The origin of phragmoplast asymmetry

The microtubule-nucleating factor MACERATOR tethers AUGMIN7 to microtubules and governs phragmoplast architecture

The plant cytokinetic microtubule array, called the phragmoplast, exhibits higher microtubule dynamics in its center (midzone) than at the periphery (distal zone). This behavior is known as the axial asymmetry. Despite being a major characteristic of the phragmoplast, little is known about regulators of this phenomenon. Here we address the role of microtubule nucleation in axial asymmetry by characterizing MACERATOR (MACET) proteins in Arabidopsis thaliana and Nicotiana benthamiana with a combination of genetic, biochemical, and live-cell imaging assays, using photo-convertible microtubule probes, and modeling. MACET paralogs accumulate at the shrinking microtubule ends and decrease the tubulin OFF rate. Loss of MACET4 and MACET5 function abrogates axial asymmetry by suppressing microtubule dynamicity in the midzone. MACET4 also narrows the microtubule nucleation angle at the phragmoplast leading edge and functions as a microtubule tethering factor for AUGMIN COMPLEX SUBUNIT 7 (AUG7). The macet4 macet5 double mutant shows diminished clustering of AUG7 in the phragmoplast distal zone. Knockout of AUG7 does not affect MACET4 localization, axial asymmetry, or microtubule nucleation angle, but increases phragmoplast length and slows down phragmoplast expansion. The mce4-1 mce5 aug7-1 triple knockout is not viable. Experimental data and modeling demonstrate that microtubule nucleation factors regulate phragmoplast architecture and axial asymmetry directly by generating new microtubules and indirectly by modulating the abundance of free tubulin.

Cell biology of primary cell wall synthesis in plants

Building a complex structure such as the cell wall, with many individual parts that need to be assembled correctly from distinct sources within the cell, is a well-orchestrated process. Additional complexity is required to mediate dynamic responses to environmental and developmental cues. Enzymes, sugars, and other cell wall components are constantly and actively transported to and from the plasma membrane during diffuse growth. Cell wall components are transported in vesicles on cytoskeletal tracks composed of microtubules and actin filaments. Many of these components, and additional proteins, vesicles, and lipids are trafficked to and from the cell plate during cytokinesis. In this review, we first discuss how the cytoskeleton is initially organized to add new cell wall material or to build a new cell wall, focusing on similarities during these processes. Next, we discuss how polysaccharides and enzymes that build the cell wall are trafficked to the correct location by motor proteins and through other interactions with the cytoskeleton. Finally, we discuss some of the special features of newly formed cell walls generated during cytokinesis.

Analysis of formin functions during cytokinesis using specific inhibitor SMIFH2

The phragmoplast separates daughter cells during cytokinesis by constructing the cell plate, which depends on interaction between cytoskeleton and membrane compartments. Proteins responsible for these interactions remain unknown, but formins can link cytoskeleton with membranes and several members of formin protein family localize to the cell plate. Progress in functional characterization of formins in cytokinesis is hindered by functional redundancies within the large formin gene family. We addressed this limitation by employing Small Molecular Inhibitor of Formin Homology 2 (SMIFH2), a small-molecule inhibitor of formins. Treatment of tobacco (Nicotiana tabacum) tissue culture cells with SMIFH2 perturbed localization of actin at the cell plate; slowed down both microtubule polymerization and phragmoplast expansion; diminished association of dynamin-related proteins with the cell plate independently of actin and microtubules; and caused cell plate swelling. Another impact of SMIFH2 was shortening of the END BINDING1b (EB1b) and EB1c comets on the growing microtubule plus ends in N. tabacum tissue culture cells and Arabidopsis thaliana cotyledon epidermis cells. The shape of the EB1 comets in the SMIFH2-treated cells resembled that of the knockdown mutant of plant Xenopus Microtubule-Associated protein of 215 kDa (XMAP215) homolog MICROTUBULE ORGANIZATION 1/GEMINI 1 (MOR1/GEM1). This outcome suggests that formins promote elongation of tubulin flares on the growing plus ends. Formins AtFH1 (A. thaliana Formin Homology 1) and AtFH8 can also interact with EB1. Besides cytokinesis, formins function in the mitotic spindle assembly and metaphase to anaphase transition. Our data suggest that during cytokinesis formins function in: (1) promoting microtubule polymerization; (2) nucleating F-actin at the cell plate; (3) retaining dynamin-related proteins at the cell plate; and (4) remodeling of the cell plate membrane.

KINESIN-12E regulates metaphase spindle flux and helps control spindle size in Arabidopsis

The bipolar mitotic spindle is a highly conserved structure among eukaryotes that mediates chromosome alignment and segregation. Spindle assembly and size control are facilitated by force-generating microtubule-dependent motor proteins known as kinesins. In animals, kinesin-12 cooperates with kinesin-5 to produce outward-directed forces necessary for spindle assembly. In plants, the relevant molecular mechanisms for spindle formation are poorly defined. While an Arabidopsis thaliana kinesin-5 ortholog has been identified, the kinesin-12 ortholog in plants remains elusive. In this study, we provide experimental evidence for the function of Arabidopsis KINESIN-12E in spindle assembly. In kinesin-12e mutants, a delay in spindle assembly is accompanied by the reduction of spindle size, demonstrating that KINESIN-12E contributes to mitotic spindle architecture. Kinesin-12E localization is mitosis-stage specific, beginning with its perinuclear accumulation during prophase. Upon nuclear envelope breakdown, KINESIN-12E decorates subpopulations of microtubules in the spindle and becomes progressively enriched in the spindle midzone. Furthermore, during cytokinesis, KINESIN-12E shares its localization at the phragmoplast midzone with several functionally diversified Arabidopsis KINESIN-12 members. Changes in the kinetochore and in prophase and metaphase spindle dynamics occur in the absence of KINESIN-12E, suggest it might play an evolutionarily conserved role during spindle formation similar to its spindle-localized animal kinesin-12 orthologs.

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.

A Novel Katanin-Tethering Machinery Accelerates Cytokinesis

Cytokinesis is fundamental for cell proliferation [1, 2]. In plants, a bipolar short-microtubule array forms the phragmoplast, which mediates vesicle transport to the midzone and guides the formation of cell walls that separate the mother cell into two daughter cells [2]. The phragmoplast centrifugally expands toward the cell cortex to guide cell-plate formation at the cortical division site [3, 4]. Several proteins in the phragmoplast midzone facilitate the anti-parallel bundling of microtubules and vesicle accumulation [5]. However, the mechanisms by which short microtubules are maintained during phragmoplast development, in particular, the behavior of microtubules at the distal zone of phragmoplasts, are poorly understood. Here, we show that a plant-specific protein, CORTICAL MICROTUBULE DISORDERING 4 (CORD4), tethers the conserved microtubule-severing protein katanin to facilitate formation of the short-microtubule array in phragmoplasts. CORD4 was specifically expressed during mitosis and localized to preprophase bands and phragmoplast microtubules. Custom-made two-photon spinning disk confocal microscopy revealed that CORD4 rapidly localized to microtubules in the distal phragmoplast zone during phragmoplast assembly at late anaphase and persisted throughout phragmoplast expansion. Loss of CORD4 caused abnormally long and oblique phragmoplast microtubules and slow expansion of phragmoplasts. The p60 katanin subunit, KTN1, localized to the distal phragmoplast zone in a CORD4-dependent manner. These results suggest that CORD4 tethers KTN1 at phragmoplasts to modulate microtubule length, thereby accelerating phragmoplast growth. This reveals the presence of a distinct machinery to accelerate cytokinesis by regulating the action of katanin.

Phragmoplast Orienting Kinesin 2 Is a Weak Motor Switching between Processive and Diffusive Modes

Plant development and morphology relies on the accurate insertion of new cell walls during cytokinesis. However, how a plant cell correctly orients a new wall is poorly understood. Two kinesin class-12 members, phragmoplast orienting kinesin 1 (POK1) and POK2, are involved in the process, but how these molecular machines work is not known. Here, we used in vivo and single-molecule in vitro measurements to determine how Arabidopsis thaliana POK2 motors function mechanically. We found that POK2 is a very weak, on average plus-end-directed, moderately fast kinesin. Interestingly, POK2 switches between processive and diffusive modes characterized by an exclusive-state mean-squared-displacement analysis. Our results support a model that POK motors push against peripheral microtubules of the phragmoplast for its guidance. This pushing model may mechanically explain the conspicuous narrowing of the division site. Together, our findings provide mechanical insight into how active motors accurately position new cell walls in plants.

Identification and characterization of the land-plant-specific microtubule nucleation factor MACET4

Here, we show that the embryophyte (land-plant)-specific protein MACERATOR4 (MACET4) binds microtubules in vitro and in vivo, promotes microtubule polymerization at sub-critical tubulin concentrations, decreases the lag phase in microtubule bulk polymerization assays, and colocalizes with microtubule nucleation sites. Furthermore, we find that MACET4 forms oligomers that induce aster formation in vitro in a manner that is similar to aster formation mediated by centrosomes and TPX2. MACET4 is expressed during cell division and accumulates at the microtubule nucleation regions of the plant-specific cytokinetic microtubule array, the phragmoplast. We found that MACET4 localizes to the preprophase band and the cortical division zone, but not the spindle. MACET4 appears as cytoplasmic foci in vivo and forms octamers in vitro Transient expression in tobacco leaf pavement cells results in labeling of shrinking plus- and minus-ends. MACET4 facilitates microtubule depolymerization by increasing the frequency of catastrophes in vivo and by suppressing rescues in vitro Microtubules formed in the presence of MACET4 in vitro are shorter, most likely due to the depletion of the free tubulin pool. Accordingly, MACET4 knockdown results in longer phragmoplasts. We conclude that the direct activity of MACET4 is in promoting microtubule nucleation.This article has an associated First Person interview with the first author of the paper.

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.

Division Plane Establishment and Cytokinesis

Plant cells divide their cytoplasmic content by forming a new membrane compartment, the cell plate, via a rerouting of the secretory pathway toward the division plane aided by a dynamic cytoskeletal apparatus known as the phragmoplast. The phragmoplast expands centrifugally and directs the cell plate to the preselected division site at the plasma membrane to fuse with the parental wall. The division site is transiently decorated by the cytoskeletal preprophase band in preprophase and prophase, whereas a number of proteins discovered over the last decade reside continuously at the division site and provide a lasting spatial reference for phragmoplast guidance. Recent studies of membrane fusion at the cell plate have revealed the contribution of functionally conserved eukaryotic proteins to distinct stages of cell plate biogenesis and emphasize the coupling of cell plate formation with phragmoplast expansion. Together with novel findings concerning preprophase band function and the setup of the division site, cytokinesis and its spatial control remain an open-ended field with outstanding and challenging questions to resolve.

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.

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.

Analysis of Phragmoplast Kinetics During Plant Cytokinesis

In plants, the partitioning of daughter cells during cytokinesis is achieved via physical insertion of a membranous cell plate within the dividing parent cell. It is a cellular process of extensive protein secretion and membrane trafficking toward the plane of cell division and the cytoskeleton is an important facilitator of this process. A specialized cytoskeletal array termed phragmoplast expands centrifugally throughout cytokinesis and directs, mostly Golgi-derived vesicles that ultimately fuse to form the developing cell plate. The function of the phragmoplast in guiding cell plate synthesis has strongly motivated many scientists to monitor its dynamic behavior. In this chapter, we present an overview of basic principles and methods concerning the live imaging of cytokinetic plant cells using confocal laser scanning microscopy (CLSM) and the analysis of phragmoplast expansion.

Phragmoplast microtubule dynamics - a game of zones

Plant morphogenesis relies on the accurate positioning of the partition (cell plate) between dividing cells during cytokinesis. The cell plate is synthetized by a specialized structure called the phragmoplast, which consists of microtubules, actin filaments, membrane compartments and associated proteins. The phragmoplast forms between daughter nuclei during the transition from anaphase to telophase. As cells are commonly larger than the originally formed phragmoplast, the construction of the cell plate requires phragmoplast expansion. This expansion depends on microtubule polymerization at the phragmoplast forefront (leading zone) and loss at the back (lagging zone). Leading and lagging zones sandwich the 'transition' zone. A population of stable microtubules in the transition zone facilitates transport of building materials to the midzone where the cell plate assembly takes place. Whereas microtubules undergo dynamic instability in all zones, the overall balance appears to be shifted towards depolymerization in the lagging zone. Polymerization of microtubules behind the lagging zone has not been reported to date, suggesting that microtubule loss there is irreversible. In this Review, we discuss: (1) the regulation of microtubule dynamics in the phragmoplast zones during expansion; (2) mechanisms of the midzone establishment and initiation of cell plate biogenesis; and (3) signaling in the phragmoplast.

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.

Dynamics of the Plant Nuclear Envelope During Cell Division

The use of suspension cultures synchronised by aphidicolin provides a method to study cell division in living plant cells. This chapter describes the use of this technique in tobacco suspension cultures expressing nuclear and nuclear envelope proteins that have been fused to fluorescent proteins. The protocol provides advice on optimizing synchrony and on real-time imaging by confocal microscopy.

Plant cytokinesis-No ring, no constriction but centrifugal construction of the partitioning membrane

Plants have evolved a unique way of partitioning the cytoplasm of dividing cells: Instead of forming a contractile ring that constricts the plasma membrane, plant cells target membrane vesicles to the plane of division where the vesicles fuse with one another to form the partitioning membrane. Plant cytokinesis starts in the centre and progresses towards the periphery, culminating in the fusion of the partitioning membrane with the parental plasma membrane. This membrane dynamics is orchestrated by a specific cytoskeletal array named phragmoplast that originates from interzone spindle remnants. Here we review the properties of the process as well as molecules that play specific roles in that process.

Arabidopsis KCBP interacts with AIR9 but stays in the cortical division zone throughout mitosis via its MyTH4-FERM domain

The preprophase band of microtubules performs the crucial function of marking the plane of cell division. Although the preprophase band depolymerises at the onset of mitosis, the division plane is ‘memorized’ by a cortical division zone to which the phragmoplast is attracted during cytokinesis. Proteins have been discovered that are part of the molecular memory but little is known about how they contribute to phragmoplast guidance. Previously, we found that the microtubule-associated protein AIR9 is found in the cortical division zone at preprophase and returns during cell plate insertion but is absent from the cortex during the intervening mitosis. To identify new components of the preprophase memory, we searched for proteins that interact with AIR9. We detected the kinesin-like calmodulin-binding protein, KCBP, which can be visualized at the predicted cortical site throughout division. A truncation study of KCBP indicates that its MyTH4-FERM domain is required for linking the motor domain to the cortex. These results suggest a mechanism by which minus-end-directed KCBP helps guide the centrifugally expanding phragmoplast to the cortical division site.

The carboxyl-terminal tail of the stalk of Arabidopsis NACK1/HINKEL kinesin is required for its localization to the cell plate formation site

Plant cytokinesis is achieved by formation of cell plates in the phragmoplast, a plant-specific cytokinetic apparatus, which consists of microtubules (MTs) and microfilaments. During cytokinesis, the cell plate is expanded centrifugally outward from the inside of cells in a process that is supported by dynamic turnover of MTs. M-phase-specific kinesin NACK1, which comprises the motor domain at the amino-terminal half to move on MT bundles and the stalk region in the carboxyl-terminal half, is a key player in the process of MT turnover. That is, the specific region in the stalk binds the MAP kinase kinase kinase to activate the whole MAP kinase cascade, which stimulates depolymerization of MTs for the MT turnover. The stalk is also responsible for recruiting the activated kinase cascade to the mid-zone of the phragmoplast, which corresponds to the cell-plate formation site. It should be crucial to uncover roles of the NACK1 kinesin stalk as well as the motor domain in the formation of cell plates in order to understand the mechanisms of cell plate formation. Using dissected Arabidopsis NACK1 (AtNACK1/HINKEL) molecules and AtNACK1-fused GFP, we showed that the C-terminal tail of the stalk in addition to the motor domain is critical for its proper localization to the site of cell plate formation in the phragmoplast, probably by affecting its motility activity.

Dynamics and organization of cortical microtubules as revealed by superresolution structured illumination microscopy

Plants employ acentrosomal mechanisms to organize cortical microtubule arrays essential for cell growth and differentiation. Using structured illumination microscopy (SIM) adopted for the optimal documentation of Arabidopsis (Arabidopsis thaliana) hypocotyl epidermal cells, dynamic cortical microtubules labeled with green fluorescent protein fused to the microtubule-binding domain of the mammalian microtubule-associated protein MAP4 and with green fluorescent protein-fused to the alpha tubulin6 were comparatively recorded in wild-type Arabidopsis plants and in the mitogen-activated protein kinase mutant mpk4 possessing the former microtubule marker. The mpk4 mutant exhibits extensive microtubule bundling, due to increased abundance and reduced phosphorylation of the microtubule-associated protein MAP65-1, thus providing a very useful genetic tool to record intrabundle microtubule dynamics at the subdiffraction level. SIM imaging revealed nano-sized defects in microtubule bundling, spatially resolved microtubule branching and release, and finally allowed the quantification of individual microtubules within cortical bundles. Time-lapse SIM imaging allowed the visualization of subdiffraction, short-lived excursions of the microtubule plus end, and dynamic instability behavior of both ends during free, intrabundle, or microtubule-templated microtubule growth and shrinkage. Finally, short, rigid, and nondynamic microtubule bundles in the mpk4 mutant were observed to glide along the parent microtubule in a tip-wise manner. In conclusion, this study demonstrates the potential of SIM for superresolution time-lapse imaging of plant cells, showing unprecedented details accompanying microtubule dynamic organization.

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.

MAPs: cellular navigators for microtubule array orientations in Arabidopsis

Microtubules are subcellular nanotubes composed of α- and β-tubulin that arise from microtubule nucleation sites, mainly composed of γ-tubulin complexes [corrected]. Cell wall encased plant cells have evolved four distinct microtubule arrays that regulate cell division and expansion. Microtubule-associated proteins, the so called MAPs, construct, destruct and reorganize microtubule arrays thus regulating their spatiotemporal transitions during the cell cycle. By physically binding to microtubules and/or modulating their functions, MAPs control microtubule dynamic instability and/or interfilament cross talk. We survey the recent analyses of Arabidopsis MAPs such as MAP65, MOR1, CLASP, katanin, TON1, FASS, TRM, TAN1 and kinesins in terms of their effects on microtubule array organizations and plant development.

The role of dynamic instability in microtubule organization

Microtubules are one of the three major cytoskeletal components in eukaryotic cells. Heterodimers composed of GTP-bound α- and β-tubulin molecules polymerize to form microtubule protofilaments, which associate laterally to form a hollow microtubule. Tubulin has GTPase activity and the GTP molecules associated with β-tubulin molecules are hydrolyzed shortly after being incorporated into the polymerizing microtubules. GTP hydrolysis alters the conformation of the tubulin molecules and drives the dynamic behavior of microtubules. Periods of rapid microtubule polymerization alternate with periods of shrinkage in a process known as dynamic instability. In plants, dynamic instability plays a key role in determining the organization of microtubules into arrays, and these arrays vary throughout the cell cycle. In this review, we describe the mechanisms that regulate microtubule dynamics and underlie dynamic instability, and discuss how dynamic instability may shape microtubule organization in plant cells.

Signaling Pathway that Controls Plant Cytokinesis

Cytokinesis is the final process of cell division cycle that properly separates cytoplasmic components and duplicated nuclei into two daughter cells. Plant cytokinesis occurs in phragmoplast, the cytokinetic machinery composed mainly of microtubule (MT) arrays. Recent studies have revealed that a plant-specific mitogen-activated protein kinase (MAPK) cascade is involved in cytokinesis. The activity of this cascade is controlled by cytokinesis-specific kinesin called NACK in tobacco and Arabidopsis, which is required for the cell plate formation in the phragmoplast. Functions of NACK are strictly controlled by cyclin-dependent kinase/cyclin B complexes so as to be activated at the correct timing for cytokinesis. Thus, this pathway constitutes a part of the regulatory system controlling the cell cycle progression. Here, we review recent advancements for understanding how the activation of this pathway can be specified in the late stage of the M phase and how this MAPK cascade can control cytokinesis through MT turnover.

The rise and fall of the phragmoplast microtubule array

The cytokinetic apparatus, the phragmoplast, contains a bipolar microtubule (MT) framework that has the MT plus ends concentrated at or near the division site. This anti-parallel MT array provides tracks for the transport of Golgi-derived vesicles toward the plus ends so that materials enclosed are subsequently deposited at the division site. Here we will discuss a proposed model of the centrifugal expansion of the phragmoplast that takes place concomitantly with the assembly of the cell plate, the ultimate product of vesicle fusion. The expansion is a result of continuous MT assembly at the phragmoplast periphery while the MTs toward the center of the phragmoplast are disassembled. These events are the result of MT-dependent MT polymerization, bundling of anti-parallel MTs coming from opposite sides of the division plane that occurs selectively at the phragmoplast periphery, positioning of the plus ends of cross-linked MTs at or near the division site by establishing a minimal MT-overlapping zone, and debundling of anti-parallel MTs that is triggered by phosphorylation of MT-associated proteins. The debundled MTs are disassembled at last by factors including the MT severing enzyme katanin.

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

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

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.

Two anti-microtubular drugs for two differential responses: a rice cell line resistant to EPC remains susceptible to oryzalin

Sensitivity to the two anti-microtubular drugs oryzalin and EPC (ethyl-N-phenylcarbamate) is shown to be uncoupled in the rice EPC-resistant ER31d cell line, derived from the corresponding ER31 mutant. The ER31d cell line grows in the presence of EPC but it remains susceptible to oryzalin. In the presence of concentrations of EPC up to 0.4 mM, ER31d cells remain viable maintaining cell anisotropy and detectable cortical microtubule array. The amount of α- and β-tubulin is also maintained high through a regulatory mechanism that operates at post-transcriptional level. In contrast, all these cellular and molecular parameters are heavily affected by the addition of 1 μM oryzalin. Also, the pattern of post-translationally modified α-tubulins changes in the ER31d cells compared to that of their Nihon-Masari wild type line of reference. The different response elicited by the two herbicides is discussed in relation to a possible differential sensitivity of the cortical MT array, that may in turn relate to their different tubulin-binding specificities and chemical structure.

Mechanism of microtubule array expansion in the cytokinetic phragmoplast

In land plants, the cell plate partitions the daughter cells at cytokinesis. The cell plate initially forms between daughter nuclei and expands centrifugally until reaching the plasma membrane. The centrifugal development of the cell plate is driven by the centrifugal expansion of the phragmoplast microtubule array, but the molecular mechanism underlying this expansion is unknown. Here, we show that the phragmoplast array comprises stable microtubule bundles and dynamic microtubules. We find that the dynamic microtubules are nucleated by γ-tubulin on stable bundles. The dynamic microtubules elongate at the plus ends and form new bundles preferentially at the leading edge of the phragmoplast. At the same time, they are moved away from the cell plate, maintaining a restricted distribution of minus ends. We propose that cycles of attachment of γ-tubulin complexes onto the microtubule bundles, microtubule nucleation and bundling, accompanied by minus-end-directed motility, drive the centrifugal development of the phragmoplast.

Plant cytokinesis: circles within circles

The current model of the plant cytokinetic apparatus, describing it as being composed of treadmilling microtubules, is challenged by a new study showing that these microtubules display dynamic instability.