Tangled1: a microtubule binding protein required for the spatial control of cytokinesis in maize

BRICK1/HSPC300 functions with SCAR and the ARP2/3 complex to regulate epidermal cell shape in Arabidopsis

The Arp2/3 complex, a highly conserved nucleator of F-actin polymerization,is essential for a variety of eukaryotic cellular processes, including epidermal cell morphogenesis in Arabidopsis thaliana. Efficient nucleation of actin filaments by the Arp2/3 complex requires the presence of an activator such as a member of the Scar/WAVE family. In mammalian cells, a multiprotein complex consisting of WAVE, PIR121/Sra-1, Nap1, Abi-2 and HSPC300 mediates responsiveness of WAVE to upstream regulators such as Rac. Essential roles in WAVE complex assembly or function have been demonstrated for PIR121/Sra-1, Nap1 and Abi-2, but the significance of HSPC300 in this complex is unclear. Plant homologs of all mammalian WAVE complex components have been identified, including HSPC300, the mammalian homolog of maize BRICK1 (BRK1). We show that, like mutations disrupting the Arabidopsis homologs of PIR121/Sra-1, Nap1 and Scar/WAVE, mutations in the Arabidopsis BRK1gene result in trichome and pavement cell morphology defects (and associated alterations in the F-actin cytoskeleton of expanding cells) similar to those caused by mutations disrupting the ARP2/3 complex itself. Analysis of double mutants provides genetic evidence that BRK1 functions in a pathway with the ARP2/3 complex. BRK1 is required for accumulation of SCAR1 protein in vivo,potentially explaining the apparently essential role of BRK1 in ARP2/3 complex function.

A maize defective-kernel mutant (longcell) characterized by tubular cells, severe morphological alterations and induction of cell death

Seeds of the longcell mutant in maize (Zea mays L) have a defective-kernel phenotype: the embryo aborts at the early coleoptilar stage and the endosperm is reduced in size. Mutant embryos have severe alterations in morphogenesis. They have a suspensor-, an embryo axis- and a scutellum-like structure, but the shoot apical meristem (SAM) is not formed. Scanning electron microscopy showed that most of the cells in longcell embryos are tubular and abnormally enlarged. The level of expression of several genes involved in basic metabolism is not severely affected during early and mid embryogenesis, but storage molecule accumulation is reduced. Genes which in normal conditions are only expressed after germination, are expressed during kernel development in the longcell seeds. Mutant embryos undergo cell death in late embryogenesis. Nuclei in dying embryos are TUNEL positive, and different genes coding for hydrolytic enzymes are up-regulated. The expression of genes related to oxidative stress is also altered in longcell embryos. These results lead us to suggest that the longcell mutant may be cytokinesis-defective.

Narrowing of the preprophase microtubule band is not required for cell division plane determination in cultured plant cells

In most higher-plant cells, cortical microtubules form a tightly focused preprophase band (PPB) that disappears with the onset of prometaphase, but whose location defines the future location of the cell plate at the end of cytokinesis. It is unclear whether the PPB microtubules themselves designate the precise area where the cell plate will insert, or rather if these microtubules are responding to a hierarchical signal(s). Here we show that narrowing of the microtubules within the PPB zone is not necessary for proper division plane determination. In cultured tobacco BY-2 cells in which PPB microtubules are depolymerized, the phragmoplast can still accurately locate and insert at the proper site. The data do not support a role for PPB microtubule narrowing in focusing the signal that is used later by the phragmoplast to position the cell plate; rather, proper phragmoplast positioning is more likely a consequence of a non-microtubule positional element. Although the PPB microtubules do not directly mark the division site, we show that they are required for accurate spindle positioning, an activity that presets the future growth trajectory of the phragmoplast and is necessary for insuring high-fidelity cell plate positioning.

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.

Dissecting defense-related and developmental transcriptional responses of maize during Ustilago maydis infection and subsequent tumor formation

Infection of maize (Zea mays) plants with the smut fungus Ustilago maydis triggers the formation of tumors on aerial parts in which the fungal life cycle is completed. A differential display screen was performed to gain insight into transcriptional changes of the host response. Some of the genes strongly up-regulated in tumors showed a pronounced developmental expression pattern with decreasing transcript levels from basal to apical shoot segments, suggesting that U. maydis has the capacity to extend the undifferentiated state of maize plants. Differentially expressed genes implicated in secondary metabolism were Bx1, involved in biosynthesis of the cyclic hydroxamic acid 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3-one, and a novel putative sesquiterpene cyclase gene U. maydis induced (Umi)2. Together with the up-regulation of Umi11 encoding a cyclotide-like protein this suggests a nonconventional induction of plant defenses. Explicitly, U. maydis was resistant to 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3-one but susceptible to its benzoxazolinone derivative 6-methoxy-2-benzoxazolinone. Infection studies of isolated leaves with U. maydis and Colletotrichum graminicola provided evidence for coregulation of Umi2 and PR-1 gene expression, with mRNA levels strongly determined by the extent of fungal colonization within tissue. However, in contrast to Umi2, transcript levels of PR-1 remained low in plants infected with wild-type U. maydis but were 8-fold elevated upon infection with an U. maydis mutant strongly attenuated in pathogenic development. This suggests that U. maydis colonization in planta suppresses a classical defense response. Furthermore, comparative expression analysis uncovered distinct transcriptional programs operating in the host in response to fungal infection and subsequent tumor formation.

Identification of a novel family of 70 kDa microtubule-associated proteins in Arabidopsis cells

Most plant microtubule-associated proteins (MAPs) have homologues across the phylogenetic spectrum. To find potential plant-specific MAPs that will have evaded bioinformatic searches we devised a low stringency method for isolating proteins from an Arabidopsis cell suspension on endogenous taxol-microtubules. By tryptic peptide mass fingerprinting we identified 55 proteins that were enriched on taxol-microtubules. Amongst a range of known MAPs, such as kinesins, MAP65 isoforms and MOR1, we detected 'unknown' 70 kDa proteins that belong to a family of five closely related Arabidopsis proteins having no known homologues amongst non-plant organisms. To verify that AtMAP70-1 associates with microtubules in vivo, it was expressed as a GFP fusion. This confirmed that the protein decorates all four microtubule arrays in both transiently infected Arabidopsis and stably transformed tobacco BY-2 suspension cells. Microtubule-directed drugs perturbed the localization of AtMAP70-1 but cytochalasin D did not. AtMAP70-1 contains four predicted coiled-coil domains and truncation studies identified a central domain that targets the fusion protein to microtubules in vivo. This study therefore introduces a novel family of plant-specific proteins that interact with microtubules.

Microtubule plus-ends reveal essential links between intracellular polarization and localized modulation of endocytosis during division-plane establishment in plant cells

BACKGROUND

A key event in plant morphogenesis is the establishment of a division plane. A plant-specific microtubular preprophase band (PPB) accurately predicts the line of cell division, whereas the phragmoplast, another plant-specific array, executes cell division by maintaining this predicted line. Although establishment of these specific arrays apparently involves intracellular repolarization events that focus cellular resources to a division site, it still remains unclear how microtubules position the cell division planes. Here we study GFP-AtEB1 decorated microtubule plus-ends to dissect events at the division plane.

RESULTS

Early mitotic events included guided growth of endoplasmic microtubules (EMTs) towards the PPB site and their coincident localization with endocytic vesicles. Consequently, an endosomal belt lay in close proximity to the microtubular PPB at its maturation and was maintained during spindle formation. During cytokinesis, EMTs radiated from the former spindle poles in a geometrical conformation correlating with cell-plate navigation and tilt-correction. Naphthylphtalamic acid (NPA), an inhibitor of polar auxin efflux, caused abnormal PPBs and shifted division planes.

CONCLUSION

Our observations reveal a spatio-temporal link between microtubules and intracellular polarization essential for localized endocytosis and precise establishment of the division plane in plants. Additionally, they implicate the growth regulator, auxin, in this important cellular event.

Plant-specific microtubule-associated protein SPIRAL2 is required for anisotropic growth in Arabidopsis

In diffusely growing plant cells, cortical microtubules play an important role in regulating the direction of cell expansion. Arabidopsis (Arabidopsis thaliana) spiral2 (spr2) mutant is defective in directional cell elongation and exhibits right-handed helical growth in longitudinally expanding organs such as root, hypocotyl, stem, petiole, and petal. The growth of spr2 roots is more sensitive to microtubule-interacting drugs than is wild-type root growth. The SPR2 gene encodes a plant-specific 94-kD protein containing HEAT-repeat motifs that are implicated in protein-protein interaction. When expressed constitutively, SPR2-green fluorescent protein fusion protein complemented the spr2 mutant phenotype and was localized to cortical microtubules as well as other mitotic microtubule arrays in transgenic plants. Recombinant SPR2 protein directly bound to taxol-stabilized microtubules in vitro. Furthermore, SPR2-specific antibody and mass spectrometry identified a tobacco (Nicotiana tabacum) SPR2 homolog in highly purified microtubule-associated protein fractions from tobacco BY-2 cell cultures. These results suggest that SPR2 is a novel microtubule-associated protein and is required for proper microtubule function involved in anisotropic growth.

Between the sheets: inter-cell-layer communication in plant development

The cells of plant meristems and embryos are arranged in an organized, and sometimes extremely beautiful, layered pattern. This pattern is maintained by the controlled orientation of cell divisions within layers. However, despite this layered structure, cell behaviour during plant development is not lineage dependent, and does not occur in a mosaic fashion. Many studies, both classical and recent, have shown that plant cell identity can be re-specified according to position, allowing plants to show remarkable developmental plasticity. However, the layered structure of meristems and the implications of this during plant development, remain subjects of some speculation. Of particular interest is the question of how cell layers communicate, and how communication between cell layers could allow coordinated developmental processes to take place. Recent research has uncovered several examples both of the molecular mechanisms by which cell layers can communicate, and of how this communication can infringe on developmental processes. A range of examples is used to illustrate the diversity of mechanisms potentially implicated in cell-layer communication during plant development.

Disorganization of cortical microtubules stimulates tangential expansion and reduces the uniformity of cellulose microfibril alignment among cells in the root of Arabidopsis

To test the role of cortical microtubules in aligning cellulose microfibrils and controlling anisotropic expansion, we exposed Arabidopsis thaliana roots to moderate levels of the microtubule inhibitor, oryzalin. After 2 d of treatment, roots grow at approximately steady state. At that time, the spatial profiles of relative expansion rate in length and diameter were quantified, and roots were cryofixed, freeze-substituted, embedded in plastic, and sectioned. The angular distribution of microtubules as a function of distance from the tip was quantified from antitubulin immunofluorescence images. In alternate sections, the overall amount of alignment among microfibrils and their mean orientation as a function of position was quantified with polarized-light microscopy. The spatial profiles of relative expansion show that the drug affects relative elongation and tangential expansion rates independently. The microtubule distributions averaged to transverse in the growth zone for all treatments, but on oryzalin the distributions became broad, indicating poorly organized arrays. At a subcellular scale, cellulose microfibrils in oryzalin-treated roots were as well aligned as in controls; however, the mean alignment direction, while consistently transverse in the controls, was increasingly variable with oryzalin concentration, meaning that microfibril orientation in one location tended to differ from that of a neighboring location. This conclusion was confirmed by direct observations of microfibrils with field-emission scanning electron microscopy. Taken together, these results suggest that cortical microtubules ensure microfibrils are aligned consistently across the organ, thereby endowing the organ with a uniform mechanical structure.

Interactions of tobacco microtubule-associated protein MAP65-1b with microtubules

Tobacco microtubule associated protein (MAP65) (NtMAP65s) constitute a family of microtubule-associated proteins with apparent molecular weight around 65 kDa that collectively induce microtubule bundling and promote microtubule assembly in vitro. They are associated with most of the tobacco microtubule arrays in situ. Recently, three NtMAP65s belonging to the NtMAP65-1 subfamily have been cloned. Here we investigated in vitro the biochemical properties of one member of this family, the tobacco NtMAP65-1b. We demonstrated that recombinant NtMAP65-1b is a microtubule-binding and a microtubule-bundling protein. NtMAP65-1b has no effect on microtubule polymerization rate and binds microtubules with an estimated equilibrium constant of dissociation (K(d)) of 0.57 micro m. Binding of NtMAP65-1b to microtubules occurs through the carboxy-terminus of tubulin, as NtMAP65-1b was no longer able to bind subtilisin-digested tubulin. In vitro, NtMAP65-1b stabilizes microtubules against depolymerization induced by cold, but not against katanin-induced destabilization. The biological implications of these results are discussed.

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.

Plant Cytoskeleton: Reinforcing Lines of Division in Plant Cells

Cytokinesis in plants has unique features concerned with defining and maintaining the line of cell division. Recent studies have identified key cytoskeletal components and events that help to ensure the fidelity of cytokinesis in higher plants.

The maize disorganized aleurone layer 1 and 2 ( dil1, dil2) mutants lack control of the mitotic division plane in the aleurone layer of developing endosperm

The maize ( Zea mays L.) endosperm consists of an epidermal like layer of isodiametric aleurone cells surrounding a central body of starchy endosperm cells. In disorgal1 ( dil1) and disorgal2 ( dil2) mutants the control of the mitotic division plane is relaxed or missing, resulting in mature grains with disorganized aleurone layers. In addition to orientation of the division plane, both the shape and size of the aleurone cells are affected, and often more than one layer of aleurone cells is present. Homozygous dil1 and dil2 grains are shrunken due to reduced accumulation of starchy endosperm and premature developmental arrest of the embryo, and mature mutant grains germinate at a very low rate and fail to develop into plants. However, homozygous mutant plants can be obtained through embryo rescue, revealing that both mutants have an irregular leaf epidermis as well as roots with a strongly reduced number of root hairs and aberrant root hair morphology. Our results suggest the presence of common regulatory mechanisms for the control of cell division orientation in the aleurone and plant epidermis.

Dynamics and regulation of plant interphase microtubules: a comparative view

Microtubule and actin cytoskeletons are fundamental to a variety of cellular activities within eukaryotic organisms. Extensive information on the dynamics and functions of microtubules, as well as on their regulatory proteins, have been revealed in fungi and animals, and corresponding pictures are now slowly emerging in plants. During interphase, plant cells contain highly dynamic cortical microtubules that organize into ordered arrays, which are apparently regulated by distinct groups of microtubule regulators. Comparison with fungal and animal microtubules highlights both conserved and unique mechanisms for the regulation of the microtubule cytoskeleton in plants.

Asymmetric division in fucoid zygotes is positioned by telophase nuclei

The relative contributions of cell polarity and nuclear position in specifying the plane of asymmetric division in fucoid zygotes were investigated. In zygotes developing normally, telophase nuclei were positioned parallel to the polar growth axis, and the division plane bisected both axes. To assess division plane specification, the colinearity of the nuclear and growth axes was uncoupled by treatment with pharmacological agents. Spatial correlations between the growth axis, telophase nuclei, and the division plane were analyzed in the treated zygotes. In all cases, cytokinesis was oriented transverse to the telophase mitotic array and was less well aligned with the growth axis. Telophase nuclei also played a predominant role in positioning the division plane in polyspermic zygotes. Microtubules from the telophase nuclei interdigitated throughout the plane of subsequent cytokinesis, and we speculate that they specify the division plane. Morphological markers of the division plane were not observed before telophase; the earliest division marker detected was a plate of actin that assembled in the zone of microtubule overlap late in telophase. These findings are consistent with division plane specification at cytoplast boundaries.

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.

Remodeling the cytoskeleton for growth and form: an overview with some new views

The cytoskeleton coordinates all aspects of growth in plant cells, including exocytosis of membrane and wall components during cell expansion. This review seeks to integrate current information about cytoskeletal components in plants and the role they play in generating cell form. Advances in genome analysis have fundamentally changed the nature of research strategies and generated an explosion of new information on the cytoskeleton-associated proteins, their regulation, and their role in signaling to the cytoskeleton. Some of these proteins appear novel to plants, but many have close homologues in other eukaryotic systems. It is becoming clear that the mechanisms behind cell growth are essentially similar across the growth continuum, which ranges from tip growth to diffuse expansion. Remodeling of the actin cytoskeleton at sites of exocytosis is an especially critical feature of polarized and may also contribute to axial growth. We evaluate the most recent work on the signaling mechanisms that continually remodel the actin cytoskeleton via the activation of actin-binding proteins (ABPs) and consider the role the microtubule cytoskeleton plays in this process.

Microtubule cytoskeleton: a track record

The plant microtubule cytoskeleton forms unique arrays during cell division and morphogenesis. Recent studies have addressed the biogenesis, turnover, spatio-temporal organisation and cellular function of microtubules. The results suggest that both conserved eukaryotic mechanisms and plant-specific modifications determine microtubule dynamics and function.

A 75-kDa plant protein isolated by tubulin-affinity chromatography is a peroxisomal matrix enzyme

The function of microtubules in plant cells relies on their interactions with various, largely unidentified, proteins. A 75-kDa polypeptide (p75) was isolated previously by tubulin affinity chromatography of tobacco BY-2 proteins and is further characterized here. We have obtained two peptide sequences of 13 and 8 amino acid residues from the p75, which have 77% and 87% similarity to two putative dTDP-glucose 4,6-dehydratase proteins of 74.9 and 75.3 kDa in Arabidopsis. The proteins contain a peroxisomal matrix targeting signal SKL or AKL near their carboxyl terminus, an epimerase-dehydratase motif, and a region of 321 amino acid residues sharing 41% similarity with a dTDP-glucose 4,6-dehydratase from Aneurinibacillus thermoaerophilus. A monoclonal antibody 4B9 against the tobacco p75 reacted with a 75-kDa polypeptide on immunoblots of tobacco BY-2 and Arabidopsis protein extracts, and in immunofluorescence microscopy it revealed small organelle-like structures in tobacco BY-2 and Arabidopsis root-tip cells. Double labelling with an antibody against the peroxisomal marker enzyme, catalase, showed that the organelles are indeed peroxisomes. The peroxisomes were in closer association with actin filaments than microtubules. This observation supports recent findings that plant peroxisomes move on actin filaments. We propose that the peroxisomal dTDP-glucose 4,6-dehydratases may be involved in plant defence responses to oxidative stress.Key words: actin, antibodies, microtubules.

Functional evidence for in vitro microtubule severing by the plant katanin homologue

Temporal and spatial assembly of microtubules in plant cells depends mainly on the activity of microtubule-interacting proteins, which either stabilize, destabilize or translocate microtubules. Recent data have revealed that the thale cress (Arabidopsis thaliana) contains a protein related to the p60 catalytic subunit of animal katanin, a microtubule-severing protein. However, effects of the plant p60 on microtubule assembly are not known. We report the first functional evidence that the recombinant A. thaliana p60 katanin subunit, Atp60, binds to microtubules and severs them in an ATP-dependent manner in vitro. ATPase activity of Atp60 is stimulated by low tubulin/katanin ratios, and is inhibited at higher ratios. Considering its properties in vitro, several functions of Atp60 in vivo are discussed.

Investigation of the role of cell-cell interactions in division plane determination during maize leaf development through mosaic analysis of the tangled mutation

Most plant cells divide in planes that can be predicted from their shapes according to simple geometrical rules, but the division planes of some cells appear to be influenced by extracellular cues. In the maize leaf, some cells divide in orientations not predicted by their shapes, raising the possibility that cell-cell communication plays a role in division plane determination in this tissue. We investigated this possibility through mosaic analysis of the tangled (tan) mutation, which causes a high frequency of cells in all tissue layers to divide in abnormal orientations. Clonal sectors of tan mutant tissue marked by a closely linked albino mutation were examined to determine the phenotypes of cells near sector boundaries. We found that tan mutant cells always showed the mutant phenotype regardless of their proximity to wild-type cells, demonstrating that the wild-type Tan gene acts cell-autonomously in both lateral and transverse leaf dimensions to promote normally oriented divisions. However, if the normal division planes of wild-type cells depend on cell-cell communication involving the products of genes other than Tan, then aberrantly dividing tan mutant cells might send abnormal signals that alter the division planes of neighboring cells. The cell-autonomy of the tan mutation allowed us to investigate this possibility by examining wild-type cells near the boundaries of tan mutant sectors for evidence of aberrantly oriented divisions. We found that wild-type cells near tan mutant cells did not divide differently from other wild-type cells. These observations argue against the idea that the division planes of proliferatively dividing maize leaf epidermal cells are governed by short-range communication with their nearest neighbors.

Cytoskeleton and plant organogenesis

The functions of microtubules and actin filaments during various processes that are essential for the growth, reproduction and survival of single plant cells have been well characterized. A large number of plant structural cytoskeletal or cytoskeleton-associated proteins, as well as genes encoding such proteins, have been identified. Although many of these genes and proteins have been partially characterized with respect to their functions, a coherent picture of how they interact to execute cytoskeletal functions in plant cells has yet to emerge. Cytoskeleton-controlled cellular processes are expected to play crucial roles during plant cell differentiation and organogenesis, but what exactly these roles are has only been investigated in a limited number of studies in the whole plant context. The intent of this review is to discuss the results of these studies in the light of what is known about the cellular functions of the plant cytoskeleton, and about the proteins and genes that are required for them. Directions are outlined for future work to advance our understanding of how the cytoskeleton contributes to plant organogenesis and development.

Cytokinesis in eukaryotes

Cytokinesis is the final event of the cell division cycle, and its completion results in irreversible partition of a mother cell into two daughter cells. Cytokinesis was one of the first cell cycle events observed by simple cell biological techniques; however, molecular characterization of cytokinesis has been slowed by its particular resistance to in vitro biochemical approaches. In recent years, the use of genetic model organisms has greatly advanced our molecular understanding of cytokinesis. While the outcome of cytokinesis is conserved in all dividing organisms, the mechanism of division varies across the major eukaryotic kingdoms. Yeasts and animals, for instance, use a contractile ring that ingresses to the cell middle in order to divide, while plant cells build new cell wall outward to the cortex. As would be expected, there is considerable conservation of molecules involved in cytokinesis between yeast and animal cells, while at first glance, plant cells seem quite different. However, in recent years, it has become clear that some aspects of division are conserved between plant, yeast, and animal cells. In this review we discuss the major recent advances in defining cytokinesis, focusing on deciding where to divide, building the division apparatus, and dividing. In addition, we discuss the complex problem of coordinating the division cycle with the nuclear cycle, which has recently become an area of intense research. In conclusion, we discuss how certain cells have utilized cytokinesis to direct development.

Xcl1 causes delayed oblique periclinal cell divisions in developing maize leaves, leading to cellular differentiation by lineage instead of position

Differentiation of plant cells is regulated by position-dependent mechanisms rather than lineage. The maize Extra cell layers1 (Xcl1) mutation causes oblique, periclinal divisions to occur in the protoderm layer. These protodermal periclinal divisions occur at the expense of normal anticlinal divisions and cause the production of extra cell layers with epidermal characteristics, indicating that cells are differentiating according to lineage instead of position. Mutant kernels have several aleurone layers instead of one, indicating that Xcl1 alters cell division orientation in cells that divide predominantly in the anticlinal plane. Dosage analysis of Xcl1 reveals that the mutant phenotype is caused by overproduction of a normal gene product. This allows cells that have already received differentiation signals to continue to divide in aberrant planes and suggests that the timing of cell division determines differentiation. Cells that divide early and in the absence of differentiation signals use positional information, while cells that divide late after perceiving differentiation signals use lineage information instead of position.

The Arabidopsis TONNEAU2 gene encodes a putative novel protein phosphatase 2A regulatory subunit essential for the control of the cortical cytoskeleton

In Arabidopsis ton2 mutants, abnormalities of the cortical microtubular cytoskeleton, such as disorganization of the interphase microtubule array and lack of the preprophase band before mitosis, markedly affect cell shape and arrangement as well as overall plant morphology. We present the molecular isolation of the TON2 gene, which is highly conserved in higher plants and has a vertebrate homolog of unknown function. It encodes a protein similar in its C-terminal part to B" regulatory subunits of type 2A protein phosphatases (PP2As). We show that the TON2 protein interacts with an Arabidopsis type A subunit of PP2A in the yeast two-hybrid system and thus likely defines a novel subclass of PP2A subunits that are possibly involved in the control of cytoskeletal structures in plants.

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.

The plant cytoskeleton: vacuoles and cell walls make the difference

The eukaryotic cytoskeleton is a dynamic filamentous network with various cellular and developmental functions. Plant cells display a singular architecture, necessitating a structurally and functionally unique cytoskeleton and plant specific control mechanisms.

Molecular aspects of microtubule dynamics in plants

Microtubules are highly dynamic structures that play a major role in a wide range of processes, including cell morphogenesis, cell division, intracellular transport and signaling. The recent identification in plants of proteins involved in microtubule organization has begun to reveal how cytoskeleton dynamics are controlled.

Of weeds and men: what genomes teach us about plant cell biology

It has generally been assumed that fundamental cellular processes are conserved at the molecular level. Genome comparisons, however, suggest that the molecular mechanisms underlying programmed cell death, defense, adaptation and development may differ considerably between the plant and animal kingdoms. Phylogenetic analyses have revealed a great deal of novelty in the plant genes that are implicated in conserved processes such as transcription, cytoskeletal dynamics and vesicle trafficking. The Arabidopsis genome highlights the highly dynamic and regulated nature of the plant cell, which is fine-tuned to light, water, nutrient availability, temperature, touch and wind.

Expansion of the phragmoplast during plant cytokinesis: a MAPK pathway may MAP it out

Plant cytokinesis involves the formation of a cell plate. This is accomplished with the help of the phragmoplast, a plant-specific cytokinetic apparatus that consists of microtubules and microfilaments. During centrifugal growth of the cell plate, the phragmoplast expands to keep its microtubules at the leading edge of the cell plate. Recent studies have revealed potential regulators of phragmoplast microtubule dynamics and the involvement of a mitogen-activated protein kinase cascade in the control of phragmoplast expansion. These studies provide new insights into the molecular mechanisms of plant cytokinesis.

A 90-kD phospholipase D from tobacco binds to microtubules and the plasma membrane

The organization of microtubule arrays in the plant cell cortex involves interactions with the plasma membrane, presumably through protein bridges. We have used immunochemistry and monoclonal antibody 6G5 against a candidate bridge protein, a 90-kD tubulin binding protein (p90) from tobacco BY-2 membranes, to characterize the protein and isolate the corresponding gene. Screening an Arabidopsis cDNA expression library with the antibody 6G5 produced a partial clone encoding phospholipase D (PLD), and a full-length gene was obtained by sequencing a corresponding expressed sequence tag clone. The predicted protein of 857 amino acids contains the active sites of a phospholipid-metabolizing enzyme and a Ca(2+)-dependent lipid binding domain and is identical to Arabidopsis PLD delta. Two amino acid sequences obtained by Edman degradation of the tobacco p90 are identical to corresponding segments of a PLD sequence from tobacco. Moreover, immunoprecipitation using the antibody 6G5 and tobacco BY-2 protein extracts gave significant PLD activity, and PLD activity of tobacco BY-2 membrane proteins was enriched 6.7-fold by tubulin-affinity chromatography. In a cosedimentation assay, p90 bound and decorated microtubules. In immunofluorescence microscopy of intact tobacco BY-2 cells or lysed protoplasts, p90 colocalized with cortical microtubules, and taxol-induced microtubule bundling was accompanied by corresponding reorganization of p90. Labeling of p90 remained along the plasma membrane when microtubules were depolymerized, although detergent extraction abolished the labeling. Therefore, p90 is a specialized PLD that associates with membranes and microtubules, possibly conveying hormonal and environmental signals to the microtubule cytoskeleton.

A 90-kD phospholipase D from tobacco binds to microtubules and the plasma membrane

The organization of microtubule arrays in the plant cell cortex involves interactions with the plasma membrane, presumably through protein bridges. We have used immunochemistry and monoclonal antibody 6G5 against a candidate bridge protein, a 90-kD tubulin binding protein (p90) from tobacco BY-2 membranes, to characterize the protein and isolate the corresponding gene. Screening an Arabidopsis cDNA expression library with the antibody 6G5 produced a partial clone encoding phospholipase D (PLD), and a full-length gene was obtained by sequencing a corresponding expressed sequence tag clone. The predicted protein of 857 amino acids contains the active sites of a phospholipid-metabolizing enzyme and a Ca(2+)-dependent lipid binding domain and is identical to Arabidopsis PLD delta. Two amino acid sequences obtained by Edman degradation of the tobacco p90 are identical to corresponding segments of a PLD sequence from tobacco. Moreover, immunoprecipitation using the antibody 6G5 and tobacco BY-2 protein extracts gave significant PLD activity, and PLD activity of tobacco BY-2 membrane proteins was enriched 6.7-fold by tubulin-affinity chromatography. In a cosedimentation assay, p90 bound and decorated microtubules. In immunofluorescence microscopy of intact tobacco BY-2 cells or lysed protoplasts, p90 colocalized with cortical microtubules, and taxol-induced microtubule bundling was accompanied by corresponding reorganization of p90. Labeling of p90 remained along the plasma membrane when microtubules were depolymerized, although detergent extraction abolished the labeling. Therefore, p90 is a specialized PLD that associates with membranes and microtubules, possibly conveying hormonal and environmental signals to the microtubule cytoskeleton.

Plant microtubule-associated proteins: the HEAT is off in temperature-sensitive mor1

Microtubules perform essential functions in plant cells and govern, with other cytoskeletal elements, cell division, formation of cell walls and morphogenesis. For microtubules to perform their roles in the cell their organization and dynamics must be regulated and microtubule-associated proteins bear the main responsibility for these activities. We are just beginning to identify these plant microtubule-regulating proteins. Biochemical, molecular and genetic procedures have identified plant homologues of known microtubule-associated proteins, such as kinesins, katanin and XMAP215, and novel classes of plant microtubule-associated proteins, such as MAP65 and MAP190. Showing how these proteins coordinate the microtubule cytoskeleton in vivo is now the challenge. The recent identification and characterization of the Arabidopsis thaliana microtubule organization mutant, mor1, begins to address this challenge and here we highlight the significance of this work.

A 90-kD phospholipase D from tobacco binds to microtubules and the plasma membrane

The organization of microtubule arrays in the plant cell cortex involves interactions with the plasma membrane, presumably through protein bridges. We have used immunochemistry and monoclonal antibody 6G5 against a candidate bridge protein, a 90-kD tubulin binding protein (p90) from tobacco BY-2 membranes, to characterize the protein and isolate the corresponding gene. Screening an Arabidopsis cDNA expression library with the antibody 6G5 produced a partial clone encoding phospholipase D (PLD), and a full-length gene was obtained by sequencing a corresponding expressed sequence tag clone. The predicted protein of 857 amino acids contains the active sites of a phospholipid-metabolizing enzyme and a Ca(2+)-dependent lipid binding domain and is identical to Arabidopsis PLD delta. Two amino acid sequences obtained by Edman degradation of the tobacco p90 are identical to corresponding segments of a PLD sequence from tobacco. Moreover, immunoprecipitation using the antibody 6G5 and tobacco BY-2 protein extracts gave significant PLD activity, and PLD activity of tobacco BY-2 membrane proteins was enriched 6.7-fold by tubulin-affinity chromatography. In a cosedimentation assay, p90 bound and decorated microtubules. In immunofluorescence microscopy of intact tobacco BY-2 cells or lysed protoplasts, p90 colocalized with cortical microtubules, and taxol-induced microtubule bundling was accompanied by corresponding reorganization of p90. Labeling of p90 remained along the plasma membrane when microtubules were depolymerized, although detergent extraction abolished the labeling. Therefore, p90 is a specialized PLD that associates with membranes and microtubules, possibly conveying hormonal and environmental signals to the microtubule cytoskeleton.

Lilliputian mutant of maize lacks cell elongation and shows defects in organization of actin cytoskeleton

The maize mutant lilliputian is characterized by miniature seedling stature, reduced cell elongation, and aberrant root anatomy. Here, we document that root cells of this mutant show several defects in the organization of actin filaments (AFs). Specifically, cells within the meristem lack dense perinuclear AF baskets and fail to redistribute AFs during mitosis. In contrast, mitotic cells of wild-type roots accumulate AFs at plasma membrane-associated domains that face the mitotic spindle poles. Both mitotic and early postmitotic mutant cells fail to assemble transverse arrays of cortical AFs, which are characteristic for wild-type root cells. In addition, early postmitotic cells show aberrant distribution of endoplasmic AF bundles that are normally organized through anchorage sites at cross-walls and nuclear surfaces. In wild-type root apices, these latter AF bundles are organized in the form of symmetrically arranged conical arrays and appear to be essential for the onset of rapid cell elongation. Exposure of wild-type and cv. Alarik maize root apices to the F-actin drugs cytochalasin D and latrunculin B mimics the phenotype of lilliputian root apices. In contrast to AFs, microtubules are more or less normally organized in root cells of lilliputian mutant. Collectively, these data suggest that the LILLIPUTIAN protein, the nature of which is still unknown, impinges on plant development via its action on the actin cytoskeleton.

Specification of bundle sheath cell fates during maize leaf development: roles of lineage and positional information evaluated through analysis of the tangled1 mutant

In leaves of the maize tangled1 (tan1) mutant, clusters of bundle sheath (BS)-like cells extend several cells distant from the veins, in association with the single layer of BS cells around the vein. We show that the BS-like cell clusters in tan1 leaves result from the continued division of cells in the procambial/BS cell lineage that do not divide further in wild-type leaves. The ectopic BS-like cells accumulate the BS marker NADP-dependent malic enzyme but not the mesophyll cell marker phosphoenolpyruvate carboxylase, and exhibit thickened walls, suggesting that they differentiate as C4-type BS cells. We propose that bundle sheath cell fate can be conferred on some derivatives of procambial cell divisions in a manner that is heritable through multiple cell divisions and is position-independent.