Cytokinesis in flowering plants: more than one way to divide a cell

Quantitative Proteomic Analysis Deciphers the Molecular Mechanism for Endosperm Nuclear Division in Early Rice Seed Development

Understanding the molecular mechanisms underlying early seed development is important in improving the grain yield and quality of crop plants. We performed a comparative label-free quantitative proteomic analysis of developing rice seeds for the WT and osctps1-2 mutant, encoding a cytidine triphosphate synthase previously reported as the endospermless 2 (enl2) mutant in rice, harvested at 0 and 1 d after pollination (DAP) to understand the molecular mechanism of early seed development. In total, 5231 proteins were identified, of which 902 changed in abundance between 0 and 1 DAP seeds. Proteins that preferentially accumulated at 1 DAP were involved in DNA replication and pyrimidine biosynthetic pathways. Notably, an increased abundance of OsCTPS1 was observed at 1 DAP; however, no such changes were observed at the transcriptional level. We further observed that the inhibition of phosphorylation increased the stability of this protein. Furthermore, in osctps1-2, minichromosome maintenance (MCM) proteins were significantly reduced compared with those in the WT at 1 DAP, and mutations in OsMCM5 caused defects in seed development. These results highlight the molecular mechanisms underlying early seed development in rice at the post-transcriptional level.

Cortical microtubules contribute to division plane positioning during telophase in maize

Cell divisions are accurately positioned to generate cells of the correct size and shape. In plant cells, the new cell wall is built in the middle of the cell by vesicles trafficked along an antiparallel microtubule and a microfilament array called the phragmoplast. The phragmoplast expands toward a specific location at the cell cortex called the division site, but how it accurately reaches the division site is unclear. We observed microtubule arrays that accumulate at the cell cortex during the telophase transition in maize (Zea mays) leaf epidermal cells. Before the phragmoplast reaches the cell cortex, these cortical-telophase microtubules transiently interact with the division site. Increased microtubule plus end capture and pausing occur when microtubules contact the division site-localized protein TANGLED1 or other closely associated proteins. Microtubule capture and pausing align the cortical microtubules perpendicular to the division site during telophase. Once the phragmoplast reaches the cell cortex, cortical-telophase microtubules are incorporated into the phragmoplast primarily by parallel bundling. The addition of microtubules into the phragmoplast promotes fine-tuning of the positioning at the division site. Our hypothesis is that division site-localized proteins such as TANGLED1 organize cortical microtubules during telophase to mediate phragmoplast positioning at the final division plane.

Redundant mechanisms in division plane positioning

Redundancies in plant cell division contribute to the maintenance of proper division plane orientation. Here we highlight three types of redundancy: 1) Temporal redundancy, or correction of earlier defects that results in proper final positioning, 2) Genetic redundancy, or functional compensation by homologous genes, and 3) Synthetic redundancy, or redundancy within or between pathways that contribute to proper division plane orientation. Understanding the types of redundant mechanisms involved provides insight into current models of division plane orientation and opens up new avenues for exploration.

Division site determination during asymmetric cell division in plants

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

The parent-of-origin lncRNA MISSEN regulates rice endosperm development

The cereal endosperm is a major factor determining seed size and shape. However, the molecular mechanisms of endosperm development are not fully understood. Long noncoding RNAs (lncRNAs) function in various biological processes. Here we show a lncRNA, MISSEN, that plays an essential role in early endosperm development in rice (Oryza sativa). MISSEN is a parent-of-origin lncRNA expressed in endosperm, and negatively regulates endosperm development, leading to a prominent dent and bulge in the seed. Mechanistically, MISSEN functions through hijacking a helicase family protein (HeFP) to regulate tubulin function during endosperm nucleus division and endosperm cellularization, resulting in abnormal cytoskeletal polymerization. Finally, we revealed that the expression of MISSEN is inhibited by histone H3 lysine 27 trimethylation (H3K27me3) modification after pollination. Therefore, MISSEN is the first lncRNA identified as a regulator in endosperm development, highlighting the potential applications in rice breeding.

Cross-talk between clathrin-dependent post-Golgi trafficking and clathrin-mediated endocytosis in Arabidopsis root cells

Coupling of post-Golgi and endocytic membrane transport ensures that the flow of materials to/from the plasma membrane (PM) is properly balanced. The mechanisms underlying the coordinated trafficking of PM proteins in plants, however, are not well understood. In plant cells, clathrin and its adaptor protein complexes, AP-2 and the TPLATE complex (TPC) at the PM, and AP-1 at the trans-Golgi network/early endosome (TGN/EE), function in clathrin-mediated endocytosis (CME) and post-Golgi trafficking. Here, we utilized mutants with defects in clathrin-dependent post-Golgi trafficking and CME, in combination with other cytological and pharmacological approaches, to further investigate the machinery behind the coordination of protein delivery and recycling to/from the TGN/EE and PM in Arabidopsis (Arabidopsis thaliana) root cells. In mutants with defective AP-2-/TPC-dependent CME, we determined that clathrin and AP-1 recruitment to the TGN/EE as well as exocytosis are significantly impaired. Likewise, defects in AP-1-dependent post-Golgi trafficking and pharmacological inhibition of exocytosis resulted in the reduced association of clathrin and AP-2/TPC subunits with the PM and a reduction in the internalization of cargoes via CME. Together, these results suggest that post-Golgi trafficking and CME are coupled via modulation of clathrin and adaptor protein complex recruitment to the TGN/EE and PM.

Cell polarity, asynchronous nuclear divisions, and bidirectional cytokinesis in male meiosis in Magnolia denudata

Magnolia, a basal angiosperm genus important for evolutionary and phylogenetic studies, is known to have male meiotic features not seen in the vast majority of angiosperms. However, knowledge about male meiosis in Magnolia is still fragmentary. Here, we report findings from an extensive investigation into male meiosis in Magnolia denudata using a combination of light and electron microscopy methods. Male meiosis in M. denudata was synchronous in prophase I but asynchronous in subsequent nuclear divisions. The polarized microspore mother cells from late prophase I onward had an elongated cell shape and thickened callose wall areas at the two smaller ends of the cell. The first nuclear division occurred along the long axis of the cell and the first callose furrow formed at the equatorial plane of the first nuclear division at the late telophase I stage. The second equatorial callose furrow formed after telophase II in a plane perpendicular to the first callose furrow. While cytokinesis occurred centripetally from the two furrows, a central callose wall island (CWI) appeared in the center of the cell and dense assemblies of vesicles and short tubules decorated the cytoplasmic regions between the furrows and the CWI. This cytokinesis mode differs from either the centripetal or the centrifugal mode of cytokinesis in microsporogenesis in the vast majority of angiosperms. As a result of this unusual cytokinesis, a large central callose mass remains in the mature tetrads. These observations may be useful to studies of cytokinetic mechanisms, evolution of microsporogenesis, and phylogenetics of angiosperms.

High-Pressure Freezing and Freeze Substitution for Transmission Electron Microscopy Imaging and Immunogold-Labeling

Electron microscopy enables the unbiased imaging of organelles and cellular structures at nano-meter scale resolution. The combination of cryofixation/freeze-substitution methods with other imaging techniques such as correlative light and electron microscopy (CLEM), electron tomography (ET), and immunogold-labeling provides unique opportunities to understand structural changes associated with cellular processes. This chapter presents the main steps in the preparation of Arabidopsis thaliana roots, cotyledons, anthers, and developing seeds by high-pressure freezing and freeze-substitution for structural analysis and immunogold-labeling using transmission electron microscopy.

Electron Tomography and Immunogold Labeling as Tools to Analyze De Novo Assembly of Plant Cell Walls

High-resolution imaging of the membranous intermediates and cytoskeletal arrays involved in the assembly of a new cell wall during plant cytokinesis requires state-of-the-art electron microscopy techniques. The combination of cryofixation/freeze-substitution methods with electron tomography (ET) has revealed amazing structural details of this unique cellular process. This chapter deals with the main steps associated with these imaging techniques: selection of samples suitable for studying plant cytokinesis, sample preparation by high-pressure freezing/freeze substitution, and ET of plastic sections. In addition, immunogold approaches for identification of proteins and polysaccharides during cell wall assembly are discussed.

The Ceratopteris (fern) developing motile gamete walls contain diverse polysaccharides, but not pectin

Unlike most plant cell walls, the five consecutive walls laid down during spermatogenesis in the model fern Ceratopteris contain sparse cellulose, lack pectin and are enriched with callose and hemicelluloses. Seed-free plants like bryophytes and pteridophytes produce swimming male gametes for sexual reproduction. During spermatogenesis, unique walls are formed that are essential to the appropriate development and maturation of the motile gametes. Other than the detection of callose and general wall polysaccharides in scattered groups, little is known about the sequence of wall formation and the composition of these walls during sperm cell differentiation in plants that produce swimming sperm. Using histochemistry and immunogold localizations, we examined the distribution of callose, cellulose, mannan and xylan-containing hemicelluloses, and homogalacturonan (HG) pectins in the special walls deposited during spermatogenesis in Ceratopteris. Five walls are produced in sequence and each has a unique fate. The first wall (W1) contains callose and sparse xylan-containing hemicelluloses. Wall two (W2) is thin and composed of cellulose crosslinked by xylan-containing hemicelluloses. The third wall (W3) is thick and composed entirely of callose, and the fourth wall (W4) is built of cellulose heavily crosslinked by galactoxyloglucan hemicelluloses. Wall five (W5) is an arabinogalactan protein (AGP)-rich matrix in which the gamete changes shape and multiple flagella elongate. We detected no esterified or unesterified HG pectins in any of the walls laid down during spermatogenesis. To consider evolutionary modifications in cell walls associated with motile gametes, comparisons are presented with male gametophyte and spermatogenous cell walls across plant groups.

Cytoplasmic MTOCs control spindle orientation for asymmetric cell division in plants

Proper orientation of the cell division axis is critical for asymmetric cell divisions that underpin cell differentiation. In animals, centrosomes are the dominant microtubule organizing centers (MTOC) and play a pivotal role in axis determination by orienting the mitotic spindle. In land plants that lack centrosomes, a critical role of a microtubular ring structure, the preprophase band (PPB), has been observed in this process; the PPB is required for orienting (before prophase) and guiding (in telophase) the mitotic apparatus. However, plants must possess additional mechanisms to control the division axis, as certain cell types or mutants do not form PPBs. Here, using live imaging of the gametophore of the moss Physcomitrella patens, we identified acentrosomal MTOCs, which we termed "gametosomes," appearing de novo and transiently in the prophase cytoplasm independent of PPB formation. We show that gametosomes are dispensable for spindle formation but required for metaphase spindle orientation. In some cells, gametosomes appeared reminiscent of the bipolar MT "polar cap" structure that forms transiently around the prophase nucleus in angiosperms. Specific disruption of the polar caps in tobacco cells misoriented the metaphase spindles and frequently altered the final division plane, indicating that they are functionally analogous to the gametosomes. These results suggest a broad use of transient MTOC structures as the spindle orientation machinery in plants, compensating for the evolutionary loss of centrosomes, to secure the initial orientation of the spindle in a spatial window that allows subsequent fine-tuning of the division plane axis by the guidance machinery.

Evolution of polymer formation within the actin superfamily

While many are familiar with actin as a well-conserved component of the eukaryotic cytoskeleton, it is less often appreciated that actin is a member of a large superfamily of structurally related protein families found throughout the tree of life. Actin-related proteins include chaperones, carbohydrate kinases, and other enzymes, as well as a staggeringly diverse set of proteins that use the energy from ATP hydrolysis to form dynamic, linear polymers. Despite differing widely from one another in filament structure and dynamics, these polymers play important roles in ordering cell space in bacteria, archaea, and eukaryotes. It is not known whether these polymers descended from a single ancestral polymer or arose multiple times by convergent evolution from monomeric actin-like proteins. In this work, we provide an overview of the structures, dynamics, and functions of this diverse set. Then, using a phylogenetic analysis to examine actin evolution, we show that the actin-related protein families that form polymers are more closely related to one another than they are to other nonpolymerizing members of the actin superfamily. Thus all the known actin-like polymers are likely to be the descendants of a single, ancestral, polymer-forming actin-like protein.

Anaphase asymmetry and dynamic repositioning of the division plane during maize meiosis

The success of an organism is contingent upon its ability to transmit genetic material through meiotic cell division. In plant meiosis I, the process begins in a large spherical cell without physical cues to guide the process. Yet, two microtubule-based structures, the spindle and phragmoplast, divide the chromosomes and the cell with extraordinary accuracy. Using a live-cell system and fluorescently labeled spindles and chromosomes, we found that the process self- corrects as meiosis proceeds. Metaphase spindles frequently initiate division off-center, and in these cases anaphase progression is asymmetric with the two masses of chromosomes traveling unequal distances on the spindle. The asymmetry is compensatory, such that the chromosomes on the side of the spindle that is farthest from the cell cortex travel a longer distance at a faster rate. The phragmoplast forms at an equidistant point between the telophase nuclei rather than at the original spindle mid-zone. This asymmetry in chromosome movement implies a structural difference between the two halves of a bipolar spindle and could allow meiotic cells to dynamically adapt to errors in metaphase and accurately divide the cell volume.

Identification of Phosphoinositide-Binding Protein PATELLIN2 as a Substrate of Arabidopsis MPK4 MAP Kinase during Septum Formation in Cytokinesis

The phosphorylation of proteins by protein kinases controls many cellular and physiological processes, which include intracellular signal transduction. However, the underlying molecular mechanisms of such controls and numerous substrates of protein kinases remain to be characterized. The mitogen-activated protein kinase (MAPK) cascade is of particular importance in a variety of extracellular and intracellular signaling processes. In plant cells, the progression of cytokinesis is an excellent example of an intracellular phenomenon that requires the MAPK cascade. However, the way in which MAPKs control downstream processes during cytokinesis in plant cells remains to be fully determined. We show here that comparisons, by two-dimensional difference gel electrophoresis, of phosphorylated proteins from wild-type Arabidopsis thaliana and mutant plants defective in a MAPK cascade allow identification of substrates of a specific MAPK. Using this method, we identified the PATELLIN2 (PATL2) protein, which has a SEC14 domain, as a substrate of MPK4 MAP kinase. PATL2 was concentrated at the cell division plane, as is MPK4, and had binding affinity for phosphoinositides. This binding affinity was altered after phosphorylation of PATL2 by MPK4, suggesting a role for the MAPK cascade in the formation of cell plates via regeneration of membranes during cytokinesis.

Analysis of gemini pollen 3 mutant suggests a broad function of AUGMIN in microtubule organization during sexual reproduction in Arabidopsis

In flowering plants, male gametes arise via meiosis of diploid pollen mother cells followed by two rounds of mitotic division. Haploid microspores undergo polar nuclear migration and asymmetric division at pollen mitosis I to segregate the male germline, followed by division of the germ cell to generate a pair of sperm cells. We previously reported two gemini pollen (gem) mutants that produced twin-celled pollen arising from polarity and cytokinesis defects at pollen mitosis I in Arabidopsis. Here, we report an independent mutant, gem3, with a similar division phenotype and severe genetic transmission defects through pollen. Cytological analyses revealed that gem3 disrupts cell division during male meiosis, at pollen mitosis I and during female gametophyte development. We show that gem3 is a hypomorphic allele (aug6-1) of AUGMIN subunit 6, encoding a conserved component in the augmin complex, which mediates microtubule (MT)-dependent MT nucleation in acentrosomal cells. We show that MT arrays are disturbed in gem3/aug6-1 during male meiosis and pollen mitosis I using fluorescent MT-markers. Our results demonstrate a broad role for the augmin complex in MT organization during sexual reproduction, and highlight gem3/aug6-1 mutants as a valuable tool for the investigation of augmin-dependent MT nucleation and dynamics in plant cells.

A Genetic Screen for Mutations Affecting Cell Division in the Arabidopsis thaliana Embryo Identifies Seven Loci Required for Cytokinesis

Cytokinesis in plants involves the formation of unique cellular structures such as the phragmoplast and the cell plate, both of which are required to divide the cell after nuclear division. In order to isolate genes that are involved in de novo cell wall formation, we performed a large-scale, microscope-based screen for Arabidopsis mutants that severely impair cytokinesis in the embryo. We recovered 35 mutations that form abnormally enlarged cells with multiple, often polyploid nuclei and incomplete cell walls. These mutants represent seven genes, four of which have previously been implicated in phragmoplast or cell plate function. Mutations in two loci show strongly reduced transmission through the haploid gametophytic generation. Molecular cloning of both corresponding genes reveals that one is represented by hypomorphic alleles of the kinesin-5 gene RADIALLY SWOLLEN 7 (homologous to tobacco kinesin-related protein TKRP125), and that the other gene corresponds to the Arabidopsis FUSED ortholog TWO-IN-ONE (originally identified based on its function in pollen development). No mutations that completely abolish the formation of cross walls in diploid cells were found. Our results support the idea that cytokinesis in the diploid and haploid generations involve similar mechanisms.

Induction of Embryogenesis in Brassica Napus Microspores Produces a Callosic Subintinal Layer and Abnormal Cell Walls with Altered Levels of Callose and Cellulose

The induction of microspore embryogenesis produces dramatic changes in different aspects of the cell physiology and structure. Changes at the cell wall level are among the most intriguing and poorly understood. In this work, we used high pressure freezing and freeze substitution, immunolocalization, confocal, and electron microscopy to analyze the structure and composition of the first cell walls formed during conventional Brassica napus microspore embryogenesis, and in cultures treated to alter the intracellular Ca(2+) levels. Our results revealed that one of the first signs of embryogenic commitment is the formation of a callose-rich, cellulose-deficient layer beneath the intine (the subintinal layer), and of irregular, incomplete cell walls. In these events, Ca(2+) may have a role. We propose that abnormal cell walls are due to a massive callose synthesis and deposition of excreted cytoplasmic material, and the parallel inhibition of cellulose synthesis. These features were absent in pollen-like structures and in microspore-derived embryos, few days after the end of the heat shock, where abnormal cell walls were no longer produced. Together, our results provide an explanation to a series of relevant aspects of microspore embryogenesis including the role of Ca(2+) and the occurrence of abnormal cell walls. In addition, our discovery may be the explanation to why nuclear fusions take place during microspore embryogenesis.

The functions of the cytoskeleton and associated proteins during mitosis and cytokinesis in plant cells

In higher plants, microtubule (MT)-based, and actin filament (AF)-based structures play important roles in mitosis and cytokinesis. Besides the mitotic spindle, the evolution of a band comprising cortical MTs and AFs, namely, the preprophase band (PPB), is evident in plant cells. This band forecasts a specific division plane before the initiation of mitosis. During cytokinesis, another plant-specific cytoskeletal structure called the phragmoplast guides vesicles in the creation of a new cell wall. In addition, a number of cytoskeleton-associated proteins are reportedly involved in the formation and function of the PPB, mitotic spindle, and phragmoplast. This review summarizes current knowledge on the cytoskeleton-associated proteins that mediate the cytoskeletal arrays during mitosis and cytokinesis in plant cells and discusses the interaction between MTs and AFs involved in mitosis and cytokinesis.

Microtubule networks for plant cell division

During cytokinesis the cytoplasm of a cell is divided to form two daughter cells. In animal cells, the existing plasma membrane is first constricted and then abscised to generate two individual plasma membranes. Plant cells on the other hand divide by forming an interior dividing wall, the so-called cell plate, which is constructed by localized deposition of membrane and cell wall material. Construction starts in the centre of the cell at the locus of the mitotic spindle and continues radially towards the existing plasma membrane. Finally the membrane of the cell plate and plasma membrane fuse to form two individual plasma membranes. Two microtubule-based cytoskeletal networks, the phragmoplast and the pre-prophase band (PPB), jointly control cytokinesis in plants. The bipolar microtubule array of the phragmoplast regulates cell plate deposition towards a cortical position that is templated by the ring-shaped microtubule array of the PPB. In contrast to most animal cells, plants do not use centrosomes as foci of microtubule growth initiation. Instead, plant microtubule networks are striking examples of self-organizing systems that emerge from physically constrained interactions of dispersed microtubules. Here we will discuss how microtubule-based activities including growth, shrinkage, severing, sliding, nucleation and bundling interrelate to jointly generate the required ordered structures. Evidence mounts that adapter proteins sense the local geometry of microtubules to locally modulate the activity of proteins involved in microtubule growth regulation and severing. Many of the proteins and mechanisms involved have roles in other microtubule assemblies as well, bestowing broader relevance to insights gained from plants.

Arabidopsis Fused kinase TWO-IN-ONE dominantly inhibits male meiotic cytokinesis

Arabidopsis Fused kinase TWO-IN-ONE (TIO) controls phragmoplast expansion through its interaction with the Kinesin-12 subfamily proteins that anchor the plus ends of interdigitating microtubules in the phragmoplast midzone. Previous analyses of loss-of-function mutants and RNA interference lines revealed that TIO positively controls both somatic and gametophytic cell cytokinesis; however, knowledge of the full spectrum of TIO functions during plant development remains incomplete. To characterize TIO functions further, we expressed TIO and a range of TIO variants under control of the TIO promoter in wild-type Arabidopsis plants. We discovered that TIO-overexpressing transgenic lines produce enlarged pollen grains, arising from incomplete cytokinesis during male meiosis, and show sporophytic abnormalities indicative of polyploidy. These phenotypes arose independently in TIO variants in which either gametophytic function or the ability of TIO to interact with Kinesin-12 subfamily proteins was abolished. Interaction assays in yeast showed TIO to bind to the AtNACK2/TETRASPORE, and plants doubly homozygous for kinesin-12a and kinesin-12b knockout mutations to produce enlarged pollen grains. Our results show TIO to dominantly inhibit male meiotic cytokinesis in a dosage-dependent manner that may involve direct binding to a component of the canonical NACK-PQR cytokinesis signaling pathway.

Identification of Arabidopsis meiotic cyclins reveals functional diversification among plant cyclin genes

Meiosis is a modified cell division in which a single S-phase is followed by two rounds of chromosome segregation resulting in the production of haploid gametes. The meiotic mode of chromosome segregation requires extensive remodeling of the basic cell cycle machinery and employment of unique regulatory mechanisms. Cyclin-dependent kinases (CDKs) and cyclins represent an ancient molecular module that drives and regulates cell cycle progression. The cyclin gene family has undergone a massive expansion in angiosperm plants, but only a few cyclins were thoroughly characterized. In this study we performed a systematic immunolocalization screen to identify Arabidopsis thaliana A- and B-type cyclins expressed in meiosis. Many of these cyclins exhibit cell-type-specific expression in vegetative tissues and distinct subcellular localization. We found six A-type cyclins and a single B-type cyclin (CYCB3;1) to be expressed in male meiosis. Mutant analysis revealed that these cyclins contribute to distinct meiosis-related processes. While A2 cyclins are important for chromosome segregation, CYCB3;1 prevents ectopic cell wall formation. We further show that cyclin SDS does not contain a D-box and is constitutively expressed throughout meiosis. Analysis of plants carrying cyclin SDS with an introduced D-box motif determined that, in addition to its function in recombination, SDS acts together with CYCB3;1 in suppressing unscheduled cell wall synthesis. Our phenotypic and expression data provide extensive evidence that multiplication of cyclins is in plants accompanied by functional diversification.

The alteration of haploid and diploid cell generations during the sexual life cycle requires meiosis, a specialized cell division that enables the formation of haploid gametes from diploid cells. Meiosis occurs only once during the life cycle, and the transition from the mitotic to meiotic mode of chromosome partitioning requires extensive remodeling of the cell cycle machinery. The cell cycle progression is driven by cyclin-dependent kinases and associated cyclins that regulate CDK activity and confer substrate specificity. Cyclin gene families have undergone a massive expansion in plants, which has raised the question of whether some of these cyclins evolved specific meiotic functions. We systematically analyzed two cyclin gene families in Arabidopsis to identify plant cyclins that are meiotically expressed. We found in total eight cyclins to be expressed in male meiotic cells, and functional characterization revealed their involvement in diverse meiotic processes. Interestingly, none of the cyclins appear to be essential for meiotic progression, indicating that plant meiosis is governed by unorthodox cell cycle regulators.

Cytokinesis in plant male meiosis

In somatic cell division, cytokinesis is the final step of the cell cycle and physically divides the mother cytoplasm into two daughter cells. In the meiotic cell division, however, pollen mother cells (PMCs) undergo two successive nuclear divisions without an intervening S-phase and consequently generate four haploid daughter nuclei out of one parental cell. In line with this, the physical separation of meiotic nuclei does not follow the conventional cytokinesis pathway, but instead is mediated by alternative processes, including polar-based phragmoplast outgrowth and RMA-mediated cell wall positioning. In this review, we outline the different cytological mechanisms of cell plate formation operating in different types of PMCs and additionally focus on some important features associated with male meiotic cytokinesis, including cytoskeletal dynamics and callose deposition. We also provide an up-to-date overview of the main molecular actors involved in PMC wall formation and additionally highlight some recent advances on the effect of cold stress on meiotic cytokinesis in plants.

Cytoskeletal and membrane dynamics during higher plant cytokinesis

Following mitosis, cytoplasm, organelles and genetic material are partitioned into daughter cells through the process of cytokinesis. In somatic cells of higher plants, two cytoskeletal arrays, the preprophase band and the phragmoplast, facilitate the positioning and de novo assembly of the plant-specific cytokinetic organelle, the cell plate, which develops across the division plane and fuses with the parental plasma membrane to yield distinct new cells. The coordination of cytoskeletal and membrane dynamics required to initiate, assemble and shape the cell plate as it grows toward the mother cell cortex is dependent upon a large array of proteins, including molecular motors, membrane tethering, fusion and restructuring factors and biosynthetic, structural and regulatory elements. This review focuses on the temporal and molecular requirements of cytokinesis in somatic cells of higher plants gleaned from recent studies using cell biology, genetics, pharmacology and biochemistry.

Asymmetric cytokinesis guide the development of pseudomonads in Rhynchospora pubera (Cyperaceae)

The late stages of microsporogenesis in the family Cyperaceae are marked by the formation of an asymmetrical tetrad, degeneration of three of the four nuclei resulting from meiosis and the formation of pseudomonads. In order to understand the cytological changes involved in the development of pseudomonads, a combination of 11 different techniques (conventional staining, cytochemistry procedures, immunofluorescence, FISH and transmission electron microscopy: TEM) were used to study the later stages of microsporogenesis in Rhynchospora pubera. The results demonstrated the occurrence of two cytoplasmic domains in the pseudomonads, one functional and the other degenerative, which are physically and asymmetrically separated by cell plate with an endomembrane system rich in polysaccharides. Other changes associated with endomembrane behaviour were observed, such as a large number of lipid droplets, vacuoles containing electron-dense material and concentric layers of endoplasmic reticulum. Concomitant with the isolation of degenerative nuclei, the tapetal cells also showed evidence of degeneration, indicating that both tissues under programmed cell death (PCD), as indicated by immunofluorescence and TEM procedures. The results are significant because they associate cellular polarisation and asymmetry with different cytoplasmic domains, and hence open new possibilities for studying cellular compartmentalisation and PCD.

Germline-specific MATH-BTB substrate adaptor MAB1 regulates spindle length and nuclei identity in maize

Germline and early embryo development constitute ideal model systems to study the establishment of polarity, cell identity, and asymmetric cell divisions (ACDs) in plants. We describe here the function of the MATH-BTB domain protein MAB1 that is exclusively expressed in the germ lineages and the zygote of maize (Zea mays). mab1 (RNA interference [RNAi]) mutant plants display chromosome segregation defects and short spindles during meiosis that cause insufficient separation and migration of nuclei. After the meiosis-to-mitosis transition, two attached nuclei of similar identity are formed in mab1 (RNAi) mutants leading to an arrest of further germline development. Transient expression studies of MAB1 in tobacco (Nicotiana tabacum) Bright Yellow-2 cells revealed a cell cycle-dependent nuclear localization pattern but no direct colocalization with the spindle apparatus. MAB1 is able to form homodimers and interacts with the E3 ubiquitin ligase component Cullin 3a (CUL3a) in the cytoplasm, likely as a substrate-specific adapter protein. The microtubule-severing subunit p60 of katanin was identified as a candidate substrate for MAB1, suggesting that MAB1 resembles the animal key ACD regulator Maternal Effect Lethal 26 (MEL-26). In summary, our findings provide further evidence for the importance of posttranslational regulation for asymmetric divisions and germline progression in plants and identified an unstable key protein that seems to be involved in regulating the stability of a spindle apparatus regulator(s).

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

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

Differentiation of endosperm transfer cells of barley: a comprehensive analysis at the micro-scale

Barley endosperm cells differentiate into transfer cells (ETCs) opposite the nucellar projection. To comprehensively analyse ETC differentiation, laser microdissection-based transcript and metabolite profiles were obtained from laser microdissected tissues and cell morphology was analysed. Flange-like secondary-wall ingrowths appeared between 5 and 7 days after pollination within the three outermost cell layers. Gene expression analysis indicated that ethylene-signalling pathways initiate ETC morphology. This is accompanied by gene activity related to cell shape control and vesicle transport, with abundant mitochondria and endomembrane structures. Gene expression analyses indicate predominant formation of hemicelluloses, glucuronoxylans and arabinoxylans, and transient formation of callose, together with proline and 4-hydroxyproline biosynthesis. Activation of the methylation cycle is probably required for biosynthesis of phospholipids, pectins and ethylene. Membrane microdomains involving sterols/sphingolipids and remorins are potentially involved in ETC development. The transcriptional activity of assimilate and micronutrient transporters suggests ETCs as the main uptake organs of solutes into the endosperm. Accordingly, the endosperm grows maximally after ETCs are fully developed. Up-regulated gene expression related to amino acid catabolism, C:N balances, carbohydrate oxidation, mitochondrial activity and starch degradation meets high demands for respiratory energy and carbohydrates, required for cell proliferation and wall synthesis. At 10 days after pollination, ETCs undergo further differentiation, potentially initiated by abscisic acid, and metabolism is reprogrammed as shown by activated storage and stress-related processes. Overall, the data provide a comprehensive view of barley ETC differentiation and development, and identify candidate genes and associated pathways.

Endosperm: food for humankind and fodder for scientific discoveries

The endosperm is an essential constituent of seeds in flowering plants. It originates from a fertilization event parallel to the fertilization that gives rise to the embryo. The endosperm nurtures embryo development and, in some species including cereals, stores the seed reserves and represents a major source of food for humankind. Endosperm biology is characterized by specific features, including idiosyncratic cellular controls of cell division and epigenetic controls associated with parental genomic imprinting. This review attempts a comprehensive summary of our current knowledge of endosperm development and highlights recent advances in this field.

Pattern of deposition of cell wall polysaccharides and transcript abundance of related cell wall synthesis genes during differentiation in barley endosperm

Immunolabeling, combined with chemical analyses and transcript profiling, have provided a comprehensive temporal and spatial picture of the deposition and modification of cell wall polysaccharides during barley (Hordeum vulgare) grain development, from endosperm cellularization at 3 d after pollination (DAP) through differentiation to the mature grain at 38 DAP. (1→3)-β-D-Glucan appears transiently during cellularization but reappears in patches in the subaleurone cell walls around 20 DAP. (1→3, 1→4)-β-Glucan, the most abundant polysaccharide of the mature barley grain, accumulates throughout development. Arabino-(1-4)-β-D-xylan is deposited significantly earlier than we previously reported. This was attributable to the initial deposition of the polysaccharide in a highly substituted form that was not recognized by antibodies commonly used to detect arabino-(1-4)-β-D-xylans in sections of plant material. The epitopes needed for antibody recognition were exposed by pretreatment of sections with α-L-arabinofuranosidase; this procedure showed that arabino-(1-4)-β-D-xylans were deposited as early as 5 DAP and highlighted their changing structures during endosperm development. By 28 DAP labeling of hetero-(1→4)-β-D-mannan is observed in the walls of the starchy endosperm but not in the aleurone walls. Although absent in mature endosperm cell walls we now show that xyloglucan is present transiently from 3 until about 6 DAP and disappears by 8 DAP. Quantitative reverse transcription-polymerase chain reaction of transcripts for GLUCAN SYNTHASE-LIKE, Cellulose Synthase, and CELLULOSE SYNTHASE-LIKE genes were consistent with the patterns of polysaccharide deposition. Transcript profiling of some members from the Carbohydrate-Active Enzymes database glycosyl transferase families GT61, GT47, and GT43, previously implicated in arabino-(1-4)-β-d-xylan biosynthesis, confirms their presence during grain development.

Universal rules for division plane selection in plants

Coordinated cell divisions and cell expansion are the key processes that command growth in all organisms. The orientation of cell divisions and the direction of cell expansion are critical for normal development. Symmetric divisions contribute to proliferation and growth, while asymmetric divisions initiate pattern formation and differentiation. In plants these processes are of particular importance since their cells are encased in cellulosic walls that determine their shape and lock their position within tissues and organs. Several recent studies have analyzed the relationship between cell shape and patterns of symmetric cell division in diverse organisms and employed biophysical and mathematical considerations to develop computer simulations that have allowed accurate prediction of cell division patterns. From these studies, a picture emerges that diverse biological systems follow simple universal rules of geometry to select their division planes and that the microtubule cytoskeleton takes a major part in sensing the geometric information and translates this information into a specific division outcome. In plant cells, the division plane is selected before mitosis, and spatial information of the division plane is preserved throughout division by the presence of reference molecules at a distinct region of the plasma membrane, the cortical division zone. The recruitment of these division zone markers occurs multiple times by several mechanisms, suggesting that the cortical division zone is a highly dynamic region.

Microtubules and mitogen-activated protein kinase signalling

Subcellular signalling by mitogen-activated protein kinases (MAPKs) was originally regarded as a means to regulate microtubule (MT) organization and dynamics, but with time MAPKs were assigned to broader roles concerning biotic and abiotic signal transductions. MAPKs, which regulate a broad spectrum of substrates including transcription factors and cytoskeletal proteins, belong to complex MAPK cascades, which are mainly involved in plant development and in plant stress responses. The fact that single MAPK can be regulated by more than a single MAPKKK/MAPKK pair make MAPK signalling modules versatile tools in the regulation of microtubule organization. Until recently, the best-studied MAPK module implicated in cytoskeletal regulation is the NACK-PQR pathway in tobacco (Nicotiana tabacum). Homologues of each constituent of this pathway were also discovered in Arabidopsis thaliana. So far, direct phosphorylation of tubulins by MAPKs has not been shown. However, the first MAPK-related substrate involved in the regulation of MT dynamics to have been identified is MT-associated protein MAP65-1.

AtMPK4 is required for male-specific meiotic cytokinesis in Arabidopsis

Mitogen-activated protein kinase (MAPK) cascades have been implicated in regulating various aspects of plant development, including somatic cytokinesis. The evolution of expanded plant MAPK gene families has enabled the diversification of potential MAPK cascades, but functionally overlapping components are also well documented. Here we report that Arabidopsis MPK4, an MAPK that was previously described as a regulator of disease resistance, can interact with and be phosphorylated by the cytokinesis-related MAP kinase kinase, AtMKK6. In mpk4 mutant plants, anthers can develop normal microspore mother cells (MMCs) and peripheral supporting tissues, but the MMCs fail to form a normal intersporal callose wall after male meiosis, and thus cannot complete meiotic cytokinesis. Nevertheless, the multinucleate mpk4 microspores subsequently proceed through mitotic cytokinesis, resulting in enlarged mature pollen grains that possess increased sets of the tricellular structure. This pollen development phenotype is reminiscent of those observed in both atnack2/tes/stud and anq1/mkk6 mutants, and protein-protein interaction analysis defines a putative signalling module linking AtNACK2/TES/STUD, AtANP3, AtMKK6 and AtMPK4 together as a cascade that facilitates male-specific meiotic cytokinesis in Arabidopsis.

SIDECAR POLLEN suggests a plant-specific regulatory network underlying asymmetric microspore division in Arabidopsis

Asymmetric cell division is a universal strategy to generate diverse cell types necessary for patterning and proliferation of all eukaryotes. The development of haploid male gametophytes (pollen grains) in flowering plants is a remarkable example in which division asymmetry governs the functional specialization and germline differentiation essential for double fertilization. The male gametophyte is patterned via two mitotic divisions resulting in three highly differentiated daughter cells at maturity, a vegetative cell and two sperm cells. The first asymmetric division segregates a unique male germ cell from an undetermined haploid microspore and is executed in an elaborate sequence of cellular events. However the molecular mechanisms governing the division asymmetry in microspores are poorly understood. Recently we studied the phenotype of sidecar pollen (scp) mutants in detail, and demonstrated a requirement of SCP for both the correct timing and orientation of microspore division. SCP is a microspore-specific member of the LOB/AS2 domain family (LBD27/ASL29) showing that a plant-specific regulator plays a key role in oriented division of polarized microspores. Identification of SCP will serve as a new platform to further explore the largely unknown molecular networks regulating division asymmetry in microspores that establishes the male germline in flowering plants.

Determination of symmetric and asymmetric division planes in plant cells

The cellular organization of plant tissues is determined by patterns of cell division and growth coupled with cellular differentiation. Cells proliferate mainly via symmetric division, whereas asymmetric divisions are associated with initiation of new developmental patterns and cell types. Division planes in both symmetrically and asymmetrically dividing cells are established through the action of a cortical preprophase band (PPB) of cytoskeletal filaments, which is disassembled upon transition to metaphase, leaving behind a cortical division site (CDS) to which the cytokinetic phragmoplast is later guided to position the cell plate. Recent progress has been made in understanding PPB formation and function as well as the nature and function of the CDS. In asymmetrically dividing cells, division plane establishment is governed by cell polarity. Recent work is beginning to shed light on polarization mechanisms in asymmetrically dividing cells, with receptor-like proteins and potential downstream effectors emerging as important players in this process.

Microtubule Reorganization during Mitosis and Cytokinesis: Lessons Learned from Developing Microgametophytes in Arabidopsis Thaliana

In angiosperms, mitosis and cytokinesis take place in the absence of structurally defined microtubule-organizing centers and the underlying mechanisms are largely unknown. In the spindle and phragmoplast, microtubule reorganization depends on microtubule-interacting factors like the γ-tubulin complex. Because of their critical functions in cell division, loss-of-function mutations in the corresponding genes are often homozygous or sporophytic lethal. However, a number of mutations like gem1, gcp2, and nedd1 can be maintained in heterozygous mutants in Arabidopsis thaliana. When mutant microspores produced by a heterozygous parent undergo pollen mitosis I, they are amenable for phenotypic characterization by fluorescence microscopy. The results would allow us to pinpoint at specific functions of particular proteins in microtubule reorganization that are characteristic to specific stages of mitosis and cytokinesis. Conclusions made in the developing microgametophytes can be extrapolated to somatic cells regarding mechanisms that regulate nuclear migration, spindle pole formation, phragmoplast assembly, and cell division plane determination.

Electron tomography and immunogold labelling as tools to analyse de novo assembly of plant cell walls

High-resolution imaging of the membranous intermediates and cytoskeletal arrays involved in the assembly of a new cell wall during plant cytokinesis requires state-of-the-art electron microscopy techniques. The combination of cryofixation/freeze-substitution methods with electron tomography (ET) has revealed amazing structural details of this unique cellular process. This chapter deals with the main steps associated with these imaging techniques: selection of samples suitable for studying plant cytokinesis, sample preparation by high-pressure freezing/freeze substitution, and ET of plastic sections. In addition, immunogold approaches for the identification of proteins and polysaccharides during cell wall assembly are discussed.

Membrane fusion process and assembly of cell wall during cytokinesis in the brown alga, Silvetia babingtonii (Fucales, Phaeophyceae)

During cytokinesis in brown algal cells, Golgi-derived vesicles (GVs) and flat cisternae (FCs) are involved in building the new cell partition membrane. In this study, we followed the membrane fusion process in Silvetia babingtonii zygotes using electron microscopy together with rapid freezing and freeze substitution. After mitosis, many FCs were formed around endoplasmic reticulum clusters and these then spread toward the future cytokinetic plane. Actin depolymerization using latrunculin B prevented the appearance of the FCs. Fusion of GVs to FCs resulted in structures that were thicker and more elongated (EFCs; expanded flat cisternae). Some complicated membranous structures (MN; membranous network) were formed by interconnection of EFCs and following the arrival of additional GVs. The MN grew into membranous sacs (MSs) as gaps between the MNs disappeared. The MSs were observed in patches along the cytokinetic plane. Neighboring MSs were united to form the new cell partition membrane. An immunocytochemical analysis indicated that fucoidan was synthesized in Golgi bodies and transported by vesicles to the future cytokinetic plane, where the vesicles fused with the FCs. Alginate was not detected until the MS phase. Incubation of sections with cellulase-gold showed that the cellulose content of the new cross wall was not comparable to that of the parent cell wall.

Cell-cell channels, viruses, and evolution: via infection, parasitism, and symbiosis toward higher levels of biological complexity

Between prokaryotic cells and eukaryotic cells there is dramatic difference in complexity which represents a problem for the current version of the cell theory, as well as for the current version of evolution theory. In the past few decades, the serial endosymbiotic theory of Lynn Margulis has been confirmed. This results in a radical departure from our understanding of living systems: the eukaryotic cell represents de facto"cells-within-cell." Higher order "cells-within-cell" situations are obvious at the eukaryotic cell level in the form of secondary and tertiary endosymbiosis, or in the male and female gametophytes of higher plants. The next challenge of the current version of the cell theory is represented by the fact that the multicellular fungi and plants are, in fact, supracellular assemblies as their cells are not physically separated from each other. Moreover, there are also examples of alliances and mergings between multicellular organisms. Infection, especially the viral one, but also bacterial and fungal infections, followed by symbiosis, is proposed to act as the major force that drives the biological evolution toward higher complexity.

Microsporogenesis variation in Codiaeum producing inaperturate pollen grain

A study of microsporogenesis (the earliest stage of pollen ontogeny) was undertaken in seven cultivars of Codiaeum variegatum var. pictum, a eudicot species that produces inaperturate pollen grains. Microsporogenesis appears highly variable for the developmental events suspected to be implicated in the determination of aperture pattern. Most eudicots have tri-aperturate pollen grains and microsporogenesis is described as highly conserved in this clade. The observed burst of variation in C. variegatum therefore appears especially remarkable. A plausible hypothesis to explain the variation is that the pollen being inaperturate, the selective forces applying on the ontogeny of the aperture pattern are relaxed.

Mechanism of action of nitrous oxide gas applied as a polyploidizing agent during meiosis in lilies

Nitrous oxide gas (N(2)O) can be used to produce polyploid plants, but the mechanism of action is unknown. The actin and microtubule cytoskeleton was observed in N(2)O-treated microsporocytes of Lilium spp 'Asiatic hybrid lilies' using fluorescence microscopy after staining with DAPI, FITC-conjugated tubulin antibody, and phalloidin-conjugated Alexa Fluor 546. Additionally, microsporocytes of L. longiflorum were observed with acetocarmine staining following N(2)O treatment. A typical metaphase I microtubule distribution was observed in control microsporocytes. After treatment with N(2)O for 24 h, microtubules were effectively depolymerized; this prevented chromosomes from moving to the poles, resulting in chromosome retention in the center of N(2)O-treated cells. Cell plate formation took place without delay, however, yielding one daughter cell with a diploid genome and another daughter without chromosomes. In addition, N(2)O treatment often induced micronuclei due to aberrant chromosome separation during cytokinesis. Actin filaments in microsporocytes are insensitive to N(2)O. These findings indicate that N(2)O mediates polyploidization by inhibiting microtubule polymerization, but not actin filament formation, during microsporocyte meiosis.

Arabidopsis kinesins HINKEL and TETRASPORE act redundantly to control cell plate expansion during cytokinesis in the male gametophyte

Asymmetric cell division at pollen mitosis I (PMI) is required to specify the differential fate of the daughter vegetative and generative cells. Cytokinesis at PMI displays specialized features, and it has been suggested that there might be distinct molecular pathways underpinning different modes of cytokinesis in plants. Activation of the NACK-PQR MAP kinase signaling pathway, which is essential for somatic cell cytokinesis in tobacco, depends upon the NACK1 and NACK2 kinesin-related proteins. Their Arabidopsis orthologs, HINKEL (HIK) and TETRASPORE (TES), were reported to be essential for cytokinesis in somatic cells and in microsporocytes, respectively. More recently, HIK and TES were shown to have a functionally redundant role in female gametophytic cytokinesis. We report here that HIK and TES are co-expressed in microspores and developing pollen, and, through analysis of microspore and pollen development in double heterozygote mutants, the occurrence of cell plate expansion defects during cytokinesis at PMI. The data demonstrate a functionally redundant role for HIK and TES in cell plate expansion during male gametophytic cytokinesis, extending the concept that different modes of cytokinesis are executed by a common signaling pathway, but reinforcing the individuality of gametophytic cytokinesis in its requirement for either TES or HIK.

Emerging molecular mechanisms that power and regulate the anastral mitotic spindle of flowering plants

Flowering plants, lacking centrosomes as well as dynein, assemble their mitotic spindle via a pathway that is distinct visually and molecularly from that of animals and yeast. The molecular components underlying mitotic spindle assembly and function in plants are beginning to be discovered. Here, we review recent evidence suggesting the preprophase band in plants functions analogously to the centrosome in animals in establishing spindle bipolarity, and we review recent progress characterizing the roles of specific motor proteins in plant mitosis. Loss of function of certain minus-end-directed KIN-14 motor proteins causes a broadening of the spindle pole; whereas, loss of function of a KIN-5 causes the formation of monopolar spindles, resembling those formed when the homologous motor protein (e.g., Eg5) is knocked out in animal cells. We present a phylogeny of the kinesin-5 motor domain, which shows deep divergence among plant sequences, highlighting possibilities for specialization. Finally, we review information concerning the roles of selected structural proteins at mitosis as well as recent findings concerning regulation of M-phase in plants. Insight into the mitotic spindle will be obtained through continued comparison of mitotic mechanisms in a diversity of cells.

The defective seed5 (des5) mutant: effects on barley seed development and HvDek1, HvCr4, and HvSal1 gene regulation

Barley, one of the major small grain crops, is especially important in climatically demanding agricultural areas of the world, with multiple uses within food, feed, and beverage. The barley endosperm is further of special scientific interest due to its three aleurone cell layers, with the potential of bringing forward the molecular understanding of seed development and cell specification from Arabidopsis and maize. Work done in Arabidopsis and maize indicate the presence of conserved seed developmental pathways where Crinkly4 (Cr4), Defective kernel1 (Dek1), and Supernumerary aleurone layer1 (Sal1) are key players. With the use of microscopy, a comprehensive phenotypic characterization of the barley defective seed5 (des5) mutant is presented here. The analysis further extends to molecular quantification of gene expression changes in the des5 mutant by qRT-PCR. Moreover, full-length genomic sequences of the barley orthologues were generated and these were annotated as HvDek1, HvCr4, and HvSal1. The most striking results in this study are the patchy reduction in number of aleurone cells, rudimentary anticlinal aleurone cell walls, and the specific change of HvCr4 expression compared to HvDek1 and HvSal1. The data presented support the involvement of Hvdes5 in establishing aleurone cells. Finally, how these results might affect the current model of aleurone and epidermal cell identity and development is discussed with a speculation regarding a possible role of Des5 in regulating cell division/ secondary cell wall building.

R1R2R3-Myb proteins positively regulate cytokinesis through activation of KNOLLE transcription in Arabidopsis thaliana

G2/M phase-specific gene transcription in tobacco cells is mediated by R1R2R3-Myb transcriptional activators, NtmybA1 and NtmybA2, which bind to mitosis-specific activator (MSA) elements. We show here that two structurally related genes, MYB3R1 and MYB3R4, which encode homologs of NtmybA1 and NtmybA2, play a partially redundant role in positively regulating cytokinesis in Arabidopsis thaliana. The myb3r1 myb3r4 double mutant often fails to complete cytokinesis, resulting in multinucleate cells with gapped walls and cell wall stubs in diverse tissues. These defects correlate with the selective reduction of transcript levels of several G2/M phase-specific genes, which include B2-type cyclin (CYCB2), CDC20.1 and KNOLLE (KN). These genes contain MSA-like motifs in their promoters and were activated by MYB3R4 in transient expression assays in tobacco cells. The KN gene encodes a cytokinesis-specific syntaxin that is essential for cell plate formation. The cytokinesis defects of myb3r1 myb3r4 double mutants were partially rescued by KN gene expression from heterologous promoters. In addition, a kn heterozygous mutation enhanced cytokinesis defects resulting from heterozygous or homozygous mutations in the MYB3R1 and MYB3R4 genes. Our results suggest that a pair of structurally related R1R2R3-Myb transcription factors may positively regulate cytokinesis mainly through transcriptional activation of the KN gene.

A divergent cellular role for the FUSED kinase family in the plant-specific cytokinetic phragmoplast

The FUSED (FU) Ser/Thr protein kinase family has a key role in the hedgehog signaling pathway known to control cell proliferation and patterning in fruit flies and humans . The genomes of Arabidopsis thaliana and rice each encode a single Fu ortholog, but their role is unknown. Here, we show that cytokinesis-defective mutants, which we named two-in-one (tio), result from mutations in Arabidopsis Fu. Phenotypic analysis of tio mutants reveals an essential role for TIO in conventional modes of cytokinesis in plant meristems and during male gametogenesis. TIO also has a key role in nonconventional modes of cytokinesis (cellularization) during female gametogenesis. We demonstrate that TIO is tightly localized to the midline of the nascent phragmoplast and remains associated with the expanding phragmoplast ring. These data reveal the evolution of a divergent role for the Fu kinase family as an essential phragmoplast-associated protein that functions in different cell type-specific modes of cytokinesis in plants.