The rise and fall of the phragmoplast microtubule array

Four-dimensional quantitative analysis of cell plate development in Arabidopsis using lattice light sheet microscopy identifies robust transition points between growth phases

Cell plate formation during cytokinesis entails multiple stages occurring concurrently and requiring orchestrated vesicle delivery, membrane remodelling, and timely deposition of polysaccharides, such as callose. Understanding such a dynamic process requires dissection in time and space; this has been a major hurdle in studying cytokinesis. Using lattice light sheet microscopy (LLSM), we studied cell plate development in four dimensions, through the behavior of yellow fluorescent protein (YFP)-tagged cytokinesis-specific GTPase RABA2a vesicles. We monitored the entire duration of cell plate development, from its first emergence, with the aid of YFP-RABA2a, in both the presence and absence of cytokinetic callose. By developing a robust cytokinetic vesicle volume analysis pipeline, we identified distinct behavioral patterns, allowing the identification of three easily trackable cell plate developmental phases. Notably, the phase transition between phase I and phase II is striking, indicating a switch from membrane accumulation to the recycling of excess membrane material. We interrogated the role of callose using pharmacological inhibition with LLSM and electron microscopy. Loss of callose inhibited the phase transitions, establishing the critical role and timing of the polysaccharide deposition in cell plate expansion and maturation. This study exemplifies the power of combining LLSM with quantitative analysis to decode and untangle such a complex process.

Arabidopsis α-Aurora kinase plays a role in cytokinesis through regulating MAP65-3 association with microtubules at phragmoplast midzone

The α-Aurora kinase is a crucial regulator of spindle microtubule organization during mitosis in plants. Here, we report a post-mitotic role for α-Aurora in reorganizing the phragmoplast microtubule array. In Arabidopsis thaliana, α-Aurora relocated from spindle poles to the phragmoplast midzone, where it interacted with the microtubule cross-linker MAP65-3. In a hypomorphic α-Aurora mutant, MAP65-3 was detected on spindle microtubules, followed by a diffuse association pattern across the phragmoplast midzone. Simultaneously, phragmoplast microtubules remained belatedly in a solid disk array before transitioning to a ring shape. Microtubules at the leading edge of the matured phragmoplast were often disengaged, accompanied by conspicuous retentions of MAP65-3 at the phragmoplast interior edge. Specifically, α-Aurora phosphorylated two residues towards the C-terminus of MAP65-3. Mutation of these residues to alanines resulted in an increased association of MAP65-3 with microtubules within the phragmoplast. Consequently, the expansion of the phragmoplast was notably slower compared to wild-type cells or cells expressing a phospho-mimetic variant of MAP65-3. Moreover, mimicking phosphorylation reinstated disrupted MAP65-3 behaviors in plants with compromised α-Aurora function. Overall, our findings reveal a mechanism in which α-Aurora facilitates cytokinesis progression through phosphorylation-dependent restriction of MAP65-3 associating with microtubules at the phragmoplast midzone.

Localized calcium transients in phragmoplast regulate cytokinesis of tobacco BY-2 cells

KEY MESSAGE

Plants exhibit a unique pattern of cytosolic Ca2+ dynamics to correlate with microtubules to regulate cytokinesis, which significantly differs from those observed in animal and yeast cells. Calcium (Ca2+) transients mediated signaling is known to be essential in cytokinesis across eukaryotic cells. However, the detailed spatiotemporal dynamics of Ca2+ during plant cytokinesis remain largely unexplored. In this study, we employed GCaMP5, a genetically encoded Ca2+ sensor, to investigate cytokinetic Ca2+ transients during cytokinesis in Nicotiana tabacum Bright Yellow-2 (BY-2) cells. We validated the effectiveness of GCaMP5 to capture fluctuations in intracellular free Ca2+ in transgenic BY-2 cells. Our results reveal that Ca2+ dynamics during BY-2 cell cytokinesis are distinctly different from those observed in embryonic and yeast cells. It is characterized by an initial significant Ca2+ spike within the phragmoplast region. This spike is followed by a decrease in Ca2+ concentration at the onset of cytokinesis in phragmoplast, which then remains elevated in comparison to the cytosolic Ca2+ until the completion of cell plate formation. At the end of cytokinesis, Ca2+ becomes uniformly distributed in the cytosol. This pattern contrasts with the typical dual waves of Ca2+ spikes observed during cytokinesis in animal embryonic cells and fission yeasts. Furthermore, applications of pharmaceutical inhibitors for either Ca2+ or microtubules revealed a close correlation between Ca2+ transients and microtubule organization in the regulation of cytokinesis. Collectively, our findings highlight the unique dynamics and crucial role of Ca2+ transients during plant cell cytokinesis, and provides new insights into plant cell division mechanisms.

D, Noriega A, Drakakaki G, Sylvester AW, Rasmussen CG. Subcellular positioning during cell division and cell plate formation in maize

Introduction

During proliferative plant cell division, the new cell wall, called the cell plate, is first built in the middle of the cell and then expands outward to complete cytokinesis. This dynamic process requires coordinated movement and arrangement of the cytoskeleton and organelles.

Methods

Here we use live-cell markers to track the dynamic reorganization of microtubules, nuclei, endoplasmic reticulum, and endomembrane compartments during division and the formation of the cell plate in maize leaf epidermal cells.

Results

The microtubule plus-end localized protein END BINDING1 (EB1) highlighted increasing microtubule dynamicity during mitosis to support rapid changes in microtubule structures. The localization of the cell-plate specific syntaxin KNOLLE, several RAB-GTPases, as well as two plasma membrane localized proteins was assessed after treatment with the cytokinesis-specific callose-deposition inhibitor Endosidin7 (ES7) and the microtubule-disrupting herbicide chlorpropham (CIPC). While ES7 caused cell plate defects in Arabidopsis thaliana, it did not alter callose accumulation, or disrupt cell plate formation in maize. In contrast, CIPC treatment of maize epidermal cells occasionally produced irregular cell plates that split or fragmented, but did not otherwise disrupt the accumulation of cell-plate localized proteins.

Discussion

Together, these markers provide a robust suite of tools to examine subcellular trafficking and organellar organization during mitosis and cell plate formation in maize.

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.

Rice kinesin-related protein STD1 and microtubule-associated protein MAP65-5 cooperatively control microtubule bundling

STD1 specifically interacts with MAP65-5 in rice and they cooperatively control microtubule bundles in phragmoplast expansion during cell division. Microtubules play critical roles during the cell cycle progression in the plant cell. We previously reported that STEMLESS DWARF 1 (STD1), a kinesin-related protein, was localized specifically to the phragmoplast midzone during telophase to regulate the lateral expansion of phragmoplast in rice (Oryza sativa). However, how STD1 regulates microtubule organization remains unknown. Here, we found that STD1 interacted directly with MAP65-5, a member of the microtubule-associated proteins (MAPs). Both STD1 and MAP65-5 could form homodimers and bundle microtubules individually. Compared with MAP65-5, the microtubules bundled by STD1 were disassembled completely into single microtubules after adding ATP. Conversely, the interaction of STD1 with MAP65-5 enhanced the microtubule bundling. These results suggest STD1 and MAP65-5 might cooperatively regulate microtubule organization in the phragmoplast at telophase.

Defects in division plane positioning in the root meristematic zone affect cell organization in the differentiation zone

Cell division plane orientation is critical for plant and animal development and growth. TANGLED1 (TAN1) and AUXIN-INDUCED-IN-ROOT-CULTURES9 (AIR9) are division-site localized microtubule-binding proteins required for division plane positioning. tan1 and air9 Arabidopsis thaliana single mutants have minor or no noticeable phenotypes but the tan1 air9 double mutant has synthetic phenotypes including stunted growth, misoriented divisions, and aberrant cell-file rotation in the root differentiation zone. These data suggest that TAN1 plays a role in nondividing cells. To determine whether TAN1 is required in elongating and differentiating cells in the tan1 air9 double mutant, we limited its expression to actively dividing cells using the G2/M-specific promoter of the syntaxin KNOLLE (pKN:TAN1-YFP). Unexpectedly, in addition to rescuing division plane defects, pKN:TAN1-YFP rescued root growth and the root differentiation zone cell file rotation defects in the tan1 air9 double mutant. This suggests that defects that occur in the meristematic zone later affect the organization of elongating and differentiating cells.

Integrated Analysis of Microarray, Small RNA, and Degradome Datasets Uncovers the Role of MicroRNAs in Temperature-Sensitive Genic Male Sterility in Wheat

Temperature-sensitive genic male sterile (TGMS) line Beijing Sterility 366 (BS366) has been utilized in hybrid breeding for a long time, but the molecular mechanism underlying male sterility remains unclear. Expression arrays, small RNA, and degradome sequencing were used in this study to explore the potential role of miRNA in the cold-induced male sterility of BS366. Microspore observation showed defective cell plates in dyads and tetrads and shrunken microspores at the vacuolated stage. Differential regulation of Golgi vesicle transport, phragmoplast formation, sporopollenin biosynthesis, pollen exine formation, and lipid metabolism were observed between cold and control conditions. Pollen development was significantly represented in the 352 antagonistic miRNA-target pairs in the integrated analysis of miRNA and mRNA profiles. The specific cleavage of ARF17 and TIR1 by miR160 and miR393 were found in the cold-treated BS366 degradome, respectively. Thus, the cold-mediated miRNAs impaired cell plate formation through repression of Golgi vesicle transport and phragmoplast formation. The repressed expression of ARF17 and TIR1 impaired pollen exine formation. The results of this study will contribute to our understanding of the roles of miRNAs in male sterility in wheat.

Plant cytokinesis and the construction of new cell wall

Cytokinesis in plants is fundamentally different from that in animals and fungi. In plant cells, a cell plate forms through the fusion of cytokinetic vesicles and then develops into the new cell wall, partitioning the cytoplasm of the dividing cell. The formation of the cell plate entails multiple stages that involve highly orchestrated vesicle accumulation, fusion, and membrane maturation, which occur concurrently with the timely deposition of polysaccharides such as callose, cellulose, and cross-linking glycans. This review summarizes the major stages in cytokinesis, endomembrane components involved in cell plate assembly and its transition to a new cell wall. An animation that can be widely used for educational purposes further summarizes the process.

Spindle Assembly and Mitosis in Plants

In contrast to well-studied fungal and animal cells, plant cells assemble bipolar spindles that exhibit a great deal of plasticity in the absence of structurally defined microtubule-organizing centers like the centrosome. While plants employ some evolutionarily conserved proteins to regulate spindle morphogenesis and remodeling, many essential spindle assembly factors found in vertebrates are either missing or not required for producing the plant bipolar microtubule array. Plants also produce proteins distantly related to their fungal and animal counterparts to regulate critical events such as the spindle assembly checkpoint. Plant spindle assembly initiates with microtubule nucleation on the nuclear envelope followed by bipolarization into the prophase spindle. After nuclear envelope breakdown, kinetochore fibers are assembled and unified into the spindle apparatus with convergent poles. Of note, compared to fungal and animal systems, relatively little is known about how plant cells remodel the spindle microtubule array during anaphase. Uncovering mitotic functions of novel proteins for spindle assembly in plants will illuminate both common and divergent mechanisms employed by different eukaryotic organisms to segregate genetic materials.

A biophysical model for plant cell plate maturation based on the contribution of a spreading force

Plant cytokinesis, a fundamental process of plant life, involves de novo formation of a "cell plate" partitioning the cytoplasm of dividing cells. Cell plate formation is directed by orchestrated delivery, fusion of cytokinetic vesicles, and membrane maturation to form a nascent cell wall by timely deposition of polysaccharides. During cell plate maturation, the fragile membrane network transitions to a fenestrated sheet and finally a young cell wall. Here, we approximated cell plate sub-structures with testable shapes and adopted the Helfrich-free energy model for membranes, including a stabilizing and spreading force, to understand the transition from a vesicular network to a fenestrated sheet and mature cell plate. Regular cell plate development in the model was possible, with suitable bending modulus, for a two-dimensional late stage spreading force of 2-6 pN/nm, an osmotic pressure difference of 2-10 kPa, and spontaneous curvature between 0 and 0.04 nm-1. With these conditions, stable membrane conformation sizes and morphologies emerged in concordance with stages of cell plate development. To reach a mature cell plate, our model required the late-stage onset of a spreading/stabilizing force coupled with a concurrent loss of spontaneous curvature. Absence of a spreading/stabilizing force predicts failure of maturation. The proposed model provides a framework to interrogate different players in late cytokinesis and potentially other membrane networks that undergo such transitions. Callose, is a polysaccharide that accumulates transiently during cell plate maturation. Callose-related observations were consistent with the proposed model's concept, suggesting that it is one of the factors involved in establishing the spreading force.

SNAREs Regulate Vesicle Trafficking During Root Growth and Development

SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins assemble to drive the final membrane fusion step of membrane trafficking. Thus, SNAREs are essential for membrane fusion and vesicular trafficking, which are fundamental mechanisms for maintaining cellular homeostasis. In plants, SNAREs have been demonstrated to be located in different subcellular compartments and involved in a variety of fundamental processes, such as cytokinesis, cytoskeleton organization, symbiosis, and biotic and abiotic stress responses. In addition, SNAREs can also contribute to the normal growth and development of Arabidopsis. Here, we review recent progress in understanding the biological functions and signaling network of SNAREs in vesicle trafficking and the regulation of root growth and development in Arabidopsis.

Comparative transcriptome and DNA methylation analysis in temperature-sensitive genic male sterile wheat BS366

Background

Known as the prerequisite component for the heterosis breeding system, the male sterile line determines the hybrid yield and seed purity. Therefore, a deep understanding of the mechanism and gene network that leads to male sterility is crucial. BS366, a temperature-sensitive genic male sterile (TGMS) line, is male sterile under cold conditions (12 °C with 12 h of daylight) but fertile under normal temperature (20 °C with 12 h of daylight).

Results

During meiosis, BS366 was defective in forming tetrads and dyads due to the abnormal cell plate. During pollen development, unusual vacuolated pollen that could not accumulate starch grains at the binucleate stage was also observed. Transcriptome analysis revealed that genes involved in the meiotic process, such as sister chromatid segregation and microtubule-based movement, were repressed, while genes involved in DNA and histone methylation were induced in BS366 under cold conditions. MethylRAD was used for reduced DNA methylation sequencing of BS366 spikes under both cold and control conditions. The differentially methylated sites (DMSs) located in the gene region were mainly involved in carbohydrate and fatty acid metabolism, lipid metabolism, and transport. Differentially expressed and methylated genes were mainly involved in cell division.

Conclusions

These results indicated that the methylation of genes involved in carbon metabolism or fatty acid metabolism might contribute to male sterility in BS366 spikes, providing novel insight into the molecular mechanism of wheat male sterility.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08163-3.

TANGLED1 mediates microtubule interactions that may promote division plane positioning in maize

The microtubule cytoskeleton serves as a dynamic structural framework for mitosis in eukaryotic cells. TANGLED1 (TAN1) is a microtubule-binding protein that localizes to the division site and mitotic microtubules and plays a critical role in division plane orientation in plants. Here, in vitro experiments demonstrate that TAN1 directly binds microtubules, mediating microtubule zippering or end-on microtubule interactions, depending on their contact angle. Maize tan1 mutant cells improperly position the preprophase band (PPB), which predicts the future division site. However, cell shape–based modeling indicates that PPB positioning defects are likely a consequence of abnormal cell shapes and not due to TAN1 absence. In telophase, colocalization of growing microtubules ends from the phragmoplast with TAN1 at the division site suggests that TAN1 interacts with microtubule tips end-on. Together, our results suggest that TAN1 contributes to microtubule organization to ensure proper division plane orientation.

Complementary Superresolution Visualization of Composite Plant Microtubule Organization and Dynamics

Microtubule bundling is an essential mechanism underlying the biased organization of interphase and mitotic microtubular systems of eukaryotes in ordered arrays. Microtubule bundle formation can be exemplified in plants, where the formation of parallel microtubule systems in the cell cortex or the spindle midzone is largely owing to the microtubule crosslinking activity of a family of microtubule associated proteins, designated as MAP65s. Among the nine members of this family in Arabidopsis thaliana, MAP65-1 and MAP65-2 are ubiquitous and functionally redundant. Crosslinked microtubules can form high-order arrays, which are difficult to track using widefield or confocal laser scanning microscopy approaches. Here, we followed spatiotemporal patterns of MAP65-2 localization in hypocotyl cells of Arabidopsis stably expressing fluorescent protein fusions of MAP65-2 and tubulin. To circumvent imaging difficulties arising from the density of cortical microtubule bundles, we use different superresolution approaches including Airyscan confocal laser scanning microscopy (ACLSM), structured illumination microscopy (SIM), total internal reflection SIM (TIRF-SIM), and photoactivation localization microscopy (PALM). We provide insights into spatiotemporal relations between microtubules and MAP65-2 crossbridges by combining SIM and ACLSM. We obtain further details on MAP65-2 distribution by single molecule localization microscopy (SMLM) imaging of either mEos3.2-MAP65-2 stochastic photoconversion, or eGFP-MAP65-2 stochastic emission fluctuations under specific illumination conditions. Time-dependent dynamics of MAP65-2 were tracked at variable time resolution using SIM, TIRF-SIM, and ACLSM and post-acquisition kymograph analysis. ACLSM imaging further allowed to track end-wise dynamics of microtubules labeled with TUA6-GFP and to correlate them with concomitant fluctuations of MAP65-2 tagged with tagRFP. All different microscopy modules examined herein are accompanied by restrictions in either the spatial resolution achieved, or in the frame rates of image acquisition. PALM imaging is compromised by speed of acquisition. This limitation was partially compensated by exploiting emission fluctuations of eGFP which allowed much higher photon counts at substantially smaller time series compared to mEos3.2. SIM, TIRF-SIM, and ACLSM were the methods of choice to follow the dynamics of MAP65-2 in bundles of different complexity. Conclusively, the combination of different superresolution methods allowed for inferences on the distribution and dynamics of MAP65-2 within microtubule bundles of living A. thaliana cells.

TPLATE Recruitment Reveals Endocytic Dynamics at Sites of Symbiotic Interface Assembly in Arbuscular Mycorrhizal Interactions

Introduction: Arbuscular mycorrhizal (AM) symbiosis between soil fungi and the majority of plants is based on a mutualistic exchange of organic and inorganic nutrients. This takes place inside root cortical cells that harbor an arbuscule: a highly branched intracellular fungal hypha enveloped by an extension of the host cell membrane-the perifungal membrane-which outlines a specialized symbiotic interface compartment. The perifungal membrane develops around each intracellular hypha as the symbiotic fungus proceeds across the root tissues; its biogenesis is the result of an extensive exocytic process and shows a few similarities with cell plate insertion which occurs at the end of somatic cytokinesis. Materials and Methods: We here analyzed the subcellular localization of a GFP fusion with TPLATE, a subunit of the endocytic TPLATE complex (TPC), a central actor in plant clathrin-mediated endocytosis with a role in cell plate anchoring with the parental plasma membrane. Results: Our observations demonstrate that Daucus carota and Medicago truncatula root organ cultures expressing a 35S::AtTPLATE-GFP construct accumulate strong fluorescent green signal at sites of symbiotic interface construction, along recently formed perifungal membranes and at sites of cell-to-cell hyphal passage between adjacent cortical cells, where the perifungal membrane fuses with the plasmalemma. Discussion: Our results strongly suggest that TPC-mediated endocytic processes are active during perifungal membrane interface biogenesis-alongside exocytic transport. This novel conclusion, which might be correlated to the accumulation of late endosomes in the vicinity of the developing interface, hints at the involvement of TPC-dependent membrane remodeling during the intracellular accommodation of AM fungi.

The γ-tubulin complex protein GCP6 is crucial for spindle morphogenesis but not essential for microtubule reorganization in Arabidopsis

γ-Tubulin typically forms a ring-shaped complex with 5 related γ-tubulin complex proteins (GCP2 to GCP6), and this γ-tubulin ring complex (γTuRC) serves as a template for microtubule (MT) nucleation in plants and animals. While the γTuRC takes part in MT nucleation in most eukaryotes, in fungi such events take place robustly with just the γ-tubulin small complex (γTuSC) assembled by γ-tubulin plus GCP2 and GCP3. To explore whether the γTuRC is the sole functional γ-tubulin complex in plants, we generated 2 mutants of the GCP6 gene encoding the largest subunit of the γTuRC in Arabidopsis thaliana Both mutants showed similar phenotypes of dwarfed vegetative growth and reduced fertility. The gcp6 mutant assembled the γTuSC, while the wild-type cells had GCP6 join other GCPs to produce the γTuRC. Although the gcp6 cells had greatly diminished γ-tubulin localization on spindle MTs, the protein was still detected there. The gcp6 cells formed spindles that lacked MT convergence and discernable poles; however, they managed to cope with the challenge of MT disorganization and were able to complete mitosis and cytokinesis. Our results reveal that the γTuRC is not the only functional form of the γ-tubulin complex for MT nucleation in plant cells, and that γ-tubulin-dependent, but γTuRC-independent, mechanisms meet the basal need of MT nucleation. Moreover, we show that the γTuRC function is more critical for the assembly of spindle MT array than for the phragmoplast. Thus, our findings provide insight into acentrosomal MT nucleation and organization.

Plant Cell Biology: Shifting CORDs to Fine-Tune Phragmoplast Microtubule Turnover

A new study provides insight into microtubule turnover during plant cell division. Using clever molecular-genetic and imaging strategies, the authors demonstrate that the recently discovered CORD4 and 5 proteins associate with phragmoplast microtubules and control recruitment and activity of the microtubule-severing protein katanin.

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.

Plant cell division - defining and finding the sweet spot for cell plate insertion

The plant microtubules form unique arrays using acentrosomal microtubule nucleation pathways, yet utilizing evolutionary conserved centrosomal proteins. In cytokinesis, a multi-component cytoskeletal apparatus, the phragmoplast mediates the biosynthesis of the new cell plate by dynamic centrifugal expansion, a process that demands exquisite coordination of microtubule turnover and endomembrane trafficking. At the same time, the phragmoplast is guided to meet with the parental wall at a cortical site that is predefined before mitotic entry and transiently marked by the preprophase band of microtubules. The cortical division zone maintains positional information of the selected division plane for the entire duration of cell division and for the guidance of the phragmoplast during cytokinesis. Its establishment is an essential requirement for normal plant organogenesis, due to the confinement of cells by rigid cell walls.

Ectopic activation of cortical cell division during the accommodation of arbuscular mycorrhizal fungi

Arbuscular mycorrhizas (AMs) between plants and soil fungi are widespread symbioses with a major role in soil nutrient uptake. In this study we investigated the induction of root cortical cell division during AM colonization by combining morphometric and gene expression analyses with promoter activation and protein localization studies of the cell-plate-associated exocytic marker TPLATE. Our results show that TPLATE promoter is activated in colonized cells of the root cortex where we also observed the appearance of cells that are half the size of the surrounding cells. Furthermore, TPLATE-green fluorescent protein recruitment to developing cell plates highlighted ectopic cell division events in the inner root cortex during early AM colonization. Lastly, transcripts of TPLATE, KNOLLE and Cyclinlike 1 (CYC1) are all upregulated in the same context, alongside endocytic markers Adaptor-Related Protein complex 2 alpha 1 subunit (AP2A1) and Clathrin Heavy Chain 2 (CHC2), known to be active during cell plate formation. This pattern of gene expression was recorded in wild-type Medicago truncatula roots, but not in a common symbiotic signalling pathway mutant where fungal colonization is blocked at the epidermal level. Altogether, these results suggest the activation of cell-division-related mechanisms by AM hosts during the accommodation of the symbiotic fungus.

Phragmoplast expansion: the four-stroke engine that powers plant cytokinesis

The phragmoplast is a plant-specific secretory module that partitions daughter cells during cytokinesis by constructing a cell plate from membranes and oligosaccharides. The cell plate is typically a long structure, which requires the phragmoplast to expand to complete cytokinesis. The phragmoplast expands by coordinating microtubule dynamics with membrane trafficking. Each step in phragmoplast expansion involves the establishment of anti-parallel microtubule overlaps that are enriched with the protein MAP65, which recruits cytokinetic vesicles through interaction with the tethering factor, TRAPPII. Cell plate assembly triggers dissolution of the anti-parallel overlaps and stabilization of microtubule plus ends through association with the cell plate assembly machinery. This opinion article discusses processes that drive phragmoplast expansion as well as highlights key questions that remain for better understanding its role in plant cell division.

Reduction of ATPase activity in the rice kinesin protein Stemless Dwarf 1 inhibits cell division and organ development

Several kinesins, the ATP-driven microtubule (MT)-based motor proteins, have been reported to be involved in many basic processes of plant development; however, little is known about the biological relevance of their ATPase activity. Here, we characterized the Oryza sativa (rice) stemless dwarf 1 (std1) mutant, showing a severely dwarfed phenotype, with no differentiation of the node and internode structure, abnormal cell shapes, a shortened leaf division zone and a reduced cell division rate. Further analysis revealed that a substantial subset of cells was arrested in the S and G2/M phases, and multinucleate cells were present in the std1 mutant. Map-based cloning revealed that STD1 encodes a phragmoplast-associated kinesin-related protein, a homolog of the Arabidopsis thaliana PAKRP2, and is mainly expressed in the actively dividing tissues. The STD1 protein is localized specifically to the phragmoplast midzone during telophase and cytokinesis. In the std1 mutant, the substitution of Val-40-Glu in the motor domain of STD1 significantly reduced its MT-dependent ATPase activity. Accordingly, the lateral expansion of phragmoplast, a key step in cell plate formation, was arrested during cytokinesis. Therefore, these results indicate that the MT-dependent ATPase activity is indispensible for STD1 in regulating normal cell division and organ development.

Dual localized kinesin-12 POK2 plays multiple roles during cell division and interacts with MAP65-3

Kinesins are versatile nano-machines that utilize variable non-motor domains to tune specific motor microtubule encounters. During plant cytokinesis, the kinesin-12 orthologs, PHRAGMOPLAST ORIENTING KINESIN (POK)1 and POK2, are essential for rapid centrifugal expansion of the cytokinetic apparatus, the phragmoplast, toward a pre-selected cell plate fusion site at the cell cortex. Here, we report on the spatio-temporal localization pattern of POK2, mediated by distinct protein domains. Functional dissection of POK2 domains revealed the association of POK2 with the site of the future cell division plane and with the phragmoplast during cytokinesis. Accumulation of POK2 at the phragmoplast midzone depends on its functional POK2 motor domain and is fine-tuned by its carboxy-terminal region that also directs POK2 to the division site. Furthermore, POK2 likely stabilizes the phragmoplast midzone via interaction with the conserved microtubule-associated protein MAP65-3/PLEIADE, a well-established microtubule cross-linker. Collectively, our results suggest that dual localized POK2 plays multiple roles during plant cell division.

Role of the BUB3 protein in phragmoplast microtubule reorganization during cytokinesis

The evolutionarily conserved WD40 protein budding uninhibited by benzimidazole 3 (BUB3) is known for its function in spindle assembly checkpoint control. In the model plant Arabidopsis thaliana, nearly identical BUB3;1 and BUB3;2 proteins decorated the phragmoplast midline through interaction with the microtubule-associated protein MAP65-3 during cytokinesis. BUB3;1 and BUB3;2 interacted with the carboxy-terminal segment of MAP65-3 (but not MAP65-1), which harbours its microtubule-binding domain for its post-mitotic localization. Reciprocally, BUB3;1 and BUB3;2 also regulated MAP65-3 localization in the phragmoplast by enhancing its microtubule association. In the bub3;1 bub3;2 double mutant, MAP65-3 localization was often dissipated away from the phragmoplast midline and abolished upon treatment of low doses of the cytokinesis inhibitory drug caffeine that were tolerated by the control plant. The phragmoplast microtubule array exhibited uncoordinated expansion pattern in the double mutant cells as the phragmoplast edge reached the parental plasma membrane at different times in different areas. Upon caffeine treatment, phragmoplast expansion was halted as if the microtubule array was frozen. As a result, cytokinesis was abolished due to failed cell plate assembly. Our findings have uncovered a novel function of the plant BUB3 in MAP65-3-dependent microtubule reorganization during cytokinesis.

An overview of plant division-plane orientation

Contents Summary 505 I. Introduction 505 II. Models of plant cell division 505 III. Establishing the division plane 506 IV. Maintaining the division plane during mitosis and cytokinesis 509 Acknowledgements 510 References 510 SUMMARY: Plants, a significant source of planet-wide biomass, have an unique type of cell division in which a new cell wall is constructed de novo inside the cell and guided towards the cell edge to complete division. The elegant control over positioning this new cell wall is essential for proper patterning and development. Plant cells, lacking migration, tightly coordinate division orientation and directed expansion to generate organized shapes. Several emerging lines of evidence suggest that the proteins required for division-plane establishment are distinct from those required for division-plane maintenance. We discuss recent shape-based computational models and mutant analyses that raise questions about, and identify unexpected connections between, the roles of well-known proteins and structures during division-plane orientation.

Tubulins, rhythms and cell walls in poplar leaves: it's all in the timing

Plant cell walls exhibit architectural and compositional changes throughout their development and in response to external cues. While tubulins are involved in cell wall biogenesis, much remains unknown about the scope of their involvement during the orchestration of this resource-demanding process. A transgenic approach coupled with cell wall compositional analysis, RNA-seq and mining of publicly available diurnal gene expression data was used to assess the involvement of tubulins in poplar leaf cell wall biogenesis. Leaf cell walls of transgenic poplar lines with constitutive overexpression of α-tubulin (TUA) exhibited an increased abundance of homogalacturonan, along with a reduction in xylose. These changes were traced to altered expression of UDP-glucuronic acid decarboxylase (GADC) in the transgenic leaves. A model is postulated by which altered diurnal control of TUA through its constitutive overexpression led to a metabolic tradeoff affecting cellular utilization of GADC substrate UDP-glucuronic acid. While there were no effects on cellulose, hemicellulose or lignin abundance, subtle effects on hemicellulose composition and associated gene expression were noted. In addition, expression and enzymatic activity of pectin methylesterase (PME) decreased in the transgenic leaves. The change is discussed in a context of increased levels of PME substrate homogalacturonan, slow stomatal kinetics and the fate of PME product methanol. Since stomatal opening and closing depend on fundamentally contrasting microtubule dynamics, the slowing of both processes in the transgenic lines as previously reported appears to be directly related to underlying cell wall compositional changes that were caused by tubulin manipulation.

Myosin XI localizes at the mitotic spindle and along the cell plate during plant cell division in Physcomitrella patens

Cell division is a fundamental biological process that has been extensively investigated in different systems. Similar to most eukaryotic cells, plant cells assemble a mitotic spindle to separate replicated chromosomes. In contrast, to complete cell division, plant cells assemble a phragmoplast, which is composed of aligned microtubules and actin filaments. This structure helps transport vesicles containing new cell wall material, which then fuse to form the cell plate; the cell plate will expand to create the new dividing cell wall. Because vesicles are known to be transported by myosin motors during interphase, we hypothesized this could also be the case during cell division and we investigated the localization of the plant homologue of myosin V - myosin XI, in cell division. In this work, we used the protonemal cells of the moss Physcomitrella patens as a model, because of its simple cellular morphology and ease to generate transgenic cell lines expressing fluorescent tagged proteins. Using a fluorescent protein fusion of myosin XI, we found that, during mitosis, this molecule appears to associate with the kinetochores immediately after nuclear envelope breakdown. Following metaphase, myosin XI stays associated with the spindle's midzone during the rest of mitosis, and when the phragmoplast is formed, it concentrates at the cell plate. Using an actin polymerization inhibitor, latrunculin B, we found that the association of myosin XI with the mitotic spindle and the phragmoplast are only partially dependent on the presence of filamentous actin. We also showed that myosin XI on the spindle partially overlaps with a v-SNARE vesicle marker but is not co-localized with the endoplasmic reticulum and a RabA vesicle marker. These observations suggest an actin-dependent and an actin-independent behavior of myosin XI during cell division, and provide novel insights to our understanding of the function of myosin XI during plant cell division.

Post-Golgi Trafficking and Transport of Cell Wall Components

The cell wall, a complex macromolecular composite structure surrounding and protecting plant cells, is essential for development, signal transduction, and disease resistance. This structure is also integral to cell expansion, as its tensile resistance is the primary balancing mechanism against internal turgor pressure. Throughout these processes, the biosynthesis, transport, deposition, and assembly of cell wall polymers are tightly regulated. The plant endomembrane system facilitates transport of polysaccharides, polysaccharide biosynthetic and modifying enzymes and glycoproteins through vesicle trafficking pathways. Although a number of enzymes involved in cell wall biosynthesis have been identified, comparatively little is known about the transport of cell wall polysaccharides and glycoproteins by the endomembrane system. This review summarizes our current understanding of trafficking of cell wall components during cell growth and cell division. Emerging technologies, such as vesicle glycomics, are also discussed as promising avenues to gain insights into the trafficking of structural polysaccharides to the apoplast.

Cytoskeletal mechanisms in positioning of the second-division spindles and meiotic restitution in tobacco (Nicotiana tabacum L.) microsporogenesis

Microsporogenesis patterns of the polyploid (2n = 4x = 96) and diploid (2n = 2x = 48) Nicotiana tabacum L. (cv. Havana Petit line SR1) plants have been analyzed and compared. Four types of abnormal positions of the second-division spindles-tripolar, parallel, proximal, and fused-have been observed. Of these abnormalities, only tripolar (2.4%) and parallel (1.4%) spindles are observable in diploid plants. As for polyploids, the increased ploidy is accompanied by an increase in the incidence of tripolar (22.8%) and parallel (8.1%) spindle orientations and emergence of two remaining abnormalities (proximal and fused spindles, 3.3%). As has been shown, the spindle position abnormalities in diploid plants have no effect on the meiotic products, whereas both dyads and triads are detectable among the tetrads in polyploid plants. Analysis of cytoskeletal remodeling has allowed for the insight into the role of interzonal radial microtubule system in spindle positioning during the second division. The reason underlying the change in spindle positioning is disturbed polymerization-depolymerization processes and interdigitation of microtubule plus ends within the interzonal cytoskeleton system in late telophase I-interkinesis and prophase II. As has been demonstrated, fused second-division spindles are formed as a result of fused cytoskeletal structures in prophase-prometaphase II in the case when the nuclei are drawn abnormally close to one another.

Alfalfa Root Growth Rate Correlates with Progression of Microtubules during Mitosis and Cytokinesis as Revealed by Environmental Light-Sheet Microscopy

Cell division and expansion are two fundamental biological processes supporting indeterminate root growth and development of plants. Quantitative evaluations of cell divisions related to root growth analyses have been performed in several model crop and non-crop plant species, but not in important legume plant Medicago sativa. Light-sheet fluorescence microscopy (LSFM) is an advanced imaging technique widely used in animal developmental biology, providing efficient fast optical sectioning under physiological conditions with considerably reduced phototoxicity and photobleaching. Long-term 4D imaging of living plants offers advantages for developmental cell biology not available in other microscopy approaches. Recently, LSFM was implemented in plant developmental biology studies, however, it is largely restricted to the model plant Arabidopsis thaliana. Cellular and subcellular events in crop species and robust plant samples have not been studied by this method yet. Therefore we performed LSFM long-term live imaging of growing root tips of transgenic alfalfa plants expressing the fluorescent molecular marker for the microtubule-binding domain (GFP-MBD), in order to study dynamic patterns of microtubule arrays during mitotic cell division. Quantitative evaluations of cell division progress in the two root tissues (epidermis and cortex) clearly indicate that root growth rate is correlated with duration of cell division in alfalfa roots. Our results favor non-invasive environmental LSFM as one of the most suitable methods for qualitative and quantitative cellular and developmental imaging of living transgenic legume crops.

Cell cycle-regulated PLEIADE/AtMAP65-3 links membrane and microtubule dynamics during plant cytokinesis

Cytokinesis, the partitioning of the cytoplasm following nuclear division, requires extensive coordination between cell cycle cues, membrane trafficking and microtubule dynamics. Plant cytokinesis occurs within a transient membrane compartment known as the cell plate, to which vesicles are delivered by a plant-specific microtubule array, the phragmoplast. While membrane proteins required for cytokinesis are known, how these are coordinated with microtubule dynamics and regulated by cell cycle cues remains unclear. Here, we document physical and genetic interactions between Transport Protein Particle II (TRAPPII) tethering factors and microtubule-associated proteins of the PLEIADE/AtMAP65 family. These interactions do not specifically affect the recruitment of either TRAPPII or MAP65 proteins to the cell plate or midzone. Rather, and based on single versus double mutant phenotypes, it appears that they are required to coordinate cytokinesis with the nuclear division cycle. As MAP65 family members are known to be targets of cell cycle-regulated kinases, our results provide a conceptual framework for how membrane and microtubule dynamics may be coordinated with each other and with the nuclear cycle during plant cytokinesis.

The Evolution of Cell Division: From Streptophyte Algae to Land Plants

The mechanism of cell division has undergone significant alterations during the evolution from aquatic streptophyte algae to land plants. Two new structures evolved, the cytokinetic phragmoplast and the preprophase band (PPB) of microtubules, whereas the ancestral mechanism of cleavage and the centrosomes disappeared. We map cell biological data onto the recently emerged phylogenetic tree of streptophytes. The tree suggests that, after the establishment of the phragmoplast mechanism, several groups independently lost their centrosomes. Surprisingly, the phragmoplast shows reductions in the Zygnematophyceae (the sister to land plants), many of which returned to cleavage. The PPB by contrast evolved stepwise and, most likely, originated in the algae. The phragmoplast/PPB mechanism established in this way served as a basis for the 3D development of land plants.

The Membrane-Associated Sec1/Munc18 KEULE is Required for Phragmoplast Microtubule Reorganization During Cytokinesis in Arabidopsis

Cytokinesis, the partitioning of the cytoplasm following nuclear division, requires extensive coordination between membrane trafficking and cytoskeletal dynamics. In plants, the onset of cytokinesis is characterized by the assembly of a bipolar microtubule array, the phragmoplast, and of a transient membrane compartment, the cell plate. Little is known about the coordination between membrane deposition at the cell plate and the dynamics of phragmoplast microtubules. In this study, we monitor the localization dynamics of microtubule and membrane markers throughout cytokinesis. Our spatiotemporal resolution is consistent with the general view that microtubule dynamics drive membrane movements. Nonetheless, we provide evidence for active sorting at the cell plate and show that this is, at least in part, mediated by the TRAPPII tethering complex. We also characterize phragmoplast microtubule organization and cell plate formation in a suite of cytokinesis-defective mutants. Of four mutant lines with defects in phragmoplast microtubule organization, only mor1 microtubule-associated mutants exhibited aberrant cell plates. Conversely, the mutants with the strongest impairment in phragmoplast microtubule reorganization are keule alleles, which have a primary defect in membrane fusion. Our findings identify the SEC1/Munc18 protein KEULE as a central regulatory node in the coordination of membrane and microtubule dynamics during plant cytokinesis.

Haspin has Multiple Functions in the Plant Cell Division Regulatory Network

Progression of cell division is controlled by various mitotic kinases. In animal cells, phosphorylation of histone H3 at Thr3 by the kinase Haspin (haploid germ cell-specific nuclear protein kinase) promotes centromeric Aurora B localization to regulate chromosome segregation. However, less is known about the function of Haspin in regulatory networks in plant cells. Here, we show that inhibition of Haspin with 5-iodotubercidin (5-ITu) in Bright Yellow-2 (BY-2) cells delayed chromosome alignment. Haspin inhibition also prevented the centromeric localization of Aurora3 kinase (AUR3) and disrupted its function. This suggested that Haspin plays a role in the specific positioning of AUR3 on chromosomes in plant cells, a function conserved in animals. The results also indicated that Haspin and AUR3 are involved in the same pathway, which regulates chromosome alignment during prometaphase/metaphase. Remarkably, Haspin inhibition by 5-ITu also led to a severe cytokinesis defect, resulting in binuclear cells with a partially formed cell plate. The 5-ITu treatment did not affect microtubules, AUR1/2 or the NACK-PQR pathway; however, it did alter the distribution of actin filaments on the cell plate. Together, these results suggested that Haspin has several functions in regulating cell division in plant cells: in the localization of AUR3 on centromeres and in regulating late cell plate expansion during cytokinesis.

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.

Characterization of Cytokinetic Mutants Using Small Fluorescent Probes

Cytokinesis is a powerful paradigm for addressing fundamental questions of plant biology including molecular mechanisms of development, cell division, cell signaling, membrane trafficking, cell wall synthesis, and cytoskeletal dynamics. Genetics was instrumental in identification of proteins regulating cytokinesis. Characterization of mutant lines generated using forward or reverse genetics includes microscopic analysis for defects in cell division. Typically, failure of cytokinesis results in appearance of multinucleate cells, formation of cell wall stubs, and isotropic cell expansion in the root elongation zone. Small fluorescent probes served as a very effective tool for the detection of cytokinetic defects. Such probes stain living or formaldehyde-fixed specimens avoiding complex preparatory steps. Although resolution of the fluorescence probes is inferior to electron microscopy, the procedure is fast, easy, and does not require expensive materials or equipment. This chapter describes techniques for staining DNA with the probes DAPI and SYTO82, for staining membranes with FM4-64, and for staining cell wall with propidium iodide.

Kinesin motors in plants: from subcellular dynamics to motility regulation

Plants produce enormous forms of the microtubule (MT)-based motor kinesins that have been inspiring plant cell biologists to uncover their functions in relation to plant growth and development. Subcellular localization of kinesin proteins detected through live-cell imaging or immunofluorescence microscopy has provided great insights into the functions of these motors. Dozens of mitotic kinesins exhibit particularly splendid localization patterns from chromosomes and kinetochores to MT arrays like the preprophase band, spindle poles, the spindle midzone, phragmoplast distal ends, and the phragmoplast midzone. Different subcellular localizations indicate distinct functions of these motors that are yet to be characterized. The localization difference between plant kinesins and their animal counterparts implies mechanistic differences in mitosis and cytokinesis between the two kingdoms. When many forms of kinesins are present simultaneously, it becomes critical that their motility is differentially regulated with spatial and temporal precision. Insights into regulatory mechanisms of motors can often be brought about by in vitro single-molecule biophysical studies. Significant advances are expected in this area in the coming years owing to rapid technological advances that are being brought to various model plants.

Endomembrane trafficking overarching cell plate formation

By contrast to other eukaryotic kingdoms, plant cytokinesis is an inside-out process. A coordinated action of cytoskeletal transitions and endomembrane trafficking events builds a novel membrane compartment, the cell plate. Deposition of cell wall polymers transforms the lumen of this membrane compartment into a new cross wall, physically separating the daughter cells. The characterization of tethering complexes acting at discrete phases during cell plate formation and upstream of vesicle fusion events, the presence of modulators directing secretion and recycling during cytokinesis, as well as the identification and temporal recruitment of the endocytic machinery, provides a starting point to dissect the transitions in endomembrane trafficking which shape this process. This review aims to integrate recent findings on endomembrane trafficking events which spatio-temporally act to construct the cell plate.

Plant cytokinesis: a tale of membrane traffic and fusion

Cytokinesis separates the forming daughter cells. Higher plants have lost the ability to constrict the plasma membrane (PM) in the division plane. Instead, trans-Golgi network (TGN)-derived membrane vesicles are targeted to the centre of the division plane and generate, by homotypic fusion, the partitioning membrane named cell plate (CP). The CP expands in a centrifugal fashion until its margin fuses with the PM at the cortical division site. Mutant screens in Arabidopsis have identified a cytokinesis-specific syntaxin named KNOLLE and an interacting Sec1/Munc18 (SM) protein named KEULE both of which are required for vesicle fusion during cytokinesis. KNOLLE is only made during M-phase, targeted to the division plane and degraded in the vacuole at the end of cytokinesis. Here we address mechanisms of KNOLLE trafficking and interaction of KNOLLE with different soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor (SNARE) partners and with SM-protein KEULE, ensuring membrane fusion in cytokinesis.

Polysaccharide deposition during cytokinesis: Challenges and future perspectives

De novo formation of a new cell wall partitions the cytoplasm of the dividing cell during plant cytokinesis. The development of the cell plate, a transient sheet-like structure, requires the accumulation of vesicles directed by the phragmoplast to the cell plate assembly matrix. Fusion and fission of the accumulated vesicles are accompanied by the deposition of polysaccharides and cell wall structural proteins; together, they are leading to the stabilization of the formed structure which after insertion into the parental wall lead to the maturation of the nascent cross wall. Callose is the most abundant polysaccharide during cell plate formation and during maturation is gradually replaced by cellulose. Matrix polysaccharides such as hemicellulose, and pectins presumably are present throughout all developmental stages, being delivered to the cell plate by secretory vesicles. The availability of novel chemical probes such as endosidin 7, which inhibits callose formation at the cell plate, has proved useful for dissecting the temporal accumulation of vesicles at the cell plate and establishing the critical role of callose during cytokinesis. The use of emerging approaches such as chemical genomics combined with live cell imaging; novel techniques of polysaccharide detection including tagged polysaccharide substrates, newly characterized polysaccharide antibodies and vesicle proteomics can be used to develop a comprehensive model of cell plate development.

Microtubules in plants

Microtubules (MTs) are highly conserved polar polymers that are key elements of the eukaryotic cytoskeleton and are essential for various cell functions. αβ-tubulin, a heterodimer containing one structural GTP and one hydrolysable and exchangeable GTP, is the building block of MTs and is formed by the sequential action of several molecular chaperones. GTP hydrolysis in the MT lattice is mechanistically coupled with MT growth, thus giving MTs a metastable and dynamic nature. MTs adopt several distinct higher-order organizations that function in cell division and cell morphogenesis. Small molecular weight compounds that bind tubulin are used as herbicides and as research tools to investigate MT functions in plant cells. The de novo formation of MTs in cells requires conserved γ-tubulin-containing complexes and targeting/activating regulatory proteins that contribute to the geometry of MT arrays. Various MT regulators and tubulin modifications control the dynamics and organization of MTs throughout the cell cycle and in response to developmental and environmental cues. Signaling pathways that converge on the regulation of versatile MT functions are being characterized.

Genetic interaction between G2/M phase-specific transcription factor MYB3R4 and MAPKKK ANP3 for execution of cytokinesis in Arabidopsis thaliana

Plant cells are surrounded by rigid cell walls, and hence, their division is associated with a plant-specific mode of cytokinesis in which the cell plate, a new cell wall, is generated and separates 2 daughter nuclei. The successful execution of cytokinesis requires the timely activation of multiple regulatory pathways, which include the AtNACK1/HINKEL kinesin-induced MAPK cascade and MYB3R1/4-mediated transcriptional activation of G2/M-specific genes. However, it remains unclear whether and how these pathways are functionally interconnected to each other. By analyzing enhancer mutations of myb3r4, here we found a close genetic interaction between the 2 pathways; a mutation in ANP3, which encodes MAPKKK (acting downstream of AtNACK1/HINKEL), strongly enhanced the defective cytokinesis observed in the myb3r4 mutant. This interaction may not be due to the direct activation of MYB3R1/4 by the MAPK cascade; rather, possibly to the downstream targets of these 2 signaling pathways, acting in close proximity. Our results showed that MYB3R1/4 may positively affect cytokinesis via multiple pathways, one of which may act independently from the KNOLLE-dependent pathway defined previously, and affect the downstream events that may also be under the control of the AtNACK1/HINKEL-mediated MAPK cascade.

Overexpression of OsKTN80a, a katanin P80 ortholog, caused the repressed cell elongation and stalled cell division mediated by microtubule apparatus defects in primary root in Oryza sativa

Katanin, a microtubule-severing enzyme, consists of two subunits: the catalytic subunit P60, and the regulatory subunit P80. In several species, P80 functions in meiotic spindle organization, the flagella biogenesis, the neuronal development, and the male gamete production. However, the P80 function in higher plants remains elusive. In this study, we found that there are three katanin P80 orthologs (OsKTN80a, OsKTN80b, and OsKTN80c) in Oryza sativa L. Overexpression of OsKTN80a caused the retarded root growth of rice seedlings. Further investigation indicates that the retained root growth was caused by the repressed cell elongation in the elongation zone and the stalled cytokinesis in the division zone in the root tip. The in vivo examination suggests that OsKTN80a acts as a microtubule stabilizer. We prove that OsKTN80a, possibly associated with OsKTN60, is involved in root growth via regulating the cell elongation and division.

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.