Isolated Plant Nuclei Nucleate Microtubule Assembly: The Nuclear Surface in Higher Plants Has Centrosome-like Activity

Division site determination during asymmetric cell division in plants

During development, both animals and plants exploit asymmetric cell division (ACD) to increase tissue complexity, a process that usually generates cells dissimilar in size, morphology, and fate. Plants lack the key regulators that control ACD in animals. Instead, plants have evolved two unique cytoskeletal structures to tackle this problem: the preprophase band (PPB) and phragmoplast. The assembly of the PPB and phragmoplast and their contributions to division plane orientation have been extensively studied. However, how the division plane is positioned off the cell center during asymmetric division is poorly understood. Over the past 20 years, emerging evidence points to a critical role for polarly localized membrane proteins in this process. Although many of these proteins are species- or cell type specific, and the molecular mechanism underlying division asymmetry is not fully understood, common features such as morphological changes in cells, cytoskeletal dynamics, and nuclear positioning have been observed. In this review, we provide updates on polarity establishment and nuclear positioning during ACD in plants. Together with previous findings about symmetrically dividing cells and the emerging roles of developmental cues, we aim to offer evolutionary insight into a common framework for asymmetric division-site determination and highlight directions for future work.

Microtubule nucleation for the assembly of acentrosomal microtubule arrays in plant cells

Contents Summary I. Introduction II. MT arrays in plant cells III. γ-Tubulin and MT nucleation IV. MT nucleation sites or flexible MTOCs in plant cells V. MT-dependent MT nucleation VI. Generating new MTs for spindle assembly VII. Generation of MTs for phragmoplast expansion during cytokinesis VIII. MT generation for the cortical MT array IX. MT nucleation: looking forward Acknowledgements References SUMMARY: Cytoskeletal microtubules (MTs) have a multitude of functions including intracellular distribution of molecules and organelles, cell morphogenesis, as well as segregation of the genetic material and separation of the cytoplasm during cell division among eukaryotic organisms. In response to internal and external cues, eukaryotic cells remodel their MT network in a regulated manner in order to assemble physiologically important arrays for cell growth, cell proliferation, or for cells to cope with biotic or abiotic stresses. Nucleation of new MTs is a critical step for MT remodeling. Although many key factors contributing to MT nucleation and organization are well conserved in different kingdoms, the centrosome, representing the most prominent microtubule organizing centers (MTOCs), disappeared during plant evolution as angiosperms lack the structure. Instead, flexible MTOCs may emerge on the plasma membrane, the nuclear envelope, and even organelles depending on types of cells and organisms and/or physiological conditions. MT-dependent MT nucleation is particularly noticeable in plant cells because it accounts for the primary source of MT generation for assembling spindle, phragmoplast, and cortical arrays when the γ-tubulin ring complex is anchored and activated by the augmin complex. It is intriguing what proteins are associated with plant-specific MTOCs and how plant cells activate or inactivate MT nucleation activities in spatiotemporally regulated manners.

Exportin Crm1 is repurposed as a docking protein to generate microtubule organizing centers at the nuclear pore

Non-centrosomal microtubule organizing centers (MTOCs) are important for microtubule organization in many cell types. In fission yeast Schizosaccharomyces pombe, the protein Mto1, together with partner protein Mto2 (Mto1/2 complex), recruits the γ-tubulin complex to multiple non-centrosomal MTOCs, including the nuclear envelope (NE). Here, we develop a comparative-interactome mass spectrometry approach to determine how Mto1 localizes to the NE. Surprisingly, we find that Mto1, a constitutively cytoplasmic protein, docks at nuclear pore complexes (NPCs), via interaction with exportin Crm1 and cytoplasmic FG-nucleoporin Nup146. Although Mto1 is not a nuclear export cargo, it binds Crm1 via a nuclear export signal-like sequence, and docking requires both Ran in the GTP-bound state and Nup146 FG repeats. In addition to determining the mechanism of MTOC formation at the NE, our results reveal a novel role for Crm1 and the nuclear export machinery in the stable docking of a cytoplasmic protein complex at NPCs.

Nuclear envelope: a new frontier in plant mechanosensing?

In animals, it is now well established that forces applied at the cell surface are propagated through the cytoskeleton to the nucleus, leading to deformations of the nuclear structure and, potentially, to modification of gene expression. Consistently, altered nuclear mechanics has been related to many genetic disorders, such as muscular dystrophy, cardiomyopathy and progeria. In plants, the integration of mechanical signals in cell and developmental biology has also made great progress. Yet, while the link between cell wall stresses and cytoskeleton is consolidated, such cortical mechanical cues have not been integrated with the nucleoskeleton. Here, we propose to take inspiration from studies on animal nuclei to identify relevant methods amenable to probing nucleus mechanics and deformation in plant cells, with a focus on microrheology. To identify potential molecular targets, we also compare the players at the nuclear envelope, namely lamina and LINC complex, in both plant and animal nuclei. Understanding how mechanical signals are transduced to the nucleus across kingdoms will likely have essential implications in development (e.g. how mechanical cues add robustness to gene expression patterns), in the nucleoskeleton-cytoskeleton nexus (e.g. how stress is propagated in turgid/walled cells), as well as in transcriptional control, chromatin biology and epigenetics.

The novel and taxonomically restricted Ah24 gene from grain amaranth (Amaranthus hypochondriacus) has a dual role in development and defense

Grain amaranths tolerate stress and produce highly nutritious seeds. We have identified several (a)biotic stress-responsive genes of unknown function in Amaranthus hypochondriacus, including the so-called Ah24 gene. Ah24 was expressed in young or developing tissues; it was also strongly induced by mechanical damage, insect herbivory and methyl jasmonate and in meristems and newly emerging leaves of severely defoliated plants. Interestingly, an in silico analysis of its 1304 bp promoter region showed a predominance of regulatory boxes involved in development, but not in defense. The Ah24 cDNA encodes a predicted cytosolic protein of 164 amino acids, the localization of which was confirmed by confocal microscopy. Additional in silico analysis identified several other Ah24 homologs, present almost exclusively in plants belonging to the Caryophyllales. The possible function of this gene in planta was examined in transgenic Ah24 overexpressing Arabidopsis thaliana and Nicotiana tabacum plants. Transformed Arabidopsis showed enhanced vegetative growth and increased leaf number with no penalty in one fitness component, such as seed yield, in experimental conditions. Transgenic tobacco plants, which grew and reproduced normally, had increased insect herbivory resistance. Modified vegetative growth in transgenic Arabidopsis coincided with significant changes in the expression of genes controlling phytohormone synthesis or signaling, whereas increased resistance to insect herbivory in transgenic tobacco coincided with higher jasmonic acid and proteinase inhibitor activity levels, plus the accumulation of nicotine and several other putative defense-related metabolites. It is proposed that the primary role of the Ah24 gene in A. hypochondriacus is to contribute to a rapid recovery post-wounding or defoliation, although its participation in defense against insect herbivory is also plausible.

Inside a plant nucleus: discovering the proteins

Nuclear proteins are a vital component of eukaryotic cell nuclei and have a profound effect on the way in which genetic information is stored, expressed, replicated, repaired, and transmitted to daughter cells and progeny. Because of the plethora of functions, nuclear proteins represent the most abundant components of cell nuclei in all eukaryotes. However, while the plant genome is well understood at the DNA level, information on plant nuclear proteins remains scarce, perhaps with the exception of histones and a few other proteins. This lack of knowledge hampers efforts to understand how the plant genome is organized in the nucleus and how it functions. This review focuses on the current state of the art of the analysis of the plant nuclear proteome. Previous proteome studies have generally been designed to search for proteins involved in plant response to various forms of stress or to identify rather a modest number of proteins. Thus, there is a need for more comprehensive and systematic studies of proteins in the nuclei obtained at individual phases of the cell cycle, or isolated from various tissue types and stages of cell and tissue differentiation. All this in combination with protein structure, predicted function, and physical localization in 3D nuclear space could provide much needed progress in our understanding of the plant nuclear proteome and its role in plant genome organization and function.

Microtubule initiation from the nuclear surface controls cortical microtubule growth polarity and orientation in Arabidopsis thaliana

The nuclear envelope in plant cells has long been known to be a microtubule organizing center (MTOC), but its influence on microtubule organization in the cell cortex has been unclear. Here we show that nuclear MTOC activity favors the formation of longitudinal cortical microtubule (CMT) arrays. We used green fluorescent protein (GFP)-tagged gamma tubulin-complex protein 2 (GCP2) to identify nuclear MTOC activity and GFP-tagged End-Binding Protein 1b (EB1b) to track microtubule growth directions. We found that microtubules initiate from nuclei and enter the cortex in two directions along the long axis of the cell, creating bipolar longitudinal CMT arrays. Such arrays were observed in all cell types showing nuclear MTOC activity, including root hairs, recently divided cells in root tips, and the leaf epidermis. In order to confirm the causal nature of nuclei in bipolar array formation, we displaced nuclei by centrifugation, which generated a corresponding shift in the bipolarity split point. We also found that bipolar CMT arrays were associated with bidirectional trafficking of vesicular components to cell ends. Together, these findings reveal a conserved function of plant nuclear MTOCs and centrosomes/spindle pole bodies in animals and fungi, wherein all structures serve to establish polarities in microtubule growth.

Functional organization and dynamics of the cell nucleus

The eukaryotic cell nucleus enclosed within the nuclear envelope harbors organized chromatin territories and various nuclear bodies as sub-nuclear compartments. This higher-order nuclear organization provides a unique environment to regulate the genome during replication, transcription, maintenance, and other processes. In this review, we focus on the plant four-dimensional nuclear organization, its dynamics and function in response to signals during development or stress.

The GIP gamma-tubulin complex-associated proteins are involved in nuclear architecture in Arabidopsis thaliana

During interphase, the microtubular cytoskeleton of cycling plant cells is organized in both cortical and perinuclear arrays. Perinuclear microtubules (MTs) are nucleated from γ-Tubulin Complexes (γ-TuCs) located at the surface of the nucleus. The molecular mechanisms of γ-TuC association to the nuclear envelope (NE) are currently unknown. The γ-TuC Protein 3 (GCP3)-Interacting Protein 1 (GIP1) is the smallest γ-TuC component identified so far. AtGIP1 and its homologous protein AtGIP2 participate in the localization of active γ-TuCs at interphasic and mitotic MT nucleation sites. Arabidopsis gip1gip2 mutants are impaired in establishing a fully functional mitotic spindle and exhibit severe developmental defects. In this study, gip1gip2 knock down mutants were further characterized at the cellular level. In addition to defects in both the localization of γ-TuC core proteins and MT fiber robustness, gip1gip2 mutants exhibited a severe alteration of the nuclear shape associated with an abnormal distribution of the nuclear pore complexes. Simultaneously, they showed a misorganization of the inner nuclear membrane protein AtSUN1. Furthermore, AtGIP1 was identified as an interacting partner of AtTSA1 which was detected, like the AtGIP proteins, at the NE. These results provide the first evidence for the involvement of a γ-TuC component in both nuclear shaping and NE organization. Functional hypotheses are discussed in order to propose a model for a GIP-dependent nucleo-cytoplasmic continuum.

Arabidopsis GCP3-interacting protein 1/MOZART 1 is an integral component of the γ-tubulin-containing microtubule nucleating complex

Microtubules in eukaryotic cells are nucleated from ring-shaped complexes that contain γ-tubulin and a family of homologous γ-tubulin complex proteins (GCPs), but the subunit composition of the complexes can vary among fungi, animals and plants. Arabidopsis GCP3-interacting protein 1 (GIP1), a small protein with no homology to the GCP family, interacts with GCP3 in vitro, and is a plant homolog of vertebrate mitotic-spindle organizing protein associated with a ring of γ-tubulin 1 (MOZART1), a recently identified component of the γ-tubulin complex in human cell lines. In this study, we characterized two closely related Arabidopsis GIP1s: GIP1a and GIP1b. Single mutants of gip1a and gip1b were indistinguishable from wild-type plants, but their double mutant was embryonic lethal, and showed impaired development of male gametophytes. Functional fusions of GIP1a with green fluorescent protein (GFP) were used to purify GIP1a-containing complexes from Arabidopsis plants, which contained all the subunits (except NEDD1) previously identified in the Arabidopsis γ-tubulin complexes. GIP1a and GIP1b interacted specifically with Arabidopsis GCP3 in yeast. GFP-GIP1a labeled mitotic microtubule arrays in a pattern largely consistent with, but partly distinct from, the localization of the γ-tubulin complex containing GCP2 or GCP3 in planta. In interphase cortical arrays, the labeled complexes were preferentially recruited to existing microtubules, from which new microtubules were efficiently nucleated. However, in contrast to complexes labeled with tagged GCP2 or GCP3, their recruitment to cortical areas with no microtubules was rarely observed. These results indicate that GIP1/MOZART1 is an integral component of a subset of the Arabidopsis γ-tubulin complexes.

The GCP3-interacting proteins GIP1 and GIP2 are required for γ-tubulin complex protein localization, spindle integrity, and chromosomal stability

Microtubules (MTs) are crucial for both the establishment of cellular polarity and the progression of all mitotic phases leading to karyokinesis and cytokinesis. MT organization and spindle formation rely on the activity of γ-tubulin and associated proteins throughout the cell cycle. To date, the molecular mechanisms modulating γ-tubulin complex location remain largely unknown. In this work, two Arabidopsis thaliana proteins interacting with gamma-tubulin complex protein3 (GCP3), GCP3-interacting protein1 (GIP1) and GIP2, have been characterized. Both GIP genes are ubiquitously expressed in all tissues analyzed. Immunolocalization studies combined with the expression of GIP-green fluorescent protein fusions have shown that GIPs colocalize with γ-tubulin, GCP3, and/or GCP4 and reorganize from the nucleus to the prospindle and the preprophase band in late G2. After nuclear envelope breakdown, they localize on spindle and phragmoplast MTs and on the reforming nuclear envelope of daughter cells. The gip1 gip2 double mutants exhibit severe growth defects and sterility. At the cellular level, they are characterized by MT misorganization and abnormal spindle polarity, resulting in ploidy defects. Altogether, our data show that during mitosis GIPs play a role in γ-tubulin complex localization, spindle stability and chromosomal segregation.

Nanoscale and geometric influences on the microtubule cytoskeleton in plants: thinking inside and outside the box

The dynamic microtubule (MT) cytoskeleton found in the cell cortex of plants drives cell expansion via cell wall modifications. In the last decade, live cell imaging studies employing green fluorescent protein have helped unravel the mechanisms behind how cells arrange cortical MTs into complex arrays and shape cell expansion. In this review, we explore the reverse scenario: how cell geometry and organelles influence and constrain the organization and behavior of cortical MTs. This newly emerging principle explains how cells perceive local nanoscale structural input from MT-organizing centers, such as the nucleus, endomembranes, and cell edges, and translate this into global cell-wide order via MT self-organization. Studies primarily using the model plant Arabidopsis thaliana and tobacco BY-2 suspension cultures have broadened our understanding of how cells form not only elegant parallel arrays but also more complex MT configurations, including the prominent MT bundles found in preprophase bands, leaf epidermal cells, and developing xylem.

The Arabidopsis TRM1-TON1 interaction reveals a recruitment network common to plant cortical microtubule arrays and eukaryotic centrosomes

Land plant cells assemble microtubule arrays without a conspicuous microtubule organizing center like a centrosome. In Arabidopsis thaliana, the TONNEAU1 (TON1) proteins, which share similarity with FOP, a human centrosomal protein, are essential for microtubule organization at the cortex. We have identified a novel superfamily of 34 proteins conserved in land plants, the TON1 Recruiting Motif (TRM) proteins, which share six short conserved motifs, including a TON1-interacting motif present in all TRMs. An archetypal member of this family, TRM1, is a microtubule-associated protein that localizes to cortical microtubules and binds microtubules in vitro. Not all TRM proteins can bind microtubules, suggesting a diversity of functions for this family. In addition, we show that TRM1 interacts in vivo with TON1 and is able to target TON1 to cortical microtubules via its C-terminal TON1 interaction motif. Interestingly, three motifs of TRMs are found in CAP350, a human centrosomal protein interacting with FOP, and the C-terminal M2 motif of CAP350 is responsible for FOP recruitment at the centrosome. Moreover, we found that TON1 can interact with the human CAP350 M2 motif in yeast. Taken together, our results suggest conservation of eukaryotic centrosomal components in plant cells.

Spores before sporophytes: hypothesizing the origin of sporogenesis at the algal-plant transition

Fossil spores from mid-Ordovician deposits (475 million yr old) are the first indication of plants on land and predate megafossils of plants by 30-50 million yr. Sporopollenin-walled spores distinguish land plants from algae, which typically have heavy-walled zygotes that germinate via meiosis into motile or protonemal cells. All land plants are embryophytes with spores produced by the sporophyte generation. It is generally assumed that retention of the zygote and delay in meiosis led to matrotrophic embryo development and intercalation of the diploid sporophyte before spore production. However, new data on the cell biology of sporogenesis in extant bryophytes suggest that spores were produced directly from zygotes in protoembryophytes. The mechanism of wall transfer from zygote to meiospores was a three-phase heterochrony involving precocious initiation of cytokinesis, acceleration of meiosis, and concomitant delay in wall deposition. In bryophyte sporogenesis, cytokinesis is typically initiated in advance of meiosis, and quadrilobing of the cytoplasm is followed by development of a bizarre quadripolar spindle that assures coordination of nuclear distribution with predetermined spore domains. This concept of the innovation of sporogenesis at the onset of terrestrialization provides a new perspective for interpreting fossil evidence and understanding the evolution of land plants.

The SIDECAR POLLEN gene encodes a microspore-specific LOB/AS2 domain protein required for the correct timing and orientation of asymmetric cell division

Cellular patterning and differentiation in plants depend on the balance of asymmetric and symmetric divisions. Patterning of the male gametophyte (pollen grains) in flowering plants requires asymmetric division of the microspore followed by a symmetric division of the germ cell to produce three highly differentiated cells: a single vegetative cell and two sperm cells. In Arabidopsis sidecar pollen (scp) mutants a proportion of microspores first divide symmetrically, and then go on to produce 'four-celled' pollen with an extra vegetative cell; however, details of the timing and origin of phenotypic defects in scp and the identity of the SCP gene have remained obscure. Comparative analysis of the original hypomorphic scp-1 allele and a T-DNA-induced null allele, scp-2, revealed that in the absence of SCP, microspores undergo normal nuclear positioning, but show delayed entry into mitosis, increased cell expansion and alterations in the orientation of nuclear division. We identified the SCP gene to encode a male gametophyte-specific LATERAL ORGAN BOUNDARIES DOMAIN/ASYMMETRIC LEAVES 2-like (LBD/ASL) protein that is expressed in microspore nuclei in a tightly regulated phase-specific manner. Therefore, our study demonstrates that the correct patterning of male gametophyte depends on the activity of a nuclear LBD/ASL family protein that is essential for the correct timing and orientation of asymmetric microspore division.

Diversity in meiotic spindle origin and determination of cytokinetic planes in sporogenesis of complex thalloid liverworts (Marchantiopsida)

As the earliest divergent land plants, bryophytes (mosses, hornworts, and liverworts) provide insight into the evolution of the unique plant process of sporogenesis by which meiosis results in heavy walled spores. New immunohistochemical data on microtubules and gamma-tubulin in four genera of complex thalloid liverworts combined with previously published data on another four genera demonstrate grades in the evolution of spindle organization in meiosis. We have discovered that all recognized forms of microtubule organizing centers (MTOCs) in plant cells (plastid MTOCs, spheroid cytoplasmic MTOCs, polar organizers, and nuclear envelope MTOCs) occur in organization of the meiotic spindle of complex thalloid liverworts. In addition, all aspects of pre-meiotic preparation for quadripartitioning of the sporocyte into a tetrad of spores occur, with the exception of pre-meiotic wall precursors found in certain simple thalloids. The preparation includes morphogenetic plastid migration, cortical bands of microtubules that mark future cytokinetic planes in pre-meiosis, quadrilobing of the cytoplasm during meiotic prophase, and quadripolar microtubule systems that are transformed into functionally bipolar metaphase I spindles. Quadripolar spindle origin is typical of bryophyte sporogenesis even though the MTOCs involved may differ. However, in certain crown taxa of complex thalloids the spindle develops with no traces of quadripolarity and placement of intersporal walls is determined after meiosis, as is typical of higher plants.

The Arabidopsis nuclear pore and nuclear envelope

The nuclear envelope is a double membrane structure that separates the eukaryotic cytoplasm from the nucleoplasm. The nuclear pores embedded in the nuclear envelope are the sole gateways for macromolecular trafficking in and out of the nucleus. The nuclear pore complexes assembled at the nuclear pores are large protein conglomerates composed of multiple units of about 30 different nucleoporins. Proteins and RNAs traffic through the nuclear pore complexes, enabled by the interacting activities of nuclear transport receptors, nucleoporins, and elements of the Ran GTPase cycle. In addition to directional and possibly selective protein and RNA nuclear import and export, the nuclear pore gains increasing prominence as a spatial organizer of cellular processes, such as sumoylation and desumoylation. Individual nucleoporins and whole nuclear pore subcomplexes traffic to specific mitotic locations and have mitotic functions, for example at the kinetochores, in spindle assembly, and in conjunction with the checkpoints. Mutants of nucleoporin genes and genes of nuclear transport components lead to a wide array of defects from human diseases to compromised plant defense responses. The nuclear envelope acts as a repository of calcium, and its inner membrane is populated by functionally unique proteins connected to both chromatin and-through the nuclear envelope lumen-the cytoplasmic cytoskeleton. Plant nuclear pore and nuclear envelope research-predominantly focusing on Arabidopsis as a model-is discovering both similarities and surprisingly unique aspects compared to the more mature model systems. This chapter gives an overview of our current knowledge in the field and of exciting areas awaiting further exploration.

Interaction of the Tobacco mosaic virus movement protein with microtubules during the cell cycle in tobacco BY-2 cells

Cell-to-cell movement of Tobacco mosaic virus (TMV) involves the interaction of virus-encoded 30-kDa movement protein (MP) with microtubules. In cells behind the infection front that accumulate high levels of MP, this activity is reflected by the formation of stabilized MP/microtubule complexes. The ability of MP to bind along and stabilize microtubules is conserved upon expression in mammalian cells. In mammalian cells, the protein also leads to inhibition of mitosis and cell division through a microtubule-independent process correlated with the loss of centrosomal gamma-tubulin and of centrosomal microtubule-nucleation activity. Since MP has the capacity to interact with plant factors involved in microtubule nucleation and dynamics, we used inducible expression in BY-2 cells to test whether MP expression inhibits mitosis and cell division also in plants. We demonstrate that MP:GFP associates with all plant microtubule arrays and, unlike in mammalian cells, does not interfere with mitosis. Thus, MP function and the interaction of MP with factors of the cytoskeleton do not entail an inhibition of mitosis in plants. We also report that the protein targets primary plasmodesmata in BY-2 cells immediately upon or during cytokinesis and that the accumulation of MP in plasmodesmata occurs in the presence of inhibitors of the cytoskeleton and the secretory pathway.

Dictyostelium discoideum CenB is a bona fide centrin essential for nuclear architecture and centrosome stability

Centrins are a family of proteins within the calcium-binding EF-hand superfamily. In addition to their archetypical role at the microtubule organizing center (MTOC), centrins have acquired multiple functionalities throughout the course of evolution. For example, centrins have been linked to different nuclear activities, including mRNA export and DNA repair. Dictyostelium discoideum centrin B is a divergent member of the centrin family. At the amino acid level, DdCenB shows 51% identity with its closest relative and only paralog, DdCenA. Phylogenetic analysis revealed that DdCenB and DdCenA form a well-supported monophyletic and divergent group within the centrin family of proteins. Interestingly, fluorescently tagged versions of DdCenB were not found at the centrosome (in whole cells or in isolated centrosomes). Instead, DdCenB localized to the nuclei of interphase cells. This localization disappeared as the cells entered mitosis, although Dictyostelium cells undergo a closed mitosis in which the nuclear envelope (NE) does not break down. DdCenB knockout cells exhibited aberrant nuclear architecture, characterized by enlarged and deformed nuclei and loss of proper centrosome-nucleus anchoring (observed as NE protrusions). At the centrosome, loss of DdCenB resulted in defects in the organization and morphology of the MTOC and supernumerary centrosomes and centrosome-related bodies. The multiple defects that the loss of DdCenB generated at the centrosome can be explained by its atypical division cycle, transitioning into the NE as it divides at mitosis. On the basis of these findings, we propose that DdCenB is required at interphase to maintain proper nuclear architecture, and before delocalizing from the nucleus, DdCenB is part of the centrosome duplication machinery.

The plant TPX2 protein regulates prospindle assembly before nuclear envelope breakdown

The Targeting Protein for Xklp2 (TPX2) is a central regulator of spindle assembly in vertebrate cells. The absence or excess of TPX2 inhibits spindle formation. We have defined a TPX2 signature motif that is present once in vertebrate sequences but twice in plants. Plant TPX2 is predominantly nuclear during interphase and is actively exported before nuclear envelope breakdown to initiate prospindle assembly. It localizes to the spindle microtubules but not to the interdigitating polar microtubules during anaphase or to the phragmoplast as it is rapidly degraded during telophase. We characterized the Arabidopsis thaliana TPX2-targeting domains and show that the protein is able to rescue microtubule assembly in TPX2-depleted Xenopus laevis egg extracts. Injection of antibodies to TPX2 into living plant cells inhibits the onset of mitosis. These results demonstrate that plant TPX2 already functions before nuclear envelope breakdown. Thus, plants have adapted nuclear-cytoplasmic shuttling of TPX2 to maintain proper spindle assembly without centrosomes.

Arabidopsis TONNEAU1 proteins are essential for preprophase band formation and interact with centrin

Plant cells have specific microtubule structures involved in cell division and elongation. The tonneau1 (ton1) mutant of Arabidopsis thaliana displays drastic defects in morphogenesis, positioning of division planes, and cellular organization. These are primarily caused by dysfunction of the cortical cytoskeleton and absence of the preprophase band of microtubules. Characterization of the ton1 insertional mutant reveals complex chromosomal rearrangements leading to simultaneous disruption of two highly similar genes in tandem, TON1a and TON1b. TON1 proteins are conserved in land plants and share sequence motifs with human centrosomal proteins. The TON1 protein associates with soluble and microsomal fractions of Arabidopsis cells, and a green fluorescent protein-TON1 fusion labels cortical cytoskeletal structures, including the preprophase band and the interphase cortical array. A yeast two-hybrid screen identified Arabidopsis centrin as a potential TON1 partner. This interaction was confirmed both in vitro and in plant cells. The similarity of TON1 with centrosomal proteins and its interaction with centrin, another key component of microtubule organizing centers, suggests that functions involved in the organization of microtubule arrays by the centrosome were conserved across the evolutionary divergence between plants and animals.

A novel function of plant histone H1: microtubule nucleation and continuous plus end association

In higher plant cells, various microtubular arrays can be seen despite of their lack of structurally defined microtubule-organizing centers (MTOCs) like centrosomes in animal cells. Little is known about the molecular properties of the microtubule-organizing centers in higher plant cells. The nuclear surface contains one of these microtubule-organizing centers and generates microtubules radially toward the cell periphery (radial microtubules). Previously, we reported that histone H1 possessed the microtubule-organizing activity, and it was suggested that histone H1 localized on the nuclear surfaces in Tobacco BY-2 cells (Nakayama, T., Ishii, T., Hotta, T., and Mizuno, K. J. Biol. Chem. (submitted)). Here we show that histone H1 forms ring-shaped complexes with tubulin, and these complexes nucleated and elongated the radial microtubules continuously (processively) associating with their proximal ends where the incorporation of tubulin occurred. Furthermore, the polarity of radial microtubules was determined to be proximal end plus. Immunofluorescence microscopy of the isolated nuclei revealed that histone H1 localized on the nuclear surfaces, distinct from that in the chromatin. These results indicate that radial microtubules are organized by a novel MTOC that is totally different from MTOCs previously found in either plant or animal cells.

Arabidopsis GCP2 and GCP3 are part of a soluble gamma-tubulin complex and have nuclear envelope targeting domains

In higher plants, microtubules (MTs) are assembled in distinctive arrays in the absence of a defined organizing center. Three MT nucleation sites have been described: the nuclear surface, the cell cortex and cortical MT branch points. The Arabidopsis thaliana (At) genome contains putative orthologues encoding all the components of characterized mammalian nucleation complexes: gamma-tubulin and gamma-tubulin complex proteins GCP2 to GCP6. We have cloned the cDNA encoding AtGCP2, and show that gamma-tubulin, AtGCP2 and AtGCP3 are part of the same tandem affinity-purified complex and are present in a large membrane-associated complex. In addition, small soluble gamma-tubulin complexes of the size expected for a gamma-tubulin core complex are recruited to isolated nuclei. Using immunogold labelling, AtGCP3 is localized to both the nuclear envelope (NE) and the plasma membrane. To identify domains that could play a role in targeting complexes to these nucleation sites, truncated AtGCP2- and AtGCP3-green fluorescent protein fusion proteins were expressed in BY-2 cells. Several domains from AtGCP2 and AtGCP3 are capable of targeting fusions to the NE. We propose that regulated recruitment of soluble gamma-tubulin-containing complexes is responsible for nucleation at dispersed sites in plant cells and contributes to the formation and organization of the various MT arrays.

3-D ultrastructure of O. tauri: electron cryotomography of an entire eukaryotic cell

The hallmark of eukaryotic cells is their segregation of key biological functions into discrete, membrane-bound organelles. Creating accurate models of their ultrastructural complexity has been difficult in part because of the limited resolution of light microscopy and the artifact-prone nature of conventional electron microscopy. Here we explored the potential of the emerging technology electron cryotomography to produce three-dimensional images of an entire eukaryotic cell in a near-native state. Ostreococcus tauri was chosen as the specimen because as a unicellular picoplankton with just one copy of each organelle, it is the smallest known eukaryote and was therefore likely to yield the highest resolution images. Whole cells were imaged at various stages of the cell cycle, yielding 3-D reconstructions of complete chloroplasts, mitochondria, endoplasmic reticula, Golgi bodies, peroxisomes, microtubules, and putative ribosome distributions in-situ. Surprisingly, the nucleus was seen to open long before mitosis, and while one microtubule (or two in some predivisional cells) was consistently present, no mitotic spindle was ever observed, prompting speculation that a single microtubule might be sufficient to segregate multiple chromosomes.

Diverse subcellular locations of cryptogein-induced reactive oxygen species production in tobacco Bright Yellow-2 cells

Reactive oxygen species (ROS) play a crucial role in many cellular responses and signaling pathways, including the oxidative burst defense response to pathogens. We have examined very early events in cryptogein-induced ROS production in tobacco (Nicotiana tabacum) Bright Yellow-2 suspension cells. Using Amplex Red and Amplex Ultra Red reagents, which report real-time H2O2 accumulation in cell populations, we show that the internal signal for H2O2 develops more rapidly than the external apoplastic signal. Subcellular accumulation of H2O2 was also followed in individual cells using the 2',7'-dichlorofluorescein diacetate fluorescent probe. Major accumulation was detected in endomembrane, cytoplasmic, and nuclear compartments. When cryptogein was added, the signal developed first in the nuclear region and, after a short delay, in the cell periphery. Interestingly, isolated nuclei were capable of producing H2O2 in a calcium-dependent manner, implying that nuclei can serve as a potential active source of ROS production. These results show complex spatial compartmentalization for ROS accumulation and an unexpected temporal sequence of events that occurs after cryptogein application, suggesting novel intricacy in ROS-signaling cascades.

Microtubule-dependent microtubule nucleation in plant cells

Regulation of microtubule nucleation sites is an essential step in microtubule organization. Cortical microtubule arrays in green plant cells at inter-phase are organized in a distinct manner--the array is formed in the absence of previously recognized organelles for microtubule nucleation, for example the centrosome and spindle pole body. Microtubules in the cortical array were recently found to be nucleated as branches on pre-existing microtubules via recruitment of cytosolic gamma-tubulin. In this review we briefly summarize the mechanism of microtubule-dependent microtubule nucleation and discuss a possible role of this mechanism in other cellular processes and their evolution.

Gamma-tubulin is essential for microtubule organization and development in Arabidopsis

The process of microtubule nucleation in plant cells is still a major question in plant cell biology. gamma-Tubulin is known as one of the key molecular players for microtubule nucleation in animal and fungal cells. Here, we provide genetic evidence that in Arabidopsis thaliana, gamma-tubulin is required for the formation of spindle, phragmoplast, and cortical microtubule arrays. We used a reverse genetics approach to investigate the role of the two Arabidopsis gamma-tubulin genes in plant development and in the formation of microtubule arrays. Isolation of mutants in each gene and analysis of two combinations of gamma-tubulin double mutants showed that the two genes have redundant functions. The first combination is lethal at the gametophytic stage. Disruption of both gamma-tubulin genes causes aberrant spindle and phragmoplast structures and alters nuclear division in gametophytes. The second combination of gamma-tubulin alleles affects late seedling development, ultimately leading to lethality 3 weeks after germination. This partially viable mutant combination enabled us to follow dynamically the effects of gamma-tubulin depletion on microtubule arrays in dividing cells using a green fluorescent protein marker. These results establish the central role of gamma-tubulin in the formation and organization of microtubule arrays in Arabidopsis.

The Origin of Centrosomes in Parthenogenetic Hymenopteran Insects

A longstanding enigma has been the origin of maternal centrosomes that facilitate parthenogenetic development in Hymenopteran insects. In young embryos, hundreds of microtubule-organizing centers (MTOCs) are assembled completely from maternal components. Two of these MTOCs join the female pronucleus to set up the first mitotic spindle in unfertilized embryos and drive their development. These MTOCs appear to be canonical centrosomes because they contain gamma-tubulin, CP190, and centrioles and they undergo duplication. Here, we present evidence that these centrosomes originate from accessory nuclei (AN), organelles derived from the oocyte nuclear envelope. In the parasitic wasps Nasonia vitripennis and Muscidifurax uniraptor, the position and number of AN in mature oocytes correspond to the position and number of maternal centrosomes in early embryos. These AN also contain high concentrations of gamma-tubulin. In the honeybee, Apis mellifera, distinct gamma-tubulin foci are present in each AN. Additionally, the Hymenopteran homolog of the Drosophila centrosomal protein Dgrip84 localizes on the outer surfaces of AN. These organelles disintegrate in the late oocyte, leaving behind small gamma-tubulin foci, which likely seed the formation of maternal centrosomes. Accessory nuclei, therefore, may have played a significant role in the evolution of haplodiploidy in Hymenopteran insects.

The fission yeast transforming acidic coiled coil-related protein Mia1p/Alp7p is required for formation and maintenance of persistent microtubule-organizing centers at the nuclear envelope

Microtubule-organizing centers (MTOCs) concentrate microtubule nucleation, attachment and bundling factors and thus restrict formation of microtubule arrays in spatial and temporal manner. How MTOCs occur remains an exciting question in cell biology. Here, we show that the transforming acidic coiled coil-related protein Mia1p/Alp7p functions in emergence of large MTOCs in interphase fission yeast cells. We found that Mia1p was a microtubule-binding protein that preferentially localized to the minus ends of microtubules and was associated with the sites of microtubule attachment to the nuclear envelope. Cells lacking Mia1p exhibited less microtubule bundles. Microtubules could be nucleated and bundled but were frequently released from the nucleation sites in mia1delta cells. Mia1p was required for stability of microtubule bundles and persistent use of nucleation sites both in interphase and postanaphase array dynamics. The gamma-tubulin-rich material was not organized in large perinuclear or microtubule-associated structures in mia1delta cells. Interestingly, absence of microtubules in dividing wild-type cells prevented appearance of large gamma-tubulin-rich MTOC structures in daughters. When microtubule polymerization was allowed, MTOCs were efficiently assembled de novo. We propose a model where MTOC emergence is a self-organizing process requiring the continuous association of microtubules with nucleation sites.

An Arabidopsis protein with a novel calcium-binding repeat sequence interacts with TONSOKU/MGOUN3/BRUSHY1 involved in meristem maintenance

TONSOKU(TSK)/MGOUN3/BRUSHY1 from Arabidopsis thaliana, which plays an important role in the maintenance of meristem organization, contains an LGN repeat motif similar to that found in animal proteins involved in asymmetric cell division. One protein that interacts with the LGN motif of TSK in a yeast two-hybrid screen, TSK-associating protein 1 (TSA1), contains a 10-fold repeat of a unique 41 amino acid sequence. The repeat sequence, with a glutamic acid-phenylalanine-glutamic acid (EFE) conserved core sequence, is enriched with acidic amino acids. TSA1 also contains an N-terminal putative signal peptide and it interacts with the LGN motif of TSK through a C-terminal region separated from the EFE repeats by a putative membrane-spanning region. The recombinant protein consisting of EFE repeats was rich in alpha-helical structure and possessed Ca2+-binding activity. Unlike nuclear localization of TSK, the TSA1 fused with green fluorescent protein (GFP) expressed in tobacco BY-2 cells was localized in small cytoplasmic vesicles during interphase. However, cellular localization of both TSA1-GFP and GFP-TSK changed dynamically during mitosis. In particular, both GFP-TSK and TSA1-GFP were concentrated in limited areas that are close to the ends of spindle microtubules ahead of separating chromatids. These results are discussed in terms of the possible involvement of TSK and TSA1 in mitosis.

Modulated targeting of GFP-AtMAP65-1 to central spindle microtubules during division

AtMAP65-1 bundles cortical microtubules and we examined how this property is regulated during division in time-lapse studies of Arabidopsis suspension cells expressing GFP-AtMAP65-1. Spindle fluorescence is diffuse during metaphase, restored to the central spindle at anaphase and then compacted at the midline during late anaphase/early telophase. However, mutagenesis of the microtubule-associated protein (MAP) consensus Cdk site to a non-phosphorylatable form allows premature decoration of microtubules traversing the central region of the metaphase spindle without affecting the timing of the subsequent compaction. This suggests that mutagenesis does not affect compaction but does affect a phosphorylation/dephosphorylation switch that normally targets AtMAP65-1 to the central spindle at the metaphase/anaphase transition. GFP-AtMAP65-1 continues to label the midline of the early phragmoplast, suggesting a structural continuity with the central spindle - both structures being composed of anti-parallel microtubules. However, once the cytokinetic apparatus expands into a ring the MAP becomes depleted at the midline. Despite this, cytokinesis is not arrested and membrane and callose are deposited at the cell plate. It is concluded that AtMAP65-1 plays a role in the central spindle at anaphase to early cytokinesis but is not essential at the midline of the phragmoplast at later stages.

Gamma-tubulin and microtubule organization during microsporogenesis in Ginkgo biloba

This is the first report on gamma-tubulin and microtubule arrays during microsporogenesis in a gymnosperm. Meiosis in Ginkgo biloba is polyplastidic, as is typical of the spermatophyte clade, and microtubule arrays are organized at various sites during meiosis and cytokinesis. In early prophase, a cluster of gamma-tubulin globules occurs in the central cytoplasm adjacent to the off-center nucleus. These globules diminish in size and spread over the surface of the nucleus. A system of microtubules focused on the gamma-tubulin forms a reticulate pattern in the cytoplasm. As the nucleus migrates to the center of the microsporocyte, gamma-tubulin becomes concentrated at several sites adjacent to the nuclear envelope. Microtubules organized at these foci of gamma-tubulin give rise to a multipolar prophase spindle. By metaphase I, the spindle has matured into a distinctly bipolar structure with pointed poles. In both first and second meiosis, gamma-tubulin becomes distributed throughout the metaphase spindles, but becomes distinctly polar again in anaphase. In telophase I, gamma-tubulin moves from polar regions to the proximal surface of chromosome groups/nuclei where interzonal microtubules are organized. No cell wall is deposited and the interzonal microtubules embrace a plate of organelles between the two nuclear cytoplasmic domains (NCDs) of the dyad. Following second meiosis, phragmoplasts that form between sister and non-sister nuclei fuse to form a complex six-sided structure that directs simultaneous cytokinesis. Gamma-tubulin becomes associated with nuclei after both meiotic divisions and is especially conspicuous in the distal hemisphere of each young microspore where an unusual encircling system of cortical microtubules develops.

Reorganization and in vivo dynamics of microtubules during Arabidopsis root hair development

Root hairs emerge from epidermal root cells (trichoblasts) and differentiate by highly localized tip growth. Microtubules (MTs) are essential for establishing and maintaining the growth polarity of root hairs. The current knowledge about the configuration of the MT cytoskeleton during root hair development is largely based on experiments on fixed material, and reorganization and in vivo dynamics of MTs during root hair development is at present unclear. This in vivo study provides new insights into the mechanisms of MT (re)organization during root hair development in Arabidopsis (Arabidopsis thaliana). Expression of a binding site of the MT-associated protein-4 tagged with green fluorescent protein enabled imaging of MT nucleation, growth, and shortening and revealed distinct MT configurations. Depending on the dynamics of the different MT populations during root hair development, either repeated two-dimensional (x, y, t) or repeated three-dimensional (x, y, z, t) scanning was performed. Furthermore, a new image evaluation tool was developed to reveal important data on MT instability. The data show how MTs reorient after apparent contact with other MTs and support a model for MT alignment based on repeated reorientation of dynamic MT growth.

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.

Disruption of actin microfilaments causes cortical microtubule disorganization and extra-phragmoplast formation at M/G1 interface in synchronized tobacco cells

The roles of actin microfilaments (MFs) in the organization of microtubules (MTs) at the M/G1 interface were investigated in transgenic tobacco BY-2 cells stably expressing a GFP-tubulin fusion protein, using the MF-disrupting agent, Bistheonellide A (BA). When MFs were disrupted by BA treatment, cortical MTs (CMTs) did not become reorganized even 3 h after phragmoplast collapse, whereas non-treated cells completed CMT reorganization within 1 h. Furthermore, in the absence of MFs, the tubulin proteins did not show appropriate recruitment but remained at the site where the phragmoplast had existed, or extra-phragmoplasts were organized. These extra-phragmoplasts could functionally form extra-cell plates. This is the first observation of the formation of multiple cell plates during one nuclear division, and of phragmoplast generation irrespective of the position of the mitotic spindle or nuclei. The significance of these observations on the role of MFs at the M/G1 interface is discussed.

Microtubules become more dynamic but not shorter during preprophase band formation: a possible "search-and-capture" mechanism for microtubule translocation

The dynamic behavior of the microtubule cytoskeleton plays a crucial role in cellular organization, but the physical mechanisms underlying microtubule (re)organization in plant cells are poorly understood. We investigated microtubule dynamics in tobacco BY-2 suspension cells during interphase and during the formation of the preprophase band (PPB), the cytoskeletal structure that defines the site of cytokinesis. Here we show that after 2 h of microtubule accumulation in the PPB and concurrent disappearance elsewhere in the cortex, the PPB is completed and starts to breakdown exponentially already 20 min before the onset of prometaphase. During formation of the PPB, the dynamic instability, i.e., the stochastic alternating between growing and shrinking phases, of the cortical microtubules outside the PPB increases significantly, but the microtubules do not become shorter. Based on this, as well as on the cross-linking of microtubules in the PPB and the lack of evidence for motor involvement, we propose a "search-and-capture" mechanism for PPB formation, in which the regulation of dynamic instability causes the cortical microtubules to become more dynamic and possibly longer, while the microtubule cross-linking activity of the developing PPB preferentially stabilizes these "searching" microtubules. Thus, microtubules gradually disappear from the cortex outside the PPB and aggregate to the forming PPB.

Gamma-tubulin in basal land plants: characterization, localization, and implication in the evolution of acentriolar microtubule organizing centers

Although seed plants have gamma-tubulin, a ubiquitous component of centrosomes associated with microtubule nucleation in algal and animal cells, they do not have discrete microtubule organizing centers (MTOCs) comparable to animal centrosomes, and the organization of microtubule arrays in plants has remained enigmatic. Spindle development in basal land plants has revealed a surprising variety of MTOCs that may represent milestones in the evolution of the typical diffuse acentrosomal plant spindle. We have isolated and characterized the gamma-tubulin gene from a liverwort, one of the extant basal land plants. Sequence similarity to the gamma-tubulin gene of higher plants suggests that the gamma-tubulin gene is highly conserved in land plants. The G9 antibody to fission yeast gamma-tubulin recognized a single band of 55 kD in immunoblots from bryophytes. Immunohistochemistry with the G9 antibody clearly documented the association of gamma-tubulin with various MTOC sites in basal land plants (e.g., discrete centrosomes with and without centrioles and the plastid surface in monoplastidic meiosis of bryophytes). Changes in the distribution of gamma-tubulin occur in a cell cycle-specific manner during monoplastidic meiosis in the liverwort Dumortiera hirsuta. gamma-Tubulin changes its localization from the plastid surface in prophase I to the spindle, from the spindle to phragmoplasts and the nuclear envelope in telophase I, and back to the plastid surfaces in prophase II. In vitro experiments show that gamma-tubulin is detectable on the surface of isolated plastids and nuclei of D. hirsuta, and microtubules can be repolymerized from the isolated plastids. gamma-Tubulin localization patterns on plastid and nuclear surfaces are not affected by the destruction of microtubules by oryzalin. We conclude that gamma-tubulin is a highly conserved protein associated with microtubule nucleation in basal land plants and that it has a cell cycle-dependent distribution essential for the orderly succession of microtubule arrays.

The plant nuclear envelope

This review summarizes our present knowledge about the composition and function of the plant nuclear envelope. Compared with animals or yeast, our molecular understanding of the nuclear envelope in higher plants is in its infancy. However, fundamental differences in the structure and function of the plant and animal nuclear envelope have already been found. Here, we compare and contrast these differences with respect to nuclear pore complexes, targeting of Ran signaling to the nuclear envelope, inner nuclear envelope proteins, and the role and fate of the nuclear envelope during mitosis. Further investigation of the emerging fundamental differences as well as the similarities between kingdoms might illuminate why there appears to be more than one blueprint for building a nucleus.

Microtubules and the shape of plants to come

Plants control the direction of cell expansion as a way of shaping growth. Since their discovery in plants 40 years ago, microtubules have been suspected of forming a template that helps to regulate the direction of growth. The detailed mechanism, however, has been elusive, especially as plants lack a microtubule-organizing centre. Developmental mutants are now beginning to show how microtubules are organized and how this affects plant morphology.

Reorganization of microtubule nucleation during muscle differentiation

Skeletal muscle differentiation involves a complete reorganization of the microtubule network. Nearly 20 years ago, Tassin et al. [1985: J Cell Biol 100:35-46] suggested a mechanism for this reorganization by showing a redistribution of the microtubule organizing center from the centrosome to the nuclear membrane. Little progress has been made since. It is still not clear whether centrosomal proteins are redistributed together, whether microtubules are nucleated at the nuclear membrane or transported there post-nucleation, and whether gamma-tubulin (gammatub) remains necessary for nucleation in myotubes. To investigate these questions, we have examined the redistribution of the centrosomal proteins pericentrin (PC), gammatub, and ninein in the C2 muscle cell line. Immunofluorescence of differentiated myotubes shows PC along the nuclear membrane whereas gammatub is only detected there after pre-fixation detergent extraction. After expression of a GFP-tagged gammatub, we observe a weak fluorescence along the nuclear membrane, confirming the presence of gammatub at a low concentration relative to PC. Microinjection of anti-gammatub antibodies into myotubes blocks microtubule growth from both nuclear membranes and centrosomal sites. The centrosomal microtubule-anchoring protein, ninein, is found at the nuclear membrane as well and its distribution appears independent of microtubule integrity. We conclude that centrosomal proteins are redistributed independently during muscle differentiation, to sites that nucleate microtubules both along the nuclear membranes and through the cytoplasm.

A kinesin mutant with an atypical bipolar spindle undergoes normal mitosis

Motor proteins have been implicated in various aspects of mitosis, including spindle assembly and chromosome segregation. Here, we show that acentrosomal Arabidopsis cells that are mutant for the kinesin, ATK1, lack microtubule accumulation at the predicted spindle poles during prophase and have reduced spindle bipolarity during prometaphase. Nonetheless, all abnormalities are rectified by anaphase and chromosome segregation appears normal. We conclude that ATK1 is required for normal microtubule accumulation at the spindle poles during prophase and possibly functions in spindle assembly during prometaphase. Because aberrant spindle morphology in these mutants is resolved by anaphase, we postulate that mitotic plant cells contain an error-correcting mechanism. Moreover, ATK1 function seems to be dosage-dependent, because cells containing one wild-type allele take significantly longer to proceed to anaphase as compared with cells containing two wild-type alleles.

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.

Dynamic changes and the role of the cytoskeleton during the cell cycle in higher plant cells

In higher plant cells microtubules (MTs) show dynamic structural changes during cell cycle progression and play significant roles in cell morphogenesis. The cortical MT (CMT), preprophase band (PPB), and phragmoplast, all of which are plant-specific MT structures, can be observed during interphase, from the late G2 phase to prophase, and from anaphase to telophase, respectively. The CMT controls cell shape, either irreversibly or reversibly, by orientating cellulose microfibril (CMF) deposition in the cell wall; the PPB is involved in determining the site of division; and the phragmoplast forms the cell plate at cytokinesis. The appearance and disappearance of these MT structures during the cell cycle have been extensively studied by immunofluorescence microscopy using highly synchronized tobacco BY-2 cells. Indeed, these studies, together with visualization of MT dynamics in living plant cells using the green fluorescent protein, have revealed much about the modes of MT structural organization, for example, of CMTs at the M/G1 interphase. The microfilaments which also show dynamic changes during the cell cycle, being similar to MTs at particular stages and different at other stages, appear to play roles in supporting MTs. In this article, we summarize our ongoing research and that of related studies of the structure and function of the plant cytoskeleton during cell cycle progression.

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.

ENDOSPERM DEVELOPMENT: Cellularization and Cell Fate Specification

The endosperm develops from the central cell of the megagametophyte after introduction of the second male gamete into the diploid central cell. Of the three forms of endosperm in angiosperms, the nuclear type is prevalent in economically important species, including the cereals. Landmarks in nuclear endosperm development are the coenocytic, cellularization, differentiation, and maturation stages. The differentiated endosperm contains four major cell types: starchy endosperm, aleurone, transfer cells, and the cells of the embryo surrounding region. Recent research has demonstrated that the first two phases of endosperm occur via mechanisms that are conserved among all groups of angiosperms, involving directed nuclear migration during the coenocytic stage and anticlinal cell wall deposition by cytoplasmic phragmoplasts formed in interzones between radial microtubular systems emanating from nuclear membranes. Complete cellularization of the endosperm coenocyte is achieved through centripetal growth of cell files, extending to the center of the endosperm cavity. Key points in cell cycle control and control of the MT (microtubular) cytoskeletal apparatus central to endosperm development are discussed. Specification of cell fates in the cereal endosperm appears to occur via positional signaling; cells in peripheral positions, except over the main vascular tissues, assume aleurone cell fate. Cells over the main vascular tissue become transfer cells and all interior cells become starchy endosperm cells. Studies in maize have implicated Crinkly4, a protein receptor kinase-like molecule, in aleurone cell fate specification.