Cytokinesis in flowering plants: more than one way to divide a cell

Two kinesins are involved in the spatial control of cytokinesis in Arabidopsis thaliana

In plant cells, the plane of division is anticipated at the onset of mitosis by the presence of a preprophase band (PPB) of microtubules and F-actin at a cortical site that circumscribes the nucleus. During cytokinesis, the microtubule- and F-actin-based phragmoplast facilitates construction of a new cell wall and is guided to the forecast division site. Proper execution of this process is essential for establishing the cellular framework of plant tissues. The microtubule binding protein TANGLED1 (TAN1) of maize is a key player in the determination of division planes . Lack of TAN1 leads to misguided phragmoplasts and mispositioned cell walls in maize. In a yeast two-hybrid screen for TAN1-interacting proteins, a pair of related kinesins was identified that shares significant sequence homology with two kinesin-12 genes in Arabidopsis thaliana (A. thaliana): PHRAGMOPLAST ORIENTING KINESIN 1 and 2 (POK1, POK2). POK1 and POK2 are expressed in tissues enriched for dividing cells. The phenotype of pok1;pok2 double mutants strongly resembles that of maize tan1 mutants, characterized by misoriented mitotic cytoskeletal arrays and misplaced cell walls. We propose that POK1 and POK2 participate in the spatial control of cytokinesis, perhaps via an interaction with the A. thaliana TAN1 homolog, ATN.

Silencing of an anther-specific zinc-finger gene, MEZ1, causes aberrant meiosis and pollen abortion in petunia

MEZ1 (MEiosis-associated Zinc-finger protein 1) was first isolated as an anther-specific cDNA from Petunia hybrida. In the present study, we report its functional characterization, including its spatial and temporal expression profiles and phenotypes in MEZ1-silenced plants. MEZ1 transcripts were specifically localized in pollen mother cells during early stages of anther development, and were later distributed in vegetative tissues in anthers. Silencing of MEZ1 by cosuppression resulted in several anomalies during male meiosis that included inability of chromosomes to condense, loss of meiotic synchrony, and premature and apparently uncontrolled cytokinetic events. Consequently, by the end of meiosis 8-10 cells, instead of the normal 4, with varying DNA contents were formed in the MEZ1-silenced meiocytes. Most of these aborted prematurely, and those that matured had a distinctive morphology. MEZ1-silenced plants were female sterile when pollinated with wild-type pollen but they infrequently produced a few seeds upon self-pollination. Resulting T1 plants had increased ploidy levels and exhibited severe anomalies during male meiosis, rendering them completely sterile. We discuss possible role of MEZ1 in the proper progression of plant meiosis.

Midbodies and phragmoplasts: analogous structures involved in cytokinesis

Cytokinesis is an event common to all organisms that involves the precise coordination of independent pathways involved in cell-cycle regulation and microtubule, membrane, actin and organelle dynamics. In animal cells, the spindle midzone/midbody with associated endo-membrane system are required for late cytokinesis events, including furrow ingression and scission. In plants, cytokinesis is mediated by the phragmoplast, an array of microtubules, actin filaments and associated molecules that act as a framework for the future cell wall. In this article (which is part of the Cytokinesis series), we discuss recent studies that highlight the increasing number of similarities in the components and function of the spindle midzone/midbody in animals and the phragmoplast in plants, suggesting that they might be analogous structures.

Asymmetric cell divisions in flowering plants - one mother, "two-many" daughters

Plant development shows a fascinating range of asymmetric cell divisions. Over the years, however, cellular differentiation has been interpreted mostly in terms of a mother cell dividing mitotically to produce two daughter cells of different fates. This popular view has masked the significance of an entirely different cell fate specification pathway, where the mother cell first becomes a coenocyte and then cellularizes to simultaneously produce more than two specialized daughter cells. The "one mother - two different daughters" pathways rely on spindle-assisted mechanisms, such as translocation of the nucleus/spindle to a specific cellular site and orientation of the spindle, which are coordinated with cell-specific allocation of cell fate determinants and cytokinesis. By contrast, during "coenocyte-cellularization" pathways, the spindle-assisted mechanisms are irrelevant since cell fate specification emerges only after the nuclear divisions are complete, and the number of specialized daughter cells produced depends on the developmental context. The key events, such as the formation of a coenocyte and migration of the nuclei to specific cellular locations, are coordinated with cellularization by unique types of cell wall formation. Both one mother - two different daughters and the coenocyte-cellularization pathways are used by higher plants in precise spatial and time windows during development. In both the pathways, epigenetic regulation of gene expression is crucial not only for cell fate specification but also for its maintenance through cell lineage. In this review, the focus is on the coenocyte-cellularization pathways in the context of our current understanding of the asymmetric cell divisions. Instances where cell differentiation does not involve an asymmetric division are also discussed to provide a comprehensive account of cell differentiation.

Quantitative analysis of changes in spatial distribution and plus-end geometry of microtubules involved in plant-cell cytokinesis

The cell plate of higher plants is formed within a ribosome-excluding cell plate assembly matrix. Phragmoplast microtubules facilitate cell-plate formation by forming a scaffold that directs Golgi-derived vesicles to the forming cell plate. Here, we analyse the effects of the cell-plate assembly matrix on phragmoplast microtubule plus-end geometry by electron tomography of cryogenically fixed Arabidopsis meristem cells. Five distinct microtubules plus-end geometries are seen - blunt, extended, horned, flared and hybrid extended/horned. We have quantified and mapped these types of plus-end morphology during the different stages of cell-plate formation and analysed the effects of cell-plate assembly matrix association on microtubule plus-end morphologies. Our results show that somatic-type phragmoplast microtubules do not interdigitate at the cell plate mid-line. The cell-plate assembly matrix is shown to stabilize microtubule plus ends, as evidenced by the fact that of these microtubules that do not terminate in such a matrix, 40-80% are horn-shaped (shrinking), whereas of those that end in such a matrix, 50-70% are blunt (metastable). Also, a third of the blunt-ended microtubules within the cell-plate assembly matrix end at a distance of ∼30 nm from the cell plate.

Plant cytokinesis: fission by fusion

Cytokinesis partitions the cytoplasm of a dividing cell. Unlike yeast and animal cells, which form cleavage furrows from the plasma membrane, cells in higher plants make a new membrane independently of the plasma membrane by homotypic fusion of vesicles. In somatic cells, a plant-specific cytoskeletal array, called a phragmoplast, is thought to deliver vesicles to the plane of division. Vesicle fusion generates a membranous network, the cell plate, which, by fusion of later-arriving vesicles with its margin, expands towards the cell periphery and eventually fuses with the plasma membrane. In this review (part of the Cytokinesis series), I describe recent studies addressing the mechanisms that underlie cell-plate formation and the coordinated dynamics of membrane fusion and cytoskeletal reorganization during progression through cytokinesis.

Localization of the microtubule end binding protein EB1 reveals alternative pathways of spindle development in Arabidopsis suspension cells

In a previous study on Arabidopsis thaliana suspension cells transiently infected with the microtubule end binding protein AtEB1a-green fluorescent protein (GFP), we reported that interphase microtubules grow from multiple sites dispersed over the cortex, with plus ends forming the characteristic comet-like pattern. In this study, AtEB1a-GFP was used to study the transitions of microtubule arrays throughout the division cycle of cells lacking a defined centrosome. During division, the dispersed origin of microtubules was replaced by a more focused pattern with the plus end comets growing away from sites associated with the nuclear periphery. The mitotic spindle then evolved in two quite distinct ways depending on the presence or absence of the preprophase band (PPB): the cells displaying outside-in as well as inside-out mitotic pathways. In those cells possessing a PPB, the fusion protein labeled material at the nuclear periphery that segregated into two polar caps, perpendicular to the PPB, before nuclear envelope breakdown (NEBD). These polar caps then marked the spindle poles upon NEBD. However, in the population of cells without PPBs, there was no prepolarization of material at the nuclear envelope before NEBD, and the bipolar spindle only emerged clearly after NEBD. Such cells had variable spindle orientations and enhanced phragmoplast mobility, suggesting that the PPB is involved in a polarization event that promotes early spindle pole morphogenesis and subsequent positional stability during division. Astral-like microtubules are not usually prominent in plant cells, but they are clearly seen in these Arabidopsis cells, and we hypothesize that they may be involved in orienting the division plane, particularly where the plane is not determined before division.

Cytokinesis in higher plants

Cytokinesis partitions the cytoplasm between two or more nuclei. In higher plants, cytokinesis is initiated by cytoskeleton-assisted targeted delivery of membrane vesicles to the plane of cell division, followed by local membrane fusion to generate tubulo-vesicular networks. This initial phase of cytokinesis is essentially the same in diverse modes of plant cytokinesis whereas the subsequent transformation of the tubulo-vesicular networks into the partitioning membrane may be different between systems. This review focuses on membrane and cytoskeleton dynamics in cell plate formation and expansion during somatic cytokinesis.

Mechanisms of Pattern Formation in Plant Embryogenesis

Many of the patterning mechanisms in plants were discovered while studying postembryonic processes and resemble mechanisms operating during animal development. The emergent role of the plant hormone auxin, however, seems to represent a plant-specific solution to multicellular patterning. This review summarizes our knowledge on how diverse mechanisms that were first dissected at the postembryonic level are now beginning to provide an understanding of plant embryogenesis.

Cytokinesis: lines of division taking shape

Plant cytokinesis requires an orchestrated interplay of membrane and cytoskeleton dynamics, which results in the formation of the membrane that partitions the cytoplasm of the dividing cell. Until recently, phragmoplast-assisted cytokinesis of somatic cells was regarded as mechanistically different from 'non-conventional' modes of cytokinesis, such as endosperm cellularisation or male meiotic cytokinesis. However, features that are similar among these diverse modes of cytokinesis have now been revealed by electron tomography, suggesting common underlying mechanisms that are also supported by genetic and molecular studies. Further insight into the complex process of cytokinesis has been gained from the identification of new components and from the analysis of known components.

Patellin1, a novel Sec14-like protein, localizes to the cell plate and binds phosphoinositides

Membrane trafficking is central to construction of the cell plate during plant cytokinesis. Consequently, a detailed understanding of the process depends on the characterization of molecules that function in the formation, transport, targeting, and fusion of membrane vesicles to the developing plate, as well as those that participate in its consolidation and maturation into a fully functional partition. Here we report the initial biochemical and functional characterization of patellin1 (PATL1), a novel cell-plate-associated protein that is related in sequence to proteins involved in membrane trafficking in other eukaryotes. Analysis of the Arabidopsis genome indicated that PATL1 is one of a small family of Arabidopsis proteins, characterized by a variable N-terminal domain followed by two domains found in other membrane-trafficking proteins (Sec14 and Golgi dynamics domains). Results from immunolocalization and biochemical fractionation studies suggested that PATL1 is recruited from the cytoplasm to the expanding and maturing cell plate. In vesicle-binding assays, PATL1 bound to specific phosphoinositides, important regulators of membrane trafficking, with a preference for phosphatidylinositol(5)P, phosphatidylinositol(4,5)P(2), and phosphatidylinositol(3)P. Taken together, these findings suggest a role for PATL1 in membrane-trafficking events associated with cell-plate expansion or maturation and point to the involvement of phosphoinositides in cell-plate biogenesis.

Occurrence, metabolism, and prospective functions of N-acylethanolamines in plants

N-Acylethanolamines (NAEs) are fatty acid amides that are derived from an N-acylated phoshatidylethanolamine presursor, a minor membrane lipid constituent of plant and animal cells. Historically, the formation of N-acylethanolamines was associated with cellular stress and tissue damage in mammals, but more recently has been shown to be part of the endocannabinoid signaling system that regulates a variety of normal physiological functions, including neurotransmission, immune responses, vasodilation, embryo development and implantation, feeding behavior, cell proliferation, etc. The widespread regulation of vertebrate physiology by this class of lipid mediators and the conservation of the mechanisms for NAE formation, perception and degradation in higher plants raises the possibility that the metabolism of NAEs represents an evolutionarily conserved lipid signaling pathway that regulates an array of physiological processes in multicellular eukaryotes. Here the recent information on NAEs in plants is reviewed in the context of the occurrence, metabolism and functions of this bioactive class of lipid mediators.

gemini pollen 2, a male and female gametophytic cytokinesis defective mutation

Gametophytic cytokinesis is essential for the development and function of the male and female gametophytes. We have previously described the isolation and characterisation of the gemini pollen 1 (gem1) that acts gametophytically to disturb asymmetric division and cytokinesis at pollen mitosis I in Arabidopsis. Here we describe the genetic and cytological analysis of an independent gametophytic mutant, gem2, with similar characteristics to gem1, but which maps to a different genetic locus. gem2 shows reduced genetic transmission through both male and female gametes and leads to the production of divided or twin-celled pollen. Developmental analysis revealed that gem2 does not affect karyokinesis at pollen mitosis I, but leads to repositioning of the cell plate and partial or complete failure of cytokinesis, resulting in symmetrical divisions or binucleate pollen grains respectively. Symmetrical divisions lead to altered pollen cell fate with both sister cells displaying vegetative cell fate. Moreover, we demonstrate that the predominant female defect in gem2 is a lack of cellularization of the embryo sac during megagametogenesis. GEM2 therefore defines an independent genetic locus that is involved in the correct specification of both male and female gametophytic cytokinesis.

Transient exposure to ethylene stimulates cell division and alters the fate and polarity of hypocotyl epidermal cells

After transient exposure to the gaseous hormone ethylene, dark-grown cucumber (Cucumis sativus) hypocotyls developed unusual features. Upon ethylene's removal, the developing epidermis showed significant increases in cell division rates, producing an abundance of guard cells and trichomes. These responses to ethylene depended on the stage of development at the time of ethylene exposure. In the upper region of the hypocotyl, where cells were least differentiated at the onset of ethylene treatment, complex, multicellular protuberances formed. Further down the hypocotyl, where stomata and trichomes were beginning to develop at the onset of ethylene exposure, an increase in the number of stomata and trichomes was observed. Stomatal complexes developing after the ethylene treatment had a significant increase in the number of stomatal subsidiary cells and the number of cells per trichome increased. Analysis of division patterns in stomatal complexes indicated that exposure to ethylene either suspended or altered cell fate. Ethylene also altered cell division polarity, resulting in aberrant stomatal complexes and branched trichomes. To our knowledge, the results of this study demonstrate for the first time that transient treatment with physiological concentrations of ethylene can alter cell fate and increase the propensity of cells to divide.

Elevated levels of N-lauroylethanolamine, an endogenous constituent of desiccated seeds, disrupt normal root development in Arabidopsis thaliana seedlings

N-Acylethanolamines (NAEs) are prevalent in desiccated seeds of various plant species, and their levels decline substantially during seed imbibition and germination. Here, seeds of Arabidopsis thaliana (L.) Heynh. were germinated in, and seedlings maintained on, micromolar concentrations of N-lauroylethanolamine (NAE 12:0). NAE 12:0 inhibited root elongation, increased radial swelling of root tips, and reduced root hair numbers in a highly selective and concentration-dependent manner. These effects were reversible when seedlings were transferred to NAE-free medium. Older seedlings (14 days old) acclimated to exogenous NAE by increased formation of lateral roots, and generally, these lateral roots did not exhibit the severe symptoms observed in primary roots. Cells of NAE-treated primary roots were swollen and irregular in shape, and in many cases showed evidence, at the light- and electron-microscope levels, of improper cell wall formation. Microtubule arrangement was disrupted in severely distorted cells close to the root tip, and endoplasmic reticulum (ER)-localized green fluorescent protein (mGFP5-ER) was more abundant, aggregated and distributed differently in NAE-treated root cells, suggesting disruption of proper cell division, endomembrane organization and vesicle trafficking. These results suggest that NAE 12:0 likely influences normal cell expansion in roots by interfering with intracellular membrane trafficking to and/or from the cell surface. The rapid metabolism of NAEs during seed imbibition/germination may be a mechanism to remove this endogenous class of lipid mediators to allow for synchronized membrane reorganization associated with cell expansion.

Acentrosomal microtubule nucleation in higher plants

Higher plants have developed a unique pathway to control their cytoskeleton assembly and dynamics. In most other eukaryotes, microtubules are nucleated in vivo at the nucleation and organizing centers and are involved in the establishment of polarity. Although the major cytoskeletal components are common to plant and animal cells, which suggests conserved regulation mechanisms, plants do not possess centrosome-like organelles. Nevertheless, they are able to build spindles and have developed their own specific cytoskeletal arrays: the cortical arrays, the preprophase band, and the phragmoplast, which all participate in basic developmental processes, as shown by defective mutants. New approaches provide essential clues to understanding the fundamental mechanisms of microtubule nucleation. Gamma-tubulin, which is considered to be the universal nucleator, is the essential component of microtubule-nucleating complexes identified as gamma-tubulin ring complexes (gamma-TuRC) in centriolar cells. A gamma-tubulin small complex (gamma-TuSC) forms a minimal nucleating unit recruited at specific sites of activity. These components--gamma-tubulin, Spc98p, and Spc97p--are present in higher plants. They play a crucial role in microtubule nucleation at the nuclear surface, which is known as the main functional plant microtubule-organizing center, and also probably at the cell cortex and at the phragmoplast, where secondary nucleation sites may exist. Surprisingly, plant gamma-tubulin is distributed along the microtubule length. As it is not associated with Spc98p, it may not be involved in microtubule nucleation, but may preferably control microtubule dynamics. Understanding the mechanisms of microtubule nucleation is the major challenge of the current research.

Alteration of microtubule dynamic instability during preprophase band formation revealed by yellow fluorescent protein-CLIP170 microtubule plus-end labeling

At the onset of mitosis, plant cells form a microtubular preprophase band that defines the plane of cell division, but the mechanism of its formation remains a mystery. Here, we describe the use of mammalian yellow fluorescent protein-tagged CLIP170 to visualize the dynamic plus ends of plant microtubules in transfected cowpea protoplasts and in stably transformed and dividing tobacco Bright Yellow 2 cells. Using plus-end labeling, we observed dynamic instability in different microtubular conformations in live plant cells. The interphase plant microtubules grow at 5 micro m/min, shrink at 20 micro m/min, and display catastrophe and rescue frequencies of 0.02 and 0.08 events/s, respectively, exhibiting faster turnover than their mammalian counterparts. Strikingly, during preprophase band formation, the growth rate and catastrophe frequency of plant microtubules double, whereas the shrinkage rate and rescue frequency remain unchanged, making microtubules shorter and more dynamic. Using these novel insights and four-dimensional time-lapse imaging data, we propose a model that can explain the mechanism by which changes in microtubule dynamic instability drive the dramatic rearrangements of microtubules during preprophase band and spindle formation in plant cells.

Plant cell cycle: cellularisation of the endoderm needs spätzle

Early development of the endosperm of flowering plants involves the formation of a syncytium through successive rounds of nuclear division without cell wall synthesis. New data reveal that cellularisation of this syncytium requires the SPATZLE protein and involves the formation of cell walls similar to those of somatic tissues.

Cellularisation in the endosperm of Arabidopsis thaliana is coupled to mitosis and shares multiple components with cytokinesis

Distinct forms of cytokinesis characterise specific phases of development in plants. In Arabidopsis, as in many other species, the endosperm that nurtures the embryo in the seed initially develops as a syncytium. This syncytial phase ends with simultaneous partitioning of the multinucleate cytoplasm into individual cells, a process referred to as cellularisation. Our in vivo observations show that, as in cytokinesis, cellularisation of the Arabidopsis endosperm is coupled to nuclear division. A genetic analysis reveals that most Arabidopsis mutations affecting cytokinesis in the embryo also impair endosperm cellularisation. These results imply that cellularisation and cytokinesis share multiple components of the same basic machinery. We further report the identification of mutations in a novel gene, SPATZLE, that specifically interfere with cellularisation of the endosperm, but not with cytokinesis in the embryo. The analysis of this mutant might identify a specific checkpoint for the onset of cellularisation.

Tubulin folding cofactors: half a dozen for a dimer

Tubulin folding cofactors control the availability of tubulin subunits and microtubule stability in eukaryotic cells. Recent work on Arabidopsis mutants has provided a new experimental system for understanding the cellular functions of tubulin folding cofactors.

Membrane trafficking during plant cytokinesis

Plant morphogenesis is regulated by cell division and expansion. Cytokinesis, the final stage of cell division, culminates in the construction of the cell plate, a unique cytokinetic membranous organelle that is assembled across the inside of the dividing cell. Both during cell-plate formation and cell expansion, the secretory pathway is highly active and is polarized toward the plane of division or toward the plasma membrane, respectively. In this review, we discuss results from recent genetic and biochemical research directed toward understanding the molecular events occurring during cytokinesis and cell expansion, including data supporting the idea that during cytokinesis one or more exocytic pathways are polarized toward the division plane. We will also highlight recent evidence for the roles of secretory vesicle transport and cytoskeletal machinery in cell-plate membrane trafficking and fusion.

Two new loci, PLEIADE and HYADE, implicate organ-specific regulation of cytokinesis in Arabidopsis

In screens for regulators of root morphogenesis in Arabidopsis we isolated six new recessive mutants with irregular cell expansion. Complementation analyses placed the mutations in two loci, PLEIADE (PLE) and HYADE (HYA). Phenotypic analyses revealed multinucleated cells, cell wall stubs, and synchronized cell divisions in incompletely separated cells that are all characteristics of defective cytokinesis. These defects were pronounced in roots and undetectable in aerial organs. In addition, fertility and germination were not affected by the mutations. Thus, the alleles that we have isolated of PLE and HYA suggest that the genes may encode organ-specific components needed primarily during root development. Analysis of microtubule arrays during cell cycle in ple and hya roots indicates that the presence of several synchronized nuclei influences the position of preprophase band, mitotic spindles, and phragmoplasts. The enhanced and synergistic phenotype of PLE/ple.hya/hya seedlings and double mutants point to a role of PLE and HYA in the same process. These mutants provide tools to elucidate the regulation of nuclear cytoskeletal interactions during cell division and cytokinesis.

Cytokinesis-defective mutants of Arabidopsis

We have identified mutations in six previously uncharacterized genes of Arabidopsis, named club, bublina, massue, rod, bloated, and bims, that are required for cytokinesis. The mutants are seedling lethal, have morphological abnormalities, and are characterized by cell wall stubs, gapped walls, and multinucleate cells. In these and other respects, the new mutants are phenotypically similar to knolle, keule, hinkel, and pleiade mutants. The mutants display a gradient of stomatal phenotypes, correlating roughly with the severity of their cytokinesis defect. Similarly, the extent to which the different mutant lines were capable of growing in tissue culture correlated well with the severity of the cytokinesis defect. Phenotypic analysis of the novel and previously characterized loci indicated that the secondary consequences of a primary defect in cytokinesis include anomalies in body organization, organ number, and cellular differentiation, as well as organ fusions and perturbations of the nuclear cycle. Two of the 10 loci are required for both cytokinesis and root hair morphogenesis. The results have implications for the identification of novel cytokinesis genes and highlight the mechanistic similarity between cytokinesis and root hair morphogenesis, two processes that result in a rapid deposition of new cell walls via polarized secretion.

The Arabidopsis PILZ group genes encode tubulin-folding cofactor orthologs required for cell division but not cell growth

Plant microtubules are organized into specific cell cycle-dependent arrays that have been implicated in diverse cellular processes, including cell division and organized cell expansion. Mutations in four Arabidopsis genes collectively called the PILZ group result in lethal embryos that consist of one or a few grossly enlarged cells. The mutant embryos lack microtubules but not actin filaments. Whereas the cytokinesis-specific syntaxin KNOLLE is not localized properly, trafficking of the putative auxin efflux carrier PIN1 to the plasma membrane is normal. The four PILZ group genes were isolated by map-based cloning and are shown to encode orthologs of mammalian tubulin-folding cofactors (TFCs) C, D, and E, and associated small G-protein Arl2 that mediate the formation of alpha/beta-tubulin heterodimers in vitro. The TFC C ortholog, PORCINO, was detected in cytosolic protein complexes and did not colocalize with microtubules. Another gene with a related, although weaker, embryo-lethal phenotype, KIESEL, was shown to encode a TFC A ortholog. Our genetic ablation of microtubules shows their requirement in cell division and vesicle trafficking during cytokinesis, whereas cell growth is mediated by microtubule-independent vesicle trafficking to the plasma membrane during interphase.

Microtubule organization in the green kingdom: chaos or self-order?

Plant microtubule arrays differ fundamentally from their animal, fungal and protistan counterparts. These differences largely reflect the requirements of plant composite polymer cell walls and probably also relate to the acquisition of chloroplasts. Plant microtubules are usually dispersed and lack conspicuous organizing centres. The key to understanding this dispersed nature is the identification of proteins that interact with and regulate the spatial and dynamic properties of microtubules. Over the past decade, a number of these proteins have been uncovered, including numerous kinesin-related proteins and a 65 kDa class of structural microtubule-associated proteins that appear to be unique to plants. Mutational analysis has identified MOR1, a probable stabilizer of microtubules that is a homologue of the TOGp-XMAP215 class of high-molecular-weight microtubule-associated proteins, and a katanin p60 subunit homologue implicated in the severing of microtubules. The identification of these two proteins provides new insights into the mechanisms controlling microtubule assembly and dynamics, particularly in the dispersed cortical array found in highly polarized plant cells.

Plant cytokinesis: motoring to the finish

Cytokinesis in plant cells involves a microtubule-containing structure, the phragmoplast, which guides the formation of new cell walls. Recent studies have identified kinesin-like proteins that appear to play a variety of roles in plant cytokinesis.

Polarized cytokinesis in vacuolate cells of Arabidopsis

The view of plant-cell cytokinesis commonly depicted in textbooks is of a symmetrical process, with the phragmoplast initiating in the center of the cell and growing outward to the parental cell membrane. In contrast to this picture, we observe that cell-plate development in Arabidopsis shoot cells is highly polarized along the plane of division. Three-dimensional live-cell imaging reveals that the mitotic spindle and phragmoplast are laterally displaced, and that the growing cell plate anchors on one side of the cell at an early stage of cytokinesis. Growth of phragmoplast across the cell creates a new partition in its wake, giving the visual effect of a curtain being pulled across the cell. Throughout this process, the advancing front of the phragmoplast is in intimate contact with the parental wall, suggesting that short-range interactions between the phragmoplast and plasma membrane may play important roles in guiding the cell plate throughout much of its development. Polarized cytokinesis was observed in a wide variety of vacuolate shoot cells and in some small root cells, implying that it is not solely a function of cell size. This mode of cytokinesis may provide a mechanically robust mechanism for cell-plate formation in large cells and suggests a simple explanation for the occurrence of cell wall stubs observed upon drug treatment or in cytokinetic mutants.

The Arabidopsis HINKEL gene encodes a kinesin-related protein involved in cytokinesis and is expressed in a cell cycle-dependent manner

Plant cytokinesis starts in the center of the division plane, with vesicle fusion generating a new membrane compartment, the cell plate, that subsequently expands laterally by continuous fusion of newly arriving vesicles to its margin. Targeted delivery of vesicles is assisted by the dynamic reorganization of a plant-specific cytoskeletal array, the phragmoplast, from a solid cylinder into an expanding ring-shaped structure. This lateral translocation is brought about by depolymerization of microtubules in the center, giving way to the expanding cell plate, and polymerization of microtubules along the edge. Whereas several components are known to mediate cytokinetic vesicle fusion [8-10], no gene function involved in phragmoplast dynamics has been identified by mutation. Mutations in the Arabidopsis HINKEL gene cause cytokinesis defects, such as enlarged cells with incomplete cell walls and multiple nuclei. Proper targeting of the cytokinesis-specific syntaxin KNOLLE [8] and lateral expansion of the phragmoplast are not affected. However, the phragmoplast microtubules appear to persist in the center, where vesicle fusion should result in cell plate formation. Molecular analysis reveals that the HINKEL gene encodes a plant-specific kinesin-related protein with a putative N-terminal motor domain and is expressed in a cell cycle-dependent manner similar to the KNOLLE gene. Our results suggest that HINKEL plays a role in the reorganization of phragmoplast microtubules during cell plate formation.

Functional characterization of the KNOLLE-interacting t-SNARE AtSNAP33 and its role in plant cytokinesis

Cytokinesis requires membrane fusion during cleavage-furrow ingression in animals and cell plate formation in plants. In Arabidopsis, the Sec1 homologue KEULE (KEU) and the cytokinesis-specific syntaxin KNOLLE (KN) cooperate to promote vesicle fusion in the cell division plane. Here, we characterize AtSNAP33, an Arabidopsis homologue of the t-SNARE SNAP25, that was identified as a KN interactor in a yeast two-hybrid screen. AtSNAP33 is a ubiquitously expressed membrane-associated protein that accumulated at the plasma membrane and during cell division colocalized with KN at the forming cell plate. A T-DNA insertion in the AtSNAP33 gene caused loss of AtSNAP33 function, resulting in a lethal dwarf phenotype. atsnap33 plantlets gradually developed large necrotic lesions on cotyledons and rosette leaves, resembling pathogen-induced cellular responses, and eventually died before flowering. In addition, mutant seedlings displayed cytokinetic defects, and atsnap33 in combination with the cytokinesis mutant keu was embryo lethal. Analysis of the Arabidopsis genome revealed two further SNAP25-like proteins that also interacted with KN in the yeast two-hybrid assay. Our results suggest that AtSNAP33, the first SNAP25 homologue characterized in plants, is involved in diverse membrane fusion processes, including cell plate formation, and that AtSNAP33 function in cytokinesis may be replaced partially by other SNAP25 homologues.

Three-dimensional analysis of syncytial-type cell plates during endosperm cellularization visualized by high resolution electron tomography

The three-dimensional architecture of syncytial-type cell plates in the endosperm of Arabidopsis has been analyzed at approximately 6-nm resolution by means of dual-axis high-voltage electron tomography of high-pressure frozen/freeze-substituted samples. Mini-phragmoplasts consisting of microtubule clusters assemble between sister and nonsister nuclei. Most Golgi-derived vesicles appear connected to these microtubules by two molecules that resemble kinesin-like motor proteins. These vesicles fuse with each other to form hourglass-shaped intermediates, which become wide (approximately 45 nm in diameter) tubules, the building blocks of wide tubular networks. New mini-phragmoplasts also are generated de novo around the margins of expanding wide tubular networks, giving rise to new foci of cell plate growth, which later become integrated into the main cell plate. Spiral-shaped rings of the dynamin-like protein ADL1A constrict but do not fission the wide tubules at irregular intervals. These rings appear to maintain the tubular geometry of the network. The wide tubular network matures into a convoluted fenestrated sheet in a process that involves increases of 45 and 130% in relative membrane surface area and volume, respectively. The proportionally larger increase in volume appears to reflect callose synthesis. Upon fusion with the parental plasma membrane, the convoluted fenestrated sheet is transformed into a planar fenestrated sheet. This transformation involves clathrin-coated vesicles that reduce the relative membrane surface area and volume by approximately 70%. A ribosome-excluding matrix encompasses the cell plate membranes from the fusion of the first vesicles until the onset of the planar fenestrated sheet formation. We postulate that this matrix contains the molecules that mediate cell plate assembly.

CYTOKINESIS AND BUILDING OF THE CELL PLATE IN PLANTS

Cytokinesis in plant cells is more complex than in animals, as it involves building a cell plate as the final step in generating two cells. The cell plate is built in the center of phragmoplast by fusion of Golgi-derived vesicles. This step imposes an architectural problem where ballooning of the fused structures has to be avoided to create a plate instead. This is apparently achieved by squeezing the vesicles into dumbbell-shaped vesicle-tubule-vesicle (VTV) structures with the help of phragmoplastin, a homolog of dynamin. These structures are fused at their ends in a star-shaped body creating a tubulovesicular "honeycomb-like" structure sandwiched between the positive ends of the phragmoplast microtubules. This review summarizes our current understanding of various mechanisms involved in budding-off of Golgi vesicles, delivery and fusion of vesicles to initiate cell plate, and the synthesis of polysaccharides at the forming cell plate. Little is known about the molecular mechanisms involved in determining the site, direction, and the point of attachment of the growing cell plate with the parental cell wall. These gaps may be filled soon, as many genes that have been identified by mutations are analyzed and functions of their products are deciphered.

SECuring the perimeter

Exquisitely regulated trafficking and fusion of vesicles is crucial for proper cell function. The molecules that regulate vesicle fusion are highly conserved among eukaryotes, but they have also undergone expansion and specialization within single genomes. With diversity comes the potential for functions in unique cell processes, and recent work in Arabidopsis reveals how a member of the SEC1 family, KEULE, functions in plant cell cytokinesis.

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.

Plant cell division: building walls in the right places

Plant cells are surrounded by walls that define their shapes and fix their positions with tissues. Consequently, establishment of a plant's cellular framework during development depends largely on the positions in which new walls are formed during cytokinesis. Experiments using various approaches are now building on classical studies to shed light on the mechanisms underlying the spatial control of cytokinesis.