Interactions of tobacco microtubule-associated protein MAP65-1b with microtubules

A possible pattern in the evolution of male meiotic cytokinesis in angiosperms

Evolution of cellular characteristics is a fundamental aspect of evolutionary biology, but knowledge about evolution at the cellular level is very limited. In particular, whether a certain intracellular characteristic evolved in angiosperms, and what significance of such evolution is to angiosperms, if it exists, are important and yet unanswered questions. We have found that bidirectional cytokinesis occurs or likely occurs in male meiosis in extant basal and near-basal angiosperm lineages, which differs from the unidirectional cytokinesis in male meiosis in monocots and eudicots. This pattern of cytokinesis in angiosperms seems to align with the distribution pattern of angiosperms with the lineages basal to monocots and eudicots living in tropical, subtropical or temperate environments and monocots and eudicots in an expanded range of environments including tropical, subtropical, temperate, subarctic and arctic environments. These two cytokinetic modes seem to result from two phragmoplast types, respectively. A phragmoplast in the bidirectional cytokinesis dynamically associates with the leading edge of a growing cell plate whereas a phragmoplast in the unidirectional cytokinesis is localized to an entire division plane. The large assembly of microtubules in the phragmoplast in unidirectional cytokinesis may be indicative of increased microtubule stability compared with that of the small microtubule assembly in the phragmoplast in bidirectional cytokinesis. Microtubules could conceivably increase their stability from evolutionary changes in tubulins and/or microtubule-associated proteins. Microtubules are very sensitive to low temperatures, which should be a reason for plants to be sensitive to low temperatures. If monocots and eudicots have more stable microtubules than other angiosperms, they will be expected to deal with low temperatures better than other angiosperms. Future investigations into the male meiotic cytokinetic directions, microtubule stability at low temperatures, and proteins affecting microtubule stability in more species may shed light on how plants evolved to inhabit cold environments.

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 plant cytoskeleton takes center stage in abiotic stress responses and resilience

Stress resilience behaviours in plants are defensive mechanisms that develop under adverse environmental conditions to promote growth, development and yield. Over the past decades, improving stress resilience, especially in crop species, has been a focus of intense research for global food security and economic growth. Plants have evolved specific mechanisms to sense external stress and transmit information to the cell interior and generate appropriate responses. Plant cytoskeleton, comprising microtubules and actin filaments, takes a center stage in stress-induced signalling pathways, either as a direct target or as a signal transducer. In the past few years, it has become apparent that the function of the plant cytoskeleton and other associated proteins are not merely limited to elementary processes of cell growth and proliferation, but they also function in stress response and resilience. This review summarizes recent advances in the role of plant cytoskeleton and associated proteins in abiotic stress management. We provide a thorough overview of the mechanisms that plant cells employ to withstand different abiotic stimuli such as hypersalinity, dehydration, high temperature and cold, among others. We also discuss the crucial role of the plant cytoskeleton in organellar positioning under the influence of high light intensity.

A rice tubulin tyrosine ligase like 12 regulates phospholipase D activity and tubulin synthesis

All plant α-tubulins encode a C-terminal tyrosine. An elusive tubulin tyrosine carboxypeptidase can cleave off, and a tubulin tyrosine ligase (TTL) re-ligate this tyrosine. The biological function of this cycle remains unclear but may correlate with microtubule stability. To get insight into the functional context of this phenomenon, we used cold-induced elimination of microtubules as experimental model. In previous work, we had analysed a rice TTL-like 12 (OsTTLL12), the only potential candidate of plant TTL. To follow the effect of OsTTLL12 upon microtubule responses in vivo, we expressed OsTTLL12-RFP into tobacco BY-2 cells stably overexpressing NtTUA3-GFP. We found that overexpression of OsTTLL12-RFP made microtubules disappear faster in response to cold stress, accompanied with more rapid Ca²⁺ influx, culminating in reduced cold tolerance. Treatment with different butanols indicated that α-tubulin detyrosination/tyrosination differently interacts with phospholipase D (PLD) dependent signalling. In fact, rice PLDα1 decorated microtubules and increased detyrosinated α-tubulin. Unexpectedly, overexpression of the two proteins (OsTTLL12-RFP, NtTUA3-GFP) mutually regulated the accumulation of their transcripts, leading us to a model, where tubulin detyrosination feeds back upon tubulin transcripts and defines a subset of microtubules for interaction with PLD dependent stress signalling.

Phragmoplast microtubule dynamics - a game of zones

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

A Mechanism for Sustained Cellulose Synthesis during Salt Stress

Abiotic stress, such as salinity, drought, and cold, causes detrimental yield losses for all major plant crop species. Understanding mechanisms that improve plants' ability to produce biomass, which largely is constituted by the plant cell wall, is therefore of upmost importance for agricultural activities. Cellulose is a principal component of the cell wall and is synthesized by microtubule-guided cellulose synthase enzymes at the plasma membrane. Here, we identified two components of the cellulose synthase complex, which we call companion of cellulose synthase (CC) proteins. The cytoplasmic tails of these membrane proteins bind to microtubules and promote microtubule dynamics. This activity supports microtubule organization, cellulose synthase localization at the plasma membrane, and renders seedlings less sensitive to stress. Our findings offer a mechanistic model for how two molecular components, the CC proteins, sustain microtubule organization and cellulose synthase localization and thus aid plant biomass production during salt stress. VIDEO ABSTRACT.

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.

Lessons from in vitro reconstitution analyses of plant microtubule-associated proteins

Plant microtubules, composed of tubulin GTPase, are irreplaceable cellular components that regulate the directions of cell expansion and cell division, chromosome segregation and cell plate formation. To accomplish these functions, plant cells organize microtubule structures by regulating microtubule dynamics. Each microtubule localizes to the proper position with repeated growth and shortening. Although it is possible to reconstitute microtubule dynamics with pure tubulin solution in vitro, many microtubule-associated proteins (MAPs) govern microtubule dynamics in cells. In plants, major MAPs are identified as microtubule stabilizers (CLASP and MAP65 etc.), microtubule destabilizers (kinesin-13, katanin, MAP18 and MDP25), and microtubule dynamics promoters (EB1, MAP215, MOR1, MAP200, SPR2). Mutant analyses with forward and reverse genetics have shown the importance of microtubules and individual MAPs in plants. However, it is difficult to understand how each MAP regulates microtubule dynamics, such as growth and shortening, through mutant analyses. In vitro reconstitution analyses with individual purified MAPs and tubulin are powerful tools to reveal how each MAP regulates microtubule dynamics at the molecular level. In this review, I summarize the results of in vitro reconstitution analyses and introduce current models of how each MAP regulates microtubule dynamic instability.

Signaling Pathway that Controls Plant Cytokinesis

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

Microtubule organization by kinesin motors and microtubule crosslinking protein MAP65

Microtubules are rigid, proteinaceous filaments required to organize and rearrange the interior of cells. They organize space by two mechanisms, including acting as the tracks for long-distance cargo transporters, such as kinesin-1, and by forming a network that supports the shape of the cell. The microtubule network is composed of microtubules and a bevy of associated proteins and enzymes that self-organize using non-equilibrium dynamic processes. In order to address the effects of self-organization of microtubules, we have utilized the filament-gliding assay with kinesin-1 motors driving microtubule motion. To further enhance the complexity of the system and determine if new patterns are formed, we added the microtubule crosslinking protein MAP65-1. MAP65-1 is a microtubule-associated protein from plants that crosslinks antiparallel microtubules, similar to mammalian PRC1 and fission yeast Ase1. We find that MAP65 can slow and halt the velocity of microtubules in gliding assays, but when pre-formed microtubule bundles are added to gliding assays, kinesin-1 motors can pull apart the bundles and reconstitute cell-like protrusions.

MAP65 coordinate microtubule growth during bundle formation

Microtubules (MTs) are highly dynamical structures that play a crucial role in cell physiology. In cooperation with microtubule-associated proteins (MAPs), MTs form bundles endowing cells with specific mechanisms to control their shape or generate forces. Whether the dynamics of MTs is affected by the lateral connections that MAPs make between MTs during bundle formation is still under debate. Using in vitro reconstitution of MT bundling, we analyzed the dynamics of MT bundles generated by two plant MAP65 (MAP65-1/4), MAP65-1 being the plant ortholog of vertebrate PRC1 and yeast Ase1. MAP65-1/4 limit the amplitude of MT bundle depolymerization and increase the elongation phases. The subsequent sustained elongation of bundles is governed by the coordination of MT growth, so that MT ends come in close vicinity. We develop a model based on the assumption that both MAP65-1/4 block MT depolymerization. Model simulations reveal that rescue frequencies are higher between parallel than between anti-parallel MTs. In consequence the polarity of bundled MTs by MAP65 controls the amplitude of bundle's growth. Our results illustrate how MAP-induced MT-bundling, which is finely tuned by MT polarity, robustly coordinates MT elongation within bundles.

Single-molecule analysis of the microtubule cross-linking protein MAP65-1 reveals a molecular mechanism for contact-angle-dependent microtubule bundling

Bundling of microtubules (MTs) is critical for the formation of complex MT arrays. In land plants, the interphase cortical MTs form bundles specifically following shallow-angle encounters between them. To investigate how cells select particular MT contact angles for bundling, we used an in vitro reconstitution approach consisting of dynamic MTs and the MT-cross-linking protein MAP65-1. We found that MAP65-1 binds to MTs as monomers and inherently targets antiparallel MTs for bundling. Dwell-time analysis showed that the affinity of MAP65-1 for antiparallel overlapping MTs is about three times higher than its affinity for single MTs and parallel overlapping MTs. We also found that purified MAP65-1 exclusively selects shallow-angle MT encounters for bundling, indicating that this activity is an intrinsic property of MAP65-1. Reconstitution experiments with mutant MAP65-1 proteins with different numbers of spectrin repeats within the N-terminal rod domain showed that the length of the rod domain is a major determinant of the range of MT bundling angles. The length of the rod domain also determined the distance between MTs within a bundle. Together, our data show that the rod domain of MAP65-1 acts both as a spacer and as a structural element that specifies the MT encounter angles that are conducive for bundling.

Cellulose synthase interactive protein 1 (CSI1) links microtubules and cellulose synthase complexes

Cellulose synthase (CESA) complexes can be observed by live-cell imaging to move with trajectories that parallel the underlying cortical microtubules. Here we report that CESA interactive protein 1 (CSI1) is a microtubule-associated protein that bridges CESA complexes and cortical microtubules. Simultaneous in vivo imaging of CSI1, CESA complexes, and microtubules demonstrates that the association of CESA complexes and cortical microtubules is dependent on CSI1. CSI1 directly binds to microtubules as demonstrated by in vitro microtubule-binding assay.

A mechanochemical model explains interactions between cortical microtubules in plants

Microtubules anchored to the two-dimensional cortex of plant cells collide through plus-end polymerization. Collisions can result in rapid depolymerization, directional plus-end entrainment, or crossover. These interactions are believed to give rise to cellwide self-organization of plant cortical microtubules arrays, which is required for proper cell wall growth. Although the cell-wide self-organization has been well studied, less emphasis has been placed on explaining the interactions mechanistically from the molecular scale. Here we present a model for microtubule-cortex anchoring and collision-based interactions between microtubules, based on a competition between cross-linker bonding, microtubule bending, and microtubule polymerization. Our model predicts a higher probability of entrainment at smaller collision angles and at longer unanchored lengths of plus-ends. This model addresses observed differences between collision resolutions in various cell types, including Arabidopsis cells and Tobacco cells.

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.

Dynamic coordination of cytoskeletal and cell wall systems during plant cell morphogenesis

Underlying the architectural complexity of plants are diverse cell types that, under the microscope, easily reveal relationships between cell structure and specialized functions. Much less obvious are the mechanisms by which the cellular growth machinery and mechanical properties of the cell interact to dictate cell shape. The recent combined use of mutants, genomic analyses, sophisticated spectroscopies, and live cell imaging is providing new insight into how cytoskeletal systems and cell wall biosynthetic activities are integrated during morphogenesis. The purpose of this review is to discuss the unique geometric properties and physical processes that regulate plant cell expansion, then to overlay on this mechanical system some of the recent discoveries about the protein machines and cellular polymers that regulate cell shape. In the end, we hope to make clear that there are many interesting opportunities to develop testable mathematical models that improve our understanding of how subcellular structures, protein motors, and extracellular polymers can exert effects at spatial scales that span cells, tissues, and organs.

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.

Spatial organization of plant cortical microtubules: close encounters of the 2D kind

The shape of plant cells depends on cortical microtubules. Their freedom from central microtubule organizing centres provides a powerful experimental system to study microtubule self-organization. New ideas have emerged from live-cell imaging of microtubules, particularly in the model system Arabidopsis thaliana, revealing the importance of encounters between microtubules in driving self-organization. Encounters are modulated by intrinsic microtubule-assembly dynamics, along with polymer activities that include cortical attachment, bundling and severing. Balancing the activities of microtubule-associated proteins (such as MOR1, CLASP, MAP65s and katanins) that control these processes is crucial for fine-tuning the organization of microtubule arrays. Too much or too little of any given activity tips the balance, with often dramatic effects on array organization, cell morphogenesis and even organ chirality.

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.

CLASP modulates microtubule-cortex interaction during self-organization of acentrosomal microtubules

CLASP proteins associate with either the plus ends or sidewalls of microtubules depending on the subcellular location and cell type. In plant cells, CLASP's distribution along the full length of microtubules corresponds with the uniform anchorage of microtubules to the cell cortex. Using live cell imaging, we show here that loss of CLASP in Arabidopsis thaliana results in partial detachment of microtubules from the cortex. The detached portions undergo extensive waving, distortion, and changes in orientation, particularly when exposed to the forces of cytoplasmic streaming. These deviations from the normal linear polymerization trajectories increase the likelihood of intermicrotubule encounters that are favorable for subsequent bundle formation. Consistent with this, cortical microtubules in clasp-1 leaf epidermal cells are hyper-parallel. On the basis of these data, we identify a novel mechanism where modulation of CLASP activity governs microtubule-cortex attachment, thereby contributing to self-organization of cortical microtubules.

Two microtubule-associated proteins of Arabidopsis MAP65s promote antiparallel microtubule bundling

The Arabidopsis MAP65s are a protein family with similarity to the microtubule-associated proteins PRC1/Ase1p that accumulate in the spindle midzone during late anaphase in mammals and yeast, respectively. Here we investigate the molecular and functional properties of AtMAP65-5 and improve our understanding of AtMAP65-1 properties. We demonstrate that, in vitro, both proteins promote the formation of a planar network of antiparallel microtubules. In vivo, we show that AtMAP65-5 selectively binds the preprophase band and the prophase spindle microtubule during prophase, whereas AtMAP65-1-GFP selectively binds the preprophase band but does not accumulate at the prophase spindle microtubules that coexists within the same cell. At later stages of mitosis, AtMAP65-1 and AtMAP65-5 differentially label the late spindle and phragmoplast. We present evidence for a mode of action for both proteins that involves the binding of monomeric units to microtubules that "zipper up" antiparallel arranged microtubules through the homodimerization of the N-terminal halves when adjacent microtubules encounter.

Straighten up and fly right: microtubule dynamics and organization of non-centrosomal arrays in higher plants

Live cell imaging and genetic studies are demonstrating that cortical microtubule arrays in plant cells are dynamic structures in which microtubule (MT) bundles play a key role in creating array organization and function. Steps important for creating and organizing these arrays include recruitment of nucleation complexes to the cell cortex and to the lattices of previously established MTs, association of newly created MTs to the cell cortex, release of MTs from sites of nucleation, transport of released MTs by polymer treadmilling, and subsequent interactions between treadmilling MTs. The results of MT interactions include induced catastrophe, severing, and the capture and reorientation of growing polymer ends by bundling interactions. Together, these properties predict a capacity for self-ordering that is likely to play an important role in establishing the parallel organization of the arrays.

Analysis of cortical arrays from Tradescantia virginiana at high resolution reveals discrete microtubule subpopulations and demonstrates that confocal images of arrays can be misleading

Cortical microtubule arrays are highly organized networks involved in directing cellulose microfibril deposition within the cell wall. Their organization results from complex interactions between individual microtubules and microtubule-associated proteins. The precise details of these interactions are often not evident using optical microscopy. Using high-resolution scanning electron microscopy, we analyzed extensive regions of cortical arrays and identified two spatially discrete microtubule subpopulations that exhibited different stabilities. Microtubules that lay adjacent to the plasma membrane were often bundled and more stable than the randomly aligned, discordant microtubules that lay deeper in the cytoplasm. Immunolabeling revealed katanin at microtubule ends, on curves, or at sites along microtubules in line with neighboring microtubule ends. End binding 1 protein also localized along microtubules, at microtubule ends or junctions between microtubules, and on the plasma membrane in direct line with microtubule ends. We show fine bands in vivo that traverse and may encircle microtubules. Comparing confocal and electron microscope images of fluorescently tagged arrays, we demonstrate that optical images are misleading, highlighting the fundamental importance of studying cortical microtubule arrays at high resolution.

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.

Arabidopsis katanin binds microtubules using a multimeric microtubule-binding domain

Katanin is a heterodimeric protein that mediates ATP-dependent destabilization of microtubules in animal cells. In plants, the catalytic subunit of Arabidopsis thaliana katanin (AtKSS, Arabidopsis thaliana Katanin Small Subunit) has been identified and its microtubule-severing activity has been demonstrated in vitro. In vivo, plant katanin plays a role in the organization of cortical microtubules, but the way it achieves this function is unknown. To go further in our understanding of the mechanisms by which katanin severs microtubules, we analyzed the functional domains of Arabidopsis katanin. We characterized the microtubule-binding domain of katanin both in vitro and in vivo. It corresponds to a poorly conserved sequence between plant and animal katanins that is located in the N-terminus of the protein. This domain interacts with cortical microtubules in vivo and has a low affinity for microtubules in vitro. We also observed that katanin microtubule-binding domain oligomerizes into trimers. These results show that, besides being involved in the interaction of katanin with microtubules, the microtubule-binding domain may also participate in the oligomerization of katanin. At the structural level, we observed that AtKSS forms ring-shaped oligomers.

Bundles of hexagonally arranged tubules in timothy grass pollen: detection of a novel pollen component using anhydrous fixation and image analysis techniques in transmission electron microscopy

Pollen from timothy grass (Phleum pratense L.) was subjected to various aqueous and non-aqueous fixation and preparation protocols for transmission electron microscopy. Only in the cytoplasm of anhydrously prepared pollen grains were conspicuous inclusions observed that range in size from less than 1 mum up to 8 or 10 mum. These bodies have so far not been described in the literature. Higher magnifications show that these inclusions consist of bundles of hexagonally arranged small tubules. In order to obtain details of the ultrastructure of this novel pollen component, TEM micrographs of ultrathin sections of hexagonally arranged tubules were analyzed using Fourier transform techniques of image analysis. It was found that the tubules form groups with quasi-periodic hexagonal arrangement, with an average centre-to-centre spacing between the neighbouring tubules of approximately 42 nm. Individual tubules are formed by 12 or 13 particles. The outer diameter of the tubules ranges between 22 and 24 nm. From our experiments, we conclude that the quasi-periodic hexagonally arranged tubules forming conspicuous cytoplasmic inclusions in dry timothy grass pollen grains are structurally similar to microtubules.

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.

AtMAP65-1 Binds to Tubulin Dimers to Promote Tubulin Assembly

In Arabidopsis thaliana, the microtubule-associated protein AtMAP65-1 shows various functions on microtubule dynamics and organizations. However, it is still an open question about whether AtMAP65-1 binds to tubulin dimers and how it regulates microtubule dynamics. In present study, the tubulin-binding activity of AtMAP65-1 was investigated. Pull-down and co-sedimentation experiments demonstrated that AtMAP65-1 bound to tubulin dimers, at a molar ratio of 1 : 1. Cross-linking experiments showed that AtMAP65-1 bound to tubulin dimers by interacting with alpha-tubulin of the tubulin heterodimer. Interfering the bundling effect of AtMAP65-1 by addition of salt and monitoring the tubulin assembly, the experiment results indicated that AtMAP65-1 promoted tubulin assembly by interacting with tubulin dimers. In addition, five truncated versions of AtMAP65-1, namely AtMAP65-1 deltaN339 (amino acids 340-587); AtMAP65-1 deltaN494 (amino acids 495-587); AtMAP65-1 340-494 (amino acids 340-494); AtMAP65-1 deltaC495 (amino acids 1-494) and AtMAP65-1 deltaC340 (amino acids 1-339), were tested for their binding activities and roles in tubulin polymerization in vitro. Four (AtMAP65-1 deltaN339, deltaN494, AtMAP65-1 340-494 and deltaC495) from the five truncated proteins were able to co-sediment with microtubules, and three (AtMAP65-1 deltaN339, deltaN494 and AtMAP65-1 340-494) of them could bind to tubulin dimers in vitro. Among the three truncated proteins, AtMAP65-1 deltaN339 showed the greatest activity to promote tubulin polymerization, AtMAP65-1 deltaN494 exhibited almost the same activity as the full length protein in promoting tubulin assembly, and AtMAP65-1 340-494 had minor activity to promote tubulin assembly. On the contrast, AtMAP65-1 deltaC495, which bound to microtubules but not to tubulin dimers, did not affect tubulin assembly. Our study suggested that AtMAP65-1 might promote tubulin assembly by binding to tubulin dimers in vivo.

WVD2 is a novel microtubule-associated protein in Arabidopsis thaliana

Arabidopsis WAVE-DAMPENED 2 (WVD2) was identified by forward genetics as an activation-tagged allele that causes plant and organ stockiness and inversion of helical root growth handedness on agar surfaces. Plants with high constitutive expression of WVD2 or other members of the WVD2-LIKE (WDL) gene family have stems and roots that are short and thick, have reduced anisotropic cell elongation, are suppressed in a root-waving phenotype, and have inverted handedness of twisting in hypocotyls and roots compared with wild-type. The wvd2-1 mutant shows aberrantly organized cortical microtubules in peripheral root cap cells as well as reduced branching of trichomes, unicellular leaf structures whose development is regulated by microtubule stability. Orthologs of the WVD2/WDL family are found widely throughout the plant kingdom, but are not similar to non-plant proteins with the exception of a C-terminal domain distantly related to the vertebrate microtubule-associated protein TPX2. in vivo, WVD2 and its closest paralog WDL1 are localized to interphase cortical microtubules in leaves, hypocotyls and roots. Recombinant glutathione-S-transferase:WVD2 or maltose binding protein:WVD2 protein bind to and bundle microtubules in vitro. We speculate that a C-terminal domain of TPX2 has been utilised by the WVD2 family for functions critical to the organization of plant microtubules.

Microtubule-associated proteins in higher plants

A variety of microtubule-associated proteins (MAPs) have been reported in higher plants. Microtubule (MT) polymerization starts from the gamma-tubulin complex (gammaTuC), a component of the MT nucleation site. MAP200/MOR1 and katanin regulate the length of the MT by promoting the dynamic instability of MTs and cutting MTs, respectively. In construction of different MT structures, MTs are bundled or are associated with other components--actin filaments, the plasma membrane, and organelles. The MAP65 family and some of kinesin family are important in bundling MTs. MT plus-end-tracking proteins (+TIPs) including end-binding protein 1 (EB1), Arabidopsis thaliana kinesin 5 (ATK5), and SPIRAL 1 (SPR1) localize to the plus end of MTs. It has been suggested that +TIPs are involved in binding of MT to other structures. Phospholipase D (PLD) is a possible candidate responsible for binding of MTs to the plasma membrane. Many candidates have been reported as actin-binding MAPs, for example calponin-homology domain (KCH) family kinesin, kinesin-like calmodulin-binding protein (KCBP), and MAP190. RNA distribution and translation depends on MT structures, and several RNA-related MAPs have been reported. This article gives an overview of predicted roles of these MAPs in higher plants.

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 cortical array organization and plant cell morphogenesis

Plant cell cortical microtubule arrays attain a high degree of order without the benefit of an organizing center such as a centrosome. New assays for molecular behaviors in living cells and gene discovery are yielding insight into the mechanisms by which acentrosomal microtubule arrays are created and organized, and how microtubule organization functions to modify cell form by regulating cellulose deposition. Surprising and potentially important behaviors of cortical microtubules include nucleation from the walls of established microtubules, and treadmilling-driven motility leading to polymer interaction, reorientation, and microtubule bundling. These behaviors suggest activities that can act to increase or decrease the local level of order in the array. The SPIRAL1 (SPR1) and SPR2 microtubule-localized proteins and the radial swollen 6 (rsw-6) locus are examples of new molecules and genes that affect both microtubule array organization and cell growth pattern. Functional tagging of cellulose synthase has now allowed the dynamic relationship between cortical microtubules and the cell-wall-synthesizing machinery to be visualized, providing direct evidence that cortical microtubules can organize cellulose synthase complexes and guide their movement through the plasma membrane as they create the cell wall.

Tobacco mosaic virus movement protein functions as a structural microtubule-associated protein

The cell-to-cell spread of Tobacco mosaic virus infection depends on virus-encoded movement protein (MP), which is believed to form a ribonucleoprotein complex with viral RNA (vRNA) and to participate in the intercellular spread of infectious particles through plasmodesmata. Previous studies in our laboratory have provided evidence that the vRNA movement process is correlated with the ability of the MP to interact with microtubules, although the exact role of this interaction during infection is not known. Here, we have used a variety of in vivo and in vitro assays to determine that the MP functions as a genuine microtubule-associated protein that binds microtubules directly and modulates microtubule stability. We demonstrate that, unlike MP in whole-cell extract, microtubule-associated MP is not ubiquitinated, which strongly argues against the hypothesis that microtubules target the MP for degradation. In addition, we found that MP interferes with kinesin motor activity in vitro, suggesting that microtubule-associated MP may interfere with kinesin-driven transport processes during infection.

Control of the AtMAP65-1 interaction with microtubules through the cell cycle

Cell division depends on the fine control of both microtubule dynamics and microtubule organisation. The microtubule bundling protein MAP65 is a ;midzone MAP' essential for the integrity of the anaphase spindle and cell division. Arabidopsis thaliana MAP65-1 (AtMAP65-1) binds and bundles microtubules by forming 25 nm cross-bridges. Moreover, as AtMAP65-1 bundles microtubules in interphase, anaphase and telophase but does not bind microtubules in prophase or metaphase, its activity through the cell cycle must be under tight control. Here we show that AtMAP65-1 is hyperphosphorylated during prometaphase and metaphase and that CDK and MAPK are involved in this phosphorylation. This phosphorylation inhibits AtMAP65-1 activity. Expression of non-phosphorylatable AtMAP65-1 has a negative effect on mitotic progression resulting in excessive accumulation of microtubules in the metaphase spindle midzone causing a delay in mitosis. We conclude that normal metaphase spindle organisation and the transition to anaphase is dependent on inactivation of AtMAP65-1.

The relationship between induction of embryogenesis and chromosome doubling in microspore cultures

The objective of this paper is to review the relationship between induction of microspore embryogenesis and chromosome doubling. It has been augmented with relative data on chromosome doubling by nuclear fusion. Some of the treatments used for induction of embryogenesis may also lead to doubling of the chromosome number, either through nuclear fusion or endomitosis. High frequencies of spontaneous chromosome doubling in cereal species appear to be induced by treatments that block cell wall formation during the first cell divisions, resulting in coenocytic cells in which the nuclei are able to fuse. The use of mannitol as a pretreatment for induction of embryogenesis in barley, wheat, and maize microspore cultures provides examples of nuclear fusion. The use of antimicrotubule agents for embryo induction via treatments during the first few hours of microspore culture has also resulted in high frequencies of chromosome doubling. Factors such as the doubling agent concentration, temperature during treatment, and duration of treatment may be critical for individual species. Actin filament as well as microtubule assembly studies related to new cell wall formation provide further evidence at the molecular level for the relationship between microspore embryogenesis and chromosome doubling.

Katanin's severing activity favors bundling of cortical microtubules in plants

Higher plant cells exhibit interphase microtubule arrays specific to plants, which are essential for their developmental program. These cortical microtubules (CMT) consist of a population of highly dynamic microtubules that are usually organized into bundles in the cortex of the cells. The organization of CMT is intimately linked to the acquisition of specialized functions, and subsequentchanges in their distribution affect their properties. The mechanisms underlying the formation and the distribution of CMT are still unclear, and little is known about the proteins that are involved in this phenomenon. Here we investigated the putative role of katanin, the only known plant microtubule-severing protein, in the organization of CMT. We generated transgenic Arabidopsis lines that overexpress katanin under the control of an ethanol-inducible promoter. In response to an induced overexpression of katanin, CMT organized into numerous and thick bundles, which ultimately depolymerized. From the analyses of CMT patterns together with recent data on CMT dynamics, we propose that, in interphase cells, katanin's main activity is to free CMT, generating motile microtubules that incorporate into bundles.

Cytoskeletal organization during xylem cell differentiation

The water and mineral conductive tube, the xylem vessel and tracheid, is a highly conspicuous tissue due to its elaborately patterned secondary-wall deposition. One constituent of the xylem vessel and tracheid, the tracheary element, is an empty dead cell that develops secondary walls in the elaborate patterns. The wall pattern is appropriately regulated according to the developmental stage of the plant. The cytoskeleton is an essential component of this regulation. In fact, the cortical microtubule is well known to participate in patterned secondary cell wall formation. The dynamic rearrangement of the microtubules and actin filaments have also been recognized in the cultured cells differentiating into tracheary elements in vitro. There has recently been considerable progress in our understanding of the dynamics and regulation of cortical microtubules, and several plant microtubule associated proteins have been identified and characterized. The microtubules have been observed during tracheary element differentiation in living Arabidopsis thaliana cells. Based on this recently acquired information on the plant cytoskeleton and tracheary element differentiation, this review discusses the role of the cytoskeleton in secondary cell wall formation.

Cortical control of plant microtubules

The cortical microtubule array of plant cells appears in early G(1) and remodels during the progression of the cell cycle and differentiation, and in response to various stimuli. Recent studies suggest that cortical microtubules are mostly formed on pre-existing microtubules and, after detachment from the initial nucleation sites, actively interact with each other to attain distinct distribution patterns. The plus end of growing microtubules is thought to accumulate protein complexes that regulate both microtubule dynamics and interactions with cortical targets. The ROP family of small GTPases and the mitogen-activated protein kinase pathways have emerged as key players that mediate the cortical control of plant microtubules.

The role of MAP65-1 in microtubule bundling during Zinnia tracheary element formation

The MAP65 family of microtubule-associated proteins performs various functions at different stages of the cell cycle and differentiation. In this study, we have investigated the synchronous transdifferentiation of Zinnia mesophyll cells into tracheary elements in vitro. This allowed us to examine the role of the microtubule-associated protein MAP65 during the characteristic bunching of cortical microtubules that underlie the developing ribs of secondarily thickened cell wall. Immunofluorescence confirmed the microtubule bundles to be decorated with anti-MAP65 antibodies. Three Zinnia MAP65 genes were examined; the expression of ZeMAP65-1 was found to match that of the differentiation marker TED2 and both were found to be upregulated upon addition of inductive hormones. We cloned the full-length sequence of ZeMAP65-1 and found it to be most similar to other MAP65 isoforms known to bundle microtubules in other plant species. However, not all MAP65 proteins crosslink cortical microtubules and so, to confirm its potential bundling capacity, ZeMAP65-1 was transiently overexpressed in Arabidopsis suspension cells. This resulted in the super-bundling of microtubules in patterns resembling those in differentiating xylem cells. These findings establish that the MAP65-1 group of proteins is responsible for the bundling of cortical microtubules during secondary cell wall formation of xylogenesis as well as during the expansion of primary cell walls.

Microtubule dynamics and organization in the plant cortical array

Live-cell studies have brought fresh insight into the organizational activities of the plant cortical array. Plant interphase arrays organize in the absence of a discrete microtubule organizing center, having plus and minus ends distributed throughout the cell cortex. Microtubule nucleation occurs at the cell cortex, frequently followed by minus-end detachment from origin sites. Microtubules associate tightly with the cell cortex, resisting lateral and axial translocation. Slow, intermitant loss of dimers from minus ends, coupled with growth-biased dynamic instability at the plus ends, results in the migration of cortically attached microtubules across the cell via polymer treadmilling. Microtubule-microtubule interactions, a direct consequence of treadmilling, result in polymer reorientation and creation of polymer bundles. The combined properties of microtubule dynamics and interactions among polymers constitute a system with predicted properties of self-organization.

Midbodies and phragmoplasts: analogous structures involved in cytokinesis

Cytokinesis is an event common to all organisms that involves the precise coordination of independent pathways involved in cell-cycle regulation and microtubule, membrane, actin and organelle dynamics. In animal cells, the spindle midzone/midbody with associated endo-membrane system are required for late cytokinesis events, including furrow ingression and scission. In plants, cytokinesis is mediated by the phragmoplast, an array of microtubules, actin filaments and associated molecules that act as a framework for the future cell wall. In this article (which is part of the Cytokinesis series), we discuss recent studies that highlight the increasing number of similarities in the components and function of the spindle midzone/midbody in animals and the phragmoplast in plants, suggesting that they might be analogous structures.

Microtubules cut and run

There is broad agreement that cells reconfigure their microtubules through rapid bouts of assembly and disassembly, as described by the mechanism known as dynamic instability. However, many cell types have complex patterns of microtubule organization that are not entirely explicable by dynamic instability. There is growing evidence that microtubules can be moved into new patterns of organization by forces generated by molecular motor proteins. Studies on several cell types support a model called 'cut and run' in which long microtubules are stationary, but relatively short microtubules are mobile. In this model, cells mobilize their microtubules by severing them into short pieces, using enzymes such as katanin and spastin that break the lattice of the microtubule polymer. After being reorganized, the short microtubules can once again elongate and lose their mobility. Microtubule severing is also crucial for a variation of 'cut and run' in which the severed microtubules are reorganized by means of treadmilling.

Systematic spatial analysis of gene expression during wheat caryopsis development

The cereal caryopsis is a complex tissue in which maternal and endosperm tissues follow distinct but coordinated developmental programs. Because of the hexaploid genome in wheat (Triticum aestivum), the identification of genes involved in key developmental processes by genetic approaches has been difficult. To bypass this limitation, we surveyed 888 genes that are expressed during caryopsis development using a novel high-throughput mRNA in situ hybridization method. This survey revealed novel distinct spatial expression patterns that either reflected the ontogeny of the developing caryopsis or indicated specialized cellular functions. We have identified both known and novel genes whose expression is cell cycle-dependent. We have identified the crease region as important in setting up the developmental patterning, because the transition from proliferation to differentiation spreads from this region to the rest of the endosperm. A comparison of this set of genes with the rice (Oryza sativa) genome shows that approximately two-thirds have rice counterparts but also suggests considerable divergence with regard to proteins involved in grain filling. We found that the wheat genes had significant homology with 350 Arabidopsis thaliana genes. At least 25 of these are already known to be essential for seed development in Arabidopsis, but many others remain to be characterized.

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.

Dynamic interaction of NtMAP65-1a with microtubules in vivo

Plant microtubules are intrinsically more dynamic than those from animals. We know little about the dynamics of the interaction of plant microtubule-associated proteins (MAPs) with microtubules. Here, we have used tobacco and Arabidopsis MAPs with relative molecular mass 65 kDa (NtMAP65-1a and AtMAP65-1), to study their interaction with microtubules in vivo. Using fluorescence recovery after photobleaching we report that the turnover of both NtMAP65-1a and AtMAP65-1 bound to microtubules is four- to fivefold faster than microtubule treadmilling (13 seconds compared with 56 seconds, respectively) and that the replacement of NtMAP65-1a on microtubules is by random association rather than by translocation along microtubules. MAP65 will only bind polymerised microtubules and not its component tubulin dimers. The turnover of NtMAP65-1a and AtMAP65-1 on microtubules is similar in the interphase cortical array, the preprophase band and the phragmoplast, strongly suggesting that their role in these arrays is the same. NtMAP65-1a and AtMAP65-1 are not observed to bind microtubules in the metaphase spindle and their rate of recovery is consistent with their cytoplasmic localisation. In addition, the dramatic reappearance of NtMAP65-1a on microtubules at the spindle midzone in anaphase B suggests that NtMAP65-1a is controlled post-translationally. We conclude that the dynamic properties of these MAPs in vivo taken together with the fact that they have been shown not to effect microtubule polymerisation in vitro, makes them ideally suited to a role in crossbridging microtubules that need to retain spatial organisation in rapidly reorganising microtubule arrays.

Two microtubule-associated proteins of the Arabidopsis MAP65 family function differently on microtubules

The organization and dynamics of microtubules are regulated by microtubule-associated proteins, or MAPs. In Arabidopsis (Arabidopsis thaliana), nine genes encode proteins of the evolutionarily conserved MAP65 family. We proposed that different MAP65s might have distinct roles in the interaction with microtubules. In this study, two AtMAP65 proteins, AtMAP65-1 and AtMAP65-6, were chosen to test this hypothesis in vitro. Although both fusion proteins were able to cosediment with microtubules in vitro, different properties on tubulin polymerization and microtubule bundling were observed. AtMAP65-1 was able to promote tubulin polymerization, enhance microtubule nucleation, and decrease the critical concentration for tubulin polymerization. It also induced the formation of large microtubule bundles by forming cross-bridges between microtubules evenly along the whole length of microtubules. In the presence of AtMAP65-1, microtubule bundles were more resistant to cold and dilution treatments. AtMAP65-6, however, demonstrated no activity in promoting tubulin polymerization and stabilizing preformed microtubules. AtMAP65-6 induced microtubules to form a mesh-like network with individual microtubules. Cross-bridge-like interactions were only found at regional sites between microtubules. The microtubule network induced by AtMAP65-6 was more resistant to high concentration of NaCl than the bundles induced by AtMAP65-1. Purified monospecific anti-AtMAP65-6 antibodies revealed that AtMAP65-6 was associated with mitochondria in Arabidopsis cells. It was concluded that these two MAP65 proteins were targeted to distinct sites, thus performing distinct functions in Arabidopsis cells.