Annual Plant Reviews, Plant Nuclear Structure, Genome Architecture and Gene Regulation: Volume 46

These advances open the way for new research into areas such as stress tolerance, plant-pathogen interactions and ultimately crop improvement and food security. Continued research into plant nuclear structure, genome architecture and gene regulation also enriches our understanding of the origin and evolution of the nucleus and its envelope. Edited by world-class researchers in plant cell biology, and comprising contributions from internationally-renowned academics, this latest volume in the prestigious Annual Plant Reviews series brings together a wealth of knowledge in the burgeoning field of plant nuclear structure and genetics.

Annual Plant Reviews, Volume Plant Nuclear Structure, Genome Architecture and Gene Regulation is a vital resource for advanced students, researchers and professionals in plant science and related disciplines. Libraries in all research establishments where plant science, biochemistry, molecular biology, genetics and genomics and agricultural science are taught and studied will find this excellent volume an essential addition to their shelf. The Best Books of Check out the top books of the year on our page Best Books of Looking for beautiful books? Visit our Beautiful Books page and find lovely books for kids, photography lovers and more.

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Annual Plant Reviews Jane Parker. Annual Plant Reviews Michael T. Annual Plant Reviews Jocelyn K. Annual Plant Reviews Keith Lindsey. Progress in Botany 71, 25— Annals of Botany , — Journal of Experimental Botany 51, — Journal of Experimental Botany 43, 31— Genomic and Breeding resources. Legume Crops and Forages, Springer, Berlin, pp — Journal of Experimental Botany 52,— Plant Journal 12, — International Journal of Systematic and Evolutionary Microbiology 52, — International Journal of Biochemistry and Cell biology 41, — Biology Direct 5, Article Number: Plant Cell 16, — Nature Reviews Genetics 8, — Simple fold composition and modular architecture of the nuclear pore complex.

Trends in Biochemical Sciences 16, — Journal of the Torrey Botanical Society , — International Journal of Plant Sciences , — Plant Journal 59, — Molecular Plant 5, — Nucleic Acids Research 40, — Biology Direct 5 Article Number: Trends in Biochemical Sciences 21, — Plant Molecular Biology 10, — Plant Journal 66, 18— Journal of Computational Biology 18, — Molecules and Cells 31, — Current Opinion in Microbiology 8, — Biochemical Journal , — American Scientist 59, — Life and Its Environment on the Early Earth.

Proceedings of the Royal Society, Series B , — Current Opinion in Plant Biology 14, — BMC Evolutionary Biology 12, Epub ahead of print PMID: Rebuild from the prokaryotic ancestors of ribosome export factors. Nucleic Acids Research 39, — FEBS Journal , — Genetic map positions and repetitive DNA structure. Genome Research 7, — Plant Physiology, , — Genes and Genetic Systems 83, — Current Opinion in Microbiology 14, — Plant Physiology , — Journal of Experimental Botany 42, — Piggy back transport of the pre-assembled complex to the nucleus.

Journal of Molecular Biology , — Journal of Biological Bhemistry , — Nucleic Acids Research 39, — EMBO Reports 6, — Plant Molecular Biology 20, — Journal of Theoretical Biology , — Journal of Biological Chemistry , — Annual Plant Reviews 46, 19—56 doi: The proteins of the nuclear envelope have been characteriszd in detail in non-plant systems, revealing a complex series of interactions between the inner and outer nuclear envelope, with the proteins of the nuclear pore complex and connecting to the nucleoskeleton and cytoskeleton.

This chapter describes the emerging networks of protein interactions at the plant nuclear envelope, together with discussion of their likely functions. In particular, the important role of plant SUN domain proteins as a key component of a linker of nucleoskeleton and cytoskeleton complex is described together with prospects for future study of plant nuclear envelope interaction networks. In consequence, it has been the subject of considerable interest and a substantial knowledge base has been created Annual Plant Reviews Volume However, in mitotic and undifferentiated cells, the nucleus is often rounded or ovoid, central and capable of movement within the cell cytoplasm.

As in metazoans, chromatin is organized into regions of hetero- and euchromatin and there are great similarities in the overall organization of the internal structure of the nucleus with heterochromatin located adjacent to the NE. The shape and structure of the plant nucleus as well as its position are likely to be maintained by the interaction of the envelope with chromatin and the nucleoskeleton and with the cytoskeleton, just as in metazoans. The proteomes of the PNER and ONM are not identical and the junction region between the two membranes diameter 2—30 nm is believed to limit protein movement Craig and Staehelin, ; Staehelin, Connections between the ONM and cytoskeleton are an important feature of the nucleus.

The second domain is the pore domain or pore membrane. The structure and function of the NPCs and their relationship with the pore domain are dealt with in chapter 3. The third domain, the INM, is closely associated with chromatin via numerous protein connections as well as with the ONM by protein bridges spanning the periplasm.

In addition, a nucleoskeletal meshwork, termed the lamina in animals, underlies the INM, thereby supporting the structure and shape of the NE and the entire nucleus. It is hypothesized that membrane-intrinsic NE membrane proteins are involved in tethering the soluble proteins to the NE but apart form RanGAP anchoring, the NE proteins and mechanisms involved in this process remain to be elucidated.

This research was originally published in Biochemical Society Transactions. Study of the plant NE reveals that while it is structurally similar, many of the key proteins providing these connections are absent. However, homologues of these are not present in the plant genome Brandizzi et al. In addition, many other proteins interlinking the INM and chromatin are absent. It may therefore be concluded that the plant NE is homologous to that of metazoans in structure and many of its functions; but is distinct in many of the protein interactions through which these features are achieved.

The work of Tamura and co-workers reveals that there are at least 29 nucleoporins present in Arabidopsis, of which 22 had not previously been described in plants Tamura et al. The major membrane anchor for the NPC found in vertebrates but not yeast , gp, is present in plants and is therefore a major component of the pore domain.

Both are WD-repeat proteins likely to be involved in multiprotein complex formation. Description of these proteins has been driven to a great extent by diseases such as progerias, skeletal defects, muscular dystrophies, lipoatrophy, and epilepsy resulting from INM protein mutations Stewart et al. Most of these functions also involve the lamina, which is tightly associated with the INM Gruenbaum et al. It has eight transmembrane domains, forms a multimeric protein complex, shows sterol reductase activity and contains a Tudor domain involved in histone binding Huang et al.

The N-terminus is located in the nucleoplasm and binds to lamin B and chromatin. Both LBR and its binding partners are absent from plants. In a recent review, Olins et al. Given the range of functions for LBR reviewed by Olins et al. The search for INM - chromatin bridges is therefore of high priority. A further major group of proteins not present in plants is the LEM domain family. They bind directly to lamins and to barrier-to-autointegration factor BAF , which binds to lamins and chromatin Gruenbaum et al.

Amino acid sensor independent ASI, another LEM family member are signal transducers that regulate expression of amino-acid permeases Zargari et al. The presence of a nucleoskeletal meshwork underneath the INM and chromatin associations at the NE suggests that the plant INM also contains proteins, which are involved in chromatin and nucleoskeletal anchorage and hence implied in controlling nuclear activities.

It is therefore of great importance to identify these components and study their functional relationships with the nucleoskeleton and chromatin. In non-plant systems it is made up of two membrane integral components; SUN domain proteins in the INM , ubiquitously present in higher and lower eukaryotes Figure 2. While SUN-domain proteins show structural and sequence conservation across species Figure 2. However, their functions in cytoskeletal and nucleoskeletal anchorage remain unclear Zhou et al.

The well characterized c-terminal SUN-domain proteins are localized at the NE in plant, animal and yeast. Their hallmark features include coiled coil domains medium grey followed by a C-terminal highly conserved SUN domain black. These proteins contain at least one transmembrane domain light grey , which anchors the protein to the INM. The c-terminal SUN domain proteins are part of protein bridges that cross-link nucleoskeletal elements and chromatin to cytoskeletal components. A second class — the mid-SUN domain proteins — have recently been described in maize and in silico data suggest the presence of these throughout the plant, animal and fungi kingdoms.

These proteins contain three transmembrane domains, at least one coiled coil domain and the highly conserved SUN domain in the centre section of the protein. While the properties and functions of these mid-SUN-domain proteins remain unknown, the maize SUN3 protein was found to be present at the nuclear periphery indicative of NE localization Murphy et al.

The SUN-domain proteins are considered in detail later in this chapter. The cytoplasmic region of the KASH proteins is highly variable, though commonly containing spectrin repeats or coiled coils. As mentioned, they also consist of a cytoplasmic coiled-coil domain, are C-terminally anchored to the ONM and have a short, nine amino acid Cterminal tail.

While this tail has no sequence homology to animal and yeast KASH tails, it contains a penultimate proline residue embedded in a VVPT motif that is conserved in other plant species Zhou et al. Knocking out either protein family results in rounded nuclei in various plant tissues Zhou et al. Whether the WIP proteins are the only plant KASH proteins and whether they mediate NE-cytoskeleton interactions like their animal and yeast counterparts, remains to be elucidated. While the lamina of animal cells has been well characterized, that of plants is much less well described.

Lamins are encoded by three genes, A, B1 and B2, which also generate the alternative splice product lamin C. A phosphorylation domain acting at the onset of NE breakdown controls association with lamin binding proteins. Plants lack sequence homologs of mammalian lamins Brandizzi et al. However, there is now considerable evidence for a protein meshwork underlying the plant INM and attached to it Minguez and Diaz de la Espina, ; Masuda et al.

The structure was also visible from the cytoplasmic face in detergent extracted nuclei and resembled the lamina meshwork of frog oocytes.

Annual Plant Reviews : Plant Nuclear Structure, Genome Architecture and Gene Regulation

There are NMCP proteins present in other dicots including Arabidopsis and Apium graveolens celery as well as monocots such as rice Table 2. As well as structurally resembling lamins, NMCP1 also has similar isoelectric point 5. Using a reverse genetics approach, Dittmer et al. In addition to the structural characteristics suggested by sequence homology with NMCP1 and 2 Table 2.

In addition, the mutant nuclei were more spherical. They remain to be functionally characterized. Ankyrin is a structural protein usually located at the plasma membrane; however, Nalp1, a 28 kDa rice ankyrin homologue, was located close to the INM, in a region termed the inner nuclear matrix region or chromatin-poor space. AtAnkyrin3, the Arabidopsis counterpart of Nalp1, shows high homology with the ankyrin repeat region of Nalp1 and is of similar size 27 kDa ; they are however, much smaller than the human ankyrins and unlikely to perform similar functions De Ruijter et al.

As well as proteins of the nucleoskeleton linking to the NE, there is also evidence for a variety of proteins involved in the interaction of the nucleoskeleton with chromatin domains. Their molecular role in maintaining nuclear architecture remains to be determined. The presence of spectrin-like proteins in pea nuclei was suggested by a study by De Ruijter et al. Western blots revealed bands of — Da, similar to erythrocyte spectrin, and a prominent 60 kDa band.

In light microscope immunolocalization, the antibodies detected proteins in puncta in the nucleoplasm and with staining increasing as the nuclear matrix was extracted progressively with detergents, DNase I and RNase A, and high salt. Spectrin-like proteins were shown to be distributed at or near the NE, associated with chromatin and at the nucleolus. In plants, researchers have shown that this antibody and various other anti-lamin antibodies recognize nuclear 60 and 65 kDa proteins McNulty and Saunders ; Minguez and Moreno Diaz de la Espina, In the most detailed of these studies Blumenthal et al.

The SUN-domain proteins occur throughout the plant kingdom. They have been characterized in detail in two species: Arabidopsis Graumann et al. Evidence that the plant SUN domain proteins are involved, directly or indirectly, in connections between the envelope and nucleo- or cytoskeleton comes from a number of experimental approaches Figure 2. Movement of the nucleus in developing root hairs was, however, not affected. As interactions between INM proteins and nucleoskeletal elements such as the lamins are known to regulate nuclei shape, it suggests that the roles of AtSUN1 and AtSUN2 in maintaining nuclear shape may also involve interactions with plant nucleoskeletal elements.

Further evidence for this comes from a study in which knock down of the lamin-like LINC proteins resulted in similarly spherical nuclei Dittmer et al. Studies of the behaviour of SUN domain proteins in mitosis also strongly points to their association with the nucleoskeleton and cytoskeleton. This process involves the breaking of protein bridges between the membrane and its associated proteins, followed by reformation of these links.

In tobacco suspension culture cells, AtSUN1 and AtSUN2 migrate to mitotic ER membranes at envelope breakdown and then rapidly accumulate in the reforming NE at the end of division, strongly suggesting direct or indirect association with chromatin. They observed microtubules both in association with the NE during NE breakdown and associated with the mitotic membranes. The SUN-domain proteins also localize to the phragmoplast — an area of high cytoskeleton and membrane activity forming the new cell wall and AtSUN2 may be interacting with other proteins here Graumann and Evans, ; Oda and Fukuda, In cytokinesis both fusion proteins are present in the expanding NE, phragmoplast and cell plate.

For colour details please see colour plate section. However, their functions and characteristics remain to be studied. Nuclear movement in response to abiotic and biotic stimuli such as bacterial, fungal and viral infections is considered later on in detail. Treatment of cells with latrunculin B or other actin depolymerizing drugs reduces nuclear movement in leaf epidermal cells expressing an NE marker Graumann et al. While the full extent of nuclear movement has yet to be described, observations of nuclei in root epidermal cells and root hairs have been made by a number of groups Chytilova et al.

Long-distance nuclear migration requires large actin bundles Iwabuchi et al. The presence of proteins for nucleating tubulin Seltzer et al. Less is known about the protein components by which the cytoskeleton is anchored to the NE. A triple knock-out mutant, which lacks all three WIP proteins results in rounded nuclei indicating these proteins are needed to shape nuclei Zhou et al. Whether this process is also dependent on cytoskeletal elements or whether other plant KASH-like proteins mediate this remains to be elucidated.

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This was thought to be due to passive diffusion, with the INM proteins retained by binding to nucleoplasmic components Figure 2. It is now clear that INM-intrinsic proteins enter the nucleus using an energy-dependent mechanism Ohba et al. They are differentiated from other membrane proteins by their transmembrane domains Saksena et al. In addition, FG nucleoporins are required for INM protein translocation and some, such as POM, are associated with, or are in close proximity to, the pore membrane Alber et al.

Targeting of NE proteins in plants is far less well understood. Two models predict how INM intrinsic protein reach their destination. There, they are retained by binding interactions, for instance with the nucleoskeleton. Importin complexes associate with the cytosolic part of the INM protein and are thought to facilitate the transport through the pore. The transport is energy dependent and may require functional and structural involvement of the NPC. On the nucleoplasmic side, the importins are thought to disassemble, similar to soluble protein import complexes.

These models are based on animal and yeast systems. Targeting of membrane proteins to the ONM appears more straightforward and does not require transfer through the pores. They are retained there by binding interactions with SUN proteins Figure 2. Whether this binding retention mechanism is used by other ONM proteins is currently not known. Membrane fusion occurs at the luminal face of the membranes to form the pore. If sensing curvature by this domain is a prerequisite for development of the pore, it implies that membrane fusion and pore formation precede the construction of the NPCs Hetzer, It is interesting to note that at this stage, as insertion of NPCs has preceded NEBD, the processes parallel those in eukaryotes with closed-cell division like yeast, where pore insertion has been characterized in detail.

Depletion of Nup and Nup results in pores that are mis-located to the INM and cytoplasm rather than creating true pores Dawson et al. In a study by Fiserova et al. In older cells, rows of NPC were observed instead. In addition, pores in three day old cells appeared simpler than those in older cells, and the cytoplasmic ring thinner, with structures similar to those observed in Xenopus leavis oocytes.

However, to date, antibody and molecular probes have not been available to permit detailed sequential analysis of NPC formation in plants and the hypothesis that the processes involved are the same as those in animals or yeast has yet to be tested. In addition to this role, Ran is also directly involved in pore assembly.

Importin has also been shown to be essential to the process by which pores assemble on chromatin Rotem et al. A role for Ran in NPC insertion in plants has yet to be considered. Open mitosis requires the detachment of the envelope from its underlying structures chromatin and the nucleoskeleton and its breakdown and separation from chromatin.

Before these events, the NPCs are removed, giving exchange between nucleoplasm and cytoplasm. Pore removal is regulated and soluble NPC components migrate to the cytoplasm or become part of the mitotic apparatus, some remaining as protein complexes Hetzer, In animal cells, this occurs in prometaphase while in plants it is earlier, in late prophase Rose, Linkage of NPC to the lamina is implied for animals as mutation, down-regulation of expression, or introduction of Fab fragments of antibodies to gp in C. Following NEBD in plants, Tpr, which is associated with the nuclear pore basket, migrates to the mitotic spindle in prometaphase Xu et al.

Tobacco Rae1 associates with mitotic microtubules including the preprophase band PPB , spindle and phragmoplast Lee et al. Deletion or down regulation of the gene for Rae1 causes defects in the spindle organization, chromatin alignment and segregation as well as decreased levels of cyclin B, cyclin-dependent kinase B CDKB and histones H3 Lee et al. Exposure of the nucleoplasm to cytoplasm initiates a series of phosphorylation events. Breakdown then progresses as NE proteins move to the mitotic ER. Dephosphorylation at the end of division reverses this Anderson and Hetzer, ; Guttinger et al.

How these kinases regulate plant NEBD and reformation remains to be established; however, aurora kinases are associated with the plant NE in interphase and localize to the mitotic spindle, centromeres and phragmoplast in division. They phosphorylate histone H3 and are involved in chromosome segregation and cytokinesis Demidov et al. Plant B1 cyclins also accumulate at the NE Rose et al. The down regulation of cyclin B and CDKB in combination with decreased mitotic activity in NbRae1 mutants suggests their involvement in plant mitosis progression Lee et al.

Loss of physical interactions within the nucleus permits physical forces to tear and open the membrane. Microtubule-dynein interactions draw membrane away from the lamina Beaudouin et al. The process in plants also involves positional information in order to establish the plane of division required in a walled structure, where the formation of a PPB of microtubules predicts the division plane.

As expected, both plant NE membrane markers and the putative plant lamin-like proteins disassemble at the beginning of mitosis Figure 2. As the nucleoskeleton is lost in celery cells, NMCP1 associates with the mitotic spindle while NMCP2 migrates to the mitotic cytoplasm and both join the reforming NE at the end of the process Kimura et al. This was also corroborated by studies using native plant proteins described below.

The SUN-domain proteins like LBR-GFP localize to spindle membranes and to tubules traversing the division zone in metaphase, while most of the spindle membranes accumulate at the spindle poles Figure 2. In contrast with metazoans, where NE proteins distribute throughout the mitotic ER leaving metaphase chromosomes devoid of membranes, membranes containing NE proteins remain in close proximity to chromatin throughout division in plants Anderson and Hetzer, ; Hetzer, ; Graumann and Evans, This has not been observed in metazoans Ellenberg et al.

This is generated along the plane of division predicted by the PPB microtubules and involves considerable secretory vesicle activity. Alternatively, it may suggest a role for these proteins in cell wall formation. It is interesting to observe that so far only in plants has spatial NE reformation from proximal to the spindle to proximal to the cell plate been observed, and it is intriguing to speculate whether this is associated with the presence of NE proteins in the cell plate.

Among other factors such as kinases, phosphatases, Ran and chromatin remodelling components, the amount of available membrane is known to affect NE reformation Webster et al. Thus the SUN-domain proteins might be involved in such regulation. Anchorage of the telomeres to the NE is mediated by the SUN-domain components of the LINC complex in animal and yeast systems and deletion of these can result in abolition of gametogenesis Tomita and Cooper, ; Ding et al.

In animal, yeast and some plants a chromosome bouquet is formed and anchored to the NE via the telomeres. In animals and yeast this is essential for homologous pairing Tomita and Cooper, ; Roberts et al. How telomeres are linked to the nuclear periphery in plants remains to be investigated but the presence of plant SUNdomain proteins suggests that similar mechanisms may exist Graumann and Evans, b. Consequently, the lipid composition of the plant NE is also non-random and highly controlled.

Plant Nuclear Structure, Genome Architecture and Gene Regulation

Most of phospholipids, sterols and sphingolipids in endomembranes are synthesized in the ER and from there transported to other internal membranes or the plasma membrane. The physical continuity between ER and NE suggests that they may share a similar lipid composition.

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In support of this, Philipp et al. The latter method is mediated through membrane contact sites and lipid transfer proteins. Evidence from yeast and mammalian systems suggests such a transport mode is present at the NE and involved in NE lipid homeostasis Jouhet et al. These sites contain lipid transfer proteins, which can shuttle individual lipids through the aqueous phase between the two membranes Jouhet et al.

The membrane contact site at the NE consists of the ONM and the tonoplast of the lytic vacuole; hence they are referred to as nucleus-vacuole junctions Kvam and Goldfarb, ; Jouhet et al. Whether they are also present in plants remains to be studied. However, one of the protein components of the nucleus-vacuole junctions has a homolog in Arabidopsis.

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The two main proteins that maintain this junction through their interaction with each other are the ONM-localized Nvj1p and the tonoplast-localized Vac8p Kvam and Goldfarb, The Arabidopsis homolog for this protein is At4g Jouhet et al. While the functions of this protein still need to be investigated in plants, the yeast homologue of Osh1 is known to bind oxysterol, an oxygenated cholesterol derivative, is linked to post-synthetic regulation of sterols and mediates lipid exchange at membrane contact sites like the nucleus-vacuole junctions Kvam and Goldfarb, ; Jouhet et al. The nucleus-vacuole junction is implied in regulating lipid homeostasis of the NE by a process called piecemeal microautophagy of the nucleus PMN.

During PMN, small portions of the nucleus, including membranes and nuclear contents, are pinched off into the vacuolar lumen by the nucleusvacuole junctions for degradation. Localization of Osh1 at these junctions is important for this process suggesting that oxysterol is here used as a signalling messenger to induce PMN Kvam and Goldfarb, Again, this process has been well studied in yeast and is also known to occur in mammalian systems.

Whether it occurs in plants is so far not known. However, microautophagy certainly takes place in plant cells and the need to regulate the composition of the NE and nucleus per se as well as the presence of a plant Osh1 homologue indicate that a process like PMN could also take place in plants. Enzymes responsible for this are phosphatases called lipins and they are involved in two different processes Sinissoglou, Firstly, they dephosphorylate phosphatidic acid into diacylglycerol and are thus involved in the triacylglycerol synthesis and phospholipid metabolism.

Secondly, they are implied in directly regulating expression of genes involved in lipid metabolism. In addition, the yeast lipin Pah1 is also required for maintaining the spherical shape of nuclei Sinissoglou, Knock-outs of Pah1 result in irregularly shaped nuclei with long stacks of NPC-containing membranes associated with the NE. Like their yeast and mammalian counterparts, they contain phosphatase activity and are essential for maintaining lipid metabolism and adaptation to phosphate starvation.

During phosphate starvation in plants, membrane lipid remodelling occurs, whereby phospholipids are converted to non-phosphorus galactolipids to free up phosphorus. However, so far it remains unclear whether either of the two proteins is localized in the plant NE and whether a single or double knock-out affects NE structure and function.

Irrespective of this, it is becoming clear that coordinating production, transport and remodelling of phospholipids is essential in maintaining the structure and function of eukaryotic membranes including the plant NE. Indeed, nuclear repositioning is one of the earliest defensive cell responses to external stimulation. Abiotic stimuli studied so far include blue light exposure and mechanical stress.

In Arabidopsis leaf cells, nuclei positioning depends on light exposure. In the dark, nuclei are located at the bottom centre of the cell adjacent to the periclinal wall. In addition to light sensitivity, nuclear movement has also been observed in response to mechanical stimulation.

Annual Plant Reviews : David Evans :

Plant cells are exposed to repeated and varied strength mechanical stimulation exerted from neighbouring cells as well as from pathogens such as fungal hyphae. It has been shown that nuclei are highly sensitive to such pressures and can react to repeated stimuli of various strength by moving towards the sites of the stimuli without any loss of velocity or sensitivity Figure 2. Pressures exerted from neighbouring cells are due to growth and cell division. If several stimuli are applied in sequence, the nucleus moves to the various sites without any loss of speed or reactivity. This is followed by the nucleus traversing the cell and thereby priming the path the invading hyphae take to traverse the cell.

More is know about the function of NE components in response to fungal and bacterial pathogens. Plants undergo symbiotic relationships with both fungi and bacteria to effectively take up phosphorous and nitrogen, respectively. Medicago truncatula is a model organism to study both of these symbiotic relationships. Gigaspora is an arbuscular mycorrhizal fungus, which infects M. During this infection, fungal hyphae penetrate root epidermal cells to spread towards the inner root cortex. The cell penetration is controlled by both plant and fungus and the plant nucleus plays a crucial role by orchestrating cellular events of this process Genre et al.

As the hyphae exert pressure onto the epidermal cell, the nucleus moves towards this area Figure 2. Cytoskeletal elements and ER membranes accumulate in the space between nucleus and appressorium and together with the nucleus form the pre-penetration apparatus Genre et al. Following the moving nucleus, endomembranes, actin and microtubule cytoskeleton reorganize to initiate the formation of a cytoplasmic column that stretches between the appressorium and the nucleus. It is inside this cytoplasmic column that the penetrating fungal hyphae traverses the plant cell Genre et al.

The bacteria release Nod factors, which trigger calcium spikes associated with the nucleus of the infected root cell. These calcium signals, in turn, activate the expression of genes involved in formation of root nodules. Rhizobial bacteria enter the root system through infection threads that stretch from the root hair to other root cell layers. Similar to the cytoplasmic tubes that contain the penetrating fungal hyphae these infection threads require the reorganization of cytoskeletal elements and endomembranes.

Notably, the growing infection thread in the root hair follows at a set distance to the migrating nucleus, which moves towards the bottom of the cell Gage, suggesting that here, too, the nucleus marks the path of infection threads. This has been studied in Vigna unguiculata cowpea during infection with cowpea rust virus.

Similar to other nuclear movement events, this repositioning is actin dependent Skalamera and Heath, During viral infection, NE components may not only play a role in nuclear movement, but the NE itself is utilized by the virus for replication and formation of mature virions. Both are RNA viruses whose replication cycles include a nuclear stage.

The exact mechanism remains to be studied, but it was found that this movement protein associates with and causes conformational changes of the NE. The N-terminus of this protein can directly interact with lipids and causes the formation of protrusions that emanate from the NE Liu et al.

These alterations of the NE are thought to be involved in integrating the protein into the membrane. The purpose of this remains hypothetical with suggestions that it is either involved in the translocation of vRNA across the NE membranes Liu et al. SYNV infection also causes NE morphological changes — nuclei are larger and numerous intranuclear membrane protrusions have been observed.

The SYN virus consists of a ribonucleoprotein core that is surrounded by a membrane layer. The membrane contains a viral glycoprotein and is thought to be attached to the viral core by a viral matrix protein. Viral replication and assembly of the core occurs inside the nucleus. The assembled core virions are thought to bud through the INM at the intranuclear membrane extensions.

By budding through the INM, the virions acquire their membrane coat. The formation of the intranuclear membranes and the presence of viral glycoprotein in the plant NE demonstrate that the viral infection interferes with nucleoskeletal and NE membrane components to effect such dramatic changes.

We have only just caught a glimpse of the functional capacity of the plant NE with many questions remaining to be answered. What are the protein components of the plant NE and what are their functional roles? How is the NE involved in chromatin anchorage and organization? Journal of Cell Science , — The Plant Journal doi: New Phytologist , — Journal of Cell Biology , — Plant Cell 20, — Molecular Biology of the Cell 11, — Nature Review Molecular Cell Biology 9, — Evaluation and characterization of novel features of the endoplasmic reticulum and associated membrane systems.

European Journal of Cell Biology 46, 81— The Journal of Cell Biology , 41— Cell Biology International 32, — The Plant Cell 17, — Cell Biology International 24, — Developmental Cell 12, — The Plant Cell 19, — Febs Letters , 44— Nature Sructural and Molecular Biology 14, The Journal of Cell Biology , — The Plant Journal 59, — Microbiol Mol Biol Rev 68, — Plant Cell 11, — Nature , — Journal of General Virology 88, — Biochemical Society Transactions 38, — Components of putative plant LINC complexes? Plant Signalling and Behaviour 5, — Biology of the Cell 99, — The Plant Journal 61, — Molecular Biology of the Cell 13, — Nature Reviews 6, 21— Nature Review Molecular Cell Biology 10, — Molecular Cell 11, — Cold Spring Harbor Perspectives in Biology 2.

Journal of Experimental Botany 54, — Plant and Cell Physiology 48, — Annual Review of Phytopathology 43, — The Plant Journal 42, — Progress in Lipid Research 46, 37— Cell Motility and the Cytoskeleton 52 1 , 22— Plant and Molecular Biology 58, 1— Plant Cell 14, — Journal of Structural Biology , — Nature Review Molecular Cell Biology 8, — Experimental Cell Research , — Nature Review Molecular Cell Biology, 5, 65— Trends in Plant Science 14, — Journal of Experimental Botany 58, 27— Current Opinion in Plant Biology 12, 87— Trends in Plant Science 13, — Current Genetics 44, — Journal of Cell Science , — Molecular Biology of the Cell 12, — SUN-domain protein gene family: BMC Plant Biology Plant Journal 66, — Current Opinion in Plant Biology 9, — Plant Physiology Preview , — Nuclear spectrin-like proteins are structural actin-binding proteins in plants.

Biology of the Cell , — The Journal of Cell Biology 68, 11— Plant Cell Reports 26, — Plant Signalling and Behaviour 3, — The Plant Journal 49, — Cytogenetic and Genome Research , — Molecular Biology of the Cell 20, — The Plant Journal 52, — Proceedings of the National Academy of Sciences , — Nature, Stuctural and Molecular Biology 13, — Trends in Biochemical Science 30, — Plant Cell 16, 45— The Plant Journal 16, — Plant J 11, — Annual Review of cell and Developmental Biology 10, — The nuclear envelope extends its reach. Plant Cell 22, — The Plant Journal 40, — Trends in Plant Science 13, 20— Current Biology 17, — Annual Plant Reviews 46, 57—92 doi: The nuclear pore complex NPC provides a highly organized pathway for selective transport between the nucleoplasm and the cytoplasm.

A newly discovered connection between nuclear pore-associated proteins and inner nuclear envelope proteins expands our knowledge of plant nuclear envelope architecture. Dynamic patterns of subcellular localization suggest mitotic functions of plant nucleoporins away from the nuclear pore. An important component of this Annual Plant Reviews Volume The nuclear pore complexes NPCs are the sole site of this transport.

NPCs are large protein complexes embedded in the NE, connecting the nucleoplasm and the cytoplasm. Here, our knowledge of the plant NPC will be discussed and compared with our more advanced understanding of the yeast and vertebrate NPC. The NPC consists of a ring of eight units arranged in eightfold radial symmetry. Two rings as viewed from both the cytoplasmic and nucleoplasmic side sandwich a core ring containing eight spokes surrounding a central channel. This ring-spoke structure makes the NPC resemble an eight-spoke wheel. The whole NPC has a diameter of — nm and a depth of 50—70 nm, depending on the organism.

Although the basic organization is conserved, NPCs of BY-2 cells can be grouped into four categories based on their conformation Fiserova et al. The presence of these categories appears to be age-related. Another feature that is possibly affected by cell age is the spatial arrangement of NPCs Fiserova et al. An NPC density variance was also observed in neuron cells of mice during postnatal development Lodin et al.

In contrast, yeast NPCs were shown to form highly mobile clusters, correlating with the absence of a lamina in yeast Belgareh and Doye, , Winey et al. Since only BY-2 suspension cell nuclei have so far been thoroughly investigated by electron microscopy, it is unclear whether the cell-age-related NPC features are also present in other plant species and play a role during plant development. However, one early study documented that the NPC density of the vegetative pollen nuclei was highly increased on the surface facing the generative cells Shi et al.

Thus, the spatial arrangement of NPCs might be developmentally regulated in plants, too. The Nups can be divided into three categories: The transmembrane Nups span the nuclear membrane at the NE equator where the NPC is embedded, forming the outmost ring-like subcomplex also known as the luminal ring, Plate 3. The scaffold Nups constitute the inner ring of the NPC and are organized in two subcomplexes in Saccharomyces cerevisiae the Nup84 and the Nic96 complexes; in vertebrate the NupNup and the NupNup complexes, Plate 3.

The Nup84 or the NupNup complex, which exhibits a Y-shape in electron micrographs, is universally conserved and has been shown to be critical for NPC assembly in different organisms Boehmer et al. Other proteins or protein complexes are shown in green. FG Nups are indicated by italic font.

In yeast, the underlined Pom and Pom34 have no homologues in vertebrates or higher plants. Positioning of plant Nups is based on their vertebrate counterparts. In most NPC models, the Y-shaped complexes are present at both the cytoplasmic and the nucleoplasmic side, sandwiching the NupNup or Nic96 complex Brohawn et al. Coiled coils often function in protein-protein interactions, and indeed, the nuclear basket and its associated factors are involved in gene expression regulation, chromatin maintenance, and mediating cell division Strambio-De-Castillia et al. In vertebrates, NPCs are further connected to the lamina through the lamin-Nup interaction at the nuclear basket not shown in Plate 3.

Based on a comparison of protein similarity and composition, the plant NPC is more similar to the NPC of vertebrates than of yeast Tamura et al. Although both Nup and Nup have FG repeats at their C-termini, bind importins, and show similar dynamics upon NE assembly, Nup contains additional domains that are missing in Nup Water, ions and metabolites fall into this category Figure 3.

Please refer to the main text for details. Karyopherins can be categorized into importins responsible for import , exportins responsible for export , and karyopherins that can function in both directions reviewed in Pemberton and Paschal, These features provide karyopherins with the ability to transport cargo molecules through the NPCs in a directional fashion Figure 3. RanGAP is present in the cytoplasm yeast Hopper et al. After cargo dissociation in the cytoplasm, exportin can re-enter the nucleus by passing through the nuclear pore. In the nucleus, RanGEF, associated with chromatin, maintains a high nuclear RanGTP concentration, which enables the formation of an exportin-cargo-RanGTP complex and triggers cargo release from an importin-cargo complex.

The recognition of cargo by transport receptors is determined by transport signals: Instead, it requires a special exportin, CAS for Cellular Apoptosis Susceptibility in order to be exported back to the cytoplasm Kutay et al. Snurportin is another nuclear import adaptor involved in the trimethylguanosine-cap-dependent nuclear import of uridine-rich small nuclear ribonucleoproteins Huber et al. Proteins with an NES can form complexes with exportins and RanGTP for nuclear export and there are also proteins that serve as export adaptors. The accessibility of these signals can be regulated, for example by phosphorylation or protein-protein interactions and can depend on external signals, the phase of cell cycle, or the developmental stage Weis, ; Terry et al.

The karyopherin-mediated Ran cycle involves another family of proteins, represented by RanBP1 for Ran-binding protein 1. However, the functions of the other AtRanBP1 proteins are largely unknown. NXT1 and Mtr2 have no sequence similarity but they are functional homologues Conti and Fribourg, Dbp5 shuttles between the cytoplasm and the nucleus but it is also enriched on the cytoplasmic side of the NPC Daneholt et al. Activation of Dbp5 requires Gle1 and the small molecule inositol hexakisphosphate York et al. This meshwork creates an entropic barrier that excludes macromolecules.

However, the detailed mechanism of this gating is still unclear and several models have been suggested reviewed in Strambio-De-Castillia et al. In this model, nonbinding molecules can only pass through the central narrow channel where the network of peptide chains is loose. None of the models has been tested in plants; however, this process might be assumed to function very similarly across kingdoms. Forward genetic approaches included screens for nodulation, innate immunity, hormone responses and different abiotic stress responses.

An example is nodulation and mycorrhiza formation, investigated in Lotus japonicus. In the mutants, nucleus-associated calcium spiking during symbiotic signal transduction is affected, which suggests that the nuclear pore might play a role in calcium signalling. Snc1 is a gain-of-function mutation of the Arabidopsis SNC1 R gene, which expresses a constitutive defence response. Some are shared with the auxin pathway, as discussed below. Zhang and Li, Zhang et al. The relationship between these molecular roles and the mutant phenotypes is not fully understood, but it is likely that impaired nucleo-cytoplasmic transport of proteins or RNAs involved in innate immunity is responsible for the suppression phenotypes.

RanGAP2 also binds the coiled-coil domains of two Rx-like proteins, Rx2 and Gpa2, the latter of which is involved in resistance against the nematode Globodera pallid Sacco et al. Reducing the expression of RanGAP2 leads to reduced Rx-mediated, as well as Gpa2-mediated resistance to their corresponding pathogens Sacco et al. A screen was developed to identify mutants with altered expression of a reporter gene induced under these conditions. The sad mutant is hypersensitive to ABA in inhibiting seed germination and seedling growth. A karyopherin-adaptor-cargo interactome study in Arabidopsis would greatly assist in testing this hypothesis.

As discussed for SAD2, reduced nuclear accumulation of important ABA regulators might be the reason for this observation. In sar1 and sar3, the morphological and molecular phenotypes of axr1 are partially restored. Another connection between the nuclear pore and auxin comes from the observation that antisense suppression of Arabidopsis RanBP1 causes auxin hypersensitivity Kim et al. The sar mutation in Nup described above sensitizes plants to chilling stress and disrupts acquired freezing tolerance Dong et al.

A common feature between sar and los is the increased accumulation of poly A RNA in the nucleus, suggestive of impaired mRNA export. If this is the primary molecular phenotype, it remains to be explained why sar and los have opposite effects on cold tolerance, while both show similar mRNA export defects. The nuclear division arrest during a mitotic stage indicates that RanGAP might play a role in mitotic cell cycle progression during female gametophyte development Rodrigo-Peiris et al.

Overexpression-based co-suppression of Nup62 also leads to similar problems with growth and development as described for the other Nup and NPC-associated protein mutants Zhao and Meier, Others that share the developmental phenotypes have not been tested for nuclear mRNA retention. The rice homologue of miR is salinity- and alkaline-stress regulated and its overexpression in rice and Arabidopsis leads to transgenic plants that were more sensitive to salt and alkali treatment Gao et al. Overexpression of miR changes the expression of several auxin signalling-related genes Zhou et al. Through regulation of the anaphase-promoting complex, miR affects pollen development in Arabidopsis Zheng et al.

Together, a number of these pathways are affected in the nuclear pore mutants. Thus, it is worth investigating whether the major developmental defects of the different mutants discussed here are a consequence of altered miRNA homeostasis, which might be more sensitive to small quantitative changes than the overall mRNA pools. Most unicellular eukaryotes, such as S. In this scenario, the cytoplasm and the nucleoplasm mix and mitotic regulators access the nucleus through passive diffusion to promote the spindle assembly.

This semi-open mitosis is thus an intermediate between the closed mitosis of budding yeast and the open mitosis of vertebrates and plants. Similarly, the NE does not completely break down in Drosophila early embryos that undergo syncytial mitosis and in the basidiomycete Ustilago maydis Kiseleva et al. During open mitosis NPCs disassemble and the nuclear envelope completely breaks down, allowing the spindle microtubules to attach to kinetochores in the cytoplasm.

Upon the NPC disassembly, Nups localize to various mitotic structures such as kinetochores, spindle pole bodies and chromatin Galy et al. There is a substantial body of evidence that these Nups exhibit new roles at these mitotic structures, indicating that organisms with open and semi-open mitosis have evolved towards mitotic functions of Nups away from the nuclear pore Osmani et al.

The siRNA-mediated depletion of human Nup revealed that it is essential for kinetochore function. In the absence of Nup, chromosome congression and segregation are severely perturbed and the assembly of other kinetochore components is strongly inhibited, leading to aberrant kinetochore structure Salina et al.

During cell division, RanGAP1 is associated with the mitotic spindle and kinetochores in a SUMO1-conjugation-dependent manner similar to its nuclear pore association and in a microtubule-dependent manner in HeLa cells Matunis et al. At this stage MEL spreads over the entire chromatin surface and then rapidly relocalizes to the NE. Another Nup, Nup, is present on kinetochores in mammalian cells Belgareh et al.

Another NPC-associated protein, Rae1, becomes preferentially associated with unattached kinetochores of prometaphase chromosomes Wang et al. It has been shown that interaction of Rae1 with NuMA nuclear mitotic apparatus protein during mitosis is critical for proper mitotic spindle formation Wong et al. Both Rae1 and Nup98 are not only involved in spindle assembly Cross and Powers, , but also have a role in proper timing of securin degradation during mitotic exit.

Additionally, the mitotic checkpoint protein Bub3 shares extensive sequence homology with Rae1 Taylor et al. Furthermore, the Drosophila and human nucleoporin, Tpr, has been shown to orchestrate the spindle checkpoint control via interaction with Mad1 and Mad2 during cell division Lee et al. Conversely, certain mitotic checkpoint proteins, such as Mad1 and Mad2, which are kinetochore associated during mitosis, are found at the nuclear face of NPCs during interphase Campbell et al.

Interestingly, a fraction of the complex also localizes to kinetochores in both C. Depletion of this complex leads to a prolonged prometaphase, chromosome missegregation and delayed onset of anaphase Zuccolo et al. For instance, the peripheral mammalian Nup, Nup, has been shown to play dual roles in both early mitotic progression and the resolution of membrane abscission during cytokinesis in HeLa cells Mackay et al. Together, these data provide a molecular basis for a potential interplay between nucleocytoplasmic transport and mitotic machinery in mammalian cells.

Nup2 is an essential gene in A. During mitosis, the An-Nup complex is located at the NE and spindle pole bodies but, unlike in vertebrate cells, does not concentrate at kinetochores Liu et al. Ndc1, one of transmembrane Nups, is essential in yeasts and is required for the duplication and NE insertion of spindle pole bodies Winey et al. However, Ndc1 is not essential in either C. Namely, An-Mad1 and An-Mlp1 relocate from the telophase matrix and associate with segregated kinetochores when mitotic exit is prevented by expression of nondegradable cyclin B De Souza et al.

In the yeast S. In contrast to A. At the onset of mitosis, the plant cortical MTs depolymerize and rearrange into the pre-prophase band PPB surrounding the nucleus. The proper formation of the PPB is crucial for the fate of a dividing cell, as this transient MT array demarcates the future cortical division zone where a cell will separate into two daughter cells Van Damme and Geelen, ; Muller et al.

RanGAP1 is anchored at the NPC and just prior to mitosis it is delivered to the PPB in an MT-dependent manner and it remains associated with the cortical division site CDS during mitosis and cytokinesis, constituting a continuous positive marker of the plant division plane Xu et al.

The silencing of RanGAP in Arabidopsis roots leads to mispositioned cell walls similar to other mutants with division plane defects. Thus, the plant Ran cycle might play roles in mitotic microtubule assembly and vesicle fusion in analogy to its functions in animal cells. NbRae1 associates with the spindle and was shown to function in the proper spindle organization and chromosome segregation, similar to its mammalian orthologue Whalen et al. Interestingly, NbRae1 is targeted to the cell plate and to other mitotic MT arrays, namely the PPB and the phragmoplast, throughout cell division, suggesting a tight linkage between the NPC components and the cytoskeleton during mitosis Lee et al.

NbRae1 silencing resulted in delayed progression of mitosis, which led to plant-growth arrest, reduced cell division activities in the shoot apex and the vascular cambium, and increased ploidy levels in mature leaves. Together, these results suggest a conserved function of Rae1 in spindle organization among eukaryotes, which is distinct from its role at the interphase NE. Apart from Rae1, other NPC proteins are localized at the cell plate as well.

Even though there has been some delay, plant NPC research is catching up in this regard. Unravelling the structure and the molecular composition of the plant NPC suggests that the overall architecture of NPCs is conserved in eukaryotes. Based on the molecular composition, plant NPCs are close to those of vertebrates, but several plantunique features also suggest a different evolutionary path.

One puzzling aspect is the diversity of phenotypes that plant NPC mutants exhibit. The notion that developmental defects do not result solely from the sensitivity of cells to perturbations in nucleocytoplasmic transport is supported by recent lines of evidence indicating that the NPC acts as a key regulator of events that occur on either side of the NE and at different developmental and cellular stages. Dissection of these NPC functions will shed light on the interplay between the nucleocytoplasmic transport and other pathways, such as mitotic cell division in plants.

Nature Cell Biology 8, — Cell Cycle 4, — Plant Cell 11, — EMBO Journal 14, — Molecular and Cellular Biology 19, — Molecular Biology of the Cell 9, — EMBO Journal 17, — Plant Cell 21, — EMBO Reports 4, — Molecular Biology of the Cell 22, — EMBO Journal 21, — Current Biology 14, — Eukaryot Cell 6, — Trends in Genetics 25, — Plant Cell 19, — Molecular and Cellular Biology 26, — Experimental Cell Research , 12— Journal of Cell Science 24 , — EMBO Journal 24, —