Division decisions and the spatial regulation of cytokinesis

Predicting Division Planes of Three-Dimensional Cells by Soap-Film Minimization

One key aspect of cell division in multicellular organisms is the orientation of the division plane. Proper division plane establishment contributes to normal plant body organization. To determine the importance of cell geometry in division plane orientation, we designed a three-dimensional probabilistic mathematical model to directly test the century-old hypothesis that cell divisions mimic soap-film minima. According to this hypothesis, daughter cells have equal volume and the division plane occurs where the surface area is at a minimum. We compared predicted division planes to a plant microtubule array that marks the division site, the preprophase band (PPB). PPB location typically matched one of the predicted divisions. Predicted divisions offset from the PPB occurred when a neighboring cell wall or PPB was directly adjacent to the predicted division site to avoid creating a potentially structurally unfavorable four-way junction. By comparing divisions of differently shaped plant cells (maize [Zea mays] epidermal cells and developing ligule cells and Arabidopsis thaliana guard cells) and animal cells (Caenorhabditis elegans embryonic cells) to divisions simulated in silico, we demonstrate the generality of this model to accurately predict in vivo division. This powerful model can be used to separate the contribution of geometry from mechanical stresses or developmental regulation in predicting division plane orientation.

Cellulose Synthase-Like D1 is integral to normal cell division, expansion, and leaf development in maize

The Cellulose Synthase-Like D (CslD) genes have important, although still poorly defined, roles in cell wall formation. Here, we show an unexpected involvement of CslD1 from maize (Zea mays) in cell division. Both division and expansion were altered in the narrow-organ and warty phenotypes of the csld1 mutants. Leaf width was reduced by 35%, due mainly to a 47% drop in the number of cell files across the blade. Width of other organs was also proportionally reduced. In leaf epidermis, the deficiency in lateral divisions was only partially compensated by a modest, uniform increase in cell width. Localized clusters of misdivided epidermal cells also led to the formation of warty lesions, with cell clusters bulging from the epidermal layer, and some cells expanding to volumes 75-fold greater than normal. The decreased cell divisions and localized epidermal expansions were not associated with detectable changes in the cell wall composition of csld1 leaf blades or epidermal peels, yet a greater abundance of thin, dense walls was indicated by high-resolution x-ray tomography of stems. Cell-level defects leading to wart formation were traced to sites of active cell division and expansion at the bases of leaf blades, where cytokinesis and cross-wall formation were disrupted. Flow cytometry confirmed a greater frequency of polyploid cells in basal zones of leaf blades, consistent with the disruption of cytokinesis and/or the cell cycle in csld1 mutants. Collectively, these data indicate a previously unrecognized role for CSLD activity in plant cell division, especially during early phases of cross-wall formation.

R1R2R3-Myb proteins positively regulate cytokinesis through activation of KNOLLE transcription in Arabidopsis thaliana

G2/M phase-specific gene transcription in tobacco cells is mediated by R1R2R3-Myb transcriptional activators, NtmybA1 and NtmybA2, which bind to mitosis-specific activator (MSA) elements. We show here that two structurally related genes, MYB3R1 and MYB3R4, which encode homologs of NtmybA1 and NtmybA2, play a partially redundant role in positively regulating cytokinesis in Arabidopsis thaliana. The myb3r1 myb3r4 double mutant often fails to complete cytokinesis, resulting in multinucleate cells with gapped walls and cell wall stubs in diverse tissues. These defects correlate with the selective reduction of transcript levels of several G2/M phase-specific genes, which include B2-type cyclin (CYCB2), CDC20.1 and KNOLLE (KN). These genes contain MSA-like motifs in their promoters and were activated by MYB3R4 in transient expression assays in tobacco cells. The KN gene encodes a cytokinesis-specific syntaxin that is essential for cell plate formation. The cytokinesis defects of myb3r1 myb3r4 double mutants were partially rescued by KN gene expression from heterologous promoters. In addition, a kn heterozygous mutation enhanced cytokinesis defects resulting from heterozygous or homozygous mutations in the MYB3R1 and MYB3R4 genes. Our results suggest that a pair of structurally related R1R2R3-Myb transcription factors may positively regulate cytokinesis mainly through transcriptional activation of the KN gene.

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.

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.

Dynamic cytoskeleton rearrangements in giant cells and syncytia of nematode-infected roots

Giant cells induced by root knot nematodes and syncytia caused by cyst nematodes are large multinucleated feeding cells containing a dense cytoplasm generated during a complex host-parasite association in plant roots. To find out whether cytoskeleton changes occurred during feeding cell development, transcriptional activity of actin (ACT) and tubulin genes and organization of the ACT filaments and of the microtubules (MTs) were analyzed in situ. The importance of changes in the cytoskeleton architecture for the proper initiation and development of galls and syncytia was demonstrated by perturbing the cytoskeleton with chemical inhibitors. The expression levels of cytoskeletal components, such as tubulins and ACTs, are proposed to be upregulated to allow the assembly of a new cytoskeleton in expanding feeding cells. However, MTs and ACT filaments failed to properly organize and appeared partially depolymerized throughout feeding site development. Both the actin and tubulin cytoskeletons were strongly disrupted in syncytia and mitotic figures were never observed. In contrast, in giant cells, an ACT and cortical MT cytokeleton, although disturbed, was still visible. In addition, a functional mitotic apparatus was present that contained multiple large spindles and arrested phragmoplasts, but no pre-prophase bands. Chemical stabilization of the microtubular cytoskeleton with taxol blocked feeding site development. On the other hand, when the ACT or MT cytoskeleton of feeding cells was depolymerized by cytochalasin D or oryzalin, nematodes could complete their life cycle. Our data suggest that the cytoskeleton rearrangements and depolymerization induced by parasitic nematodes may be essential for a successful feeding process.

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.

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.

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.

Plant cytokinesis: KNOLLE joins the club

Dividing plant cells assemble a new intracellular compartment, the cell plate, which grows centrifugally by vesicle fusion to partition the cytoplasm. Genetic studies in Arabidopsis are revealing the molecular signals that specify this special membrane transport pathway.

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.

Novel patterns of ectopic cell plate growth and lipid body distribution in the Arabidopsis gemini pollen1 mutant

The nature of aberrant gametophytic cell divisions and altered pollen cell fate in the gemini pollen1 (gem1) mutant was investigated through ultrastructural analysis. The earliest noticeable defect in gem1 was the appearance of extended membrane profiles at the early bicellular stage. These were replaced by ectopic internal walls, which divided the cytoplasm into twin or multiple cell compartments. Complete or partial internal walls were callosic with highly complex profiles, indicating failed guidance or deregulated cell plate growth. Extended membrane profiles and delayed callose synthesis at division sites further suggested a novel pattern of cell plate assembly in gem1. Multiple cell compartments in gem1 adopted vegetative cell fate with regard to lipid body distribution. In the wild type, lipid bodies appear specifically in the vegetative cell, whereas in gem1, lipid bodies accumulated in all cytoplasmic compartments. Our results support the hypothesis that altered pollen cell fate in gem1 results from abnormal inheritance of cell fate determinants as a result of disturbed cytokinesis.

Molecular motors and their functions in plants

Molecular motors that hydrolyze ATP and use the derived energy to generate force are involved in a variety of diverse cellular functions. Genetic, biochemical, and cellular localization data have implicated motors in a variety of functions such as vesicle and organelle transport, cytoskeleton dynamics, morphogenesis, polarized growth, cell movements, spindle formation, chromosome movement, nuclear fusion, and signal transduction. In non-plant systems three families of molecular motors (kinesins, dyneins, and myosins) have been well characterized. These motors use microtubules (in the case of kinesines and dyneins) or actin filaments (in the case of myosins) as tracks to transport cargo materials intracellularly. During the last decade tremendous progress has been made in understanding the structure and function of various motors in animals. These studies are yielding interesting insights into the functions of molecular motors and the origin of different families of motors. Furthermore, the paradigm that motors bind cargo and move along cytoskeletal tracks does not explain the functions of some of the motors. Relatively little is known about the molecular motors and their roles in plants. In recent years, by using biochemical, cell biological, molecular, and genetic approaches a few molecular motors have been isolated and characterized from plants. These studies indicate that some of the motors in plants have novel features and regulatory mechanisms. The role of molecular motors in plant cell division, cell expansion, cytoplasmic streaming, cell-to-cell communication, membrane trafficking, and morphogenesis is beginning to be understood. Analyses of the Arabidopsis genome sequence database (51% of genome) with conserved motor domains of kinesin and myosin families indicates the presence of a large number (about 40) of molecular motors and the functions of many of these motors remain to be discovered. It is likely that many more motors with novel regulatory mechanisms that perform plant-specific functions are yet to be discovered. Although the identification of motors in plants, especially in Arabidopsis, is progressing at a rapid pace because of the ongoing plant genome sequencing projects, only a few plant motors have been characterized in any detail. Elucidation of function and regulation of this multitude of motors in a given species is going to be a challenging and exciting area of research in plant cell biology. Structural features of some plant motors suggest calcium, through calmodulin, is likely to play a key role in regulating the function of both microtubule- and actin-based motors in plants.

Fruit-developmental regulation of bell pepper knolle gene (cakn) expression(1)

A cDNA, cakn, orthologous to the Arabidopsis KN gene, which is involved in vesicle fusion during cell plate formation, was isolated from bell pepper fruit. cakn seems to be monogenic and its expression mainly transcriptionally regulated. During fruit development, transcript and protein levels increase significantly in the early stages in which numerous cell divisions occur, but in the stages corresponding to fruit growth by cell enlargement, whereas the messengers are undetectable, proteins are still faintly present, suggesting a different stability rate for the two types of macromolecules.

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.

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

Several different cytokinetic mechanisms operate in flowering plants. During 'conventional' somatic cytokinesis, the mitotic spindle remnants give rise to a phragmoplast that serves as a framework for the assembly of the cell plate. Cell plates fuse with the parental plasma membrane at specific cortical sites previously defined by the preprophase band of microtubules. In nuclear endosperms, meiocytes, and gametophytic cells, cytokinesis occurs without preprophase bands. The position of the new cell walls is determined instead by interacting arrays of microtubules that radiate from the nuclear envelope surfaces. The nuclear cytoplasmic domains defined by these microtubule arrays demarcate the boundaries of the future cells. Recent studies have provided new insights into the ultrastructural similarities and dissimilarities between conventional and non-conventional cytokinesis. Numerous proteins have also been localized to cytokinesis-related cytoskeletal arrays and cell plates but the functions of most of them have yet to be elucidated.

Identification of a phragmoplast-associated kinesin-related protein in higher plants

The phragmoplast executes cytokinesis in higher plants. The major components of the phragmoplast are microtubules, which are arranged in two mirror-image arrays perpendicular to the division plane [1]. The plus ends of these microtubules are located near the site of the future cell plate. Golgi-derived vesicles are transported along microtubules towards the plus ends to deliver materials bound for the cell plate [2] [3]. During cell division, rapid microtubule reorganization in the phragmoplast requires the orchestrated activities of microtubule motor proteins such as kinesins. We isolated an Arabidopsis cDNA clone of a gene encoding an amino-terminal motor kinesin, AtPAKRP1, and have determined the partial sequence of its rice homolog. Immunofluorescence experiments with two sets of specific antibodies revealed consistent localization of AtPAKRP1 and its homolog in Arabidopsis and rice cells undergoing anaphase, telophase and cytokinesis. AtPAKRP1 started to accumulate along microtubules towards the spindle midzone during late anaphase. Once the phragmoplast microtubule array was established, AtPAKRP1 conspicuously localized to microtubules near the future cell plate. Our results provide evidence that AtPAKRP1 is a hitherto unknown motor that may take part in the establishment and/or maintenance of the phragmoplast microtubule array.