Genetic dissection of cytokinesis

The evo-devo of multinucleate cells, tissues, and organisms, and an alternative route to multicellularity

Multinucleate cells, tissues, or organisms occur in 60 families of land plants and in five otherwise diverse algal lineages (Rhodophyceae, Xanthophyceae, Chlorophyceae, Ulvophyceae, and Charophyceae). Inspection of a morphospace constructed out of eight developmental processes reveals a large number of possible variants of multinucleate cells and organisms that, with two exceptions, are represented by one or more plant species in one or more clades. Thus, most of these permutations of developmental processes exist in nature. Inspection of the morphospace also shows how the siphonous body plan (a multinucleate cell with the capacity for indeterminate growth in size) can theoretically serve as the direct progenitor of a multicellular organism by a process similar to segregative cell division observed in siphonocladean algae. Using molecular phylogenies of algal clades, different evolutionary scenarios are compared to see how the multicellular condition may have evolved from a multinucleate unicellular progenitor. We also show that the siphonous progenitor of a multicellular organism has previously passed through the alignment-of-fitness phase (in which genetic similarity among cells/nuclei minimizes internal genomic conflict) and the export-of-fitness phase (in which genetically similar cells/nuclei collaborate to achieve a reproductively integrated multicellular organism). All that is theoretically required is the evolutionary acquisition of the capacity to compartmentalize its cytoplasm.

Arabidopsis Fused kinase and the Kinesin-12 subfamily constitute a signalling module required for phragmoplast expansion

The conserved Fused kinase plays vital but divergent roles in many organisms from Hedgehog signalling in Drosophila to polarization and chemotaxis in Dictyostelium. Previously we have shown that Arabidopsis Fused kinase termed TWO-IN-ONE (TIO) is essential for cytokinesis in both sporophytic and gametophytic cell types. Here using in vivo imaging of GFP-tagged microtubules in dividing microspores we show that TIO is required for expansion of the phragmoplast. We identify the phragmoplast-associated kinesins, PAKRP1/Kinesin-12A and PAKRP1L/Kinesin-12B, as TIO-interacting proteins and determine TIO-Kinesin-12 interaction domains and their requirement in male gametophytic cytokinesis. Our results support the role of TIO as a functional protein kinase that interacts with Kinesin-12 subfamily members mainly through the C-terminal ARM repeat domain, but with a contribution from the N-terminal kinase domain. The interaction of TIO with Kinesin proteins and the functional requirement of their interaction domains support the operation of a Fused kinase signalling module in phragmoplast expansion that depends upon conserved structural features in diverse Fused kinases.

The Arabidopsis cell division cycle

Plant cells have evolved a complex circuitry to regulate cell division. In many aspects, the plant cell cycle follows a basic strategy similar to other eukaryotes. However, several key issues are unique to plant cells. In this chapter, both the conserved and unique cellular and molecular properties of the plant cell cycle are reviewed. In addition to division of individual cells, the specific characteristic of plant organogenesis and development make that cell proliferation control is of primary importance during development. Therefore, special attention should be given to consider plant cell division control in a developmental context. Proper organogenesis depends on the formation of different cell types. In plants, many of the processes leading to cell differentiation rely on the occurrence of a different cycle, termed the endoreplication cycle, whereby cells undergo repeated full genome duplication events in the absence of mitosis and increase their ploidy. Recent findings are focusing on the relevance of changes in chromatin organization for a correct cell cycle progression and, conversely, in the relevance of a correct functioning of chromatin remodelling complexes to prevent alterations in both the cell cycle and the endocycle.

ENDOSPERM DEFECTIVE1 Is a Novel Microtubule-Associated Protein Essential for Seed Development in Arabidopsis

Early endosperm development involves a series of rapid nuclear divisions in the absence of cytokinesis; thus, many endosperm mutants reveal genes whose functions are essential for mitosis. This work finds that the endosperm of Arabidopsis thaliana endosperm-defective1 (ede1) mutants never cellularizes, contains a reduced number of enlarged polyploid nuclei, and features an aberrant microtubule cytoskeleton, where the specialized radial microtubule systems and cytokinetic phragmoplasts are absent. Early embryo development is substantially normal, although occasional cytokinesis defects are observed. The EDE1 gene was cloned using a map-based approach and represents the pioneer member of a conserved plant-specific family of genes of previously unknown function. EDE1 is expressed in the endosperm and embryo of developing seeds, and its expression is tightly regulated during cell cycle progression. EDE1 protein accumulates in nuclear caps in premitotic cells, colocalizes along microtubules of the spindle and phragmoplast, and binds microtubules in vitro. We conclude that EDE1 is a novel plant-specific microtubule-associated protein essential for microtubule function during the mitotic and cytokinetic stages that generate the Arabidopsis endosperm and embryo.

The first zygotic division in Arabidopsis requires de novo transcription of thymidylate kinase

Re-activation of cell division after fertilization involves the specific regulation of a set of genes. To identify genes involved in the gametophytic to sporophytic transition, we screened Arabidopsis T-DNA insertion lines for early seed abortion at the zygote (zeus) or one-cell embryo stages (cyclops), and characterized a sporophytic zygote-lethal mutation, zeus1. ZEUS1 encodes a thymidylate kinase (AtTMPK) that synthesizes dTDP and is involved in the regulation of DNA replication. Unlike in yeast and animals, the single AtTMPK gene is capable of producing two proteins by alternative splicing; the longer isoform is targeted to the mitochondria, the shorter to the cytosol. Transcription of AtTMPK is activated during the G(1)/S-phase transition of the cell cycle, similarly to yeast and mammalian orthologues. In AtTMPK:GUS plants, the reporter gene was preferentially expressed in cells undergoing division, but was not detected during the male and female gametophytic mitoses. GUS expression was observed in mature embryo sacs prior to fertilization, and this expression may indicate the time of synchronization of the gamete cell-cycle phases. Identification of ZEU1 emphasizes the importance of control of the metabolism of DNA in the regulation of the G(1)/S-phase transition at fertilization.

Meiosis-driven genome variation in plants

Meiosis includes two successive divisions of the nucleus with one round of DNA replication and leads to the formation of gametes with half of the chromosomes of the mother cell during sexual reproduction. It provides a cytological basis for gametogenesis and nheritance in eukaryotes. Meiotic cell division is a complex and dynamic process that involves a number of molecular and cellular events, such as DNA and chromosome replication, chromosome pairing, synapsis and recombination, chromosome segregation, and cytokinesis. Meiosis maintains genome stability and integrity over sexual life cycles. On the other hand, meiosis generates genome variations in several ways. Variant meiotic recombination resulting from specific genome structures induces deletions, duplications, and other rearrangements within the genic and non-genic genomic regions and has been considered a major driving force for gene and genome evolution in nature. Meiotic abnormalities in chromosome segregation lead to chromosomally imbalanced gametes and aneuploidy. Meiotic restitution due to failure of the first or second meiotic division gives rise to unreduced gametes, which triggers polyploidization and genome expansion. This paper reviews research regarding meiosis-driven genome variation, including deletion and duplication of genomic regions, aneuploidy, and polyploidization, and discusses the effect of related meiotic events on genome variation and evolution in plants. Knowledge of various meiosis-driven genome variations provides insight into genome evolution and genetic variability in plants and facilitates plant genome research.

QQT proteins colocalize with microtubules and are essential for early embryo development in Arabidopsis

During Arabidopsis embryogenesis, the control of division between daughter cells is critical for pattern formation. Two embryo-defective (emb) mutant lines named quatre-quart (qqt) were characterized by forward and reverse genetics. The terminal arrest of qqt1 and qqt2 embryos was at the octant stage, just prior to the round of periclinal divisions that establishes the dermatogen stage . Homozygous embryos of a weaker allele of qqt1 were able to divide further, resulting in aberrant periclinal divisions. These phenotypic analyses support an essential role of the QQT proteins in the correct formation of the tangential divisions. That an important proportion of qqt1 embryos were arrested prior to the octant stage indicated a more general role in cell division. The analysis of QQT1 and QQT2 genes revealed that they belong to a small subgroup of the large family encoding ATP/GTP binding proteins, and are widely conserved among plants, vertebrates and Archaea. We showed that QQT1 and QQT2 proteins interact with each other in a yeast two-hybrid system, and that QQT1 and QQT2 tagged by distinct fluorescent probes colocalize with microtubules during mitosis, in agreement with their potential role in cell division and their mutant phenotype. We propose that QQT1 and QQT2 proteins participate in the organization of microtubules during cell division, and that this function is essential for the correct development of the early embryo.

Disruption of the Arabidopsis SMC4 gene, AtCAP-C, compromises gametogenesis and embryogenesis

In higher eukaryotes, the condensin complex is a multisubunit apparatus that plays a pivotal role in the coordinated condensation of chromatin during mitosis. The catalytic subunits, CAP-E and CAP-C, members of the SMC family of ATPases, form a heterodimer, the activity of which is controlled by the non-SMC subunits CAP-D2, CAP-G and CAP-H. Here, we report the characterization of a T-DNA insertion mutant of the Arabidopsis CAP-C gene. Analysis of the progeny of selfed heterozygotes revealed that the homozygous null genotype is embryo lethal, with arrest occurring at or before the globular stage of development. Patterning defects associated with altered planes of cytokinesis were found in both the embryo and the suspensor. Crosses of heterozygotes with wild type plants revealed both male and female gametophytic defects. Stretched chromatin was observed between segregating mitotic chromosomes in pollen produced by selfed heterozygotes. Additionally, some plants heterozygous for the T-DNA insertion exhibited loss of apical dominance and mild fasciation, indicating a semi-dominant effect of the mutation. These results reveal a critical role for AtCAP-C during cell division and, unlike our previous studies on the AtCAP-E genes, suggest that no redundant factors for AtCAP-C exist in the Arabidopsis genome.

The origin of the eukaryotic cell based on conservation of existing interfaces

Current theories about the origin of the eukaryotic cell all assume that during evolution a prokaryotic cell acquired a nucleus. Here, it is shown that a scenario in which the nucleus acquired a plasma membrane is inherently less complex because existing interfaces remain intact during evolution. Using this scenario, the evolution to the first eukaryotic cell can be modeled in three steps, based on the self-assembly of cellular membranes by lipid-protein interactions. First, the inclusion of chromosomes in a nuclear membrane is mediated by interactions between laminar proteins and lipid vesicles. Second, the formation of a primitive endoplasmic reticulum, or exomembrane, is induced by the expression of intrinsic membrane proteins. Third, a plasma membrane is formed by fusion of exomembrane vesicles on the cytoskeletal protein scaffold. All three self-assembly processes occur both in vivo and in vitro. This new model provides a gradual Darwinistic evolutionary model of the origins of the eukaryotic cell and suggests an inherent ability of an ancestral, primitive genome to induce its own inclusion in a membrane.

The cell biology of neurogenesis

During the development of the mammalian central nervous system, neural stem cells and their derivative progenitor cells generate neurons by asymmetric and symmetric divisions. The proliferation versus differentiation of these cells and the type of division are closely linked to their epithelial characteristics, notably, their apical-basal polarity and cell-cycle length. Here, we discuss how these features change during development from neuroepithelial to radial glial cells, and how this transition affects cell fate and neurogenesis.

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.

The AtNACK1/HINKEL and STUD/TETRASPORE/AtNACK2 genes, which encode functionally redundant kinesins, are essential for cytokinesis in Arabidopsis

Cytokinesis is the critical step during which daughter cells are separated. We showed previously that a protein complex that consists of NACK1 (and NACK2) kinesin-like protein and NPK1 MAPKKK and its substrate NQK1 MAPKK are required for progression of cytokinesis in Nicotiana tabacum. The genome of Arabidopsis thaliana encodes homologues of NACK1 and NACK2, namely, AtNACK1/HINKEL and STUD/TETRASPORE/AtNACK2, respectively. Loss-of-function mutations in AtNACK1/HINKEL and STUD/TETRASPORE/AtNACK2 result in the occasional failure of somatic and male-meiotic cytokinesis, respectively. However, it is likely that these genes function redundantly to some extent in somatic tissues and female gametogenesis. We describe the phenotypes of Arabidopsis plants that have mutations in both the AtNACK1/HINKEL and STUD/TETRASPORE/AtNACK2 genes. These phenotypes suggest that the two genes are essential during both male and female gametogenesis. Female gametes with atnack1 atnack2 double mutations failed to cellularize and to generate a central cell, synergids and the egg cells. Male gametes with atnack1 atnack2 mutations were also not transmitted to the next generation. The AtNACK1/HINKEL and STUD/TETRASPORE/AtNACK2 genes for kinesin-like proteins have overlapping functions that are essential for gametogenetic cytokinesis. They appear to be essential components of a MAP kinase cascade that promotes cytokinesis of plant cells in both gametophytic (haploid) and sporophytic (diploid) proliferation.

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.

Arabidopsis WPP-domain proteins are developmentally associated with the nuclear envelope and promote cell division

The nuclear envelope (NE) acts as a selective barrier to macromolecule trafficking between the nucleus and the cytoplasm and undergoes a complex reorganization during mitosis. Different eukaryotic kingdoms show specializations in NE function and composition. In contrast with vertebrates, the protein composition of the NE and the function of NE proteins are barely understood in plants. MFP1 attachment factor 1 (MAF1) is a plant-specific NE-associated protein first identified in tomato (Lycopersicon esculentum). Here, we demonstrate that two Arabidopsis thaliana MAF1 homologs, WPP1 and WPP2, are associated with the NE specifically in undifferentiated cells of the root tip. Reentry into cell cycle after callus induction from differentiated root segments reprograms their NE association. Based on green fluorescent protein fusions and immunogold labeling data, the proteins are associated with the outer NE and the nuclear pores in interphase cells and with the immature cell plate during cytokinesis. RNA interference-based suppression of the Arabidopsis WPP family causes shorter primary roots, a reduced number of lateral roots, and reduced mitotic activity of the root meristem. Together, these data demonstrate the existence of regulated NE targeting in plants and identify a class of plant-specific NE proteins involved in mitotic activity.

Forespore membrane assembly in yeast: coordinating SPBs and membrane trafficking

In the yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae, sporulation involves de novo synthesis of forespore membrane (FSM) within the cytoplasm of mother cells. The FSM ultimately becomes the plasma membrane of the developing ascospores. Several protein components of the FSM have been identified. Visualization of these proteins has demonstrated the dynamic nature of the genesis and development of the FSM. It begins to develop at the differentiated outer plaque of the spindle pole bodies (SPBs) and extends outwards, encapsulating each of the haploid nuclei produced by meiosis. Several coiled-coil proteins are specifically recruited to the SPBs and play indispensable roles in FSM assembly. Temporal and spatial coordination of meiotic nuclear divisions and membrane assembly is of special importance. Comparison of the processes of FSM assembly in these yeasts shows that the basic mechanism has been conserved, even though the individual proteins involved are often different. Understanding these dynamic aspects of yeast sporulation will help to elucidate a general mechanism for the cellularization of cytoplasm containing multiple nuclei.

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.

Electron tomographic analysis of somatic cell plate formation in meristematic cells of Arabidopsis preserved by high-pressure freezing

We have investigated the process of somatic-type cytokinesis in Arabidopsis (Arabidopsis thaliana) meristem cells with a three-dimensional resolution of approximately 7 nm by electron tomography of high-pressure frozen/freeze-substituted samples. Our data demonstrate that this process can be divided into four phases: phragmoplast initials, solid phragmoplast, transitional phragmoplast, and ring-shaped phragmoplast. Phragmoplast initials arise from clusters of polar microtubules (MTs) during late anaphase. At their equatorial planes, cell plate assembly sites are formed, consisting of a filamentous ribosome-excluding cell plate assembly matrix (CPAM) and Golgi-derived vesicles. The CPAM, which is found only around growing cell plate regions, is suggested to be responsible for regulating cell plate growth. Virtually all phragmoplast MTs terminate inside the CPAM. This association directs vesicles to the CPAM and thereby to the growing cell plate. Cell plate formation within the CPAM appears to be initiated by the tethering of vesicles by exocyst-like complexes. After vesicle fusion, hourglass-shaped vesicle intermediates are stretched to dumbbells by a mechanism that appears to involve the expansion of dynamin-like springs. This stretching process reduces vesicle volume by approximately 50%. At the same time, the lateral expansion of the phragmoplast initials and their CPAMs gives rise to the solid phragmoplast. Later arriving vesicles begin to fuse to the bulbous ends of the dumbbells, giving rise to the tubulo-vesicular membrane network (TVN). During the transitional phragmoplast stage, the CPAM and MTs disassemble and then reform in a peripheral ring phragmoplast configuration. This creates the centrifugally expanding peripheral cell plate growth zone, which leads to cell plate fusion with the cell wall. Simultaneously, the central TVN begins to mature into a tubular network, and ultimately into a planar fenestrated sheet (PFS), through the removal of membrane via clathrin-coated vesicles and by callose synthesis. Small secondary CPAMs with attached MTs arise de novo over remaining large fenestrae to focus local growth to these regions. When all of the fenestrae are closed, the new cell wall is complete. Few endoplasmic reticulum (ER) membranes are seen associated with the phragmoplast initials and with the TVN cell plate that is formed within the solid phragmoplast. ER progressively accumulates thereafter, reaching a maximum during the late PFS stage, when most cell plate growth is completed.

Genetic interaction of an origin recognition complex subunit and the Polycomb group gene MEDEA during seed development

The eukaryotic origin recognition complex (ORC) is made up of six subunits and functions in nuclear DNA replication, chromatin structure, and gene silencing in both fungi and metazoans. We demonstrate that disruption of a plant ORC subunit homolog, AtORC2 of Arabidopsis (Arabidopsis thaliana), causes a zygotic lethal mutant phenotype (orc2). Seeds of orc2 abort early, typically producing embryos with up to eight cells. Nuclear division in the endosperm is arrested at an earlier developmental stage: only approximately four nuclei are detected in orc2 endosperm. The endosperm nuclei in orc2 are dramatically enlarged, a phenotype that is most similar to class B titan mutants, which include mutants in structural maintenance of chromosomes (SMC) cohesins. The highest levels of ORC2 gene expression were found in preglobular embryos, coinciding with the stage at which homozygous orc2 mutant seeds arrest. The homologs of the other five Arabidopsis ORC subunits are also expressed at this developmental stage. The orc2 mutant phenotype is partly suppressed by a mutation in the Polycomb group gene MEDEA. In double mutants between orc2 and medea (mea), orc2 homozygotes arrest later with a phenotype intermediate between those of mea and orc2 single mutants. Either alterations in chromatin structure or the release of cell cycle checkpoints by the mea mutation may allow more cell and nuclear divisions to occur in orc2 homozygous seeds.

SCD1 is required for cytokinesis and polarized cell expansion in Arabidopsis thaliana [corrected]

In the leaf epidermis, guard mother cells undergo a stereotyped symmetric division to form the guard cells of stomata. We have identified a temperature-sensitive Arabidopsis mutant, stomatal cytokinesis-defective 1-1 (scd1-1), which affects this specialized division. At the non-permissive temperature, 22 degrees C, defective scd1-1 guard cells are binucleate, and the formation of their ventral cell walls is incomplete. Cytokinesis was also disrupted in other types of epidermal cells such as pavement cells. Further phenotypic analysis of scd1-1 indicated a role for SCD1 in seedling growth, root elongation and flower morphogenesis. More severe scd1 T-DNA insertion alleles (scd1-2 and scd1-3) markedly affect polar cell expansion, most notably in trichomes and root hairs. SCD1 is a unique gene in Arabidopsis that encodes a protein related to animal proteins that regulate intracellular protein transport and/or mitogen-activated protein kinase signaling pathways. Consistent with a role for SCD1 in membrane trafficking, secretory vesicles were found to accumulate in cytokinesis-defective scd1 cells. In addition the scd1 mutant phenotype was enhanced by low doses of inhibitors of cell plate consolidation and vesicle secretion. We propose that SCD1 functions in polarized vesicle trafficking during plant cytokinesis and cell expansion.

TETRASPORE encodes a kinesin required for male meiotic cytokinesis in Arabidopsis

A key step in pollen formation is the segregation of the products of male meiosis into a tetrad of microspores, each of which develops into a pollen grain. Separation of microspores does not occur in tetraspore (tes) mutants of Arabidopsis thaliana, owing to the failure of male meiotic cytokinesis. tes mutants thus generate large 'tetraspores' containing all the products of a single meiosis. Here, we report the positional cloning of the TES locus and details of the role played by the TES product in male cytokinesis. The predicted TES protein includes an N-terminal domain homologous to kinesin motors and a C-terminus with little similarity to other proteins except for a small number of plant kinesins. These include the Arabidopsis HINKEL protein and NACK1 and two from tobacco (Nishihama et al., 2002), which are involved in microtubule organization during mitotic cytokinesis. Immunocytochemistry shows that the characteristic radial arrays of microtubules associated with male meiotic cytokinesis fail to form in tes mutants. The TES protein therefore is likely to function as a microtubule-associated motor, playing a part either in the formation of the radial arrays that establish spore domains following meiosis, or in maintaining their stability.

Members of the Arabidopsis dynamin-like gene family, ADL1, are essential for plant cytokinesis and polarized cell growth

Polarized membrane trafficking during plant cytokinesis and cell expansion are critical for plant morphogenesis, yet very little is known about the molecular mechanisms that guide this process. Dynamin and dynamin-related proteins are large GTP binding proteins that are involved in membrane trafficking. Here, we show that two functionally redundant members of the Arabidopsis dynamin-related protein family, ADL1A and ADL1E, are essential for polar cell expansion and cell plate biogenesis. adl1A-2 adl1E-1 double mutants show defects in cell plate assembly, cell wall formation, and plasma membrane recycling. Using a functional green fluorescent protein fusion protein, we show that the distribution of ADL1A is dynamic and that the protein is localized asymmetrically to the plasma membrane of newly formed and mature root cells. We propose that ADL1-mediated membrane recycling is essential for plasma membrane formation and maintenance in plants.

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.

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.

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 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.

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.

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.

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.

The protein kinases AtMAP3Kepsilon1 and BnMAP3Kepsilon1 are functional homologues of S. pombe cdc7p and may be involved in cell division

We identified an Arabidopsis thaliana gene, AtMAP3Kepsilon1, and a Brassica napus cDNA, BnMAP3Kepsilon1, encoding functional protein serine/threonine kinases closely related to cdc7p and Cdc15p from Schizosaccharomyces pombe and Saccharomyces cerevisiae, respectively. This is the first report of cdc7-related genes in non-fungal eukaryotes; no such genes have as yet been identified in Metazoans. The B. napus protein is able to partially complement a cdc7 loss of function mutation in S. pombe. RT-PCR and in situ hybridisation revealed that the A. thaliana and B. napus genes are expressed in both the sporophytic and the gametophytic tissues of the respective plant species and revealed further that expression is highest in dividing cells. Moreover, AtMAP3Kepsilon1 gene expression is cell cycle-regulated, with higher expression in G2-M phases. Our results strongly suggest that the plant cdc7p-related protein kinases are involved in a signal transduction pathway similar to the SIN pathway, which positively regulates cytokinesis in S. pombe.