Roles for kinesin and myosin during cytokinesis

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.

Cytoskeletal regulation of primary plant cell wall assembly

The plant cell wall is an extracellular matrix that envelopes cells, gives them structure and shape, constitutes the interface with symbionts, and defends plants against external biotic and abiotic stress factors. The assembly of this matrix is regulated and mediated by the cytoskeleton. Cytoskeletal elements define where new cell wall material is added and how fibrillar macromolecules are oriented in the wall. Inversely, the cytoskeleton is also key in the perception of mechanical cues generated by structural changes in the cell wall as well as the mediation of intracellular responses. We review the delivery processes of the cell wall precursors that are required for the cell wall assembly process and the structural continuity between the inside and the outside of the cell. We provide an overview of the different morphogenetic processes for which cell wall assembly is a crucial element and elaborate on relevant feedback mechanisms.

The role of the nuclear envelope in the regulation of chromatin dynamics during cell division

The nuclear envelope delineates the eukaryotic cell nucleus. The membrane system of the nuclear envelope consists of an outer nuclear membrane and an inner nuclear membrane separated by a perinuclear space. It serves as more than just a static barrier, since it regulates the communication between the nucleoplasm and the cytoplasm and provides the anchoring points where chromatin is attached. Fewer nuclear envelope proteins have been identified in plants in comparison with animals and yeasts. Here, we review the current state of knowledge of the nuclear envelope in plants, focusing on its role as a chromatin organizer and regulator of gene expression, as well as on the modifications that it undergoes to be efficiently disassembled and reassembled with each cell division. Advances in knowledge concerning the mitotic role of some nuclear envelope constituents are also presented. In addition, we summarize recent progress on the contribution of the nuclear envelope elements to telomere tethering and chromosome dynamics during the meiotic division in different plant species.

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.

Vesicle Dynamics during Plant Cell Cytokinesis Reveals Distinct Developmental Phases

Cell division in plant cells requires the deposition of a new cell wall between the two daughter cells. The assembly of this plate requires the coordinated movement of cargo vesicles whose size is below the diffraction-limited resolution of the optical microscope. We combined high spatial and temporal resolution confocal laser scanning microscopy with advanced image-processing tools and fluorescence fluctuation methods and distinguished three distinct phases during cell plate expansion in tobacco (Nicotiana tabacum) 'Bright Yellow-2' cells: massive delivery of preexisting vesicles to a disk-shaped region at the equatorial plane precedes a primary rapid expansion phase followed by a secondary, slow expansion phase during which the extremity of the circular plate seeks contact with the mother wall and brings about the separation of the two portions of cytoplasm. Different effects of pharmacological inhibition emphasize the distinct nature of the assembly and expansion mechanisms characterizing these phases.

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 conserved chimeric transcript UPGRADE2 is associated with unreduced pollen formation and is exclusively found in apomictic Boechera species

In apomictic Boechera spp., meiotic diplospory leads to the circumvention of meiosis and the suppression of recombination to produce unreduced male and female gametes (i.e. apomeiosis). Here, we have established an early flower developmental staging system and have performed microarray-based comparative gene expression analyses of the pollen mother cell stage in seven diploid sexual and seven diploid apomictic genotypes to identify candidate factors for unreduced pollen formation. We identified a transcript unique to apomictic Boechera spp. called UPGRADE2 (BspUPG2), which is highly up-regulated in their pollen mother cells. BspUPG2 is highly conserved among apomictic Boechera spp. genotypes but has no homolog in sexual Boechera spp. or in any other taxa. BspUPG2 undergoes posttranscriptional processing but lacks a prominent open reading frame. Together with the potential of stably forming microRNA-like secondary structures, we hypothesize that BspUPG2 functions as a long regulatory noncoding messenger RNA-like RNA. BspUPG2 has apparently arisen through a three-step process initiated by ancestral gene duplication of the original BspUPG1 locus, followed by sequential insertions of segmentally duplicated gene fragments, with final exonization of its sequence structure. Its genesis reflects the hybridization history that characterizes the genus Boechera.

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.

Formins: emerging players in the dynamic plant cell cortex

Formins (FH2 proteins) are an evolutionarily conserved family of eukaryotic proteins, sharing the common FH2 domain. While they have been, until recently, understood mainly as actin nucleators, formins are also engaged in various additional aspects of cytoskeletal organization and signaling, including, but not limited to, the crosstalk between the actin and microtubule networks. A surprising diversity of domain organizations has been discovered among the FH2 proteins, and specific domain setups have been found in plants. Seed plants have two clades of formins, one of them (Class I) containing mostly transmembrane proteins, while members of the other one (Class II) may be anchored to membranes via a putative membrane-binding domain related to the PTEN antioncogene. Thus, plant formins present good candidates for possible mediators of coordination of the cortical actin and microtubule cytoskeletons, as well as their attachment to the plasma membrane, that is, aspects of cell cortex organization likely to be important for cell and tissue morphogenesis. Although experimental studies of plant formin function are hampered by the large number of formin genes and their functional redundancy, recent experimental work has already resulted in some remarkable insights into the function of FH2 proteins in plants.

Inhibitors of myosin, but not actin, alter transport through Tradescantia plasmodesmata

Actin and myosin are components of plasmodesmata, the cytoplasmic channels between plant cells, but their role in regulating these channels is unclear. Here, we investigated the role of myosin in regulating plasmodesmata in a well-studied, simple system comprising single filaments of cells which form stamen hairs in Tradescantia virginiana flowers. Effects of myosin inhibitors were assessed by analysing cell-to-cell movement of fluorescent tracers microinjected into treated cells. Incubation in the myosin inhibitor, 2,3-butanedione monoxime (BDM) or injection of anti-myosin antibodies increased cell-cell transport of fluorescent dextrans, while treatment with the myosin inhibitor N-ethylmaleimide (NEM) decreased cell-cell transport. Pretreatment with the callose synthesis inhibitor, deoxy-D: -glucose (DDG), enhanced transport induced by BDM treatment or injection of myosin antibodies but did not relieve NEM-induced reduction in transport. In contrast to the myosin inhibitors, cell-to-cell transport was unaffected by treatment with the actin polymerisation inhibitor, latrunculin B, after controlling for callose synthesis with DDG. Transport was increased following azide treatment, and reduced after injection of ATP, as in previous studies. We propose that myosin detachment from actin, induced by BDM, opens T. virginiana plasmodesmata whereas the firm attachment of myosin to actin, promoted by NEM, closes them.

Tungsten affects the cortical microtubules of Pisum sativum root cells: experiments on tungsten-molybdenum antagonism

Tungsten (W) is increasingly shown to be toxic to various organisms, including plants. Apart from inactivation of molybdo-enzymes, other potential targets of W toxicity in plants, especially at the cellular level, have not yet been revealed. In the present study, the effect of W on the cortical microtubule array of interphase root tip cells was investigated, in combination with the possible antagonism of W for the pathway of molybdenum (Mo). Pisum sativum seedlings were treated with W, Mo or a combination of the two, and cortical microtubules were examined using tubulin immunofluorescnce and TEM. Treatments with anti-microtubule (oryzalin, colchicine and taxol) or anti-actomyosin (cytochalasin D, BDM or ML-7) drugs and W were also performed. W-affected cortical microtubules were low in number, short, not uniformly arranged and were resistant to anti-microtubule drugs. Cells pre-treated with oryzalin or colchicine and then treated with W displayed W-affected microtubules, while cortical microtubules pre-stabilized with taxol were resistant to W. Treatment with Mo and anti-actomyosin drugs prevented W from affecting cortical microtubules. Cortical microtubule recovery after W treatment was faster in Mo solution than in water. The results indicate that cortical microtubules of plant cells are indirectly affected by W, most probably through a mechanism depending on the in vivo antagonism of W for the Mo-binding site of Cnx1 protein.

The Arabidopsis cell division cycle

Plant cells have evolved a complex circuitry to regulate cell division. In many aspects, the plant cell cycle follows a basic strategy similar to other eukaryotes. However, several key issues are unique to plant cells. In this chapter, both the conserved and unique cellular and molecular properties of the plant cell cycle are reviewed. In addition to division of individual cells, the specific characteristic of plant organogenesis and development make that cell proliferation control is of primary importance during development. Therefore, special attention should be given to consider plant cell division control in a developmental context. Proper organogenesis depends on the formation of different cell types. In plants, many of the processes leading to cell differentiation rely on the occurrence of a different cycle, termed the endoreplication cycle, whereby cells undergo repeated full genome duplication events in the absence of mitosis and increase their ploidy. Recent findings are focusing on the relevance of changes in chromatin organization for a correct cell cycle progression and, conversely, in the relevance of a correct functioning of chromatin remodelling complexes to prevent alterations in both the cell cycle and the endocycle.

Differential positioning of C(4) mesophyll and bundle sheath chloroplasts: recovery of chloroplast positioning requires the actomyosin system

In C(4) plants, bundle sheath (BS) chloroplasts are arranged in the centripetal position or in the centrifugal position, although mesophyll (M) chloroplasts are evenly distributed along cell membranes. To examine the molecular mechanism for the intracellular disposition of these chloroplasts, we observed the distribution of actin filaments in BS and M cells of the C(4) plants finger millet (Eleusine coracana) and maize (Zea mays) using immunofluorescence. Fine actin filaments encircled chloroplasts in both cell types, and an actin network was observed adjacent to plasma membranes. The intracellular disposition of both chloroplasts in finger millet was disrupted by centrifugal force but recovered within 2 h in the dark. Actin filaments remained associated with chloroplasts during recovery. We also examined the effects of inhibitors on the rearrangement of chloroplasts. Inhibitors of actin polymerization, myosin-based activities and cytosolic protein synthesis blocked migration of chloroplasts. In contrast, a microtubule-depolymerizing drug had no effect. These results show that C(4) plants possess a mechanism for keeping chloroplasts in the home position which is dependent on the actomyosin system and cytosolic protein synthesis but not tubulin or light.

Orientation of microtubules suggests a role in mRNA transportation in fertilized eggs of Chinese pine (Pinus tabulaeformis)

Polysomes become associated with microtubules (MTs) in egg cells of Chinese pine upon fertilization, providing direct evidence for MT-based intracellular mRNA and polysome localization. We have investigated by immunoelectron microscopy the orientation and spatial distribution of MTs and their association with polysomes in the fertilized egg cells. There is a perinuclear accumulation of MTs and polysomes in the zygote soon after fertilization. At this time, some of the MTs are perpendicular to the nuclear envelope and directly connected to the outer membrane or nuclear-pore complexes (NPC) at one end, and the other ends reach to the outer tier or cortical MTs that are parallel to the long axis of the zygote. The polysomes in the perinuclear region show the same spatial and temporal pattern as the MTs. Immunolocalization of the mRNA-binding protein hnRNP indicates that the mRNAs are loaded onto the nucleus-associated MTs immediately after their export from the nuclear-pore complexes. The polysomes and mRNAs are then transported from these MTs to the outer tier and/or cortical MTs, where they further localize to the polar region of the cell.

A kinesin-like calmodulin-binding protein in Chlamydomonas: evidence for a role in cell division and flagellar functions

Kinesin-like calmodulin-binding protein, KCBP, is a novel member of the C-kinesin superfamily first discovered in flowering plants. This minus-end-directed kinesin exhibits Ca(2+)-calmodulin-sensitive motor activity in vitro and has been implicated in trichome morphogenesis and cell division. A homologue of KCBP is also found in the unicellular, biflagellate green alga Chlamydomonas reinhardtii (CrKCBP). Unlike plant cells, Chlamydomonas cells do not form trichomes and do not assemble a phragmoplast before cell division. To test whether CrKCBP is involved in additional microtubule-based processes not observed in plants, we generated antibodies against the putative calmodulin-binding domain and used these antibodies in biochemical and localization studies. In interphase cells CrKCBP primarily localizes near the base of the flagella, although surprisingly, a small fraction also localizes along the length of the flagella. CrKCBP is bound to isolated axonemes in an ATP-dependent fashion and is not a component of the dynein arms, radial spokes or central apparatus. During mitosis, CrKCBP appears concentrated at the centrosomes during prophase and metaphase. However, during telophase and cytokinesis CrKCBP co-localizes with the microtubules associated with the phycoplast. These studies implicate CrKCBP in flagellar functions as well as cell division.

Plum pox virus induces differential gene expression in the partially resistant stone fruit tree Prunus armeniaca cv. Goldrich

We investigated the changes in the expression profiles of the partially resistant apricot (Prunus armeniaca L.) cultivar Goldrich following inoculation with Plum pox virus (PPV) using cDNA-amplification fragment length polymorphism (AFLP). Altered expression patterns were detected and twenty-one differentially expressed cDNA had homologies with genes in databases coding for proteins involved in metabolism, signal transduction, defense, stress and intra/intercellular connections. Seven of the modified expressed patterns were further investigated by semi-quantitative RT-PCR or Northern blotting. The expression patterns of five of these genes were confirmed in the partially resistant P. armeniaca cv. 'Goldrich' and assessed in a susceptible genotype. One of these cDNAs, coding for a putative class III chitinase, appeared to be repressed in infected plants of the partially resistant genotype and expressed in the susceptible one which could be related to the partially resistant phenotype. On the contrary, the expression patterns of the genes coding for a transketolase, a kinesin-like and an ankyrin-like protein, were clearly linked to the susceptible interaction. These candidate genes could play a role either in the compatible interaction leading to virus invasion or to the quantitative resistance of apricot to PPV.

Plant cytokinesis: fission by fusion

Cytokinesis partitions the cytoplasm of a dividing cell. Unlike yeast and animal cells, which form cleavage furrows from the plasma membrane, cells in higher plants make a new membrane independently of the plasma membrane by homotypic fusion of vesicles. In somatic cells, a plant-specific cytoskeletal array, called a phragmoplast, is thought to deliver vesicles to the plane of division. Vesicle fusion generates a membranous network, the cell plate, which, by fusion of later-arriving vesicles with its margin, expands towards the cell periphery and eventually fuses with the plasma membrane. In this review (part of the Cytokinesis series), I describe recent studies addressing the mechanisms that underlie cell-plate formation and the coordinated dynamics of membrane fusion and cytoskeletal reorganization during progression through cytokinesis.

Cytokinesis in higher plants

Cytokinesis partitions the cytoplasm between two or more nuclei. In higher plants, cytokinesis is initiated by cytoskeleton-assisted targeted delivery of membrane vesicles to the plane of cell division, followed by local membrane fusion to generate tubulo-vesicular networks. This initial phase of cytokinesis is essentially the same in diverse modes of plant cytokinesis whereas the subsequent transformation of the tubulo-vesicular networks into the partitioning membrane may be different between systems. This review focuses on membrane and cytoskeleton dynamics in cell plate formation and expansion during somatic cytokinesis.

Arabidopsis WPP-domain proteins are developmentally associated with the nuclear envelope and promote cell division

The nuclear envelope (NE) acts as a selective barrier to macromolecule trafficking between the nucleus and the cytoplasm and undergoes a complex reorganization during mitosis. Different eukaryotic kingdoms show specializations in NE function and composition. In contrast with vertebrates, the protein composition of the NE and the function of NE proteins are barely understood in plants. MFP1 attachment factor 1 (MAF1) is a plant-specific NE-associated protein first identified in tomato (Lycopersicon esculentum). Here, we demonstrate that two Arabidopsis thaliana MAF1 homologs, WPP1 and WPP2, are associated with the NE specifically in undifferentiated cells of the root tip. Reentry into cell cycle after callus induction from differentiated root segments reprograms their NE association. Based on green fluorescent protein fusions and immunogold labeling data, the proteins are associated with the outer NE and the nuclear pores in interphase cells and with the immature cell plate during cytokinesis. RNA interference-based suppression of the Arabidopsis WPP family causes shorter primary roots, a reduced number of lateral roots, and reduced mitotic activity of the root meristem. Together, these data demonstrate the existence of regulated NE targeting in plants and identify a class of plant-specific NE proteins involved in mitotic activity.

A cortical cytoplasmic ring predicts the division plane in vacuolated cells of Coleus: the role of actomyosin and microtubules in the establishment and function of the division site

•  The role of actomyosin and microtubules (MTs) in division plane establishment and cell plate expansion was studied in vacuolated cells of Coleus. •  Fluorescence and conventional microscopy were applied on control material and after treatment with anti-MT and anti-actin filament (AF) drugs as well as myosin inhibitors. •  The division plane is predicted by a cortical cytoplasmic ring (CCR), rich in AFs and endoplasmic reticulum, formed at interphase. The nucleus migrates to the CCR before its entrance in the phragmosome. The latter consists of transvacuolar cytoplasmic strands connecting the perinuclear cytoplasm with the CCR. During preprophase, a preprophase MT band is organized in the CCR. AF disruption and myosin inhibition destroy the CCR and cytoplasmic strands, arrest migration of the nucleus and affect cell plate expansion. MT disorganization inhibits anchoring of the cytoplasmic strands into the division plane. •  These observations support that CCR establishment, formation of transvacuolar cytoplasmic strands, migration of the nucleus and lateral cell plate expansion depend on actomyosin. The MTs guide and anchor the cytoplasmic strands of the phragmosome into the predetermined division plane.

Electron tomographic analysis of somatic cell plate formation in meristematic cells of Arabidopsis preserved by high-pressure freezing

We have investigated the process of somatic-type cytokinesis in Arabidopsis (Arabidopsis thaliana) meristem cells with a three-dimensional resolution of approximately 7 nm by electron tomography of high-pressure frozen/freeze-substituted samples. Our data demonstrate that this process can be divided into four phases: phragmoplast initials, solid phragmoplast, transitional phragmoplast, and ring-shaped phragmoplast. Phragmoplast initials arise from clusters of polar microtubules (MTs) during late anaphase. At their equatorial planes, cell plate assembly sites are formed, consisting of a filamentous ribosome-excluding cell plate assembly matrix (CPAM) and Golgi-derived vesicles. The CPAM, which is found only around growing cell plate regions, is suggested to be responsible for regulating cell plate growth. Virtually all phragmoplast MTs terminate inside the CPAM. This association directs vesicles to the CPAM and thereby to the growing cell plate. Cell plate formation within the CPAM appears to be initiated by the tethering of vesicles by exocyst-like complexes. After vesicle fusion, hourglass-shaped vesicle intermediates are stretched to dumbbells by a mechanism that appears to involve the expansion of dynamin-like springs. This stretching process reduces vesicle volume by approximately 50%. At the same time, the lateral expansion of the phragmoplast initials and their CPAMs gives rise to the solid phragmoplast. Later arriving vesicles begin to fuse to the bulbous ends of the dumbbells, giving rise to the tubulo-vesicular membrane network (TVN). During the transitional phragmoplast stage, the CPAM and MTs disassemble and then reform in a peripheral ring phragmoplast configuration. This creates the centrifugally expanding peripheral cell plate growth zone, which leads to cell plate fusion with the cell wall. Simultaneously, the central TVN begins to mature into a tubular network, and ultimately into a planar fenestrated sheet (PFS), through the removal of membrane via clathrin-coated vesicles and by callose synthesis. Small secondary CPAMs with attached MTs arise de novo over remaining large fenestrae to focus local growth to these regions. When all of the fenestrae are closed, the new cell wall is complete. Few endoplasmic reticulum (ER) membranes are seen associated with the phragmoplast initials and with the TVN cell plate that is formed within the solid phragmoplast. ER progressively accumulates thereafter, reaching a maximum during the late PFS stage, when most cell plate growth is completed.

The role of the cytoskeleton in the morphogenesis and function of stomatal complexes

Microtubules (MTs) and actin filaments (AFs) form highly organized arrays in stomatal cells that play key roles in the morphogenesis of stomatal complexes. The cortical MTs controlling the orientation of the depositing cellulose microfibrils (CMs) and affecting the pattern of local wall thickenings define the mechanical properties of the walls of stomatal cells, thus regulating accurately their shape. Besides, they are involved in determination of the cell division plane. Substomatal cavity and stomatal pore formation are also MT-dependent processes. Among the cortical MT arrays, the radial ones lining the periclinal walls are of particular morphogenetic importance. Putative MT organizing centers (MTOCs) function at their focal regions, at least in guard cells (GCs), or alternatively, these regions either organize or nucleate cortical MTs. AFs are involved in cell polarization preceding asymmetrical divisions, in determination of the cell division plane and final cell plate alignment and probably in transduction of stimuli implicated in stomatal complex morphogenesis. Mature kidney-shaped GCs display radial AF arrays, undergoing definite organization cycles during stomatal movement. They are involved in stomatal movement, probably by controlling plasmalemma ion-channel activities. Radial MT arrays also persist in mature GCs, but a role in stomatal function cannot yet be attributed to them. Contents Summary 613 I. Introduction 614 II. Cytoskeleton and development of the stomatal complexes 614 III. Cytoskeleton and stomatal cell shaping 620 IV. Stomatal pore formation 624 V. Substomatal cavity formation 625 VI. Stomatal complex morphogenesis in mutants 626 VII. Cytoskeleton dynamics in functioning stomata 628 VIII. Mechanisms of microtubule organization in stomatal cells 631 IX. Conclusions-perspectives 634 References 635.

Microtubule cytoskeleton: a track record

The plant microtubule cytoskeleton forms unique arrays during cell division and morphogenesis. Recent studies have addressed the biogenesis, turnover, spatio-temporal organisation and cellular function of microtubules. The results suggest that both conserved eukaryotic mechanisms and plant-specific modifications determine microtubule dynamics and function.

Introduction

This is an auspicious time for plant cell biology. We have the recent genome sequence of the model plant Arabidopsis, together with powerful new sets of tools that include functional genomics and the dynamic imaging of green fluorescent protein (GFP)–conjugated proteins by confocal microscopy. We are well placed indeed for uncovering the ways in which all genes act out their roles within individual cells. It is the combinatorial properties of these individual cells that underpin both the higherorder processes of plant growth and development and of plant evolution. At a crucial level in the hierarchy of explanations and structures, between the linear DNA sequence at one extreme and whole plant phenomena at the other, lies the cell. It is to this cellular level, to the irreducible unit of life, that this collection of papers is addressed, literally ‘between genome and plant’. Most of the major conceptual advances in our understanding of cell biology have come from work with animal and fungal systems. The ways in which membranes create dynamic compartments within cells and the role of the cytoskeleton in providing an ordering framework are familiar from the textbooks. But plant cells have special features.