Identification and preliminary characterization of a 65 kDa higher-plant microtubule-associated protein

Characterization of Soybean Events with Enhanced Expression of the Microtubule-Associated Protein 65-1 (MAP65-1)

Microtubule-associated protein 65-1 (MAP65-1) protein plays an essential role in plant cellular dynamics through impacting stabilization of the cytoskeleton by serving as a crosslinker of microtubules. The role of MAP65-1 in plants has been associated with phenotypic outcomes in response to various environmental stresses. The Arabidopsis MAP65-1 (AtMAP65-1) is a known virulence target of plant bacterial pathogens and is thus a component of plant immunity. Soybean events were generated that carry transgenic alleles for both AtMAP65-1 and GmMAP65-1, the soybean AtMAP65-1 homolog, under control of cauliflower mosaic virus 35S promoter. Both AtMAP65-1 and GmMAP65-1 transgenic soybeans are more resistant to challenges by the soybean bacterial pathogen Pseudomonas syringae pv. glycinea and the oomycete pathogen Phytophthora sojae, but not the soybean cyst nematode, Heterodera glycines. Soybean plants expressing AtMAP65-1 and GmMAP65-1 also display a tolerance to the herbicide oryzalin, which has a mode of action to destabilize microtubules. In addition, GmMAP65-1-expressing soybean plants show reduced cytosol ion leakage under freezing conditions, hinting that ectopic expression of GmMAP65-1 may enhance cold tolerance in soybean. Taken together, overexpression of AtMAP65-1 and GmMAP65-1 confers tolerance of soybean plants to various biotic and abiotic stresses. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Comprehensive analyses of microtubule-associated protein MAP65 family genes in Cucurbitaceae and CsaMAP65s expression profiles in cucumber

MAP65 is a microtubule-binding protein family in plants and plays crucial roles in regulating cell growth and development, intercellular communication, and plant responses to various environmental stresses. However, MAP65s in Cucurbitaceae are still less understood. In this study, a total of 40 MAP65s were identified from six Cucurbitaceae species (Cucumis sativus L., Citrullus lanatus, Cucumis melo L., Cucurbita moschata, Lagenaria siceraria, and Benincasa hispida) and classified into five groups by phylogenetic analysis according to gene structures and conserved domains. A conserved domain (MAP65_ASE1) was found in all MAP65 proteins. In cucumber, we isolated six CsaMAP65s with different expression patterns in tissues including root, stem, leaf, female flower, male flower, and fruit. Subcellular localizations of CsaMAP65s verified that all CsaMAP65s were localized in microtubule and microfilament. Analyses of the promoter regions of CsaMAP65s have screened different cis-acting regulatory elements involved in growth and development and responses to hormone and stresses. In addition, CsaMAP65-5 in leaves was significantly upregulated by salt stress, and this promotion effect was higher in cucumber cultivars with salt tolerant than that without salt tolerant. CsaMAP65-1 in leaves was significantly upregulated by cold stress, and this promotion was higher in cold-tolerant cultivar than intolerant cultivar. With the genome-wide characterization and phylogenetic analysis of Cucurbitaceae MAP65s, and the expression profile of CsaMAP65s in cucumber, this study laid a foundation for further study on MAP65 functions in developmental processes and responses to abiotic stress in Cucurbitaceae species.

The phosphoinositide signature guides the final step of plant cytokinesis

Plant cytokinesis, which fundamentally differs from that in animals, requires the outward expansion of a plasma membrane precursor named the cell plate. How the transition from a cell plate to a plasma membrane occurs remains poorly understood. Here, we report that the acquisition of plasma membrane identity occurs through lateral patterning of the phosphatidylinositol 4,5-bisphosphate PI(4,5)P2 at the newly formed cell plate membrane. There, the phosphoinositide phosphatase SAC9 emerges as a key regulator, colocalizing with and regulating the function of the microtubule-associated protein MAP65-3 at the cell plate leading zone. In sac9-3 mutant, the polar distribution of PI(4,5)P2 at the cell plate is altered, leading to ectopic recruitment of the cytokinesis apparatus and formation of an additional cell plate insertion site. We propose that at the cell plate, SAC9 drives the depletion of PI(4,5)P2, which acts as a polar cue to spatially separate cell plate expansion from the acquisition of plasma membrane identity during final step of cytokinesis.

Redundant mechanisms in division plane positioning

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

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.

Quantification of Microtubule-Bundling Activity of MAPs Using TIRF Microscopy

Cross-linking of microtubules by microtubule-associated proteins (MAPs) results in the formation of microtubule bundles. It has been shown that a majority of microtubules in interphase plant cells are bundled. Bundling can contribute to maintaining structural stability and sustaining spatial organization of microtubule arrays. While bundling can be readily detected by an electron or fluorescent microscope, quantifying this activity remains technically challenging. Here we describe a method for quantifying microtubule-bundling in vitro using green and red stable microtubules. Furthermore, this method distinguishes between different types of microtubule-microtubule interactions: bundling, annealing, and branching. Our technique can be used to compare bundling activity of different MAPs and generate parameters for modeling their contribution to organization and dynamics of microtubule arrays.

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.

Mechanical stress effects on transcriptional regulation of genes encoding microtubule- and actin-associated proteins

Plant cytoskeleton regulation has been studied using a new approach based on both (1) pharmacological analysis of tubulin and actin inhibitors and (2) mechanical stimulation achieved by using a slow-rotating (2 rpm) clinostat in combination with transcriptional analysis of genes encoding TUA6, ACT2, MAP65-1, CLASP, PLDδ, FH4 and FH1 proteins in Arabidopsis thaliana seedling roots. The obtained data suggest feedback between the organization of microtubule (MT) and actin filament (AF) networks and the expression of the ACT2, TUA6, MAP65-1, CLASP and FH1/FH4 genes. Different regulation of feedback between MT/AF organization and TUA6, ACT2, MAP65-1, CLASP, FH4 and FH1 gene expression was noted during slow clinorotation, possibly due to altered mechanical impact on the cortical cytoskeleton. For the first time, the expression of the tubulin-associated gene MAP65-1 was shown to be dependent upon the organization of AFs. TUA6, MAP65-1, CLASP, FH1 and FH4 likely participate in mechanical signal transduction. Our work demonstrated that slow clinorotation is able to cause mechanical stress.

Mechanisms of the Ase1/PRC1/MAP65 family in central spindle assembly

During cytokinesis, the organization of the spindle midzone and chromosome segregation is controlled by the central spindle, a microtubule cytoskeleton containing kinesin motors and non-motor microtubule-associated proteins. The anaphase spindle elongation 1/protein regulator of cytokinesis 1/microtubule associated protein 65 (Ase1/PRC1/MAP65) family of microtubule-bundling proteins are key regulators of central spindle assembly, mediating microtubule crosslinking and spindle elongation in the midzone. Ase1/PRC1/MAP65 serves as a complex regulatory platform for the recruitment of other midzone proteins at the spindle midzone. Herein, we summarize recent advances in understanding of the structural domains and molecular kinetics of the Ase1/PRC1/MAP65 family. We summarize the regulatory network involved in post-translational modifications of Ase1/PRC1 by cyclin-dependent kinase 1 (Cdk1), cell division cycle 14 (Cdc14) and Polo-like kinase 1 (Plk1) and also highlight multiple functions of Ase1/PRC1 in central spindle organization, spindle elongation and cytokinesis during cell division.

Phosphorylation of Plant Microtubule-Associated Proteins During Cell Division

Progression of mitosis and cytokinesis depends on the reorganization of cytoskeleton, with microtubules driving the segregation of chromosomes and their partitioning to two daughter cells. In dividing plant cells, microtubules undergo global reorganization throughout mitosis and cytokinesis, and with the aid of various microtubule-associated proteins (MAPs), they form unique systems such as the preprophase band (PPB), the acentrosomal mitotic spindle, and the phragmoplast. Such proteins include nucleators of de novo microtubule formation, plus end binding proteins involved in the regulation of microtubule dynamics, crosslinking proteins underlying microtubule bundle formation and members of the kinesin superfamily with microtubule-dependent motor activities. The coordinated function of such proteins not only drives the continuous remodeling of microtubules during mitosis and cytokinesis but also assists the positioning of the PPB, the mitotic spindle, and the phragmoplast, affecting tissue patterning by controlling cell division plane (CDP) orientation. The affinity and the function of such proteins is variably regulated by reversible phosphorylation of serine and threonine residues within the microtubule binding domain through a number of protein kinases and phosphatases which are differentially involved throughout cell division. The purpose of the present review is to provide an overview of the function of protein kinases and protein phosphatases involved in cell division regulation and to identify cytoskeletal substrates relevant to the progression of mitosis and cytokinesis and the regulation of CDP orientation.

Modification of growth anisotropy and cortical microtubule dynamics in Arabidopsis hypocotyls grown under microgravity conditions in space

We carried out a space experiment, denoted as Aniso Tubule, to examine the effects of microgravity on the growth anisotropy and cortical microtubule dynamics in Arabidopsis hypocotyls, using lines in which microtubules are visualized by labeling tubulin or microtubule-associated proteins (MAPs) with green fluorescent protein (GFP). In all lines, GFP-tubulin6 (TUB6)-, basic proline-rich protein1 (BPP1)-GFP- and spira1-like3 (SP1L3)-GFP-expressing using a constitutive promoter, and spiral2 (SPR2)-GFP- and GFP-65 kDa MAP-1 (MAP65-1)-expressing using a native promoter, the length of hypocotyls grown under microgravity conditions in space was longer than that grown at 1 g conditions on the ground. In contrast, the diameter of hypocotyls grown under microgravity conditions was smaller than that of the hypocotyls grown at 1 g. The percentage of cells with transverse microtubules was increased under microgravity conditions, irrespective of the lines. Also, the average angle of the microtubules with respect to the transverse cell axis was decreased in hypocotyls grown under microgravity conditions. When GFP fluorescence was quantified in hypocotyls of GFP-MAP65-1 and SPR2-GFP lines, microgravity increased the levels of MAP65-1, which appears to be involved in the maintenance of transverse microtubule orientation. However, the levels of SPR2 under microgravity conditions were comparable to those at 1 g. These results suggest that the microgravity-induced increase in the levels of MAP65-1 is involved in increase in the transverse microtubules, which may lead to modification of growth anisotropy, thereby developing longer and thinner hypocotyls under microgravity conditions in space.

Arabidopsis MAP65-4 plays a role in phragmoplast microtubule organization and marks the cortical cell division site

The evolutionarily conserved MAP65 family proteins bundle anti-parallel microtubules (MTs). In Arabidopsis thaliana, mutations in the MAP65-3 gene lead to serious defects in MT organization in the phragmoplast and cause failures in cytokinesis. However, the functions of other ArabidopsisMAP65 isoforms are largely unknown. MAP65 functions were analyzed based on genetic interactions among different map65 mutations. Live-cell imaging and immunolocalization experiments revealed dynamic activities of two closely related MAP65 proteins in dividing cells. The map65-4 mutation caused synthetic lethality with map65-3 although map65-4 alone did not cause a noticeable phenotype. Furthermore, the introduction of an extra copy of the MAP65-4 gene significantly suppressed defects in cytokinesis and seedling growth caused by map65-3 because of restoring MT engagement in the spindle midzone. During mitosis, MAP65-4 first appeared at the preprophase band and persisted at the cortical division site afterwards. It was also concentrated on MTs in the spindle midzone and the phragmoplast. In the absence of MAP65-3, MAP65-4 exhibited greatly enhanced localization in the midzone of developing phragmoplast. Therefore, we have uncovered redundant but differential contributions of MAP65-3 and MAP65-4 to engaging and bundling anti-parallel MTs in the phragmoplast and disclosed a novel action of MAP65-4 at the cortical cell division site.

Proteins immunologically related to MAP65-1 accumulate and localize differentially during bud development in Vitis vinifera L

Various arrays of microtubules are present throughout the plant cell cycle and are involved in distinct functions. Microtubule-associated proteins (MAPs) regulate microtubule dynamics by acting as stabilizers, destabilizers, and promoters of microtubule dynamics. The MAP65 family is a specific group of cross-linkers required for structural maintenance of microtubules. In plants, different isoforms of MAP65 are differentially expressed according to their developmental program. In this work, we analyzed the differential distribution of proteins immunologically related to MAP65-1 during bud development in grapevine (Vitis vinifera L.). First, we annotated the MAP65 genes present in the Vitis genome in order to compare the number and sequence of genes to other species. Subsequently, we focused on a specific isoform (MAP65-1) by characterizing its accumulation and distribution. Proteins were extracted from different organs of Vitis (buds, leaves, flowers, and tendrils), were separated by two-dimensional electrophoresis (2-DE), and were probed by immunoblot with a specific antiserum. We found seven spots immunologically related to MAP65-1, grouped in two distinct clusters, which accumulate differentially according to the developmental stage. In addition, we analyzed the localization of MAP65-1 during three different stages of bud development. Implication of data on the use of different isotypes of MAP65-1 during Vitis development is discussed.

Cell Biology: Narrowing the Great Divide

Construction of the cell plate during plant cell division requires the precise insertion of materials around the circumferentially growing phragmoplast. New work shows that two kinesin-4 motor proteins act to shorten the domain of overlapping microtubules at the phragmoplast perimeter, limiting the site of material deposition.

Isolation of Microtubules and Microtubule-Associated Proteins

Microtubules are essential cellular structures in plant cells. They are polymerized from tubulin dimers and are regulated by microtubule-associated proteins (MAPs). Here, we describe a protocol for purifying tubulin dimers and MAPs from plant cells. The protocol involves preparing vacuole-free mini-protoplasts, a high quality cytoplasmic extract, cycles of microtubule polymerization and depolymerization to increase tubulin and MAP concentration, separation of tubulin and MAPs by column chromatography. We also present tubulin purification methods for biochemical assays.

Genome-wide analysis of the TPX2 family proteins in Eucalyptus grandis

BACKGROUND

The Xklp2 (TPX2) proteins belong to the microtubule-associated (MAP) family of proteins. All members of the family contain the conserved TPX2 motif, which can interact with microtubules, regulate microtubule dynamics or assist with different microtubule functions, for example, maintenance of cell morphology or regulation of cell growth and development. However, the role of members of the TPX family have not been studied in the model tree species Eucalyptus to date. Here, we report the identification of the members of the TPX2 family in Eucalyptus grandis (Eg) and analyse the expression patterns and functions of these genes.

RESULTS

In present study, a comprehensive analysis of the plant TPX2 family proteins was performed. Phylogenetic analyses indicated that the genes can be classified into 6 distinct subfamilies. A genome-wide survey identified 12 members of the TPX2 family in the sequenced genome of Eucalyptus grandis. The basic genetic properties of the TPX2 family in Eucalyptus were analysed. Our results suggest that the TPX2 family proteins within different sub-groups are relatively conserved but there are important differences between groups. Quantitative real-time PCR (qRT-PCR) was performed to confirm the expression levels of the genes in different tissues. The results showed that in the whole plant, the levels of EgWDL5 transcript are the highest, followed by those of EgWDL4. Compared with other tissues, the level of the EgMAP20 transcript is the highest in the root. Over-expression of EgMAP20 in Arabidopsis resulted in organ twisting. The cotyledon petioles showed left-handed twisting while the hypocotyl epidermal cells produced right-handed helical twisting. Finally, EgMAP20, EgWDL3 and EgWDL3L were all able to decorate microtubules.

CONCLUSIONS

Plant TPX2 family proteins were systematically analysed using bioinformatics methods. There are 12 TPX2 family proteins in Eucalyptus. We have performed an initial characterization of the functions of several members of the TPX2 family. We found that the gene products are localized to the microtubule cytoskeleton. Our results lay the foundation for future efforts to reveal the biological significance of TPX2 family proteins in Eucalyptus.

Multifunctional Microtubule-Associated Proteins in Plants

Microtubules (MTs) are involved in key processes in plant cells, including cell division, growth and development. MT-interacting proteins modulate MT dynamics and organization, mediating functional and structural interaction of MTs with other cell structures. In addition to conventional microtubule-associated proteins (MAPs) in plants, there are many other MT-binding proteins whose primary function is not related to the regulation of MTs. This review focuses on enzymes, chaperones, or proteins primarily involved in other processes that also bind to MTs. The MT-binding activity of these multifunctional MAPs is often performed only under specific environmental or physiological conditions, or they bind to MTs only as components of a larger MT-binding protein complex. The involvement of multifunctional MAPs in these interactions may underlie physiological and morphogenetic events, e.g., under specific environmental or developmental conditions. Uncovering MT-binding activity of these proteins, although challenging, may contribute to understanding of the novel functions of the MT cytoskeleton in plant biological processes.

ABA Affects Brassinosteroid-Induced Antioxidant Defense via ZmMAP65-1a in Maize Plants

Brassinosteroids (BRs) and ABA co-ordinately regulate water deficit tolerance in maize leaves. ZmMAP65-1a, a maize microtubule-associated protein (MAP) which plays an essential role in BR-induced antioxidant defense, has been characterized previously. However, the interactions among BR, ABA and ZmMAP65-1a in water deficit tolerance remain unexplored. In this study, we demonstrated that ABA was required for BR-induced antioxidant defense via ZmMAP65-1a by using biochemical blocking and ABA biosynthetic mutants. The expression of ZmMAP65-1a in maize leaves and mesophyll protoplasts could be increased under polyethylene glycol- (PEG) stimulated water deficit and ABA treatments. Furthermore, the importance of ABA in the early pathway of BR-induced water deficit tolerance was demonstrated by limiting ABA availability. Blocking ABA biosynthesis biochemically or by a null mutation inhibited the downstream gene expression of ZmMAP65-1a and the activity of ZmMAPK5 in the pathway. It also affected the activities of BR-induced antioxidant defense-related enzymes, namely ascorbate peroxidase (APX), catalase (CAT), glutathione reductase (GR), superoxide dismutase (SOD) and NADPH oxidase. In addition, combining results from transiently overexpressed or silenced ZmMAP65-1a in mesophyll protoplasts, we discovered that ZmMAP65-1a mediated the ABA-induced gene expression and activities of APX and SOD. Surprisingly, silencing of ZmMAP65-1a in mesophyll protoplasts did not alter the gene expression of ZmCCaMK and vice versa in response to ABA. Taken together, our data indicate that water deficit-induced ABA is a key mediator in BR-induced antioxidant defense via ZmMAP65-1a in maize.

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.

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.

Microtubule organization and microtubule-associated proteins in plant cells

Plants have unique microtubule (MT) arrays, cortical MTs, preprophase band, mitotic spindle, and phragmoplast, in the processes of evolution. These MT arrays control the directions of cell division and expansion especially in plants and are essential for plant morphogenesis and developments. Organizations and functions of these MT arrays are accomplished by diverse MT-associated proteins (MAPs). This review introduces 10 of conserved MAPs in eukaryote such as γ-TuC, augmin, katanin, kinesin, EB1, CLASP, MOR1/MAP215, MAP65, TPX2, formin, and several plant-specific MAPs such as CSI1, SPR2, MAP70, WVD2/WDL, RIP/MIDD, SPR1, MAP18/PCaP, EDE1, and MAP190. Most of the studies cited in this review have been analyzed in the particular model plant, Arabidopsis thaliana. The significant knowledge of A. thaliana is the important established base to understand MT organizations and functions in plants.

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.

Purification and characterization of novel microtubule-associated proteins from Arabidopsis cell suspension cultures

Plant microtubules (MTs) play essential roles in cell division, anisotropic cell expansion, and overall organ morphology. Microtubule-associated proteins (MAPs) bind to MTs and regulate their dynamics, stability, and organization. Identifying the full set of MAPs in plants would greatly enhance our understanding of how diverse MT arrays are formed and function; however, few proteomics studies have characterized plant MAPs. Using liquid chromatography-tandem mass spectrometry, we identified hundreds of proteins from MAP-enriched preparations derived from cell suspension cultures of Arabidopsis (Arabidopsis thaliana). Previously reported MAPs, MT regulators, kinesins, dynamins, peroxisome-resident enzymes, and proteins implicated in replication, transcription, and translation were highly enriched. Dozens of proteins of unknown function were identified, among which 12 were tagged with green fluorescent protein (GFP) and examined for their ability to colocalize with MTs when transiently expressed in plant cells. Six proteins did indeed colocalize with cortical MTs in planta. We further characterized one of these MAPs, designated as BASIC PROLINE-RICH PROTEIN1 (BPP1), which belongs to a seven-member family in Arabidopsis. BPP1-GFP decorated interphase and mitotic MT arrays in transgenic Arabidopsis plants. A highly basic, conserved region was responsible for the in vivo MT association. Overexpression of BPP1-GFP stabilized MTs, caused right-handed helical growth in rapidly elongating tissues, promoted the formation of transverse MT arrays, and resulted in the outgrowth of epidermal cells in light-grown hypocotyls. Our high-quality proteome database of Arabidopsis MAP-enriched preparations is a useful resource for identifying novel MT regulators and evaluating potential MT associations of proteins known to have other cellular functions.

Resistance of plants to gravitational force

Developing resistance to gravitational force is a critical response for terrestrial plants to survive under 1 × g conditions. We have termed this reaction "gravity resistance" and have analyzed its nature and mechanisms using hypergravity conditions produced by centrifugation and microgravity conditions in space. Our results indicate that plants develop a short and thick body and increase cell wall rigidity to resist gravitational force. The modification of body shape is brought about by the rapid reorientation of cortical microtubules that is caused by the action of microtubule-associated proteins in response to the magnitude of the gravitational force. The modification of cell wall rigidity is regulated by changes in cell wall metabolism that are caused by alterations in the levels of cell wall enzymes and in the pH of apoplastic fluid (cell wall fluid). Mechanoreceptors on the plasma membrane may be involved in the perception of the gravitational force. In this review, we discuss methods for altering gravitational conditions and describe the nature and mechanisms of gravity resistance in plants.

MAP65-1a positively regulates H2O2 amplification and enhances brassinosteroid-induced antioxidant defence in maize

Brassinosteroid (BR)-induced antioxidant defence has been shown to enhance stress tolerance. In this study, the role of the maize 65 kDa microtubule-associated protein (MAP65), ZmMAP65-1a, in BR-induced antioxidant defence was investigated. Treatment with BR increased the expression of ZmMAP65-1a in maize (Zea mays) leaves and mesophyll protoplasts. Transient expression and RNA interference silencing of ZmMAP65-1a in mesophyll protoplasts further revealed that ZmMAP65-1a is required for the BR-induced increase in expression and activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX). Both exogenous and BR-induced endogenous H2O2 increased the expression of ZmMAP65-1a. Conversely, transient expression of ZmMAP65-1a in maize mesophyll protoplasts enhanced BR-induced H2O2 accumulation, while transient silencing of ZmMAP65-1a blocked the BR-induced expression of NADPH oxidase genes and inhibited BR-induced H2O2 accumulation. Inhibiting the activity and gene expression of ZmMPK5 significantly prevented the BR-induced expression of ZmMAP65-1a. Likewise, transient expression of ZmMPK5 enhanced BR-induced activities of the antioxidant defence enzymes SOD and APX in a ZmMAP65- 1a-dependent manner. ZmMPK5 directly interacted with ZmMAP65-1a in vivo and phosphorylated ZmMAP65-1a in vitro. These results suggest that BR-induced antioxidant defence in maize operates through the interaction of ZmMPK5 with ZmMAP65-1a. Furthermore, ZmMAP65-1a functions in H2O2 self-propagation via regulation of the expression of NADPH oxidase genes in BR signalling.

Characterization and functional analysis of the potato pollen-specific microtubule-associated protein SBgLR in tobacco

Microtubule-associated proteins play a crucial role in the regulation of microtubule dynamics, and are very important for plant cell and organ development. SBgLR is a potato pollen-specific protein, with five imperfect V-V-E-K-K-N/E-E repetitive motifs that are responsible for microtubule binding activity. In present study, SBgLR showed typical microtubule-associated protein characteristics; it bound tubulin and microtubules, and colocalized with microtubules in vitro. We also found that SBgLR could form oligomers, and that both the SBgLR monomers and oligomers bundle microtubules in vitro. Constitutive expression of SBgLR in tobacco caused curving and right-handed twisting root growth, abnormal directional cell expansion and cell layer arrangement, and pollen abortion. Immunofluorescence staining assays revealed that microtubule organization is altered in root epidermal cells in SBgLR-overexpressing lines. These suggest that SBgLR functions as a microtubule-associated protein in pollen development. Our results indicate that normal organization of MTs may be crucial for pollen development.

Interaction of antiparallel microtubules in the phragmoplast is mediated by the microtubule-associated protein MAP65-3 in Arabidopsis

In plant cells, microtubules (MTs) in the cytokinetic apparatus phragmoplast exhibit an antiparallel array and transport Golgi-derived vesicles toward MT plus ends located at or near the division site. By transmission electron microscopy, we observed that certain antiparallel phragmoplast MTs overlapped and were bridged by electron-dense materials in Arabidopsis thaliana. Robust MT polymerization, reported by fluorescently tagged End Binding1c (EB1c), took place in the phragmoplast midline. The engagement of antiparallel MTs in the central spindle and phragmoplast was largely abolished in mutant cells lacking the MT-associated protein, MAP65-3. We found that endogenous MAP65-3 was selectively detected on the middle segments of the central spindle MTs at late anaphase. When MTs exhibited a bipolar appearance with their plus ends placed in the middle, MAP65-3 exclusively decorated the phragmoplast midline. A bacterially expressed MAP65-3 protein was able to establish the interdigitation of MTs in vitro. MAP65-3 interacted with antiparallel microtubules before motor Kinesin-12 did during the establishment of the phragmoplast MT array. Thus, MAP65-3 selectively cross-linked interdigitating MTs (IMTs) to allow antiparallel MTs to be closely engaged in the phragmoplast. Although the presence of IMTs was not essential for vesicle trafficking, they were required for the phragmoplast-specific motors Kinesin-12 and Phragmoplast-Associated Kinesin-Related Protein2 to interact with MT plus ends. In conclusion, we suggest that the phragmoplast contains IMTs and highly dynamic noninterdigitating MTs, which work in concert to bring about cytokinesis in plant cells.

TMBP200, a XMAP215 homologue of tobacco BY-2 cells, has an essential role in plant mitosis

TMBP200 from tobacco BY-2 cells is a member of the highly conserved family of microtubule-associated proteins that includes Xenopus XMAP215, human TOGp, and Arabidopsis MOR1/GEM1. XMAP215 homologues have an essential role in spindle assembly and function in animals and yeast, but their role in plant mitosis is not fully clarified. Here, we show by immunoblot analysis that TMBP200 levels in synchronously cultured BY-2 cells increased when the cells entered mitosis, thus indicating that TMBP200 plays an important role in mitosis in tobacco. To investigate the role of TMBP200 in mitosis, we employed inducible RNA interference to silence TMBP200 expression in BY-2 cells. The resulting depletion of TMBP200 caused severe defects in bipolar spindle formation and resulted in the appearance of multinucleated cells with variable-sized nuclei. This finding indicates that TMBP200 has an essential role in bipolar spindle formation and function.

Microtubule-associated proteins MAP65-1 and MAP65-2 positively regulate axial cell growth in etiolated Arabidopsis hypocotyls

The Arabidopsis thaliana MAP65-1 and MAP65-2 genes are members of the larger eukaryotic MAP65/ASE1/PRC gene family of microtubule-associated proteins. We created fluorescent protein fusions driven by native promoters that colocalized MAP65-1 and MAP65-2 to a subset of interphase microtubule bundles in all epidermal hypocotyl cells. MAP65-1 and MAP65-2 labeling was highly dynamic within microtubule bundles, showing episodes of linear extension and retraction coincident with microtubule growth and shortening. Dynamic colocalization of MAP65-1/2 with polymerizing microtubules provides in vivo evidence that plant cortical microtubules bundle through a microtubule-microtubule templating mechanism. Analysis of etiolated hypocotyl length in map65-1 and map65-2 mutants revealed a critical role for MAP65-2 in modulating axial cell growth. Double map65-1 map65-2 mutants showed significant growth retardation with no obvious cell swelling, twisting, or morphological defects. Surprisingly, interphase microtubules formed coaligned arrays transverse to the plant growth axis in dark-grown and GA(4)-treated light-grown map65-1 map65-2 mutant plants. We conclude that MAP65-1 and MAP65-2 play a critical role in the microtubule-dependent mechanism for specifying axial cell growth in the expanding hypocotyl, independent of any mechanical role in microtubule array organization.

BENT UPPERMOST INTERNODE1 encodes the class II formin FH5 crucial for actin organization and rice development

The actin cytoskeleton is an important regulator of cell expansion and morphogenesis in plants. However, the molecular mechanisms linking the actin cytoskeleton to these processes remain largely unknown. Here, we report the functional analysis of rice (Oryza sativa) FH5/BENT UPPERMOST INTERNODE1 (BUI1), which encodes a formin-type actin nucleation factor and affects cell expansion and plant morphogenesis in rice. The bui1 mutant displayed pleiotropic phenotypes, including bent uppermost internode, dwarfism, wavy panicle rachis, and enhanced gravitropic response. Cytological observation indicated that the growth defects of bui1 were caused mainly by inhibition of cell expansion. Map-based cloning revealed that BUI1 encodes the class II formin FH5. FH5 contains a phosphatase tensin-like domain at its amino terminus and two highly conserved formin-homology domains, FH1 and FH2. In vitro biochemical analyses indicated that FH5 is capable of nucleating actin assembly from free or profilin-bound monomeric actin. FH5 also interacts with the barbed end of actin filaments and prevents the addition and loss of actin subunits from the same end. Interestingly, the FH2 domain of FH5 could bundle actin filaments directly and stabilize actin filaments in vitro. Consistent with these in vitro biochemical activities of FH5/BUI1, the amount of filamentous actin decreased, and the longitudinal actin cables almost disappeared in bui1 cells. The FH2 or FH1FH2 domains of FH5 could also bind to and bundle microtubules in vitro. Thus, our study identified a rice formin protein that regulates de novo actin nucleation and spatial organization of the actin filaments, which are important for proper cell expansion and rice morphogenesis.

Tobacco microtubule-associated protein, MAP65-1c, bundles and stabilizes microtubules

Three genes that encode MAP65-1 family proteins have been identified in the Nicotiana tabacum genome. In this study, NtMAP65-1c fusion protein was shown to bind and bundle microtubules (MTs). Further in vitro investigations demonstrated that NtMAP65-1c not only alters MT assembly and nucleation, but also exhibits high MT stabilizing activity against cold or katanin-induced destabilization. Analysis of NtMAP65-1c-GFP expressing BY-2 cells clearly demonstrated that NtMAP65-1c was able to bind to MTs during specific stages of the cell cycle. Furthermore, in vivo, NtMAP65-1c-GFP-bound cortical MTs displayed an increase in resistance against the MT-disrupting drug, propyzamide, as well as against cold temperatures. Taken together, these results strongly suggest that NtMAP65-1c stabilizes MTs and is involved in the regulation of MT organization and cellular dynamics.

MAP65 in tubulin/colchicine paracrystals of Vigna sinensis root cells: possible role in the assembly and stabilization of atypical tubulin polymers

Members of the MAP65 family, colocalizing with microtubule arrays, have been identified in Vigna sinensis root cells by Western blotting and immunofluorescence. MAP65 proteins were also found in tubulin/colchicine paracrystals, which were formed during colchicine treatment by both immunofluorescence and immunogold microscopy. During recovery from colchicine, MAP65 signal was depleted from disintegrating paracrystals appearing in the reinstating microtubule arrays. MAP65-free perinuclear tubulin/colchicine aggregates were observed in plasmolyzed colchicine-treated cells. Deplasmolysis of the above cells resulted in the formation of MAP65-decorated paracrystals. As confirmed by appropriate biochemical assays with the Phos-tag reagent, MAP65 proteins underwent phosphorylation during plasmolysis, which was reversible by deplasmolysis. According to the effect of the mitogen-activated protein kinase (MAPK) inhibitor UO126, the phosphorylation status of MAP65, as well as its presence in tubulin/colchicine polymers is probably controlled by MAPK-mediated phosphorylation. According to the above, it seems likely that apart from binding to microtubules, MAP65 proteins may act as "tubulin associated proteins" in a broader manner, promoting the polymerization and/or stabilization of atypical polymers such as tubulin/colchicine paracrystals.

A MAP for bundling microtubules

Microtubules assemble into arrays of bundled filaments that are critical for multiple steps in cell division, including anaphase and cytokinesis. Recent structural and functional studies, including two papers in this issue of Cell (Bieling et al., 2010; Subramanian et al., 2010), demonstrate how the MAP65 protein PRC1 crosslinks microtubules and cooperates with kinesin motors to control the dynamics and size of bundled regions.

Strategies of actin reorganisation in plant cells

Spatial-temporal flexibility of the actin filament network (F-actin) is essential for all basic cellular functions and is governed by a stochastic dynamic model. In this model, actin filaments that randomly polymerise from a pool of free actin are bundled with other filaments and severed by ADF/cofilin. The fate of the severed fragments is not known. It has been proposed that the fragments are disassembled and the monomeric actin recycled for the polymerisation of new filaments. Here, we have generated tobacco cell lines and Arabidopsis plants expressing the actin marker Lifeact to address the mechanisms of F-actin reorganisation in vivo. We found that F-actin is more dynamic in isotropically expanding cells and that the density of the network changes with a periodicity of 70 seconds. The depolymerisation rate, but not the polymerisation rate, of F-actin increases when microtubules are destabilised. New filaments can be assembled from shorter free cytoplasmic fragments, from the products of F-actin severing and by polymerisation from the ends of extant filaments. Thus, remodelling of F-actin might not require bulk depolymerisation of the entire network, but could occur via severing and end-joining of existing polymers.

The putative RNA-processing protein, THO2, is a microtubule-associated protein in tobacco

THO2 is a component of the THO-TREX (transcription and export factor) complex that participates in mRNA metabolism and export from the nucleus in yeast and animal cells. Here we report that tobacco putative THO2-related protein (NtTHO2) is a microtubule-associated protein, which directly binds to microtubules in vitro and co-localizes with cortical microtubules in vivo. We purified endogenous NtTHO2 by cycles of microtubule polymerization-depolymerization from crude extracts of tobacco BY-2 miniprotoplasts. Purified NtTHO2 sedimented with microtubules in vitro. Immunofluorescence revealed that NtTHO2 was present in both the nucleus and cytoplasm. In interphase, cytoplasmic NtTHO2 was localized along cortical microtubules. In the mitotic phase, NtTHO2 was localized to the mitotic spindle but not to either the preprophase band or the phragmoplast. In mature cells of seedling roots, and in BY-2 cells in which proliferation was stopped by removing 2,4-D, NtTHO2 staining was confined mainly to the nucleolus. These results suggest that NtTHO2 is a multifunctional protein that participates in mRNA metabolism, and also functions within the cortical microtubules and mitotic spindle.

Division plane control in plants: new players in the band

Unique mechanisms are used to orient cell division planes in plants. A cortical ring of cytoskeletal filaments called the preprophase band (PPB) predicts the future division plane during G2 and is disassembled as the mitotic spindle forms, leaving behind a 'cortical division site' (CDS) that guides the placement of the new cell wall (cell plate) during cytokinesis. The molecular features of the CDS have remained elusive for decades. Recently, a few proteins have at last been identified that are specifically localized to or excluded from the CDS and that participate in the orientation, attachment or maturation of cell plates. Recent progress has also been made in identifying proteins needed for PPB formation and thus for division plane establishment.

Arabidopsis microtubule-associated protein AtMAP65-2 acts as a microtubule stabilizer

Nine genes that encode proteins of the MAP65 family have been identified in the Arabidopsis thaliana genome. In this study, we reported that AtMAP65-2, a member of the AtMAP65 family, could strongly stabilize microtubules (MTs). Bacterially-expressed AtMAP65-2 fusion proteins induced the formation of large MT bundles in vitro. Although AtMAP65-2 showed little effect on MT assembly or nucleation, AtMAP65-2 greatly stabilized MTs that were subjected to low-temperature treatment in vitro. Analyses of truncated versions of AtMAP65-2 indicated that the region that encompassed amino acids 495-578, which formed a flexible extended loop, played a crucial role in the stabilization of MTs. Analysis of suspension-cultured Arabidopsis cells that expressed the AtMAP65-2-GFP fusion protein showed that AtMAP65-2 co-localized with MTs throughout the cell cycle. Cortical MTs that were decorated with AtMAP65-2-GFP were more resistant to the MT-disrupting drug propyzamide and to ice treatment in vivo. The results of this study demonstrate that AtMAP65-2 strongly stabilizes MTs and is involved in the regulation of MT organization and dynamics.

Evaluating the microtubule cytoskeleton and its interacting proteins in monocots by mining the rice genome

BACKGROUND

Microtubules (MTs) are assembled by heterodimers of alpha- and beta-tubulins, which provide tracks for directional transport and frameworks for the spindle apparatus and the phragmoplast. MT nucleation and dynamics are regulated by components such as the gamma-tubulin complex which are conserved among eukaryotes, and other components which are unique to plants. Following remarkable progress made in the model plant Arabidopsis thaliana toward revealing key components regulating MT activities, the completed rice (Oryza sativa) genome has prompted a survey of the MT cytoskeleton in this important crop as a model for monocots.

SCOPE

The rice genome contains three alpha-tubulin genes, eight beta-tubulin genes and a single gamma-tubulin gene. A functional gamma-tubulin ring complex is expected to form in rice as genes encoding all components of the complex are present. Among proteins that interact with MTs, compared with A. thaliana, rice has more genes encoding some members such as the MAP65/Ase1p/PRC1 family, but fewer for the motor kinesins, the end-binding protein EB1 and the mitotic kinase Aurora. Although most known MT-interacting factors have apparent orthologues in rice, no orthologues of arabidopsis RIC1 and MAP18 have been identified in rice. Among all proteins surveyed here, only a few have had their functions characterized by genetic means in rice. Elucidating functions of proteins of the rice MT cytoskeleton, aided by recent technical advances made in this model monocot, will greatly advance our knowledge of how monocots employ their MTs to regulate their growth and form.

The C-terminal variable region specifies the dynamic properties of Arabidopsis microtubule-associated protein MAP65 isotypes

The microtubule-associated protein, MAP65, is a member of a family of divergent microtubule-associated proteins from different organisms generally involved in maintaining the integrity of the central spindle in mitosis. The dicotyledon Arabidopsis thaliana and the monocotyledon rice (Oryza sativa) genomes contain 9 and 11 MAP65 genes, respectively. In this work, we show that the majority of these proteins fall into five phylogenetic clades, with the greatest variation between clades being in the C-terminal random coil domain. At least one Arabidopsis and one rice isotype is within each clade, indicating a functional specification for the C terminus. In At MAP65-1, the C-terminal domain is a microtubule binding region (MTB2) harboring the phosphorylation sites that control its activity. The At MAP65 isotypes show differential localization to microtubule arrays and promote microtubule polymerization with variable efficiency in a MTB2-dependent manner. In vivo studies demonstrate that the dynamics of the association and dissociation of different MAP65 isotypes with microtubules can vary up to 10-fold and that this correlates with their ability to promote microtubule polymerization. Our data demonstrate that the C-terminal variable region, MTB2, determines the dynamic properties of individual isotypes and suggest that slower turnover is conditional for more efficient microtubule polymerization.

MAP65-3 microtubule-associated protein is essential for nematode-induced giant cell ontogenesis in Arabidopsis

The infection of plants by obligate parasitic nematodes constitutes an interesting model for investigating plant cytoskeleton functions. Root knot nematodes have evolved the ability to manipulate host functions to their own advantage by redifferentiating root cells into multinucleate and hypertrophied feeding cells. These giant cells result from repeated rounds of karyokinesis without cell division. Detailed functional analyses demonstrated that Arabidopsis thaliana Microtubule-Associated Protein65-3 (MAP65-3) was essential for giant cell ontogenesis and that cytokinesis was initiated but not completed in giant cells. In developing giant cells, MAP65-3 was associated with a novel kind of cell plate-the giant cell mini cell plate-that separates daughter nuclei. In the absence of functional MAP65-3, giant cells developed but failed to fully differentiate and were eventually destroyed. These defects in giant cells impaired the maturation of nematode larvae. Thus, MAP65-3 is essential for giant cell development during root knot nematode infection. Subcellular localization of MAP65-3 and analysis of microtubule organization in the dyc283 T-DNA map65-3 mutant demonstrated that MAP65-3 played a critical role in organizing the mitotic microtubule array during both early and late mitosis in all plant organs. Here, we propose a model for the role of MAP65-3 in giant cell ontogenesis.

SB401, a pollen-specific protein from Solanum berthaultii, binds to and bundles microtubules and F-actin

We characterize a novel, pollen-specific, microtubule-associated protein, SB401, found in Solanum berthaultii. This protein binds to and bundles taxol-stabilized microtubules and enhances tubulin polymerization in a concentration-dependent manner, particularly at lower temperatures. Electron microscopy revealed that the protein decorates the entire length of microtubules. Cross-linking and electrophoresis studies showed that SB401 protein forms dimers, and suggest that dimerization could account for bundling. Double immunofluorescent staining of pollen tubes of S. berthaultii showed that SB401 protein co-localized with cortical microtubule bundles. SB401 protein also binds to and bundles actin filaments, and could connect actin filaments to microtubules. SB401 protein had a much higher affinity for microtubules than for actin filaments. In the presence of both cytoskeletal elements, the protein preferentially bound microtubules to form bundles. These results demonstrate that SB401 protein may have important roles in organizing the cytoskeleton in pollen tubes.

The AtMAP65-1 Cross-Bridge Between Microtubules is Formed by One Dimer

The microtubule-associated protein AtMAP65-1 from Arabidopsis thaliana dimerizes and forms 25 nm cross-bridges between microtubules, but the exact mechanism is unknown. Here, we used the predicted three-dimensional structure of AtMAP65-1 as a basis for analyzing the actual cross-bridging in detail. Fold-recognition predicts that AtMAP65-1 contains four coiled-coil domains and a flexible extended loop. The length of these coiled-coil domains is about 25 nm, suggesting that one molecule could span the gap, hence forming an antiparallel overlapping dimer instead of an end-to-end dimer. We then tested this model by using truncations of AtMAP65-1. EDC {[3-(dimethylamino) propyl] carbodiimide} cross-linking analysis indicated that the N-terminus of the rod domain of AtMAP65-1 (amino acids 1-339) binds to the C-terminus of the rod domain (amino acids 340-494) and also participates in connecting the two antiparallel proteins in the cross-bridge. Nevertheless, microtubules can still form bundles in the presence of AtMAP65-1 340-587 (amino acids 340-587) or AtMAP65-1 1-494 (amino acids 1-494). Comparing the cold stability of microtubule bundles induced by full-length AtMAP65-1 with that of AtMAP65-1 340-587 or AtMAP65-1 1-494, we conclude that AtMAP65-1 495-587 acts as a flexible extended loop, playing a crucial role in binding to and stabilizing microtubules in the AtMAP65-1 cross-bridge.

MAP65: a bridge linking a MAP kinase to microtubule turnover

After the segregation of chromosomes, animal and plant cells build a central spindle (midbody) and a phragmoplast, respectively, that are mainly composed of aligned microtubules and microfilaments. These microtubule-based structures are highly dynamic and play an essential role in cytokinesis. Recent studies using model organisms have shed light on the involvement of common molecules in the regulatory mechanisms of cytokinesis, including microtubule dynamics, in a variety of species. Among these molecules, members of the MAP65 protein family, a microtubule-associated protein family, appear to be key regulators of both the maintenance and dynamics of central spindles and phragmoplasts.

Microtubule organization in three-dimensional confined geometries: evaluating the role of elasticity through a combined in vitro and modeling approach

Microtubules or microtubule bundles in cells often grow longer than the size of the cell, which causes their shape and organization to adapt to constraints imposed by the cell geometry. We test the reciprocal role of elasticity and confinement in the organization of growing microtubules in a confining box-like geometry, in the absence of other (active) microtubule organizing processes. This is inspired, for example, by the cortical microtubule array of elongating plant cells, where microtubules are typically organized in an aligned array transverse to the cell elongation axis. The method we adopt is a combination of analytical calculations, in which the polymers are modeled as inextensible filaments with bending elasticity confined to a two-dimensional surface that defines the limits of a three-dimensional space, and in vitro experiments, in which microtubules are polymerized from nucleation seeds in microfabricated chambers. We show that these features are sufficient to organize the polymers in aligned, coiling configurations as for example observed in plant cells. Though elasticity can account for the regularity of these arrays, it cannot account for a transverse orientation of microtubules to the cell's long axis. We therefore conclude that an additional active, force-generating process is necessary to create a coiling configuration perpendicular to the long axis of the cell.

Phosphorylation of NtMAP65-1 by a MAP kinase down-regulates its activity of microtubule bundling and stimulates progression of cytokinesis of tobacco cells

The tobacco mitogen-activated protein kinase (MAPK) cascade, which includes MAPK NRK1/NTF6, positively regulates expansion of the cytokinetic machinery known as the phragmoplast, which is followed by the synthesis of cell plates for completion of cell division. However, molecular events lying between the MAPK and phragmoplast expansion were not known. Here, we show that NRK1/NTF6 phosphorylates the threonine residue at position 579 in NtMAP65-1a, a microtubule-associated (MT-associated) protein. Levels of phosphorylated NtMAP65-1 increase during late M phase of the cell cycle, when NRK1/NTF6 is activated. Phosphorylated NtMAP65-1 is concentrated at the equator of phragmoplast, as is NRK1/NTF6. Overexpression of mutant forms of NtMAP65-1a that cannot be phosphorylated by NRK1 delays progression of the M phase and phragmoplast expansion, also rendering phragmoplast structures resistant to an MT-depolymerizing drug. Phosphorylation of NtMAP65-1 by NRK1/NTF6 down-regulates its MT-bundling activity in vitro. These results suggest that phosphorylation of NtMAP65-1 by NRK1/NTF6 also reduces its MT-bundling activity in vivo, which enhances destabilization and turnover of MTs at the phragmoplast equator, perhaps facilitating phragmoplast expansion.

Not so divided: the common basis of plant and animal cell division

Plant cells do not have centrioles and their mitosis is frequently likened to the chromosome-based mechanism seen in acentriolar animal cells. However, this is a false analogy. Although plants can use this mechanism, they generally divide by a method that uses bipolar mitotic caps, which is more similar to the canonical centrosome-based method of animals.

Modulated targeting of GFP-AtMAP65-1 to central spindle microtubules during division

AtMAP65-1 bundles cortical microtubules and we examined how this property is regulated during division in time-lapse studies of Arabidopsis suspension cells expressing GFP-AtMAP65-1. Spindle fluorescence is diffuse during metaphase, restored to the central spindle at anaphase and then compacted at the midline during late anaphase/early telophase. However, mutagenesis of the microtubule-associated protein (MAP) consensus Cdk site to a non-phosphorylatable form allows premature decoration of microtubules traversing the central region of the metaphase spindle without affecting the timing of the subsequent compaction. This suggests that mutagenesis does not affect compaction but does affect a phosphorylation/dephosphorylation switch that normally targets AtMAP65-1 to the central spindle at the metaphase/anaphase transition. GFP-AtMAP65-1 continues to label the midline of the early phragmoplast, suggesting a structural continuity with the central spindle - both structures being composed of anti-parallel microtubules. However, once the cytokinetic apparatus expands into a ring the MAP becomes depleted at the midline. Despite this, cytokinesis is not arrested and membrane and callose are deposited at the cell plate. It is concluded that AtMAP65-1 plays a role in the central spindle at anaphase to early cytokinesis but is not essential at the midline of the phragmoplast at later stages.