of auxin transport proteins in the actin mutants revealed that ACT7 specifically functions in the root meristem to facilitate the trafficking of auxin efflux carriers PIN1 and PIN2, and consequently the transport of auxin. Compared with act7, the act7act8 double mutant exhibited slightly enhanced phenotypic response and altered intracellular trafficking. The altered distribution of auxin in act7 and act7act8 affects the response of the roots to ethylene, but not to cytokinin. Collectively, our results suggest that ACT7-dependent auxin-ethylene response plays a key role in controlling Arabidopsis root meristem development.

Tools for studying the cytoskeleton during plant cell division

The plant cytoskeleton regulates fundamental biological processes, including cell division. How to experimentally perturb the cytoskeleton is a key question if one wants to understand the role of both actin filaments (AFs) and microtubules (MTs) in a given biological process. While a myriad of mutants are available, knock-out in cytoskeleton regulators, when nonlethal, often produce little or no phenotypic perturbation because such regulators are often part of a large family, leading to functional redundancy. In this review, alternative techniques to modify the plant cytoskeleton during plant cell division are outlined. The different pharmacological and genetic approaches already developed in cell culture, transient assays, or in whole organisms are presented. Perspectives on the use of optogenetics to perturb the plant cytoskeleton are also discussed.

Dynamic apico-basal enrichment of the F-actin during cytokinesis in Arabidopsis cells embedded in their tissues

Cell division is a tightly regulated mechanism, notably in tissues where malfunctions can lead to tumour formation or developmental defects. This is particularly true in land plants, where cells cannot relocate and therefore cytokinesis determines tissue topology. In plants, cell division is executed in radically different manners than in animals, with the appearance of new structures and the disappearance of ancestral mechanisms. Whilst F-actin and microtubules closely co-exist, recent studies mainly focused on the involvement of microtubules in this key process. Here, we used a root tracking system to image the spatio-temporal dynamics of both F-actin reporters and cell division markers in dividing cells embedded in their tissues. In addition to the F-actin accumulation at the phragmoplast, we observed and quantified a dynamic apico-basal enrichment of F-actin from the prophase/metaphase transition until the end of the cytokinesis.

Methods to Visualize the Actin Cytoskeleton During Plant Cell Division

Cell division in plants consists of separating the mother cell in two daughter cells by the centrifugal growth of a new wall. This process involves the reorganization of the structural elements of the cell, namely the microtubules and actin cytoskeleton which allow the coordination, the orientation, and the progression of mitosis. In addition to its implication in those plant-specific structures, the actin cytoskeleton, in close association with the plasma membrane, exhibits specific patterning at the cortex of the dividing cells, and might act as a signaling component. This review proposes an overview of the techniques available to visualize the actin cytoskeleton in fixed tissues or living cells during division, including electron, fluorescent, and super-resolution microscopy techniques.

Actin Filament Disruption Alters Phragmoplast Microtubule Dynamics during the Initial Phase of Plant Cytokinesis

Plant growth and development relies on the accurate positioning of the cell plate between dividing cells during cytokinesis. The cell plate is synthetized by a specialized structure called the phragmoplast, which contains bipolar microtubules that polymerize to form a framework with the plus ends at or near the division site. This allows the transport of Golgi-derived vesicles toward the plus ends to form and expand the cell plate. Actin filaments play important roles in cell plate expansion and guidance in plant cytokinesis at the late phase, but whether they are involved at the early phase is unknown. To investigate this further, we disrupted the actin filaments in cell cycle-synchronized tobacco BY-2 cells with latrunculin B (LatB), an actin polymerization inhibitor. We observed the cells under a transmission electron microscope or a spinning-disk confocal laser scanning microscope. We found that disruption of actin filaments by LatB caused the membrane vesicles at the equatorial plane of the cell plate to be dispersed rather than form clusters as they did in the untreated cells. The midzone constriction of phragmoplast microtubules also was perturbed in LatB-treated cells. The live cell imaging and kymograph analysis showed that disruption of actin filaments also changed the accumulation timing of NACK1 kinesin, which plays a crucial role in cell plate expansion. This suggests that there are two functionally different types of microtubules in the phragmoplast. Together, our results show that actin filaments regulate phragmoplast microtubules at the initial phase of plant cytokinesis.

The functions of the cytoskeleton and associated proteins during mitosis and cytokinesis in plant cells

In higher plants, microtubule (MT)-based, and actin filament (AF)-based structures play important roles in mitosis and cytokinesis. Besides the mitotic spindle, the evolution of a band comprising cortical MTs and AFs, namely, the preprophase band (PPB), is evident in plant cells. This band forecasts a specific division plane before the initiation of mitosis. During cytokinesis, another plant-specific cytoskeletal structure called the phragmoplast guides vesicles in the creation of a new cell wall. In addition, a number of cytoskeleton-associated proteins are reportedly involved in the formation and function of the PPB, mitotic spindle, and phragmoplast. This review summarizes current knowledge on the cytoskeleton-associated proteins that mediate the cytoskeletal arrays during mitosis and cytokinesis in plant cells and discusses the interaction between MTs and AFs involved in mitosis and cytokinesis.

Myosin VIII associates with microtubule ends and together with actin plays a role in guiding plant cell division

Plant cells divide using the phragmoplast, a microtubule-based structure that directs vesicles secretion to the nascent cell plate. The phragmoplast forms at the cell center and expands to reach a specified site at the cell periphery, tens or hundreds of microns distant. The mechanism responsible for guiding the phragmoplast remains largely unknown. Here, using both moss and tobacco, we show that myosin VIII associates with the ends of phragmoplast microtubules and together with actin plays a role in guiding phragmoplast expansion to the cortical division site. Our data lead to a model whereby myosin VIII links phragmoplast microtubules to the cortical division site via actin filaments. Myosin VIII's motor activity along actin provides a molecular mechanism for steering phragmoplast expansion.

DOI:http://dx.doi.org/10.7554/eLife.03498.001

Plant cells are surrounded by a membrane, which controls what enters and leaves the cell, and a cell wall, which provides rigidity. When a plant cell is ready to divide, it needs to produce two new cell membranes, with a new cell wall sandwiched between them, to split the cell contents into two daughter cells.

During the division process the cell builds a scaffold called the phragmoplast that guides the delivery of the materials that are needed to make the new cell wall and membranes. The phragmoplast—which is made of rod-like proteins called microtubules and actin filaments—starts at the centre of the cell and expands towards a pre-determined site on the existing cell wall. The question is: how does the phragmoplast target this site, which can be tens or hundreds of microns away?

Wu and Bezanilla have now found that a protein called myosin VIII has a central role in guiding the growing phragmoplast to the cell wall. Myosin VIII is a motor protein that moves along actin filaments. Wu and Bezanilla propose that myosin VIII can guide the expansion of the phragmoplast by pulling microtubules along the actin filaments.

The experiments were carried out on two distantly-related plant species, tobacco and a moss called Physcomitrella patens. Similar results were found in both species so it is possible that myosin VIII may play the same role in cell division in all plants.

DOI:http://dx.doi.org/10.7554/eLife.03498.002

Organization of actin cytoskeleton during meiosis I in a wheat thermo-sensitive genic male sterile line

BS366 is a thermo-sensitive male sterile line of wheat (Triticum aestivum L.) for two-line hybrid breeding, which exhibits aberrant meiotic cytokinesis under low temperature. Through transcriptome analysis, a possible regulatory role for plant actin cytoskeleton was suggested. However, the organization of actin cytoskeleton in meiosis has been poorly understood so far. Here, fixed microsporocytes during meiosis were labeled with tetramethylrhodamine isothiocyanate-phalloidin and 4',6-diamidino-2-phenylindole. Quantities of fluorescent micrographs were captured using a confocal microscope, including the transient state from metaphase to telophase. We observed that actin filaments were abundant in typical kariokinetic spindle, central spindle (parallel microtubules or actin fibers between two separated chromosomes in anaphase), and phragmoplast. Interestingly, we identified the Chinese lantern-shaped actin phragmoplast in wheat meiosis for the first time. Under low temperature, phragmoplast actin filaments were chaotic and normal cell plate failed to form. These data provide new insights into the organization of actin filaments during male meiosis of plant and support a role of actin cytoskeleton in bringing about thermo-sensitive male sterility in wheat.

Strategies of actin reorganisation in plant cells

Spatial-temporal flexibility of the actin filament network (F-actin) is essential for all basic cellular functions and is governed by a stochastic dynamic model. In this model, actin filaments that randomly polymerise from a pool of free actin are bundled with other filaments and severed by ADF/cofilin. The fate of the severed fragments is not known. It has been proposed that the fragments are disassembled and the monomeric actin recycled for the polymerisation of new filaments. Here, we have generated tobacco cell lines and Arabidopsis plants expressing the actin marker Lifeact to address the mechanisms of F-actin reorganisation in vivo. We found that F-actin is more dynamic in isotropically expanding cells and that the density of the network changes with a periodicity of 70 seconds. The depolymerisation rate, but not the polymerisation rate, of F-actin increases when microtubules are destabilised. New filaments can be assembled from shorter free cytoplasmic fragments, from the products of F-actin severing and by polymerisation from the ends of extant filaments. Thus, remodelling of F-actin might not require bulk depolymerisation of the entire network, but could occur via severing and end-joining of existing polymers.

Quantitative analysis of changes in actin microfilament contribution to cell plate development in plant cytokinesis

BACKGROUND

Plant cells divide by the formation of new cross walls, known as cell plates, from the center to periphery of each dividing cell. Formation of the cell plate occurs in the phragmoplast, a complex structure composed of membranes, microtubules (MTs) and actin microfilaments (MFs). Disruption of phragmoplast MTs was previously found to completely inhibit cell plate formation and expansion, indicative of their crucial role in the transport of cell plate membranes and materials. In contrast, disruption of MFs only delays cell plate expansion but does not completely inhibit cell plate formation. Despite such findings, the significance and molecular mechanisms of MTs and MFs remain largely unknown.

RESULTS

Time-sequential changes in MF-distribution were monitored by live imaging of tobacco BY-2 cells stably expressing the GFP-actin binding domain 2 (GFP-ABD2) fusion protein, which vitally co-stained with the endocytic tracer, FM4-64, that labels the cell plate. During cytokinesis, MFs accumulated near the newly-separated daughter nuclei towards the emerging cell plate, and subsequently approached the expanding cell plate edges. Treatment with an actin polymerization inhibitor caused a decrease in the cell plate expansion rate, which was quantified using time-lapse imaging and regression analysis. Our results demonstrated time-sequential changes in the contribution of MFs to cell plate expansion; MF-disruption caused about a 10% decrease in the cell plate expansion rate at the early phase of cytokinesis, but about 25% at the late phase. MF-disruption also caused malformation of the emerging cell plate at the early phase, indicative of MF involvement in early cell plate formation and expansion. The dynamic movement of endosomes around the cell plate was also inhibited by treatment with an actin polymerization inhibitor and a myosin ATPase inhibitor, respectively. Furthermore, time-lapse imaging of the endoplasmic reticulum (ER) revealed that MFs were involved in ER accumulation in the phragmoplast at the late phase.

CONCLUSION

By expression of GFP-ABD2 and vital staining with FM4-64, the dynamics of MFs and the cell plate could be followed throughout plant cytokinesis in living cells. Pharmacological treatment and live imaging analysis also allowed us to quantify MF contribution to cell plate expansion during cytokinesis. Our results suggest that MFs play significant roles in cell plate formation and expansion via regulation of endomembrane dynamics.

Cortical actin filaments at the division site of mitotic plant cells: a reconsideration of the 'actin-depleted zone'

The preprophase bands of microtubules and F-actin are primary markers of the division site for most plant cells. After preprophase band breakdown, the division site has been considered to be 'negatively' memorized by the local absence of cortical actin filaments. However, there have been reports of cortical F-actin at the division site of mitotic plant cells, calling into question its distribution and possible role there. In this article, previous and recent data on this issue are reviewed. It is proposed that the division site of mitotic plant cells is not devoid of F-actin but is traversed by scarce cortical actin filaments. The description of the division site as an 'actin exclusion zone' might therefore be attributed to a failure to preserve and/or image the notoriously sensitive actin filaments. Accordingly, the 'actin-depleted zone' should be considered as a site with fewer actin filaments than the rest of the cortical cytoplasm. Taking into account recent molecular data on division site components, a possible role for the scarcity of cortical actin filaments in establishing a zone of minimum mobility is also proposed.

Cell and Molecular Biology of the Spindle Matrix

The concept of a spindle matrix has long been proposed to account for incompletely understood features of microtubule spindle dynamics and force production during mitosis. In its simplest formulation, the spindle matrix is hypothesized to provide a stationary or elastic molecular matrix that can provide a substrate for motor molecules to interact with during microtubule sliding and which can stabilize the spindle during force production. Although this is an attractive concept with the potential to greatly simplify current models of microtubule spindle behavior, definitive evidence for the molecular nature of a spindle matrix or for its direct role in microtubule spindle function has been lagging. However, as reviewed here multiple studies spanning the evolutionary spectrum from lower eukaryotes to vertebrates have provided new and intriguing evidence that a spindle matrix may be a general feature of mitosis.

Localization of actin filaments on mitotic apparatus in tobacco BY-2 cells

Actin filaments are among the major components of the cytoskeleton, and participate in various cellular dynamic processes. However, conflicting results had been obtained on the localization of actin filaments on the mitotic apparatus and their participation in the process of chromosome segregation. We demonstrated by using rhodamine-phalloidin staining, the localization of actin filaments on the mitotic spindles of tobacco BY-2 cells when the cells were treated with cytochalasin D. At prophase, several clear spots were observed at or near the kinetochores of the chromosomes. At anaphase, the actin filaments that appeared to be pulling chromosomes toward the division poles were demonstrated. However, as there was a slight possibility that these results might have been the artifacts of cytochalasin D treatment or the phalloidin staining, we analyzed the localization of actin filaments at the mitotic apparatus immunologically. We cloned a novel BY-2 alpha-type actin cDNA and prepared a BY-2 actin antibody. The fluorescence of the anti-BY-2 actin antibody was clearly observed at the mitotic apparatus in both non-treated and cytochalasin D-treated BY-2 cells during mitosis. The facts that similar results were obtained in both actin staining with rhodamine-phalloidin and immunostaining with actin antibody strongly indicate the participation of actin in the organization of the spindle body or in the process of chromosome segregation. Furthermore, both filamentous actin and spindle bodies disappeared in the cells treated with propyzamide, which depolymerizes microtubules, supporting the notion that actin filaments are associated with microtubules organizing the spindle body.

Structure of kinetochore fibres in crane-fly spermatocytes after irradiation with an ultraviolet microbeam: Neither microtubules nor actin filaments remain in the irradiated region

We studied chromosome movement after kinetochore microtubules were severed. Severing a kinetochore fibre in living crane-fly spermatocytes with an ultraviolet microbeam creates a kinetochore stub, a birefringent remnant of the spindle fibre connected to the kinetochore and extending only to the edge of the irradiated region. After the irradiation, anaphase chromosomes either move poleward led by their stubs or temporarily stop moving. We examined actin and/or microtubules in irradiated cells by means of confocal fluorescence microscopy or serial-section reconstructions from electron microscopy. For each cell thus examined, chromosome movement had been recorded continuously until the moment of fixation. Kinetochore microtubules were completely severed by the ultraviolet microbeam in cells in which chromosomes continued to move poleward after the irradiation: none were seen in the irradiated regions. Similarly, actin filaments normally present in kinetochore fibres were severed by the ultraviolet microbeam irradiations: the irradiated regions contained no actin filaments and only local spots of non-filamentous actin. There was no difference in irradiated regions when the associated chromosomes continued to move versus when they stopped moving. Thus, one cannot explain motion with severed kinetochore microtubules in terms of either microtubules or actin-filaments bridging the irradiated region. The data seem to negate current models for anaphase chromosome movement and support a model in which poleward chromosome movement results from forces generated within the spindle matrix that propel kinetochore fibres or kinetochore stubs poleward.

Phallacidin stains the kinetochore region in the mitotic spindle of the green algae Oedogonium spp

We found previously that in living cells of Oedogonium cardiacum and O. donnellii, mitosis is blocked by the drug cytochalasin D (CD). We now report on the staining observed in these spindles with fluorescently actin-labeling reagents, particularly Bodipy FL phallacidin. Normal mitotic cells exhibited spots of staining associated with chromosomes; frequently the spots appeared in pairs during prometaphase-metaphase. During later anaphase and telophase, the staining was confined to the region between chromosomes and poles. The texture of the staining appeared to be somewhat dispersed by CD treatment but it was still present, particularly after shorter (< 2 h) exposure. Electron microscopy of CD-treated cells revealed numerous spindle microtubules (MTs); many kinetochores had MTs associated with them, often laterally and some even terminating in the kinetochore as normal, but the usual bundle of kinetochore MTs was never present. As treatment with CD became prolonged, the kinetochores became shrunken and sunk into the chromosomes. These results support the possibility that actin is present in the kinetochore of Oedogonium spp. The previous observations on living cells suggest that it is a functional component of the kinetochore-MT complex involved in the correct attachment of chromosomes to the spindle.

Microtubule-associated proteins in plants--why we need a MAP

Plants have four main microtubule assemblies. Three are involved in arranging when and where the cell wall is laid down and have no direct homologues in animals. Microtubule-associated proteins are important components of these assemblies, and we are now starting to uncover what these proteins are and how they might work.

CYTOSKELETAL PERSPECTIVES ON ROOT GROWTH AND MORPHOGENESIS

Growth and development of all plant cells and organs relies on a fully functional cytoskeleton comprised principally of microtubules and microfilaments. These two polymeric macromolecules, because of their location within the cell, confer structure upon, and convey information to, the peripheral regions of the cytoplasm where much of cellular growth is controlled and the formation of cellular identity takes place. Other ancillary molecules, such as motor proteins, are also important in assisting the cytoskeleton to participate in this front-line work of cellular development. Roots provide not only a ready source of cells for fundamental analyses of the cytoskeleton, but the formative zone at their apices also provides a locale whereby experimental studies can be made of how the cytoskeleton permits cells to communicate between themselves and to cooperate with growth-regulating information supplied from the apoplasm.

Actin cytoskeleton in plants: from transport networks to signaling networks

The plant actin cytoskeleton is characterized by a high diversity in regard to gene families, isoforms, and degree of polymerization. In addition to the most abundant F-actin assemblies like filaments and their bundles, G-actin obviously assembles in the form of actin oligomers composed of a few actin molecules which can be extensively cross-linked into complex dynamic meshworks. The role of the actomyosin complex as a force generating system - based on principles operating as in muscle cells - is clearly established for long-range mass transport in large algal cells and specialized cell types of higher plants. Extended F-actin networks, mainly composed of F-actin bundles, are the structural basis for this cytoplasmic streaming of high velocities On the other hand, evidence is accumulating that delicate meshworks built of short F-actin oligomers are critical for events occurring at the plasma membrane, e.g., actin interventions into activities of ion channels and hormone carriers, signaling pathways based on phospholipids, and exo- and endocytotic processes. These unique F-actin arrays, constructed by polymerization-depolymerization processes propelled via synergistic actions of actin-binding proteins such as profilin and actin depolymerizing factor (ADF)/cofilin are supposed to be engaged in diverse aspects of plant morphogenesis. Finally, rapid rearrangements of F-actin meshworks interconnecting endocellular membranes turn out to be especially important for perception-signaling purposes of plant cells, e.g., in association with guard cell movements, mechano- and gravity-sensing, plant host-pathogen interactions, and wound-healing.

Microtubule-microfilament synergy in the cytoskeleton

This review describes examples of structural and functional synergy of the microtubule and actin filament cytoskeleton. An analysis of basal body (centriole)-associated fibrillar networks includes studies of ciliated epithelium, neurosensory epithelium, centrosomes, and ciliated protozoa. Microtubule and actin filament interactions in cell division and development are illustrated by centrosome motility, cleavage furrow positioning, centriole migration, nuclear migration, dynamics in the phragmoplast, growth cone motility, syncytial organization, and ring canals. Model systems currently used for studies on organelle transport are described in relation to mitochondrial transport in axons and vesicular transport in polarized epithelium. Evidence that both anterograde and retrograde motors are associated with one organelle is also discussed. The final section reviews proteins that bind both microtubules and actin filaments and are possible regulators of microtubule-microfilament interactions. Regulatory roles for posttranslational modifications, microtubule and microfilament dynamics, and multisubunit complexes are considered.

Actin dynamics during the cell cycle in Chlamydomonas reinhardtii

We have used two monoclonal antibodies to demonstrate the presence and localization of actin in interphase and mitotic vegetative cells of the green alga Chlamydomonas reinhardtii. Commercially available monoclonal antibodies raised against smooth muscle actin (Lessard: Cell Motil. Cytoskeleton 10:349-362, 1988; Lin: Proc. Natl. Acad. Sci. USA 78:2335-2339, 1981) identify Chlamydomonas actin as a approximately 43,000-M(r) protein by Western immunoblot procedures. In an earlier study, Detmers and coworkers (Cell Motil. 5:415-430, 1985) first identified Chlamydomonas actin using NBD-phallacidin and an antibody raised against Dictyostelium actin; they demonstrated that F-actin is localized in the fertilization tubule of mating gametes. Here, we show by immunofluorescence that vegetative Chlamydomonas cells have an array of actin that surrounds the nucleus in interphase cells and undergoes dramatic reorganization during mitosis and cytokinesis. This includes the following: reorganization of actin to the anterior of the cell during preprophase; the formation of a cruciate actin band in prophase; reorganization to a single anterior actin band in metaphase; rearrangement forming a focus of actin anterior to the metaphase plate; reextension of the actin band in anaphase; presence of actin in the forming cleavage furrow during telophase and cytokinesis; and finally reestablishment of the interphase actin array. The studies presented here do not allow us to discriminate between G and F-actin. None the less, our observations, demonstrating dynamic reorganization of actin during the cell cycle, suggest a role for actin that may include the movement of basal bodies toward the spindle poles in mitosis and the formation of the cleavage furrow during cytokinesis.

Organization of the actin cytoskeleton during pollen development inGasteria verrucosa (Mill.) H. Duval visualized with rhodamine-phalloidin

The three-dimensional organization of the microfilamental cytoskeleton of developingGasteria pollen was investigated by light microscopy using whole cells and fluorescently labelled phalloidin. Cells were not fixed chemically but their walls were permeabilized with dimethylsulphoxide and Nonidet P-40 at premicrospore stages or with dimethylsulphoxide, Nonidet P-40 and 4-methylmorpholinoxide-monohydrate at free-microspore and pollen stages to dissolve the intine.Four strikingly different microfilamentous configurations were distinguished. (i) Actin filaments were observed in the central cytoplasm throughout the successive stages of pollen development. The network was commonly composed of thin bundles ramifying throughout the cytoplasm at interphase stages but as thick bundles encaging the nucleus prior to the first and second meiotic division. (ii) In released microspores and pollen, F-actin filaments formed remarkably parallel arrays in the peripheral cytoplasm. (iii) In the first and second meiotic spindles there was an apparent localization of massive arrays of phalloidin-reactive material. Fluorescently labelled F-actin was present in kinetochore fibers and pole-to-pole fibers during metaphase and anaphase. (iv) At telophase, microfilaments radiated from the nuclear envelopes and after karyokinesis in the second meiotic division, F-actin was observed in phragmoplasts.We did not observe rhodamine-phalloidin-labelled filaments in the cytoplasm after cytochalasin-B treatment whereas F-actin persisted in the spindle. Incubation at 4° C did not influence the existence of cytoplasmic microfilaments whereas spindle filaments disappeared. This points to a close interdependence of spindle microfilaments and spindle tubules.Based on present data and earlier observations on the configuration of microtubules during pollen development in the same species (Van Lammeren et al., 1985, Planta165, 1-11) there appear to be apparent codistributions of F-actin and microtubules during various stages of male meiosis inGasteria verrucosa.

Microtubule-binding proteins from carrot : I. Initial characterization and microtubule bundling

Microtubules (MTs) participate in several processes of fundamental importance to growth and development in higher plants, yet little is known about the proteins with which they associate. Information about these molecules is important because they probably play a role in mediating functional and structural differences between various MT arrays. As a first step in gaining insight into this problem, we have isolated, from suspension-cultured cells of carrot (Daucus carota L.), non-tubulin proteins which bind to and affect microtubules (MTs) in vitro. These proteins were isolated using taxol-stabilized neuronal MTs as an affinity substrate. They cause MT bundling at substoichiometric concentrations, support the assembly of tubulin in vitro, and at low concentrations, decorate single MTs in a periodic fashion. The bundled MTs formed in vitro share similarities with those seen in situ in a variety of plant cells, including a center-center spacing of 34 nm, cold stability, resistance to anti-microtubule drugs, and sensitivity to calcium. The bundling activity is specific; other cationic proteins, as well as poly-L-lysine, do not behave in a similar manner. The bundling activity is insensitive to ATP. By assaying bundling activity with dark-field microscopy and employing standard biochemical procedures, a small number of polypeptides involved in the bundling process were identified. Affinity-isolated antibodies to one of these polypeptides (Mr=76000) were found to co-localize with MTs in the cortical array of protoplasts. Our findings are discussed with reference to the importance of these proteins in the cell and to their relationship to microtubule-associated proteins in other eukaryotes.

Microfilament bundles of F-actin inSpirogyra observed by fluorescence microscopy

Microfilament bundles (MFBs) of F-actin were observed by fluorescence microscopy in cells ofSpirogyra treated with rhodamine-phalloidin. Four types of MFBs could be recognized on the basis of locality and appearance: those dispersed in the cytoplasm near the cell surface; those beneath the plasma membrane running parallel to each other; those at the edges of the chloroplast; and those surrounding the nucleus. Each type exhibited a unique behavior during the cell cycle. Microfilament bundles dispersed in the cytoplasm came together at the middle of the cell to form a fibril ring at the mitotic prophase. The fibril ring decreased in diameter, causing the development of a furrow in the protoplast that progressed from the outside to the inside. After the completion of furrowing, the MFBs in the fibril ring dispersed beneath the plasma membrane. Microfilament bundles surrounding the nucleus formed a net-like cage which became invisible at the mitotic anaphase, while MFBs seen at the chloroplast edges persisted there during the cell cycle without changing their position. Parallel MFBs running perpendicular to the long axis of the cell were seen at all stages in the cell cycle.

Three-dimensional localization and redistribution of F-actin in higher plant mitosis and cell plate formation

The distribution of F-actin cables in dividing endosperm cells of a higher plant, Haemanthus, was visualized with the immunogold-silver-enhanced method and compared with the arrangement of immunogold-stained microtubules in the same cells. The three-dimensional distribution of F-actin cables and microtubules during mitosis and cell plate formation was analyzed using ultrathin optical sectioning of whole mounts in polarized light video microscopy. F-actin cables form a loose irregular network in the interphase cytoplasm. Much of this network remains outside of the spindle during mitosis. A few F-actin cables were detected within the spindle. Their pronounced rearrangement during mitosis appears to be related to the presence and growth of microtubule arrays. During prometaphase, actin cables located on the spindle surface and those present within the spindle tend to arrange parallel to the long axis of the spindle. Cables outside the spindle do not reorient, except those at the polar region, where they appear to be compressed by the elongating spindle. Beginning with mid-anaphase, shorter actin cables oriented in various directions accumulate at the equator. Some of them are incorporated into the phragmoplast and cell plate and are gradually fragmented as the cell plate is formed and ages. Actin cables adjacent to microtubule arrays often show a regular punctate staining pattern. Such a pattern is seldom observed in the peripheral cytoplasm, which contains few microtubules. The rearrangement of F-actin cables mimicks the behavior of spindle inclusions, such as starch grains, mitochondria, etc., implying that F-actin is redistributed passively by microtubule growth or microtubule-related transport. Thus F-actin or actomyosin-based motility does not appear to be directly involved in mitosis and cytokinesis in higher plants.

Characterization and dynamics of cytoplasmic F-actin in higher plant endosperm cells during interphase, mitosis, and cytokinesis

We have identified an F-actin cytoskeletal network that remains throughout interphase, mitosis, and cytokinesis of higher plant endosperm cells. Fluorescent labeling was obtained using actin monoclonal antibodies and/or rhodamine-phalloidin. Video-enhanced microscopy and ultrastructural observations of immunogold-labeled preparations illustrated microfilament-microtubule co-distribution and interactions. Actin was also identified in cell crude extract with Western blotting. During interphase, microfilament and microtubule arrays formed two distinct networks that intermingled. At the onset of mitosis, when microtubules rearranged into the mitotic spindle, microfilaments were redistributed to the cell cortex, while few microfilaments remained in the spindle. During mitosis, the cortical actin network remained as an elastic cage around the mitotic apparatus and was stretched parallel to the spindle axis during poleward movement of chromosomes. This suggested the presence of dynamic cross-links that rearrange when they are submitted to slow and regular mitotic forces. At the poles, the regular network is maintained. After midanaphase, new, short microfilaments invaded the equator when interzonal vesicles were transported along the phragmoplast microtubules. Colchicine did not affect actin distribution, and cytochalasin B or D did not inhibit chromosome transport. Our data on endosperm cells suggested that plant cytoplasmic actin has an important role in the cell cortex integrity and in the structural dynamics of the poorly understood cytoplasm-mitotic spindle interface. F-actin may contribute to the regulatory mechanisms of microtubule-dependent or guided transport of vesicles during mitosis and cytokinesis in higher plant cells.

An actin network is present in the cytoplasm throughout the cell cycle of carrot cells and associates with the dividing nucleus

We have studied the F-actin network in cycling suspension culture cells of carrot (Daucus carota L.) using rhodaminyl lysine phallotoxin (RLP). In addition to conventional fixation with formaldehyde, we have used two different nonfixation methods before adding RLP: extracting cells in a stabilizing buffer; inducing transient pores in the plasma membrane with pulses of direct current (electroporation). These alternative methods for introducing RLP revealed additional features of the actin network not seen in aldehyde-fixed cells. The three-dimensional organization of this network in nonflattened cells was demonstrated by projecting stereopairs derived from through-focal series of computer-enhanced images. F-actin is present in interphase cells in four interconnected configurations: a meshwork surrounding the nucleus; thick cables in transvacuolar strands and deep in the cytoplasm; a finer network of bundles within the cortical cytoplasm; even finer filaments that run in ordered transverse array around the cell periphery. The actin network is organized differently during division but it does not disappear as do the cortical microtubules. RLP stains a central filamentous cortical band as the chromatin begins to condense (preprophase); it stains the mitotic spindle (as recently shown by Seagull et al. [Seagull, R. W., M. Falconer, and C. A. Weerdenburg, 1987, J. Cell Biol., 104:995-1004] for aldehyde fixed suspension cells) and the cytokinetic apparatus (as shown by Clayton, L., and C. W. Lloyd, 1985, Exp. Cell Res., 156:231-238). However, it is now shown that an additional network of F-actin persists in the cytoplasm throughout division associating in turn with the preprophase band, the mitotic spindle, and the cytokinetic phragmoplast.

Actin in the preprophase band of Allium cepa

F-actin has been identified in the preprophase band of Allium cepa. Cells attached to subbed slides were obtained from formaldehyde-fixed root tips digested in EGTA and Cellulysin. The air-dried cells were extracted in Triton X-100, treated with rhodamine-phalloidin, rinsed briefly in PBS, and viewed in the fluorescence microscope. Interphase cells contain a network of actin fibers that extends into all areas of the cytoplasm. During preprophase, the network is replaced by a band of fibers aligned in the position of the preprophase band. Colocalization of F-actin with rhodamine-phalloidin and microtubules with tubulin immunocytochemistry confirms that the two bands are coincident. The actin appears to comprise a thin layer of fibers next to the plasmalemma. Like the microtubule preprophase band, the actin band narrows as preprophase progresses and disappears by midprophase. Fluorescent actin bands are not seen in fixed cells pretreated with excess unlabeled phalloidin before staining. They are also absent in roots exposed to cytochalasins B and D before fixation, but preprophase band microtubules at all stages of aggregation are still present. Colchicine treatment leads to the loss of both preprophase band microtubules and actin. The possible function of preprophase band actin is discussed.

Microfilaments: dynamic arrays in higher plant cells

By using fluorescently labeled phalloidin we have examined, at the light microscope level, the three-dimensional distribution and reorganization of actin-like microfilaments (mfs) during plant cell cycle and differentiation. At interphase, mfs are organized into three distinct yet interconnected arrays: fine peripheral networks close to the plasma membrane; large axially oriented cables in the subcortical region; a nuclear "basket" of mfs extending into the transvacuolar strands. All these arrays, beginning with the peripheral network, disappear at the onset of mitosis and reappear, beginning with the nuclear basket, after cytokinesis. During mitotic and cytokinetic events, mfs are associated with the spindle and phragmoplast. Actin staining in the spindle is localized between the chromosomes and the spindle poles and changes in a functionally specific manner. The nuclear region appears to be the center for mf organization and/or initiation. During differentiation from rapid cell division to cell elongation, mf arrays switch from an axial to a transverse orientation, thus paralleling the microtubules. This change in orientation reflects a shift in the direction of cytoplasmic streaming. These observations show for the first time that actin-like mfs form intricate and dynamic arrays in plant cells which may be involved in many as yet undescribed cell functions.

Monoclonal antibodies against plant proteins recognise animal intermediate filaments

Four monoclonal antibodies were raised against polypeptides present in a high-salt detergent-insoluble fraction from cells of Chlamydomonas reinhardtii. Indirect immunofluorescence microscopy of fibroblasts and epithelial cells grown in culture using these plant antibodies revealed staining arrays identical to those obtained with well characterised antibodies to animal intermediate filaments. Immunofluorescence microscopy of Chlamydomonas with these monoclonal antibodies and a monoclonal antibody that recognises all animal intermediate filaments (anti-IFA) gave a diffuse, patchy cytoplasmic staining pattern. Both the plant antibodies and anti-IFA stained interphase onion root tip cells in a diffuse perinuclear pattern. In metaphase through to telophase, the labelling patterns colocalised with those of microtubules. Labelling of the phragmoplast was also detected but not staining of the preprophase band. On Western blots of various animal cell lines and tissues, all the antibodies labelled known intermediate filament proteins. On Western blots of whole Chlamydomonas proteins, all the antibodies labelled a broad band in the 57,000 Mr range, and three antibodies labelled bands around 66,000 and 140,000 Mr but with variable intensities. On Western blots of whole onion root tip proteins, all the antibodies labelled 50,000 Mr (two to three bands) polypeptides and a diffuse band around 60,000 Mr and three of the antibodies also labelled several polypeptides in the 90,000-200,000 Mr range. The consistent labelling of these different bands by several different monoclonal antibodies recognising animal intermediate filaments makes these polypeptides putative plant intermediate filament proteins.

Effects of microfilament disrupters on microfilament distribution and morphology in maize root cells

Maize root tip cells were examined for the distribution of actin microfilaments in various cell types and to determine the effects of microfilament disrupters. Fluorescence microscopy on fixed, stabilized, squashed cells using the F-actin specific probe, rhodamine-labelled phalloidin, allowed for a three-dimensional visualization of actin microfilaments. Microfilaments were observed as long, meandering structures in root cap cells and meristematic cells, while those in immature vascular parenchyma were abundant in the thin band of cytoplasm and were long and less curved. By modifying standard electron microscopic fixation procedures, microfilaments in plant cells could be easily detected in all cell types. Treatment with cytochalasin B, cytochalasin D and lead acetate, compounds that interfere with microfilament related processes, re-organized the microfilaments into abnormal crossed and highly condensed masses. All the treatments affected not only the microfilaments but also the accumulation of secretory vesicles. The vivid demonstration of the effects of all of these microfilament disrupters on the number and size of Golgi vesicles indicates that these vesicles may depend on microfilaments for intracellular movement.

Cytodifferentiation of polar plant cells: use of anti-microtubular agents during the differentiation of statocytes from cress roots (Lepidium sativum L.)

The development of the structural polarity of statocytes from cress roots (Lepidium sativum L.) was studied in a time- and stage-dependent manner. outgrowing radicles had statocytes with abundant lipid droplets, sparsely developed endoplasmic reticulum (ER) and nuclei located at the proximal cell poles. During differentiation, coincidentally the lipid droplets disappeared while rough ER increased in length. The ER was translocated into the distal cell pole to establish a complex of stacked ER. Microtubules occurred first at the distal cell edges. As a second step, ER was produced in the vicinity of the nucleus and was also translocated distally. By application of the antimicrotubular agents heavy water (90%), colchicine (10(-4) mol l-1) and triethyl lead chloride (20 micromoles l-1), the involvement of microtubules in these events was studied. Triethyl lead chloride led to a complete cessation of differentiation; root-cap cells remained at a stage without polar arrangement of the ER. Colchicine affected the development of structural polarity slightly, as shown by a higher density of cortical ER cisternae. Heavy water inhibited the translocation of ER almost completely and yielded ER located also in the cell center. All anti-microtubular agents inhibited cell division and the differentiation of the distal cell layer of the dermatocalyptrogen into statocytes. It is hypothesized that microtubules serve as anchoring sites for microfilaments, which actually mediate the translocation of the ER. Hence, an intact system of microtubules and microfilaments is necessary for the expression of structural polarity.

Distribution of calmodulin in pea seedlings: Immunocytochemical localization in plumules and root apices

Immunofluorescence techniques have been used to study the distribution of calmodulin in several tissues in young etiolated pea (Pisum sativum L.) seedlings. A fairly uniform staining was seen in the nucleoplasm and background cytoplasm of most cell types. Cell walls and nucleoli were not stained. In addition, patterned staining reactions were seen in many cells. In cells of the plumule, punctate staining of the cytoplasm was common, and in part this stain appeared to be associated with the plastids. A very distinctive staining of amyloplasts was seen in the columella of the root cap. Staining associated with cytoskeletal elements could be shown in division stages. By metaphase, staining of the spindle region was quite evident. In epidermal cells of the stem and along the underside of the leaf there was an intense staining of the vacuolar contents. Guard cells lacked this vacuolar stain. Vacuolar staining was sometimes seen in cells of the stele, but the most distinctive pattern in the stele was associated with young conducting cells of the xylem. These staining patterns are consistent with the idea that the interactions of plastids and the cytoskeletal system may be one of the Ca(2+)-mediated steps in the response of plants to environmental stimuli. Nuclear functions may also be controlled, at least in part, by Ca(2+).

The distribution of cytoplasmic microtubules throughout the cell cycle of the centric diatom Stephanopyxis turris: their role in nuclear migration and positioning the mitotic spindle during cytokinesis

The cell cycle of the marine centric diatom Stephanopyxis turris consists of a series of spatially and temporally well-ordered events. We have used immunofluorescence microscopy to examine the role of cytoplasmic microtubules in these events. At interphase, microtubules radiate out from the microtubule-organizing center, forming a network around the nucleus and extending much of the length and breadth of the cell. As the cell enters mitosis, this network breaks down and a highly ordered mitotic spindle is formed. Peripheral microtubule bundles radiate out from each spindle pole and swing out and away from the central spindle during anaphase. Treatment of synchronized cells with 2.5 X 10(-8) M Nocodazole reversibly inhibited nuclear migration concurrent with the disappearance of the extensive cytoplasmic microtubule arrays associated with migrating nuclei. Microtubule arrays and mitotic spindles that reformed after the drug was washed out appeared normal. In contrast, cells treated with 5.0 X 10(-8) M Nocodazole were not able to complete nuclear migration after the drug was washed out and the mitotic spindles that formed were multipolar. Normal and multipolar spindles that were displaced toward one end of the cell by the drug treatment had no effect on the plane of division during cytokinesis. The cleavage furrow always bisected the cell regardless of the position of the mitotic spindle, resulting in binucleate/anucleate daughter cells. This suggests that in S. turris, unlike animal cells, the location of the plane of division is cortically determined before mitosis.

The cytoskeleton of Cobaea seed hairs: : Patterning during cell-wall differentiation

The cell wall of Cobaea scandens seed hairs developed in a characteristic sequence, with the deposition of a cellulose thread onto a pectic swelling layer was the final event. The cellulose thread was intracellularly accompanied by a band of 10-18 microtubules. During the formation of the swelling layer the microtubules were homogeneously distributed; they ran circumferentially normal to the cell axis. When cellulose-thread formation started, the microtubules became arranged in a helical band. The density of the microtubules varied during the different phases of development. The highest density was observed before cellulosethread formation and ranged from 6-15 μm·μm(-2). The length of the microtubules, 20-30 μm, was determined by direct measurements, as well as estimated from the total microtubular length in a given area and the counted free ends. With the indirect immunofluorescence technique the microtubules of the band stained inhomogeneously. Those which were located at the edges of the band fluoresced more intensely than those of the central part. Attempts to visualize actin filaments in the hair cells with rhodaminyl-conjugated phalloidin resulted in a homogeneous staining of the area of the microtubular band, indicating that actin filaments may be present in this region. Though, in thin sections and dry-cleaved cells, filamentous structures were observed between the microtubules, caution is expressed that the observed fluorescence was, indeed, due to actin filaments. The role of the filamentous structures is discussed with respect to formation and maintenance of the microtubular band. Microtubules apparently did not cross coated pits which were visualized in the plasma membrane through the dry-cleaving technique.

Monoclonal antibody to intermediate filament antigen cross-reacts with higher plant cells

The monoclonal antibody (anti-IFA) raised (Pruss et al., 1981, Cell 27:419-428) against an intermediate filament antigen, which is widespread throughout phylogeny, has been shown here to cross-react with higher plants. On immunoblotting, anti-IFA cross-reacted with proteins in homogenates of carrot suspension cells and of meristematic cells from onion root tips. A 50-kD cross-reactive protein was enriched in a fraction that consisted of detergent-insoluble bundles of 7-nm fibrils from carrot protoplasts (Powell et al., 1982, J. Cell Sci. 56:319-335). By use of indirect immunofluorescence, anti-IFA stained formaldehyde-fixed onion meristematic cells and carrot protoplasts in patterns approximating those obtained with monoclonal anti-tubulins. That anti-IFA was not recognizing plant tubulins was established by use of immunoblots of two-dimensional gels on which the proteins that comprised isolated fibrillar bundles and taxol-purified carrot tubulins had been separated. The two groups of proteins had different positional coordinates: anti-IFA recognized the fibrillar bundle proteins, and anti-tubulins recognized plant microtubule proteins with no cross-reaction to the heterologous proteins. Likewise, formaldehyde-fixed taxol microtubules from carrot cells could be stained with anti-tubulin but not with anti-IFA. It is concluded that an epitope common to intermediate filaments from animals co-distributes with microtubules in higher plant cells.