Dynamics of the plant nuclear envelope and nuclear pore

Life-cycle-coupled evolution of mitosis in close relatives of animals

Eukaryotes have evolved towards one of two extremes along a spectrum of strategies for remodelling the nuclear envelope during cell division: disassembling the nuclear envelope in an open mitosis or constructing an intranuclear spindle in a closed mitosis1,2. Both classes of mitotic remodelling involve key differences in the core division machinery but the evolutionary reasons for adopting a specific mechanism are unclear. Here we use an integrated comparative genomics and ultrastructural imaging approach to investigate mitotic strategies in Ichthyosporea, close relatives of animals and fungi. We show that species in this clade have diverged towards either a fungal-like closed mitosis or an animal-like open mitosis, probably to support distinct multinucleated or uninucleated states. Our results indicate that multinucleated life cycles favour the evolution of closed mitosis.

Nuclear envelope dynamics in connection to chromatin remodeling

The nucleus is a central organelle of eukaryotic cells undergoing dynamic structural changes during cellular fundamental processes such as proliferation and differentiation. These changes rely on the integration of developmental and stress signals at the nuclear envelope (NE), orchestrating responses at the nucleo-cytoplasmic interface for efficient genomic functions such as DNA transcription, replication and repair. While in animals, correlation has already been established between NE dynamics and chromatin remodeling using last-generation tools and cutting-edge technologies, this topic is just emerging in plants, especially in response to mechanical cues. This review summarizes recent data obtained in this field with more emphasis on the mechanical stress response. It also highlights similarities/differences between animal and plant cells at multiples scales, from the structural organization of the nucleo-cytoplasmic continuum to the functional impacts of NE dynamics.

Characterisation of the trans-membrane nucleoporins GP210 and NDC1 in Arabidopsis thaliana

The nuclear pore is structurally conserved across eukaryotes as are many of the pore's constituent proteins. The transmembrane nuclear pore proteins GP210 and NDC1 span the nuclear envelope holding the nuclear pore in place. Orthologues of GP210 and NDC1 in Arabidopsis were investigated through characterisation of T-DNA insertional mutants. While the T-DNA insert into GP210 reduced expression of the gene, the insert in the NDC1 gene resulted in increased expression in both the ndc1 mutant as well as the ndc1/gp210 double mutant. The ndc1 and gp210 individual mutants showed little phenotypic difference from wild-type plants, but the ndc1/gp210 mutant showed a range of phenotypic effects. As with many plant nuclear pore protein mutants, these effects included non-nuclear phenotypes such as reduced pollen viability, reduced growth and glabrous leaves in mature plants. Importantly, however, ndc1/gp210 exhibited nuclear-specific effects including modifications to nuclear shape in different cell types. We also observed functional changes to nuclear transport in ndc1/gp210 plants, with low levels of cytoplasmic fluorescence observed in cells expressing nuclear-targeted GFP. The lack of phenotypes in individual insertional lines, and the relatively mild phenotype suggests that additional transmembrane nucleoporins, such as the recently-discovered CPR5, likely compensate for their loss.

A dynamic intron retention program regulates the expression of several hundred genes during pollen meiosis

Intron retention is a stage-specific mechanism of functional attenuation of a subset of co-regulated, functionally related genes during early stages of pollen development. To improve our understanding of the gene regulatory mechanisms that drive developmental processes, we performed a genome-wide study of alternative splicing and isoform switching during five key stages of pollen development in field mustard, Brassica rapa. Surprisingly, for several hundred genes (12.3% of the genes analysed), isoform switching results in stage-specific expression of intron-retaining transcripts at the meiotic stage of pollen development. In such cases, we report temporally regulated switching between expression of a canonical, translatable isoform and an intron-retaining transcript that is predicted to produce a truncated and presumably inactive protein. The results suggest a new pervasive mechanism underlying modulation of protein levels in a plant developmental program. The effect is not based on gene expression induction but on the type of transcript produced. We conclude that intron retention is a stage-specific mechanism of functional attenuation of a subset of co-regulated, functionally related genes during meiosis, especially genes related to ribosome biogenesis, mRNA transport and nuclear envelope architecture. We also propose that stage-specific expression of a non-functional isoform of Brassica rapa BrSDG8, a non-redundant member of histone methyltransferase gene family, linked to alternative splicing regulation, may contribute to the intron retention observed.

The role of the nuclear envelope in the regulation of chromatin dynamics during cell division

The nuclear envelope delineates the eukaryotic cell nucleus. The membrane system of the nuclear envelope consists of an outer nuclear membrane and an inner nuclear membrane separated by a perinuclear space. It serves as more than just a static barrier, since it regulates the communication between the nucleoplasm and the cytoplasm and provides the anchoring points where chromatin is attached. Fewer nuclear envelope proteins have been identified in plants in comparison with animals and yeasts. Here, we review the current state of knowledge of the nuclear envelope in plants, focusing on its role as a chromatin organizer and regulator of gene expression, as well as on the modifications that it undergoes to be efficiently disassembled and reassembled with each cell division. Advances in knowledge concerning the mitotic role of some nuclear envelope constituents are also presented. In addition, we summarize recent progress on the contribution of the nuclear envelope elements to telomere tethering and chromosome dynamics during the meiotic division in different plant species.

Nup96 and HOS1 Are Mutually Stabilized and Gate CONSTANS Protein Level, Conferring Long-Day Photoperiodic Flowering Regulation in Arabidopsis

The nuclear pore complex profoundly affects the timing of flowering; however, the underlying mechanisms are poorly understood. Here, we report that Nucleoporin96 (Nup96) acts as a negative regulator of long-day photoperiodic flowering in Arabidopsis (Arabidopsis thaliana). Through multiple approaches, we identified the E3 ubiquitin ligase HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE1 (HOS1) and demonstrated its interaction in vivo with Nup96. Nup96 and HOS1 mainly localize and interact on the nuclear membrane. Loss of function of Nup96 leads to destruction of HOS1 proteins without a change in their mRNA abundance, which results in overaccumulation of the key activator of long-day photoperiodic flowering, CONSTANS (CO) proteins, as previously reported in hos1 mutants. Unexpectedly, mutation of HOS1 strikingly diminishes Nup96 protein level, suggesting that Nup96 and HOS1 are mutually stabilized and thus form a novel repressive module that regulates CO protein turnover. Therefore, the nup96 and hos1 single and nup96 hos1 double mutants have highly similar early-flowering phenotypes and overlapping transcriptome changes. Together, this study reveals a repression mechanism in which the Nup96-HOS1 repressive module gates the level of CO proteins and thereby prevents precocious flowering in long-day conditions.

Identification and characterization of genes encoding the nuclear envelope LINC complex in the monocot species Zea mays

The linker of nucleoskeleton to cytoskeleton (LINC) complex is an essential multi-protein structure spanning the nuclear envelope. It connects the cytoplasm to the nucleoplasm, functions to maintain nuclear shape and architecture and regulates chromosome dynamics during cell division. Knowledge of LINC complex composition and function in the plant kingdom is primarily limited to Arabidopsis, but critically missing from the evolutionarily distant monocots, which include grasses, the most important agronomic crops worldwide. To fill this knowledge gap, we identified and characterized 22 maize genes, including a new grass-specific KASH gene family. By using bioinformatic, biochemical and cell biological approaches, we provide evidence that representative KASH candidates localize to the nuclear periphery and interact with Zea mays (Zm)SUN2 in vivo FRAP experiments using domain deletion constructs verified that this SUN-KASH interaction was dependent on the SUN but not the coiled-coil domain of ZmSUN2. A summary working model is proposed for the entire maize LINC complex encoded by conserved and divergent gene families. These findings expand our knowledge of the plant nuclear envelope in a model grass species, with implications for both basic and applied cellular research.This article has an associated First Person interview with the first author of the paper.

In situ architecture of the algal nuclear pore complex

Nuclear pore complexes (NPCs) span the nuclear envelope and mediate nucleocytoplasmic exchange. They are a hallmark of eukaryotes and deeply rooted in the evolutionary origin of cellular compartmentalization. NPCs have an elaborate architecture that has been well studied in vertebrates. Whether this architecture is unique or varies significantly in other eukaryotic kingdoms remains unknown, predominantly due to missing in situ structural data. Here, we report the architecture of the algal NPC from the early branching eukaryote Chlamydomonas reinhardtii and compare it to the human NPC. We find that the inner ring of the Chlamydomonas NPC has an unexpectedly large diameter, and the outer rings exhibit an asymmetric oligomeric state that has not been observed or predicted previously. Our study provides evidence that the NPC is subject to substantial structural variation between species. The divergent and conserved features of NPC architecture provide insights into the evolution of the nucleocytoplasmic transport machinery.

Human Cytomegalovirus Nuclear Egress Proteins Ectopically Expressed in the Heterologous Environment of Plant Cells are Strictly Targeted to the Nuclear Envelope

In all eukaryotic cells, the nucleus forms a prominent cellular compartment containing the cell's nuclear genome. Although structurally similar, animal and plant nuclei differ substantially in details of their architecture. One example is the nuclear lamina, a layer of tightly interconnected filament proteins (lamins) underlying the nuclear envelope of metazoans. So far no orthologous lamin genes could be detected in plant genomes and putative lamin-like proteins are only poorly described in plants. To probe for potentially conserved features of metazoan and plant nuclear envelopes, we ectopically expressed the core nuclear egress proteins of human cytomegalovirus pUL50 and pUL53 in plant cells. pUL50 localizes to the inner envelope of metazoan nuclei and recruits the nuclear localized pUL53 to it, forming heterodimers. Upon expression in plant cells, a very similar localization pattern of both proteins could be determined. Notably, pUL50 is specifically targeted to the plant nuclear envelope in a rim-like fashion, a location to which coexpressed pUL53 becomes strictly corecruited from its initial nucleoplasmic distribution. Using pUL50 as bait in a yeast two-hybrid screening, the cytoplasmic re-initiation supporting protein RISP could be identified. Interaction of pUL50 and RISP could be confirmed by coexpression and coimmunoprecipitation in mammalian cells and by confocal laser scanning microscopy in plant cells, demonstrating partial pUL50-RISP colocalization in areas of the nuclear rim and other intracellular compartments. Thus, our study provides strong evidence for conserved structural features of plant and metazoan nuclear envelops and identifies RISP as a potential pUL50-interacting plant protein.

Dynamic Distribution and Interaction of the Arabidopsis SRSF1 Subfamily Splicing Factors

Ser/Arg-rich (SR) proteins are essential nucleus-localized splicing factors. Our prior studies showed that Arabidopsis (Arabidopsis thaliana) RSZ22, a homolog of the human SRSF7 SR factor, exits the nucleus through two pathways, either dependent or independent on the XPO1 receptor. Here, we examined the expression profiles and shuttling dynamics of the Arabidopsis SRSF1 subfamily (SR30, SR34, SR34a, and SR34b) under control of their endogenous promoter in Arabidopsis and in transient expression assay. Due to its rapid nucleocytoplasmic shuttling and high expression level in transient assay, we analyzed the multiple determinants that regulate the localization and shuttling dynamics of SR34. By site-directed mutagenesis of SR34 RNA-binding sequences and Arg/Ser-rich (RS) domain, we further show that functional RRM1 or RRM2 are dispensable for the exclusive protein nuclear localization and speckle-like distribution. However, mutations of both RRMs induced aggregation of the protein whereas mutation in the RS domain decreased the stability of the protein and suppressed its nuclear accumulation. Furthermore, the RNA-binding motif mutants are defective for their export through the XPO1 (CRM1/Exportin-1) receptor pathway, but retain nucleocytoplasmic mobility. We performed a yeast two hybrid screen with SR34 as bait and discovered SR45 as a new interactor. SR45 is an unusual SR splicing factor bearing two RS domains. These interactions were confirmed in planta by FLIM-FRET and BiFC and the roles of SR34 domains in protein-protein interactions were further studied. Altogether, our report extends our understanding of shuttling dynamics of Arabidopsis SR splicing factors.

GIP Contributions to the Regulation of Centromere at the Interface Between the Nuclear Envelope and the Nucleoplasm

Centromeres are known as specific chromatin domains without which eukaryotic cells cannot divide properly during mitosis. Despite the considerable efforts to understand the centromere/kinetochore assembly during mitosis, until recently, comparatively few studies have dealt with the regulation of centromere during interphase. Here, we briefly review and discuss past and recent advances about the architecture of centromeres and their regulation during the cell cycle. Furthermore, we highlight and discuss new findings and hypotheses regarding the specific regulation of centromeres in both plant and animal nuclei, especially with GIP proteins at the interface between the nuclear envelope and the nucleoplasm.

Mitotic lifecycle of chromosomal 3xHMG-box proteins and the role of their N-terminal domain in the association with rDNA loci and proteolysis

The high mobility group (HMG)-box is a DNA-binding domain characteristic of various eukaryotic DNA-binding proteins. 3xHMG-box proteins (containing three copies of the HMG-box domain and a unique basic N-terminal domain) are specific for plants and the Arabidopsis genome encodes two versions termed 3xHMG-box1 and 3xHMG-box2, whose expression is cell cycle-dependent, peaking during mitosis. Here, we analysed in detail the spatiotemporal expression, subcellular localisation and chromosome association of the Arabidopsis thaliana 3xHMG-box proteins. Live cell imaging and structured illumination microscopy revealed that the expression of the 3xHMG-box proteins is induced in late G2 phase of the cell cycle and upon nuclear envelope breakdown in prophase they rapidly associate with the chromosomes. 3xHMG-box1 associates preferentially with 45S rDNA loci and the basic N-terminal domain is involved in the targeting of rDNA loci. Shortly after mitosis the 3xHMG-box proteins are degraded and an N-terminal destruction-box mediates the proteolysis. Ectopic expression/localisation of 3xHMG-box1 in interphase nuclei results in reduced plant growth and various developmental defects including early bolting and abnormal flower morphology. The remarkable conservation of 3xHMG-box proteins within the plant kingdom, their characteristic expression during mitosis, and their striking association with chromosomes, suggest that they play a role in the organisation of plant mitotic chromosomes.

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.

The plant LINC complex at the nuclear envelope

Significant advances in understanding the plant nuclear envelope have been made over the past few years; indeed, knowledge of the protein network at the nuclear envelope is rapidly growing. One such network, the linker of nucleoskeleton and cytoskeleton (LINC) complex, is known in animals to connect chromatin to the cytoskeleton through the nuclear envelope. The LINC complex is made of Sad1/Unc84 (SUN) and Klarsicht/Anc1/Syne1 homology (KASH) proteins which have been recently characterized in plants. SUN proteins are located within the inner nuclear membrane, while the KASH proteins are included into the outer nuclear membrane. SUN and KASH domains interact and bridge the two nuclear membranes. In Arabidopsis, KASH proteins also interact with the tryptophan-proline-proline (WPP) domain-interacting tail-anchored protein 1 (WIT1), associated with the nuclear pore complex and with myosin XI-i which directly interacts with the actin cytoskeleton. Although evidence for a plant LINC complex connecting the nucleus to the cytoskeleton is growing, its interaction with chromatin is still unknown, but knowledge gained from animal models strongly suggests its existence in plants. Possible functions of the plant LINC complex in cell division, nuclear shape, and chromatin organization are discussed.

Polarized endoplasmic reticulum aggregations in the establishing division plane of protodermal cells of the fern Asplenium nidus

The determination of the division plane in protodermal cells of the fern Asplenium nidus occurs during interphase with the formation of the phragmosome, the organization of which is controlled by the actomyosin system. Usually, the phragmosomes between adjacent cells were oriented on the same plane. In the phragmosomal cortical cytoplasm, an interphase microtubule (MT) ring was formed and large quantities of endoplasmic reticulum (ER) membranes were gathered, forming an interphase U-like ER bundle. During preprophase/prophase, the interphase MT ring and the U-like ER bundle were transformed into a MT and an ER preprophase band (PPB), respectively. Parts of the ER-PPB were maintained during mitosis. Furthermore, the plasmalemma as well as the nuclear envelope displayed local polarization on the phragmosome plane, while the cytoplasm between them was occupied by distinct ER aggregations. These consistent findings suggest that Α. nidus protodermal cells constitute a unique system in which three elements of the endomembrane system (ER, plasmalemma, and nuclear envelope) show specific characteristics in the establishing division plane. Our experimental data support that the organization of the U-like ER bundle is controlled on a cellular level by the actomyosin system and intercellularly by factors emitted from the leaf apex. The possible role of the above endomembrane system elements on the mechanism that coordinates the determination of the division plane between adjacent cells in protodermal tissue of A. nidus is discussed.

Nuclear architecture and chromatin dynamics in interphase nuclei of Arabidopsis thaliana

The interphase cell nucleus is extraordinarily complex, ordered, and dynamic. In the last decade, remarkable progress has been made in deciphering the functional organisation of the cell nucleus, and intricate relationships between genome functions (transcription, DNA repair, or replication) and various nuclear compartments have been revealed. In this review, we describe the architecture of the Arabidopsis thaliana interphase cell nucleus and discuss the dynamic nature of its organisation. We underline the need for further developments in quantitative and modelling approaches to nuclear organization.

Plant nuclear photorelocation movement

Organelle movement and positioning are essential for proper cellular function. A nucleus moves dynamically during cell division and differentiation and in response to environmental changes in animal, fungal, and plant cells. Nuclear movement is well-studied and the mechanisms have been mostly elucidated in animal and fungal cells, but not in plant cells. In prothallial cells of the fern Adiantum capillus-veneris and leaf cells of the flowering plant Arabidopsis thaliana, light induces nuclear movement and nuclei change their position according to wavelength, intensity, and direction of light. This nuclear photorelocation movement shows some common features with the photorelocation movement of chloroplasts, which is one of the best-characterized plant organelle movements. This review summarizes nuclear movement and positioning in plant cells, especially plant-specific nuclear photorelocation movement and discusses the relationship between nuclear photorelocation movement and chloroplast photorelocation movement.

Deformation of nuclei and abnormal spindles assembly in the second male meiosis of polyploid tobacco plants

The bipolar spindle is a major cytoskeletal structure, which ensures an equal chromosome distribution between the daughter nuclei. The spindle formation in animal cells depends on centrosomes activity. In flowering plant cells the centrosomes have not been identified as definite structures. The absence of these structures suggests that plants assemble their spindle via novel mechanisms. Nonetheless, the cellular and molecular mechanisms controlling the cytoskeleton remodeling during the spindle development in plants are still insufficiently clear. This article describes the results of a comparative analysis of the microtubular cytoskeleton dynamics during assembly of the second division spindle in tobacco microsporocytes with the normal and deformed nuclei. According to our observations, the bipolar spindle fibres are formed from short arrays of the disintegrated perinuclear cytoskeleton system, the perinuclear microtubular band. The microsporocytes of polyploid tobacco plants with deformed nuclei entirely lack this cytoskeleton structure. In such type of cells the overall prometaphase events are blocked, and the assembly of second division spindles is completely arrested.

Functional identification of multiple nucleocytoplasmic trafficking signals in the broad-spectrum resistance protein RPW8.2

Nuclear localization signals (NLSs) and nuclear export signals (NESs) are important intramolecular regulatory elements for protein nucleocytoplasmic trafficking. This regulation confers spatial specificity to signal initiation and transduction in eukaryotic cells and thus is fundamental to the viability of all eukaryotic organisms. Here, we developed a simple and rapid method in which activity of putative NLSs or NESs was reported by subcellular localization of two tandem fluorescent proteins in fusion with the respective NLSs or NESs after agroinfiltration-mediated transient expression in leaves of Nicotiana benthamiana (Nb). We further demonstrated that the predicted NES from amino acid residue (aa) 9 to 22 and the NLS from aa91 to 101 in the broad-spectrum disease resistance protein RPW8.2 possess nuclear export and import activity, respectively. Additionally, by testing overlapping fragments covering the full length of RPW8.2, we identified another NLS from aa65 to 74 with strong nuclear import activity and two tandem non-canonical NESs in the C-terminus with strong nuclear export activity. Taken together, our results demonstrated the utility of a simple method to evaluate potential NLSs and NESs in plant cells and suggested that RPW8.2 may be subject to opposing nucleocytoplasmic trafficking forces for its subcellular localization and functional execution.

Organization of perinuclear actin in live tobacco cells observed by PALM with optical sectioning

Actin performs a wide variety of different tasks. This functional diversity may be accomplished either by the formation of different isotypes or by suitable protein decoration that regulates structure and dynamics of actin filaments. To probe for such a potential differential decoration, the actin-binding peptide Lifeact was fused to different photoactivatable fluorescent proteins. These fusions were stably expressed in Nicotiana tabacum L. cv. Bright Yellow 2 cells to follow dynamic reorganization of the actin cytoskeleton during the cell cycle. The Lifeact-monomeric variant of IrisFP fusion protein was observed to indiscriminately label both, central and cortical, actin filaments, whereas the tetrameric Lifeact-photoswitchable red fluorescent protein fusion construct selectively labeled only a specific perinuclear sub-population of actin. By using photoactivated localization microscopy, we acquired super-resolution images with optical sectioning to obtain a 3D model of perinuclear actin. This novel approach revealed that the perinuclear actin basket wraps around the nuclear envelope in a lamellar fashion and repartitions toward the leading edge of the migrating nucleus. Based on these data, we suggest that actin that forms the perinuclear basket differs from other actin assemblies by a reduced decoration with actin binding proteins, which is consistent with the differential decoration model.

Enriching the pore: splendid complexity from humble origins

The nucleus is the defining intracellular organelle of eukaryotic cells and represents a major structural innovation that differentiates the eukaryotic and prokaryotic cellular form. The presence of a nuclear envelope (NE) encapsulating the nucleus necessitates a mechanism for interchange between the contents of the nuclear interior and the cytoplasm, which is mediated via the nuclear pore complex (NPC), a large protein assembly residing in nuclear pores in the NE. Recent advances have begun to map the structure and functions of the NPC in multiple organisms, and to allow reconstruction of some of the evolutionary events that underpin the modern NPC form, highlighting common and differential NPC features across the eukaryotes. Here we discuss some of these advances and the questions being pursued, consider how the evolution of the NPC has been constrained, and finally propose a model for how the NPC evolved.

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.

Towards the Physics of Calcium Signalling in Plants

Calcium is an abundant element with a wide variety of important roles within cells. Calcium ions are inter- and intra-cellular messengers that are involved in numerous signalling pathways. Fluctuating compartment-specific calcium ion concentrations can lead to localised and even plant-wide oscillations that can regulate downstream events. Understanding the mechanisms that give rise to these complex patterns that vary both in space and time can be challenging, even in cases for which individual components have been identified. Taking a systems biology approach, mathematical and computational techniques can be employed to produce models that recapitulate experimental observations and capture our current understanding of the system. Useful models make novel predictions that can be investigated and falsified experimentally. This review brings together recent work on the modelling of calcium signalling in plants, from the scale of ion channels through to plant-wide responses to external stimuli. Some in silico results that have informed later experiments are highlighted.

Organisation of the endosperm and endosperm-placenta syncytia in bladderworts (Utricularia, Lentibulariaceae) with emphasis on the microtubule arrangement

Multinucleate cells play an important role in higher plants, especially during reproduction; however, the configurations of their cytoskeletons, which are formed as a result of mitosis without cytokinesis, have mainly been studied in coenocytes. Previous authors have proposed that in spite of their developmental origin (cell fusion or mitosis without cytokinesis), in multinucleate plant cells, radiating microtubules determine the regular spacing of individual nuclei. However, with the exception of specific syncytia induced by parasitic nematodes, there is no information about the microtubular cytoskeleton in plant heterokaryotic syncytia, i.e. when the nuclei of fused cells come from different cell pools. In this paper, we describe the arrangement of microtubules in the endosperm and special endosperm-placenta syncytia in two Utricularia species. These syncytia arise from different progenitor cells, i.e. cells of the maternal sporophytic nutritive tissue and the micropylar endosperm haustorium (both maternal and paternal genetic material). The development of the endosperm in the two species studied was very similar. We describe microtubule configurations in the three functional endosperm domains: the micropylar syncytium, the endosperm proper and the chalazal haustorium. In contrast to plant syncytia that are induced by parasitic nematodes, the syncytia of Utricularia had an extensive microtubular network. Within each syncytium, two giant nuclei, coming from endosperm cells, were surrounded by a three-dimensional cage of microtubules, which formed a huge cytoplasmic domain. At the periphery of the syncytium, where new protoplasts of the nutritive cells join the syncytium, the microtubules formed a network which surrounded small nuclei from nutritive tissue cells and were also distributed through the cytoplasm. Thus, in the Utricularia syncytium, there were different sized cytoplasmic domains, whose architecture depended on the source and size of the nuclei. The endosperm proper was isolated from maternal (ovule) tissues by a cuticle layer, so the syncytium and chalazal haustorium were the only way for nutrients to be transported from the maternal tissue towards the developing embryo.

Microtubule nucleation and establishment of the mitotic spindle in vascular plant cells

The microtubular cytoskeleton plays a major role in cellular organization and proliferation. The first step in construction of a microtubule is microtubule nucleation. Individual microtubules then participate in organization of more complex microtubule arrays. A strong body of evidence suggests that the underlying molecular mechanisms involve protein complexes that are conserved among eukaryotes. However, plant cell specificities, mainly characterized by the presence of a cell wall and the absence of centrosomes, must be taken into account to understand their mitotic processes. The goal of this review is to summarize and discuss current knowledge regarding the mechanisms involved in plant spindle assembly during early mitotic events. The functions of the proteins currently characterized at microtubule nucleation sites and involved in spindle assembly are considered during cell-cycle progression from G2 phase to metaphase.

The molecular architecture of the plant nuclear pore complex

The nucleus contains the cell's genetic material, which directs cellular activity via gene regulation. The physical barrier of the nuclear envelope needs to be permeable to a variety of macromolecules and signals. The most prominent gateways for the transport of macromolecules are the nuclear pore complexes (NPCs). The NPC is the largest multiprotein complex in the cell, and is composed of multiple copies of ~30 different proteins called nucleoporins. Although much progress has been made in dissecting the NPC structure in vertebrates and yeast, the molecular architecture and physiological function of nucleoporins in plants remain poorly understood. In this review, we summarize the current knowledge regarding the plant NPC proteome and address structural and functional aspects of plant nucleoporins, which support the fundamental cellular machinery.

Nuclear pore complex composition: a new regulator of tissue-specific and developmental functions

Nuclear pore complexes (NPCs) are multiprotein aqueous channels that penetrate the nuclear envelope connecting the nucleus and the cytoplasm. NPCs consist of multiple copies of roughly 30 different proteins known as nucleoporins (NUPs). Due to their essential role in controlling nucleocytoplasmic transport, NPCs have traditionally been considered as structures of ubiquitous composition. The overall structure of the NPC is indeed conserved in all cells, but new evidence suggests that the protein composition of NPCs varies among cell types and tissues. Moreover, mutations in various nucleoporins result in tissue-specific diseases. These findings point towards a heterogeneity in NPC composition and function. This unexpected heterogeneity suggests that cells use a combination of different nucleoporins to assemble NPCs with distinct properties and specialized functions.

New insights into the dynamics of plant cell nuclei and chromosomes

The plant lamin-like protein NMCP/AtLINC and orthologues of the SUN-KASH complex across the nuclear envelope (NE) show the universality of nuclear structure in eukaryotes. However, depletion of components in the connection complex of the NE in plants does not induce severe defects, unlike in animals. Appearance of the Rabl configuration is not dependent on genome size in plant species. Topoisomerase II and condensin II are not essential for plant chromosome condensation. Plant endoreduplication shares several common characteristics with animals, including involvement of cyclin-dependent kinases and E2F transcription factors. Recent finding regarding endomitosis regulator GIG1 shed light on the suppression mechanism of endomitosis in plants. The robustness of plants, compared with animals, is reflected in their genome redundancy. Spatiotemporal functional analyses using chromophore-assisted light inactivation, super-resolution microscopy, and 4D (3D plus time) imaging will reveal new insights into plant nuclear and chromosomal dynamics.

New insights into the role of the small ubiquitin-like modifier (SUMO) in plants

Small ubiquitin-like modifier (SUMO) is a small (∼12kDa) protein that occurs in all eukaryotes and participates in the reversible posttranslational modification of target cellular proteins. The three-dimensional structure of SUMO and ubiquitin (Ub) are superimposable although there is very little similarity in their primary amino acid sequences. In all organisms, conjugation and deconjugation of Ub and SUMO proceed by the same reactions while using pathway-specific enzymes. SUMO conjugation in plants is a part of the controls governing important biological processes such as growth, development, flowering, environmental (abiotic) stress responses, and response to pathogen infection. Most of the evidence for this comes from genetic analyses. Recent efforts to dissect the function of sumoylation have focused on uncovering targets of SUMO conjugation by using either a yeast two-hybrid screen employing components of the SUMO cycle as bait or by using affinity purification of SUMO-conjugated proteins followed by identification of these proteins by mass spectrometry. This chapter reviews the current knowledge regarding sumoylation in plants, with special focus on the model plant Arabidopsis thaliana.

A comparison between nuclear dismantling during plant and animal programmed cell death

Programmed cell death (PCD) is a process of organized destruction of cells, essential for the development and maintenance of cellular homeostasis of multicellular organisms. Cells undergoing PCD begin a degenerative process in response to internal or external signals, whereby the nucleus becomes one of the targets. The process of nuclear dismantling includes events affecting the nuclear envelope, such as formation of lobes at the nuclear surface, selective proteolysis of nucleoporins and nuclear pore complex clustering. In addition, chromatin condensation increases in coordination with DNA fragmentation. These processes have been largely studied in animals, but remain poorly understood in plants. The overall process of cell death has different morphological and biochemical features in plants and animals. However, recent advances suggest that nuclear dismantling in plant cells progresses with morphological and biochemical characteristics similar to those in apoptotic animal cells. In this review, we summarize nuclear dismantling in plant PCD, focusing on the similarities and differences with their animal counterparts.

A role for phosphoinositides in regulating plant nuclear functions

Nuclear localized inositol phospholipids and inositol phosphates are important for regulating many essential processes in animal and yeast cells such as DNA replication, recombination, RNA processing, mRNA export and cell cycle progression. An overview of the current literature indicates the presence of a plant nuclear phosphoinositide (PI) pathway. Inositol phospholipids, inositol phosphates, and enzymes of the PI pathway have been identified in plant nuclei and are implicated in DNA replication, chromatin remodeling, stress responses and hormone signaling. In this review, the potential functions of the nuclear PI pathway in plants are discussed within the context of the animal and yeast literature. It is anticipated that future research will help shed light on the functional significance of the nuclear PI pathway in plants.