RanGAP is required for post-meiotic mitosis in female gametophyte development in Arabidopsis thaliana

Identification of nuclear membrane SUN proteins and components associated with wheat fungal stress responses

In eukaryotes, the nuclear membrane that encapsulates genomic DNA is composed of an inner nuclear membrane (INM), an outer nuclear membrane (ONM), and a perinuclear space. SUN proteins located in the INM and KASH proteins in the ONM form the SUN-KASH NM-bridge, which functions as the junction of the nucleocytoplasmic complex junction. Proteins containing the SUN domain showed the highest correlation with differentially accumulated proteins (DAPs) in the wheat response to fungal stress. To understand the characteristics of SUN and its associated proteins in wheat responding to pathogen stress, here we investigated and comprehensive analyzed SUN- and KASH-related proteins among the DAPs under fungi infection based on their conserved motifs. In total, four SUN proteins, one WPP domain-interacting protein (WIP), four WPP domain-interacting tail-anchored proteins (WIT), two WPP proteins and one Ran GTPase activating protein (RanGAP) were identified. Following transient expression of Nicotiana benthamiana, TaSUN2, TaRanGAP2, TaWIT1 and TaWIP1 were identified as nuclear membrane proteins, while TaWPP1 and TaWPP2 were expressed in both the nucleus and cell membrane. RT-qPCR analysis demonstrated that the transcription of TaSUN2, TaRanGAP2 and TaWPP1 were strongly upregulated in response to fungal infection. Furthermore, using the bimolecular fluorescence complementation, the luciferase complementation and a nuclear and split-ubiquitin-based membrane yeast two-hybrid systems, we substantiated the interaction between TaSUN2 and TaWIP1, as well as TaWIP1/WIT1 and TaWPP1/WPP2. Silencing of TaSUN2, TaRanGAP2 and TaWPP1 in wheat leaves promoted powdery mildew infection and hyphal growth, and reduced the expression of TaBRI1, TaBAK1 and Ta14-3-3, indicating that these NM proteins play a positive role in resistance to fungal stress. Our study reveals the characteristics of NM proteins and propose the preliminary construction of SUN-WIP-WPP-RanGAP complex in wheat, which represents a foundation for detail elucidating their functions in wheat in future.

Genomic structure analysis and construction of DNA fingerprint for four sheep populations

The Yongdeng Qishan sheep (QS) is a sheep population found locally in China. To gain in-depth knowledge of its population characteristics, three control groups were chosen, comprising the Lanzhou fat-tailed sheep (LFT), TAN sheep (TAN), and Minxian black fur sheep (MBF), inhabiting the nearby environments. This study genotyped a total of 120 individuals from four sheep populations: QS, LFT, TAN, and MBF. Using Specific-Locus Amplified Fragment Sequencing, we conducted genetic diversity, population structure, and selective sweep analysis, and constructed the fingerprint of each population. In total, there were 782 535 single nucleotide polymorphism (SNP) variations identified, with most being situated within regions that are intergenic or intronic. The genetic diversity analysis revealed that the QS population exhibited lower genetic diversity compared to the other three populations. Consistent results were obtained from the principal component, phylogenetic tree, and population structure analysis, indicating significant genetic differences between QS and the other three populations. However, a certain degree of differentiation was observed within the QS population. The linkage disequilibrium (LD) patterns among the four populations showed clear distinctions, with the QS group demonstrating the most rapid LD decline. Kinship analysis supported the findings of population structure, dividing the 90 QS individuals into two subgroups consisting of 23 and 67 individuals. Selective sweep analysis identified a range of genes associated with reproduction, immunity, and adaptation to high-altitude hypoxia. These genes hold potential as candidate genes for marker-assisted selection breeding. Additionally, a total of 86 523 runs of homozygosity (ROHs) were detected, showing non-uniform distribution across chromosomes, with chromosome 1 having the highest coverage percentage and chromosome 26 the lowest. In the high-frequency ROH islands, 79 candidate genes were associated with biological processes such as reproduction and fat digestion and absorption. Furthermore, a DNA fingerprint was constructed for the four populations using 349 highly polymorphic SNPs. In summary, our research delves into the genetic diversity and population structure of QS population. The construction of DNA fingerprint profiles for each population can provide valuable references for the identification of sheep breeds both domestically and internationally.

Emerging role of small GTPases and their interactome in plants to combat abiotic and biotic stress

Plants are frequently subjected to abiotic and biotic stress which causes major impediments in their growth and development. It is emerging that small guanosine triphosphatases (small GTPases), also known as monomeric GTP-binding proteins, assist plants in managing environmental stress. Small GTPases function as tightly regulated molecular switches that get activated with the aid of guanosine triphosphate (GTP) and deactivated by the subsequent hydrolysis of GTP to guanosine diphosphate (GDP). All small GTPases except Rat sarcoma (Ras) are found in plants, including Ras-like in brain (Rab), Rho of plant (Rop), ADP-ribosylation factor (Arf) and Ras-like nuclear (Ran). The members of small GTPases in plants interact with several downstream effectors to counteract the negative effects of environmental stress and disease-causing pathogens. In this review, we describe processes of stress alleviation by developing pathways involving several small GTPases and their associated proteins which are important for neutralizing fungal infections, stomatal regulation, and activation of abiotic stress-tolerant genes in plants. Previous reviews on small GTPases in plants were primarily focused on Rab GTPases, abiotic stress, and membrane trafficking, whereas this review seeks to improve our understanding of the role of all small GTPases in plants as well as their interactome in regulating mechanisms to combat abiotic and biotic stress. This review brings to the attention of scientists recent research on small GTPases so that they can employ genome editing tools to precisely engineer economically important plants through the overexpression/knock-out/knock-in of stress-related small GTPase genes.

Ran-GTP/-GDP-dependent nuclear accumulation of NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 and TGACG-BINDING FACTOR2 controls salicylic acid-induced leaf senescence

Leaf senescence is the final stage of leaf development and can be triggered by various external factors, such as hormones and light deprivation. In this study, we demonstrate that the overexpression of the GTP-bound form of Arabidopsis (Arabidopsis thaliana) Ran1 (a Ras-related nuclear small G-protein, AtRan1) efficiently promotes age-dependent and dark-triggered leaf senescence, while Ran-GDP has the opposite effect. Transcriptome analysis comparing AtRan1-GDP- and AtRan1-GTP-overexpressing transgenic plants (Ran1T27Nox and Ran1G22Vox, respectively) revealed that differentially expressed genes (DEGs) related to the senescence-promoting hormones salicylic acid (SA), jasmonic acid, abscisic acid, and ethylene (ET) were significantly upregulated in dark-triggered senescing leaves of Ran1G22Vox, indicating that these hormones are actively involved in Ran-GTP/-GDP-dependent, dark-triggered leaf senescence. Bioinformatic analysis of the promoter regions of DEGs identified diverse consensus motifs, including the bZIP motif, a common binding site for TGACG-BINDING FACTOR (TGA) transcription factors. Interestingly, TGA2 and its interactor, NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1), which are two positive transcriptional regulators of SA signaling, differed in their extent of accumulation in the nucleus versus cytoplasm of Ran1T27Nox and Ran1G22Vox plants. Moreover, SA-induced, Ran-GTP-/-GDP-dependent functions of NPR1 included genome-wide global transcriptional reprogramming of genes involved in cell death, aging, and chloroplast organization. Furthermore, the expression of AtRan1-GTP in SA signaling-defective npr1 and SA biosynthesis-deficient SA-induction deficient2 genetic backgrounds abolished the effects of AtRan1-GTP, thus retarding age-promoted leaf senescence. However, ET-induced leaf senescence was not mediated by Ran machinery-dependent nuclear shuttling of ETHYLENE-INSENSITIVE3 and ETHYLENE-INSENSITIVE3-LIKE1 proteins. We conclude that Ran-GTP/-GDP-dependent nuclear accumulation of NPR1 and TGA2 represents another regulatory node for SA-induced leaf senescence.

Transcriptome Profiling Identifies Candidate Genes Contributing to Male and Female Gamete Development in Synthetic Brassica Allohexaploids

Polyploidy plays a crucial role in plant evolution and speciation. The development of male and female gametes is essential to the reproductive capacity of polyploids, but their gene expression pattern has not been fully explored in newly established polyploids. The present study aimed to reveal a detailed atlas of gene expression for gamete development in newly synthetic Brassica allohexaploids that are not naturally existing species. Comparative transcriptome profiling between developing anthers (staged from meiosis to mature pollen) and ovules (staged from meiosis to mature embryo sac) was performed using RNA-Seq analysis. A total of 8676, 9775 and 4553 upregulated differentially expressed genes (DEGs) were identified for the development of both gametes, for male-only, and for female-only gamete development, respectively, in the synthetic Brassica allohexaploids. By combining gene ontology (GO) biological process analysis and data from the published literature, we identified 37 candidate genes for DNA double-strand break formation, synapsis and the crossover of homologous recombination during male and female meiosis and 51 candidate genes for tapetum development, sporopollenin biosynthesis and pollen wall development in male gamete development. Furthermore, 23 candidate genes for mitotic progression, nuclear positioning and cell specification and development were enriched in female gamete development. This study lays a good foundation for revealing the molecular regulation of genes related to male and female gamete development in Brassica allohexaploids and provides more resourceful genetic information on the reproductive biology of Brassica polyploid breeding.

Arabidopsis RAN GTPases are critical for mitosis during male and female gametogenesis

The development of male and female gametophytes is a prerequisite for successful propagation of angiosperms. The small GTPases RAN play fundamental roles in numerous cellular processes. Although RAN GTPases have been characterized in plants, their roles in cellular processes are far from understood. We report here that RAN GTPases in Arabidopsis are critical for gametophytic development. RAN1 loss-of-function showed no defects in gametophytic development likely due to redundancy. However, the expression of a dominant negative or constitutively active RAN1 resulted in gametophytic lethality. Genetic interference of RAN GTPases caused the arrest of pollen mitosis I and of mitosis of functional megaspores, implying a key role of properly regulated RAN activity in mitosis during gametophytic development.

DEFECTIVE EMBRYO AND MERISTEMS genes are required for cell division and gamete viability in Arabidopsis

The DEFECTIVE EMBRYO AND MERISTEMS 1 (DEM1) gene encodes a protein of unknown biochemical function required for meristem formation and seedling development in tomato, but it was unclear whether DEM1’s primary role was in cell division or alternatively, in defining the identity of meristematic cells. Genome sequence analysis indicates that flowering plants possess at least two DEM genes. Arabidopsis has two DEM genes, DEM1 and DEM2, which we show are expressed in developing embryos and meristems in a punctate pattern that is typical of genes involved in cell division. Homozygous dem1 dem2 double mutants were not recovered, and plants carrying a single functional DEM1 allele and no functional copies of DEM2, i.e. DEM1/dem1 dem2/dem2 plants, exhibit normal development through to the time of flowering but during male reproductive development, chromosomes fail to align on the metaphase plate at meiosis II and result in abnormal numbers of daughter cells following meiosis. Additionally, these plants show defects in both pollen and embryo sac development, and produce defective male and female gametes. In contrast, dem1/dem1 DEM2/dem2 plants showed normal levels of fertility, indicating that DEM2 plays a more important role than DEM1 in gamete viability. The increased importance of DEM2 in gamete viability correlated with higher mRNA levels of DEM2 compared to DEM1 in most tissues examined and particularly in the vegetative shoot apex, developing siliques, pollen and sperm. We also demonstrate that gamete viability depends not only on the number of functional DEM alleles inherited following meiosis, but also on the number of functional DEM alleles in the parent plant that undergoes meiosis. Furthermore, DEM1 interacts with RAS-RELATED NUCLEAR PROTEIN 1 (RAN1) in yeast two-hybrid and pull-down binding assays, and we show that fluorescent proteins fused to DEM1 and RAN1 co-localize transiently during male meiosis and pollen development. In eukaryotes, RAN is a highly conserved GTPase that plays key roles in cell cycle progression, spindle assembly during cell division, reformation of the nuclear envelope following cell division, and nucleocytoplasmic transport. Our results demonstrate that DEM proteins play an essential role in cell division in plants, most likely through an interaction with RAN1.

Up to half of the genes predicted from genome projects lack a known biological and biochemical function. Many of these genes are likely to play essential roles but it is difficult to reveal their function because minor changes in the genetic sequence can result in lethality and genetic redundancy can obscure analysis. Genome projects predict that flowering plants have at least two DEM genes that encode a protein of unknown cellular and biochemical function. In this paper, we use multiple combinations of dem mutants in Arabidopsis to show that DEM genes are essential for cell division and gamete viability. Interestingly, gamete viability depends not only on the number of functional copies of DEM genes in the gametes, but also on the number of functional copies of DEM genes in the parent plant that produces the gametes. We also show that DEM proteins interact with RAN, a highly conserved protein that controls cell division in all eukaryotic organisms.

Heat Stress-Dependent Association of Membrane Trafficking Proteins With mRNPs Is Selective

The Arabidopsis AAA ATPase SKD1 is essential for ESCRT-dependent endosomal sorting by mediating the disassembly of the ESCRTIII complex in an ATP-dependent manner. In this study, we show that SKD1 localizes to messenger ribonucleoprotein complexes upon heat stress. Consistent with this, the interactome of SKD1 revealed differential interactions under normal and stress conditions and included membrane transport proteins as well as proteins associated with RNA metabolism. Localization studies with selected interactome proteins revealed that not only RNA associated proteins but also several ESCRTIII and membrane trafficking proteins were recruited to messenger ribonucleoprotein granules after heat stress.

The Small GTPase Superfamily in Plants: A Conserved Regulatory Module with Novel Functions

Small GTP-binding proteins represent a highly conserved signaling module in eukaryotes that regulates diverse cellular processes such as signal transduction, cytoskeletal organization and cell polarity, cell proliferation and differentiation, intracellular membrane trafficking and transport vesicle formation, and nucleocytoplasmic transport. These proteins function as molecular switches that cycle between active and inactive states, and this cycle is linked to GTP binding and hydrolysis. In this review, the roles of the plant complement of small GTP-binding proteins in these cellular processes are described, as well as accessory proteins that control their activity, and current understanding of the functions of individual members of these families in plants-with a focus on the model organism Arabidopsis-is presented. Some potential novel roles of these GTPases in plants, relative to their established roles in yeast and/or animal systems, are also discussed.

Reproductive Multitasking: The Female Gametophyte

Fertilization of flowering plants requires the organization of complex tasks, many of which become integrated by the female gametophyte (FG). The FG is a few-celled haploid structure that orchestrates division of labor to coordinate successful interaction with the sperm cells and their transport vehicle, the pollen tube. As reproductive outcome is directly coupled to evolutionary success, the underlying mechanisms are under robust molecular control, including integrity check and repair mechanisms. Here, we review progress on understanding the development and function of the FG, starting with the functional megaspore, which represents the haploid founder cell of the FG. We highlight recent achievements that have greatly advanced our understanding of pollen tube attraction strategies and the mechanisms that regulate plant hybridization and gamete fusion. In addition, we discuss novel insights into plant polyploidization strategies that expand current concepts on the evolution of flowering plants.

Protein methylome analysis in Arabidopsis reveals regulation in RNA-related processes

Protein methylation has been proposed as an important post-translational modification, which occurs predominantly on lysine and arginine residues. Recent discoveries have revealed that protein methylation is also present on non-histones besides histones, and plays critical roles in regulating protein stability and function. However, proteome-wide identification of methylated proteins in plants remains unexplored. Here, we present the first global survey of monomethyl arginine, symmetric and asymmetric dimethyl arginine, and monomethyl, dimethyl, trimethyl lysine modifications in the proteomes of 10-day-old Arabidopsis seedlings through a combination of immunoaffinity purification and mass spectrometry analysis. In total, we identified 617 methylation sites which mapped to 412 proteins, with 263 proteins harboring 381 lysine methylation sites and 149 proteins harboring 236 arginine methylation sites. Among them, 607 methylation sites on 408 proteins were novel findings. Motif analysis revealed that glycine preferentially flanked methylated arginine residues, whereas aspartate and glutamate enriched around mono- and dimethylated lysine sites. Methylated proteins were involved in a variety of metabolic processes, showing significant enrichment in RNA-related metabolic pathways including spliceosome, RNA transport, and ribosome. Our data provide a global view of methylated non-histone proteins in Arabidopsis, laying foundations for elucidating the biological function of protein methylation in plants. SIGNIFICANCE: Protein methylation has emerged as a common and important modification both in eukaryotes and prokaryotes. The identification of methylated sites/peptides is fundamental for further functional analysis of protein methylation. This study was the first proteome-scale identification of lysine and arginine methylation in plants. We found that methylation occurred widely on non-histone proteins in Arabidopsis and was involved in diverse biological functions. The results provide foundations for the investigation of the protein methylome in Arabidopsis and provide powerful resources for the functional analysis of protein methylation in plants.

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

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

Genome-wide discovery of DNA polymorphisms among chickpea cultivars with contrasting seed size/weight and their functional relevance

Seed size/weight is a major agronomic trait which determine crop productivity in legumes. To understand the genetic basis of seed size determination, we sought to identify DNA polymorphisms between two small (Himchana 1 and Pusa 362) and two large-seeded (JGK 3 and PG 0515) chickpea cultivars via whole genome resequencing. We identified a total of 75535 single nucleotide polymorphisms (SNPs), 6486 insertions and deletions (InDels), 1938 multi-nucleotide polymorphisms (MNPs) and 5025 complex variants between the two small and two large-seeded chickpea cultivars. Our analysis revealed 814, 244 and 72 seed-specific genes harboring DNA polymorphisms in promoter or non-synonymous and large-effect DNA polymorphisms, respectively. Gene ontology analysis revealed enrichment of cell growth and division related terms in these genes. Among them, at least 22 genes associated with quantitative trait loci, and those involved in cell growth and division and encoding transcription factors harbored promoter and/or large-effect/non-synonymous DNA polymorphisms. These also showed higher expression at late-embryogenesis and/or mid-maturation stages of seed development in the large-seeded cultivar, suggesting their role in seed size/weight determination in chickpea. Altogether, this study provided a valuable resource for large-scale genotyping applications and a few putative candidate genes that might play crucial role in governing seed size/weight in chickpea.

GAP Activity, but Not Subcellular Targeting, Is Required for Arabidopsis RanGAP Cellular and Developmental Functions

The Ran GTPase activating protein (RanGAP) is important to Ran signaling involved in nucleocytoplasmic transport, spindle organization, and postmitotic nuclear assembly. Unlike vertebrate and yeast RanGAP, plant RanGAP has an N-terminal WPP domain, required for nuclear envelope association and several mitotic locations of Arabidopsis thaliana RanGAP1. A double null mutant of the two Arabidopsis RanGAP homologs is gametophyte lethal. Here, we created a series of mutants with various reductions in RanGAP levels by combining a RanGAP1 null allele with different RanGAP2 alleles. As RanGAP level decreases, the severity of developmental phenotypes increases, but nuclear import is unaffected. To dissect whether the GAP activity and/or the subcellular localization of RanGAP are responsible for the observed phenotypes, this series of rangap mutants were transformed with RanGAP1 variants carrying point mutations abolishing the GAP activity and/or the WPP-dependent subcellular localization. The data show that plant development is differentially affected by RanGAP mutant allele combinations of increasing severity and requires the GAP activity of RanGAP, while the subcellular positioning of RanGAP is dispensable. In addition, our results indicate that nucleocytoplasmic trafficking can tolerate both partial depletion of RanGAP and delocalization of RanGAP from the nuclear envelope.

Mechanisms of plant cell division

Plant cells are confined by a network of cellulosic walls that imposes rigid control over the selection of division plane orientations, crucial for morphogenesis and genetically regulated. While in animal cells and yeast, the actin cytoskeleton is instrumental in the execution of cytokinesis, in plant cells the microtubule cytoskeleton is taking the lead in spatially controlling and executing cytokinesis by the formation of two unique, plant-specific arrays, the preprophase band (PPB) and the phragmoplast. The formation of microtubule arrays in plant cells is contingent on acentrosomal microtubule nucleation. At the onset of mitosis, the PPB defines the plane of cell division where the partitioning cell wall is later constructed by the cytokinetic phragmoplast, imposing a spatio-temporal relationship between the two processes. Current research progress in the field of plant cell division focuses on identifying and tying the links between early and late events in spatial control of cytokinesis and how microtubule array formation is regulated in plant cells.

The plant nuclear envelope and regulation of gene expression

The nuclear envelope (NE) separates the key mechanisms of transcription and translation, and as such is a critical control point in all eukaryotic cells. In plants, the proteins of the NE influence a number of processes including the control of nucleo-cytoplasmic transport of RNA and protein, chromatin localization to the nuclear periphery, and direct chromatin binding by members of the nuclear pore complex (NPC). In this review I attempt to bring these roles under the umbrella of their effect on gene expression, even though the complex nature of this cellular environment means there is considerable overlap of effects. Although the volume of research in plant cells has greatly improved over recent years, it is clear that our understanding of how the components of the NE either directly or indirectly influence gene expression is still in its infancy.

Proteomic profiling reveals insights into Triticeae stigma development and function

To our knowledge, this study represents the first high-throughput characterization of a stigma proteome in the Triticeae. A total of 2184 triticale mature stigma proteins were identified using three different gel-based approaches combined with mass spectrometry. The great majority of these proteins are described in a Triticeae stigma for the first time. These results revealed many proteins likely to play important roles in stigma development and pollen-stigma interactions, as well as protection against biotic and abiotic stresses. Quantitative comparison of the triticale stigma transcriptome and proteome showed poor correlation, highlighting the importance of having both types of analysis. This work makes a significant contribution towards the elucidation of the Triticeae stigma proteome and provides novel insights into its role in stigma development and function.

Efficient plant male fertility depends on vegetative nuclear movement mediated by two families of plant outer nuclear membrane proteins

Increasing evidence suggests that nuclear migration is important for eukaryotic development. Although nuclear migration is conserved in plants, its importance for plant development has not yet been established. The most extraordinary plant nuclear migration events involve plant fertilization, which is starkly different from that of animals. Instead of evolving self-propelled sperm cells (SCs), plants use pollen tubes to deliver SCs, in which the pollen vegetative nucleus (VN) and the SCs migrate as a unit toward the ovules, a fundamental but barely understood process. Here, we report that WPP domain-interacting proteins (WIPs) and their binding partners the WPP domain-interacting tail-anchored proteins (WITs) are essential for pollen nuclear migration. Loss-of-function mutations in WIT and/or WIP gene families resulted in impaired VN movement, inefficient SC delivery, and defects in pollen tube reception. WIPs are Klarsicht/ANC-1/Syne-1 Homology (KASH) analogs in plants. KASH proteins are key players in animal nuclear migration. Thus, this study not only reveals an important nuclear migration mechanism in plant fertilization but also, suggests that similar nuclear migration machinery is conserved between plants and animals.

The Phragmoplast-Orienting Kinesin-12 Class Proteins Translate the Positional Information of the Preprophase Band to Establish the Cortical Division Zone in Arabidopsis thaliana

The preprophase band (PPB) is a faithful but transient predictor of the division plane in somatic cell divisions. Throughout mitosis the PPBs positional information is preserved by factors that continuously mark the division plane at the cell cortex, the cortical division zone, by their distinct spatio-temporal localization patterns. However, the mechanism maintaining these identity factors at the plasma membrane after PPB disassembly remains obscure. The pair of kinesin-12 class proteins PHRAGMOPLAST ORIENTING KINESIN1 (POK1) and POK2 are key players in division plane maintenance. Here, we show that POK1 is continuously present at the cell cortex, providing a spatial reference for the site formerly occupied by the PPB. Fluorescence recovery after photobleaching analysis combined with microtubule destabilization revealed dynamic microtubule-dependent recruitment of POK1 to the PPB during prophase, while POK1 retention at the cortical division zone in the absence of cortical microtubules appeared static. POK function is strictly required to maintain the division plane identity factor TANGLED (TAN) after PPB disassembly, although POK1 and TAN recruitment to the PPB occur independently during prophase. Together, our data suggest that POKs represent fundamental early anchoring components of the cortical division zone, translating and preserving the positional information of the PPB by maintaining downstream identity markers.

Ran GTPase-activating protein 1 is a therapeutic target in diffuse large B-cell lymphoma

Lymphoma-specific biomarkers contribute to therapeutic strategies and the study of tumorigenesis. Diffuse large B-cell lymphoma (DLBCL) is the most common type of malignant lymphoma. However, only 50% of patients experience long-term survival after current treatment; therefore, developing novel therapeutic strategies is warranted. Comparative proteomic analysis of two DLBCL lines with a B-lymphoblastoid cell line (LCL) showed differential expression of Ran GTPase-activating protein 1 (RanGAP1) between them, which was confirmed using immunoblotting. Immunostaining showed that the majority of DLBCLs (92%, 46/50) were RanGAP1(+), while reactive lymphoid hyperplasia (n = 12) was RanGAP1(+) predominantly in germinal centers. RanGAP1 was also highly expressed in other B-cell lymphomas (BCL, n = 180) with brisk mitotic activity (B-lymphoblastic lymphoma/leukemia: 93%, and Burkitt lymphoma: 95%) or cell-cycle dysregulation (mantle cell lymphoma: 83%, and Hodgkin's lymphoma 91%). Interestingly, serum RanGAP1 level was higher in patients with high-grade BCL (1.71 ± 2.28 ng/mL, n = 62) than in low-grade BCL (0.75 ± 2.12 ng/mL, n = 52) and healthy controls (0.55 ± 1.58 ng/mL, n = 75) (high-grade BCL vs. low-grade BCL, p = 0.002; high-grade BCL vs. control, p < 0.001, Mann-Whitney U test). In vitro, RNA interference of RanGAP1 showed no effect on LCL but enhanced DLBCL cell death (41% vs. 60%; p = 0.035) and cell-cycle arrest (G0/G1: 39% vs. 49%, G2/M: 19.0% vs. 7.5%; p = 0.030) along with decreased expression of TPX2 and Aurora kinases, the central regulators of mitotic cell division. Furthermore, ON 01910.Na (Estybon), a multikinase inhibitor induced cell death, mitotic cell arrest, and hyperphosphorylation of RanGAP1 in DLBCL cell lines but no effects in normal B and T cells. Therefore, RanGAP1 is a promising marker and therapeutic target for aggressive B-cell lymphoma, especially DLBCL.

The role of the cytoskeleton and associated proteins in determination of the plant cell division plane

In plants, as in all eukaryotic organisms, microtubule- and actin-filament based structures play fundamental roles during cell division. In addition to the mitotic spindle, plant cells have evolved a unique cytoskeletal structure that designates a specific division plane before the onset of mitosis via formation of a cortical band of microtubules and actin filaments called the preprophase band. During cytokinesis, a second plant-specific microtubule and actin filament structure called the phragmoplast directs vesicles to create the new cell wall. In response to intrinsic and extrinsic cues, many plant cells form a preprophase band in G2 , then the preprophase band recruits specific proteins to populate the cortical division site prior to disassembly of the preprophase band in prometaphase. These proteins are thought to act as a spatial reminder that actively guides the phragmoplast towards the cortical division site during cytokinesis. A number of proteins involved in determination and maintenance of the plane of cell division have been identified. Our current understanding of the molecular interactions of these proteins and their regulation of microtubules is incomplete, but advanced imaging techniques and computer simulations have validated some early concepts of division site determination.

Assessing the function of the plant nuclear pore complex and the search for specificity

Plant cells encounter a wide variety of molecules that influence their gene expression and development. A key component of most signal transduction pathways involves the regulated movement of molecules into and out of the nucleus. The plant nuclear pore complex (NPC) is a critical controlling element in this nucleocytoplasmic movement of protein and RNA. The NPC is comprised of approximately 30 nucleoporin proteins arranged in radial symmetry around the central pore. Over recent years our understanding of how the NPC impacts different signalling pathways has increased following the identification of a range of nucleoporin mutant plants. These mutants allow us to gain insight into how the response to hormonal, abiotic, and biotic stresses are effected by changes in nuclear transport. Importantly we have little information regarding the specific molecules whose nuclear transport is altered in these processes and the identification of these proteins is a significant challenge. Here is presented an overview as to how the members of the plant NPC affect signalling pathways, highlighting the progress and difficulties within this research area.

The R2R3 MYB transcription factors FOUR LIPS and MYB88 regulate female reproductive development

Gamete formation is an important step in the life cycle of sexually reproducing organisms. In flowering plants, haploid spores are formed after the meiotic division of spore mother cells. These spores develop into male and female gametophytes containing gametes after undergoing mitotic divisions. In the female, the megaspore mother cell undergoes meiosis forming four megaspores, of which one is functional and three degenerate. The megaspore then undergoes three mitotic cycles thus generating an embryo sac with eight nuclei. The embryo sac undergoes cellularization to form the mature seven-celled female gametophyte. Entry into and progression through meiosis is essential for megasporogenesis and subsequent megagametogenesis, but control of this process is not well understood. FOUR LIPS (FLP) and its paralogue MYB88, encoding R2R3 MYB transcription factors, have been extensively studied for their role in limiting the terminal division in stomatal development by direct regulation of the expression of cell cycle genes. Here it is demonstrated that FLP and MYB88 also regulate female reproduction. Both FLP and MYB88 are expressed during ovule development and their loss significantly increases the number of ovules produced by the placenta. Despite the presence of excess ovules, single and double mutants exhibit reduced seed set due to reduced female fertility. The sterility results at least in part from defective meiotic entry and progression. Therefore, FLP and MYB88 are important regulators of entry into megasporogenesis, and probably act via the regulation of cell cycle genes.