The plant cytoskeleton: recent advances in the study of the plant microtubule-associated proteins MAP-65, MAP-190 and the Xenopus MAP215-like protein, MOR1

Ionic strength alters crosslinker-driven self-organization of microtubules

The microtubule cytoskeleton is a major structural element inside cells that directs self-organization using microtubule-associated proteins and motors. It has been shown that finite-sized, spindle-like microtubule organizations, called "tactoids," can form in vitro spontaneously from mixtures of tubulin and the antiparallel crosslinker, MAP65, from the MAP65/PRC1/Ase family. Here, we probe the ability of MAP65 to form tactoids as a function of the ionic strength of the buffer to attempt to break the electrostatic interactions binding MAP65 to microtubules and inter-MAP65 binding. We observe that, with increasing monovalent salts, the organizations change from finite tactoids to unbounded length bundles, yet the MAP65 binding and crosslinking appear to stay intact. We further explore the effects of ionic strength on the dissociation constant of MAP65 using both microtubule pelleting and single-molecule binding assays. We find that salt can reduce the binding, yet salt never negates it. Instead, we believe that the salt is affecting the ability of the MAP65 to form phase-separated droplets, which cause the nucleation and growth of tactoids, as recently demonstrated.

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.

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.

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

Background

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

Results

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

Conclusions

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

Supplementary Information

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

The Protein Phosphatase PP2A Plays Multiple Roles in Plant Development by Regulation of Vesicle Traffic-Facts and Questions

The protein phosphatase PP2A is essential for the control of integrated eukaryotic cell functioning. Several cellular and developmental events, e.g., plant growth regulator (PGR) mediated signaling pathways are regulated by reversible phosphorylation of vesicle traffic proteins. Reviewing present knowledge on the relevant role of PP2A is timely. We discuss three aspects: (1) PP2A regulates microtubule-mediated vesicle delivery during cell plate assembly. PP2A dephosphorylates members of the microtubule associated protein family MAP65, promoting their binding to microtubules. Regulation of phosphatase activity leads to changes in microtubule organization, which affects vesicle traffic towards cell plate and vesicle fusion to build the new cell wall between dividing cells. (2) PP2A-mediated inhibition of target of rapamycin complex (TORC) dependent signaling pathways contributes to autophagy and this has possible connections to the brassinosteroid signaling pathway. (3) Transcytosis of vesicles transporting PIN auxin efflux carriers. PP2A regulates vesicle localization and recycling of PINs related to GNOM (a GTP-GDP exchange factor) mediated pathways. The proper intracellular traffic of PINs is essential for auxin distribution in the plant body, thus in whole plant development. Overall, PP2A has essential roles in membrane interactions of plant cell and it is crucial for plant development and stress responses.

Phase separation of Arabidopsis EMB1579 controls transcription, mRNA splicing, and development

Tight regulation of gene transcription and mRNA splicing is essential for plant growth and development. Here we demonstrate that a plant-specific protein, EMBRYO DEFECTIVE 1579 (EMB1579), controls multiple growth and developmental processes in Arabidopsis. We demonstrate that EMB1579 forms liquid-like condensates both in vitro and in vivo, and the formation of normal-sized EMB1579 condensates is crucial for its cellular functions. We found that some chromosomal and RNA-related proteins interact with EMB1579 compartments, and loss of function of EMB1579 affects global gene transcription and mRNA splicing. Using floral transition as a physiological process, we demonstrate that EMB1579 is involved in FLOWERING LOCUS C (FLC)-mediated repression of flowering. Interestingly, we found that EMB1579 physically interacts with a homologue of Drosophila nucleosome remodeling factor 55-kDa (p55) called MULTIPLE SUPPRESSOR OF IRA 4 (MSI4), which has been implicated in repressing the expression of FLC by forming a complex with DNA Damage Binding Protein 1 (DDB1) and Cullin 4 (CUL4). This complex, named CUL4-DDB1^MSI4, physically associates with a CURLY LEAF (CLF)-containing Polycomb Repressive Complex 2 (CLF-PRC2). We further demonstrate that EMB1579 interacts with CUL4 and DDB1, and EMB1579 condensates can recruit and condense MSI4 and DDB1. Furthermore, emb1579 phenocopies msi4 in terms of the level of H3K27 trimethylation on FLC. This allows us to propose that EMB1579 condensates recruit and condense CUL4-DDB1^MSI4 complex, which facilitates the interaction of CUL4-DDB1^MSI4 with CLF-PRC2 and promotes the role of CLF-PRC2 in establishing and/or maintaining the level of H3K27 trimethylation on FLC. Thus, we report a new mechanism for regulating plant gene transcription, mRNA splicing, and growth and development.

This study reveals that a plant-specific protein, EMB1579, controls multiple growth and developmental processes in Arabidopsis thaliana by regulating gene transcription and mRNA splicing through the formation of liquid-like droplets via liquid-liquid phase separation.

Insights of Molecular Mechanism of Xylem Development in Five Black Poplar Cultivars

Black poplar (Populus deltoides, P. nigra, and their hybrids) is the main poplar cultivars in China. It offers interesting options of large-scale biomass production for bioenergy due to its rapid growth and high yield. Poplar wood properties were associated with chemical components and physical structures during wood formation. In this study, five poplar cultivars, P. euramericana 'Zhonglin46' (Pe1), P. euramericana 'Guariento' (Pe2), P. nigra 'N179' (Pn1), P. deltoides 'Danhong' (Pd1), and P. deltoides 'Nanyang' (Pd2), were used to explore the molecular mechanism of xylem development. We analyzed the structural differences of developing xylem in the five cultivars and profiled the transcriptome-wide gene expression patterns through RNA sequencing. The cross sections of the developing xylem showed that the cell wall thickness of developed fiber in Pd1 was thickest and the number of xylem vessels of Pn1 was the least. A total of 10,331 differentially expressed genes were identified among 10 pairwise comparisons of the five cultivars, most of them were related to programmed cell death and secondary cell wall thickening. K-means cluster analysis and Gene Ontology enrichment analysis showed that the genes highly expressed in Pd1 were related to nucleotide decomposition, metabolic process, transferase, and microtubule cytoskeleton; whereas the genes highly expressed in Pn1 were involved in cell wall macromolecule decomposition and polysaccharide binding processes. Based on a weighted gene co-expression network analysis, a large number of candidate regulators for xylem development were identified. And their potential regulatory roles to cell wall biosynthesis genes were validated by a transient overexpression system. This study provides a set of promising candidate regulators for genetic engineering to improve feedstock and enhance biofuel conversion in the bioenergy crop Populus.

Complementary Superresolution Visualization of Composite Plant Microtubule Organization and Dynamics

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

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.

Microtubule bundling by MAP65-1 protects against severing by inhibiting the binding of katanin

The microtubule-severing enzyme katanin (KTN1) regulates the organization and turnover of microtubule arrays by the localized breakdown of microtubule polymers. In land plants, KTN1 activity is essential for the formation of linearly organized cortical microtubule arrays that determine the axis of cell expansion. Cell biological studies have shown that even though KTN1 binds to the sidewalls of single and bundled microtubules, severing activity is restricted to microtubule cross-over and nucleation sites, indicating that cells contain protective mechanisms to prevent indiscriminate microtubule severing. Here, we show that the microtubule-bundling protein MAP65-1 inhibits KTN1-mediated microtubule severing in vitro. Severing is inhibited at bundled microtubule segments and the severing rate of nonbundled microtubules is reduced by MAP65-1 in a concentration-dependent manner. Using various MAP65-1 mutant proteins, we demonstrate that efficient cross-linking of microtubules is crucial for this protective effect and that microtubule binding alone is not sufficient. Reduced severing due to microtubule bundling by MAP65-1 correlated to decreased binding of KTN1 to these microtubules. Taken together, our work reveals that cross-linking of microtubules by MAP65-1 confers resistance to severing by inhibiting the binding of KTN1 and identifies the structural features of MAP65-1 that are important for this activity.

The microtubule cytoskeleton acts as a sensor for stress response signaling in plants

Stress tolerance pathways are protective mechanisms that have evolved to protect plant growth and increase production under various environmental stress conditions. Enhancing stress tolerance in crop plants has become an area of intense study with aims of increasing crop production and enhancing economic benefits. A growing number of studies suggest that in addition to playing vital roles in mechanical architecture and cell division, microtubules are also involved the adaptation to severe environmental conditions in plants. However, the mechanisms that integrate microtubule regulation, cellular metabolism and cell signaling in plant stress responses remain unclear. Recent studies suggest that microtubules act as sensors for different abiotic stresses and maintain mechanical stability by forming bundles. Characterizing the diverse roles of plant microtubules is vital to furthering our understanding of stress tolerance in plants.

Microtubule bundling by MAP65-1 protects against severing by inhibiting the binding of katanin

The microtubule-severing enzyme katanin (KTN1) regulates the organization and turnover of microtubule arrays by the localized breakdown of microtubule polymers. In land plants, KTN1 activity is essential for the formation of linearly organized cortical microtubule arrays that determine the axis of cell expansion. Cell biological studies have shown that even though KTN1 binds to the sidewalls of single and bundled microtubules, severing activity is restricted to microtubule cross-over and nucleation sites, indicating that cells contain protective mechanisms to prevent indiscriminate microtubule severing. Here, we show that the microtubule-bundling protein MAP65-1 inhibits KTN1-mediated microtubule severing in vitro. Severing is inhibited at bundled microtubule segments and the severing rate of nonbundled microtubules is reduced by MAP65-1 in a concentration-dependent manner. Using various MAP65-1 mutant proteins, we demonstrate that efficient cross-linking of microtubules is crucial for this protective effect and that microtubule binding alone is not sufficient. Reduced severing due to microtubule bundling by MAP65-1 correlated to decreased binding of KTN1 to these microtubules. Taken together, our work reveals that cross-linking of microtubules by MAP65-1 confers resistance to severing by inhibiting the binding of KTN1 and identifies the structural features of MAP65-1 that are important for this activity.

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.

Comparative Transcriptomics of Root Development in Wild and Cultivated Carrots

The carrot is the most popular root vegetable worldwide. The genetic makeup underlying the development of the edible storage root are fragmentary. Here, we report the first comparative transcriptome analysis between wild and cultivated carrot roots at multiple developmental stages. Overall, 3285, 4637, and 570 genes were differentially expressed in the cultivated carrot in comparisons made for young plants versus developing roots, young plants versus mature roots, and developing roots versus mature roots, respectively. Of those, 1916, 2645, and 475, respectively, were retained after filtering out genes showing similar profiles of expression in the wild carrot. They were assumed to be of special interest with respect to the development of the storage root. Among them, transcription factors and genes encoding proteins involved in post-translational modifications (signal transduction and ubiquitination) were mostly upregulated, while those involved in redox signaling were mostly downregulated. Also, genes encoding proteins regulating cell cycle, involved in cell divisions, development of vascular tissue, water transport, and sugar metabolism were enriched in the upregulated clusters. Genes encoding components of photosystem I and II, together with genes involved in carotenoid biosynthesis, were upregulated in the cultivated roots, as opposed to the wild roots; however, they were largely downregulated in the mature storage root, as compared with the young and developing root. The experiment produced robust resources for future investigations on the regulation of storage root formation in carrot and Apiaceae.

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.

Dose-dependent interactions between two loci trigger altered shoot growth in BG-5 × Krotzenburg-0 (Kro-0) hybrids of Arabidopsis thaliana

Hybrids occasionally exhibit genetic interactions resulting in reduced fitness in comparison to their parents. Studies of Arabidopsis thaliana have highlighted the role of immune conflicts, but less is known about the role of other factors in hybrid incompatibility in plants. Here, we present a new hybrid incompatibility phenomenon in this species. We have characterized a new case of F₁ hybrid incompatibility from a cross between the A. thaliana accessions Krotzenburg-0 (Kro-0) and BG-5, by conducting transcript, metabolite and hormone analyses, and identified the causal loci through genetic mapping. The F₁ hybrids showed arrested growth of the main stem, altered shoot architecture, and altered concentrations of hormones in comparison to parents. The F₁ phenotype could be rescued in a developmental-stage-dependent manner by shifting to a higher growth temperature. These F₁ phenotypes were linked to two loci, one on chromosome 2 and one on chromosome 3. The F₂ generation segregated plants with more severe phenotypes which were linked to the same loci as those in the F₁ . This study provides novel insights into how previously unknown mechanisms controlling shoot branching and stem growth can result in hybrid incompatibility.

MAP65-1 is required for the depolymerization and reorganization of cortical microtubules in the response to salt stress in Arabidopsis

Microtubules (MTs) are highly dynamical structures that play crucial roles in plant development and in response to environmental signals and stress conditions. MT-associated proteins (MAPs) play important roles in regulating the organization of MT arrays. MAP65 is a family of plant MT-bundling proteins. Here, we determined the role of MAP65-1 in the response to salt stress. MAP65-1 is involved not only in regulating the depolymerization, but also in the following reorganization of cortical MTs in salt stress responses. In addition, the depolymerization of the cortical MTs affected the survival of seedlings during salt stress, and map65-1 mutants had enhanced salt hypersensitivity levels. MAP65-1 interacted with mitogen-activated protein kinase (MPK) 3 and 6; however, only the mpk6 mutant exhibited hypersensitivity to salt stress, and MPK6 was involved in regulating the salt stress-induced depolymerization of cortical MTs. Thus, MAP65-1 plays a critical role in the response to salt stress and is required for regulating the rapid depolymerization and reorganization of cortical MTs. MAP65-1 interacts with MPK6, not MPK3, affecting the MT's dynamic instability which is critical for plant salt-stress tolerance.

Structural disorder in plant proteins: where plasticity meets sessility

Plants are sessile organisms. This intriguing nature provokes the question of how they survive despite the continual perturbations caused by their constantly changing environment. The large amount of knowledge accumulated to date demonstrates the fascinating dynamic and plastic mechanisms, which underpin the diverse strategies selected in plants in response to the fluctuating environment. This phenotypic plasticity requires an efficient integration of external cues to their growth and developmental programs that can only be achieved through the dynamic and interactive coordination of various signaling networks. Given the versatility of intrinsic structural disorder within proteins, this feature appears as one of the leading characters of such complex functional circuits, critical for plant adaptation and survival in their wild habitats. In this review, we present information of those intrinsically disordered proteins (IDPs) from plants for which their high level of predicted structural disorder has been correlated with a particular function, or where there is experimental evidence linking this structural feature with its protein function. Using examples of plant IDPs involved in the control of cell cycle, metabolism, hormonal signaling and regulation of gene expression, development and responses to stress, we demonstrate the critical importance of IDPs throughout the life of the plant.

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.

Proteomic responses of drought-tolerant and drought-sensitive cotton varieties to drought stress

Drought, one of the most widespread factors reducing agricultural crop productivity, affects biological processes such as development, architecture, flowering and senescence. Although protein analysis techniques and genome sequencing have made facilitated the proteomic study of cotton, information on genetic differences associated with proteomic changes in response to drought between different cotton genotypes is lacking. To determine the effects of drought stress on cotton seedlings, we used two-dimensional polyacrylamide gel electrophoresis (2-DE) and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry to comparatively analyze proteome of drought-responsive proteins during the seedling stage in two cotton (Gossypium hirsutum L.) cultivars, drought-tolerant KK1543 and drought-sensitive Xinluzao26. A total of 110 protein spots were detected on 2-DE maps, of which 56 were identified by MALDI-TOF and MALDI-TOF/TOF mass spectrometry. The identified proteins were mainly associated with metabolism (46.4 %), antioxidants (14.2 %), and transport and cellular structure (23.2 %). Some key proteins had significantly different expression patterns between the two genotypes. In particular, 5-methyltetrahydropteroyltriglutamate-homocysteine methyltransferase, UDP-D-glucose pyrophosphorylase and ascorbate peroxidase were up-regulated in KK1543 compared with Xinluzao26. Under drought stress conditions, the vacuolar H(+)-ATPase catalytic subunit, a 14-3-3g protein, translation initiation factor 5A and pathogenesis-related protein 10 were up-regulated in KK1543, whereas ribosomal protein S12, actin, cytosolic copper/zinc superoxide dismutase, protein disulfide isomerase, S-adenosylmethionine synthase and cysteine synthase were down-regulated in Xinluzao26. This work represents the first characterization of proteomic changes that occur in response to drought in roots of cotton plants. These differentially expressed proteins may be related to biochemical pathways responsible for drought tolerance in KK1543. Although further studies are needed, this proteomic analysis underlines the role of post-translational events. The differentially expressed proteins and their corresponding genes may be used as markers for the breeding of drought tolerance in cotton.

The Arabidopsis microtubule-associated protein MAP65-3 supports infection by filamentous biotrophic pathogens by down-regulating salicylic acid-dependent defenses

The oomycete Hyaloperonospora arabidopsidis and the ascomycete Erysiphe cruciferarum are obligate biotrophic pathogens causing downy mildew and powdery mildew, respectively, on Arabidopsis. Upon infection, the filamentous pathogens induce the formation of intracellular bulbous structures called haustoria, which are required for the biotrophic lifestyle. We previously showed that the microtubule-associated protein AtMAP65-3 plays a critical role in organizing cytoskeleton microtubule arrays during mitosis and cytokinesis. This renders the protein essential for the development of giant cells, which are the feeding sites induced by root knot nematodes. Here, we show that AtMAP65-3 expression is also induced in leaves upon infection by the downy mildew oomycete and the powdery mildew fungus. Loss of AtMAP65-3 function in the map65-3 mutant dramatically reduced infection by both pathogens, predominantly at the stages of leaf penetration. Whole-transcriptome analysis showed an over-represented, constitutive activation of genes involved in salicylic acid (SA) biosynthesis, signaling, and defense execution in map65-3, whereas jasmonic acid (JA)-mediated signaling was down-regulated. Preventing SA synthesis and accumulation in map65-3 rescued plant susceptibility to pathogens, but not the developmental phenotype caused by cytoskeleton defaults. AtMAP65-3 thus has a dual role. It positively regulates cytokinesis, thus plant growth and development, and negatively interferes with plant defense against filamentous biotrophs. Our data suggest that downy mildew and powdery mildew stimulate AtMAP65-3 expression to down-regulate SA signaling for infection.

Microtubule encounter-based catastrophe in Arabidopsis cortical microtubule arrays

BACKGROUND

The cortical microtubules (CMTs) that line the plasma membrane of interphase plant cells are extensively studied owing to their importance in forming cell walls, and their usefulness as a model system for the study of MT dynamic instability and acentrosomal MT organization. CMTs influence the orientation and structure of cellulose microfibrils in the cell wall by cooperatively forming arrays of varied patterns from parallel to netted. These CMT patterns are controlled by the combined activities of MT dynamic instability and MT-MT interactions. However, it is an open question as to how CMT patterns may feedback to influence CMT dynamics and interactions.

RESULTS

To address this question, we investigated the effects of CMT array patterning on encounter-based CMT catastrophe, which occurs when one CMT grows into another and is unable to cross over. We hypothesized that the varied CMT angles present in disordered (mixed CMTs) arrays will create more opportunities for MT-MT interactions, and thus increase encounter-based catastrophe rates and distribution. Using live-cell imaging of Arabidopsis cotyledon and leaf epidermal cells, we found that roughly 87% of catastrophes occur via the encounter-based mechanism, with the remainder occurring without encounter (free). When comparing ordered (parallel) and disordered (mixed orientation) CMT arrays, we found that disordered configurations show higher proportions of encounter-based catastrophe relative to free. Similarly, disordered CMT arrays have more catastrophes in general than ordered arrays. Encounter-based catastrophes were associated with frequent and sustained periods of pause prior to depolymerization, and CMTs with tight anchoring to the plasma membrane were more prone to undergo encounter-based catastrophe than weakly-attached ones. This suggests that encounter-based catastrophe has a mechanical basis, wherein MTs form physical barriers to one another. Lastly, we show that the commonly used measure of catastrophe frequencies (Fcat) can also be influenced by CMT ordering and plasma membrane anchoring.

CONCLUSIONS

Our observations add a new layer of complexity to our current understanding of MT organization in plants, showing that not only do individual CMT dynamics influence CMT array organization, but that CMT organization itself has a strong effect on the behavior of individual MTs.

Hypothesis: NDL proteins function in stress responses by regulating microtubule organization

N-MYC DOWNREGULATED-LIKE proteins (NDL), members of the alpha/beta hydrolase superfamily were recently rediscovered as interactors of G-protein signaling in Arabidopsis thaliana. Although the precise molecular function of NDL proteins is still elusive, in animals these proteins play protective role in hypoxia and expression is induced by hypoxia and nickel, indicating role in stress. Homology of NDL1 with animal counterpart N-MYC DOWNREGULATED GENE (NDRG) suggests similar functions in animals and plants. It is well established that stress responses leads to the microtubule depolymerization and reorganization which is crucial for stress tolerance. NDRG is a microtubule-associated protein which mediates the microtubule organization in animals by causing acetylation and increases the stability of α-tubulin. As NDL1 is highly homologous to NDRG, involvement of NDL1 in the microtubule organization during plant stress can also be expected. Discovery of interaction of NDL with protein kinesin light chain- related 1, enodomembrane family protein 70, syntaxin-23, tubulin alpha-2 chain, as a part of G protein interactome initiative encourages us to postulate microtubule stabilizing functions for NDL family in plants. Our search for NDL interactors in G protein interactome also predicts the role of NDL proteins in abiotic stress tolerance management. Based on published report in animals and predicted interacting partners for NDL in G protein interactome lead us to hypothesize involvement of NDL in the microtubule organization during abiotic stress management in plants.

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.

The connection of cytoskeletal network with plasma membrane and the cell wall

The cell wall provides external support of the plant cells, while the cytoskeletons including the microtubules and the actin filaments constitute an internal framework. The cytoskeletons contribute to the cell wall biosynthesis by spatially and temporarily regulating the transportation and deposition of cell wall components. This tight control is achieved by the dynamic behavior of the cytoskeletons, but also through the tethering of these structures to the plasma membrane. This tethering may also extend beyond the plasma membrane and impact on the cell wall, possibly in the form of a feedback loop. In this review, we discuss the linking components between the cytoskeletons and the plasma membrane, and/or the cell wall. We also discuss the prospective roles of these components in cell wall biosynthesis and modifications, and aim to provide a platform for further studies in this field.

Determination of phosphorylation sites in microtubule associated protein MAP65-1

Reorganization of microtubules during cell cycle depends on the modulation of activity of microtubule-associated proteins. MAP65 is one of the main microtubule structural proteins in plants responsible for the formation of bundles of parallel and antiparallel microtubules. A member of MAP65 protein family, MAP65-1, binds to microtubules of preprophase band during early stages of cell division and later to the midzone of anaphase spindle and the phragmoplast, but exhibits no or reduced microtubule binding during metaphase. Artificially induced interaction of MAP65-1 with microtubules during metaphase promotes excessive formation of pole-to-pole microtubule bundles and causes delay of anaphase onset. The exact mechanism of this delay is not known, but it was suggested that microtubule bundles induced by MAP65 impose spatial constraints on the chromosome movement obstructing their alignment in the metaphase plate. Interaction of MAP65-1 with microtubules is controlled by phosphorylation. This chapter describes a strategy for the identification of phosphorylation residues responsible for the cell-cycle control of MAP65-1 activity.

Three BUB1 and BUBR1/MAD3-related spindle assembly checkpoint proteins are required for accurate mitosis in Arabidopsis

The spindle assembly checkpoint (SAC) is a refined surveillance mechanism which ensures that chromosomes undergoing mitosis do not segregate until they are properly attached to the spindle microtubules (MT). The SAC has been extensively studied in metazoans and yeast, but little is known about its role in plants. We identified proteins interacting with a MT-associated protein MAP65-3, which plays a critical role in organising mitotic MT arrays, and carried out a functional analysis of previously and newly identified SAC components. We show that Arabidopsis SAC proteins BUB3.1, MAD2, BUBR1/MAD3s and BRK1 interact with each other and with MAP65-3. We found that two BUBR1/MAD3s interacted specifically at centromeres. When stably expressed in Arabidopsis, BRK1 localised to the kinetochores during all stages of the mitotic cell cycle. Early in mitosis, BUB3.1 and BUBR1/MAD3.1 localise to the mitotic spindle, where MAP65-3 organises spindle MTs. A double-knockout mad3.1 mad3.2 mutant presented spindle MT abnormalities, chromosome misalignments on the metaphase plate and the production of lagging chromosomes and micronuclei during mitosis. We conclude that BRK1 and BUBR1/MAD3-related proteins play a key role in ensuring faithful chromosome segregation during mitosis and that their interaction with MAP65-3 may be important for the regulation of MT-chromosome attachment.

MAPs: cellular navigators for microtubule array orientations in Arabidopsis

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

The role of dynamic instability in microtubule organization

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

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.

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.

Microtubule nucleation and establishment of the mitotic spindle in vascular plant cells

The microtubular cytoskeleton plays a major role in cellular organization and proliferation. The first step in construction of a microtubule is microtubule nucleation. Individual microtubules then participate in organization of more complex microtubule arrays. A strong body of evidence suggests that the underlying molecular mechanisms involve protein complexes that are conserved among eukaryotes. However, plant cell specificities, mainly characterized by the presence of a cell wall and the absence of centrosomes, must be taken into account to understand their mitotic processes. The goal of this review is to summarize and discuss current knowledge regarding the mechanisms involved in plant spindle assembly during early mitotic events. The functions of the proteins currently characterized at microtubule nucleation sites and involved in spindle assembly are considered during cell-cycle progression from G2 phase to metaphase.

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

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

The microtubule-associated kinase-like protein RUNKEL functions in somatic and syncytial cytokinesis

The microtubule (MT)-associated putative kinase RUNKEL (RUK) is an important component of the phragmoplast machinery involved in cell plate formation in Arabidopsis somatic cytokinesis. Since loss-of-function ruk mutants display seedling lethality, it was previously not known whether RUK functions in mature sporophytes or during gametophyte development. In this study we utilized RUK proteins that lack the N-terminal kinase domain to further examine biological processes related to RUK function. Truncated RUK proteins when expressed in wild-type Arabidopsis plants cause cellularization defects not only in seedlings and adult tissues but also during male meiocyte development, resulting in abnormal pollen and reduced fertility. Ultrastructural analysis of male tetrads revealed irregular and incomplete or absent intersporal cell walls, caused by disorganized radial MT arrays. Moreover, in ruk mutants endosperm cellularization defects were also caused by disorganized radial MT arrays. Intriguingly, in seedlings expressing truncated RUK proteins, the kinesin HINKEL, which is required for the activation of a mitogen-activated protein kinase signaling pathway regulating phragmoplast expansion, was mislocalized. Together, these observations support a common role for RUK in both phragmoplast-based cytokinesis in somatic cells and syncytial cytokinesis in reproductive cells.

MAP65/Ase1 promote microtubule flexibility

Microtubules (MTs) are dynamic cytoskeletal elements involved in numerous cellular processes. Although they are highly rigid polymers with a persistence length of 1-8 mm, they may exhibit a curved shape at a scale of few micrometers within cells, depending on their biological functions. However, how MT flexural rigidity in cells is regulated remains poorly understood. Here we ask whether MT-associated proteins (MAPs) could locally control the mechanical properties of MTs. We show that two major cross-linkers of the conserved MAP65/PRC1/Ase1 family drastically decrease MT rigidity. Their MT-binding domain mediates this effect. Remarkably, the softening effect of MAP65 observed on single MTs is maintained when MTs are cross-linked. By reconstituting physical collisions between growing MTs/MT bundles, we further show that the decrease in MT stiffness induced by MAP65 proteins is responsible for the sharp bending deformations observed in cells when they coalign at a steep angle to create bundles. Taken together, these data provide new insights into how MAP65, by modifying MT mechanical properties, may regulate the formation of complex MT arrays.

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.

MAP65-1 and MAP65-2 promote cell proliferation and axial growth in Arabidopsis roots

We investigated the role of the Arabidopsis microtubule associated proteins 65-1 and 65-2 (MAP65-1 and MAP65-2) in the control of axial root growth. Transgenic plants expressing fluorescent fusion proteins from native promoters indicated exactly overlapping accumulation of MAP65-1 and MAP65-2 in the root tip and elongation zone. Nearly identical protein accumulation patterns were observed when MAP65-1 and MAP65-2 were expressed behind a constitutive CaMV 35S promoter, suggesting a level of post-transcriptional control that restricts these proteins to rapidly growing portions of the root. Co-expression of MAP65-1 and MAP65-2 fusion proteins showed precise co-localization to interphase and cytokinetic microtubule arrays. In interphase root tip cells, the fluorescent protein fusions labeled microtubules that were organized into a variety of different array patterns. In the rapidly growing cells of the root elongation zone, we found MAP65-1 and MAP65-2 co-localized exclusively to the lateral faces of cells that were axially extending. Genetic analysis showed that MAP65-1 and MAP65-2 are coordinately required for proper root elongation. Double map65-1-1 map65-2-2 mutant roots from dark-grown plants contained 50% fewer cells per file than wild-type roots, but we found no evidence that cytokinesis was disrupted. We additionally discovered that cell length was significantly shorter in the mature regions of the root beyond the zone where MAP65-1 and MAP65-2 accumulated. Our data indicate that MAP65-1 and MAP65-2 play a critical role in root growth by promoting cell proliferation and axial extension.

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.

Microtubules and mitogen-activated protein kinase signalling

Subcellular signalling by mitogen-activated protein kinases (MAPKs) was originally regarded as a means to regulate microtubule (MT) organization and dynamics, but with time MAPKs were assigned to broader roles concerning biotic and abiotic signal transductions. MAPKs, which regulate a broad spectrum of substrates including transcription factors and cytoskeletal proteins, belong to complex MAPK cascades, which are mainly involved in plant development and in plant stress responses. The fact that single MAPK can be regulated by more than a single MAPKKK/MAPKK pair make MAPK signalling modules versatile tools in the regulation of microtubule organization. Until recently, the best-studied MAPK module implicated in cytoskeletal regulation is the NACK-PQR pathway in tobacco (Nicotiana tabacum). Homologues of each constituent of this pathway were also discovered in Arabidopsis thaliana. So far, direct phosphorylation of tubulins by MAPKs has not been shown. However, the first MAPK-related substrate involved in the regulation of MT dynamics to have been identified is MT-associated protein MAP65-1.

A role for katanin in plant cell division: microtubule organization in dividing root cells of fra2 and lue1Arabidopsis thaliana mutants

Severing of microtubules by katanin has proven to be crucial for cortical microtubule organization in elongating and differentiating plant cells. On the contrary, katanin is currently not considered essential during cell division in plants as it is in animals. However, defects in cell patterning have been observed in katanin mutants, implying a role for it in dividing plant cells. Therefore, microtubule organization was studied in detail by immunofluorescence in dividing root cells of fra2 and lue1 katanin mutants of Arabidopsis thaliana. In both, early preprophase bands consisted of poorly aligned microtubules, prophase spindles were multipolar, and the microtubules of expanding phragmoplasts were elongated, bended toward and connected to the surface of daughter nuclei. Accordingly, severing by katanin seems to be necessary for the proper organization of these microtubule arrays. In both fra2 and lue1, metaphase/anaphase spindles and initiating phragmoplasts exhibited typical organization. However, they were obliquely oriented more frequently than in the wild type. It is proposed that this oblique orientation may be due to prophase spindle multipolarity and results in a failure of the cell plate to follow the predetermined division plane, during cytokinesis, producing oblique cell walls in the roots of both mutants. It is therefore concluded that, like in animal cells, katanin is important for plant cell division, influencing the organization of several microtubule arrays. Moreover, failure in microtubule severing indirectly affects the orientation of the division plane.

Plant organelle proteomics: collaborating for optimal cell function

Organelle proteomics describes the study of proteins present in organelle at a particular instance during the whole period of their life cycle in a cell. Organelles are specialized membrane bound structures within a cell that function by interacting with cytosolic and luminal soluble proteins making the protein composition of each organelle dynamic. Depending on organism, the total number of organelles within a cell varies, indicating their evolution with respect to protein number and function. For example, one of the striking differences between plant and animal cells is the plastids in plants. Organelles have their own proteins, and few organelles like mitochondria and chloroplast have their own genome to synthesize proteins for specific function and also require nuclear-encoded proteins. Enormous work has been performed on animal organelle proteomics. However, plant organelle proteomics has seen limited work mainly due to: (i) inter-plant and inter-tissue complexity, (ii) difficulties in isolation of subcellular compartments, and (iii) their enrichment and purity. Despite these concerns, the field of organelle proteomics is growing in plants, such as Arabidopsis, rice and maize. The available data are beginning to help better understand organelles and their distinct and/or overlapping functions in different plant tissues, organs or cell types, and more importantly, how protein components of organelles behave during development and with surrounding environments. Studies on organelles have provided a few good reviews, but none of them are comprehensive. Here, we present a comprehensive review on plant organelle proteomics starting from the significance of organelle in cells, to organelle isolation, to protein identification and to biology and beyond. To put together such a systematic, in-depth review and to translate acquired knowledge in a proper and adequate form, we join minds to provide discussion and viewpoints on the collaborative nature of organelles in cell, their proper function and evolution.

An ungrouped plant kinesin accumulates at the preprophase band in a cell cycle-dependent manner

Past phylogenic studies have identified a plant-specific, ungrouped family of kinesins in which the motor domain does not group to one of the fourteen recognized families. Members of this family contain an N-terminal motor domain, a C-terminal armadillo repeat domain and a conserved destruction box (D-BOX) motif. This domain architecture is unique to plants and to a subset of protists. Further characterization of one representative member from Arabidopsis, Arabidopsis thaliana KINESIN ungrouped clade, gene A (AtKINUa), was completed to ascertain its functional role in plants. Fluorescence confocal microscopy revealed an accumulation of ATKINUA:GFP at the preprophase band (PPB) in a cell cycle-dependent manner in Arabidopsis epidermal cells and tobacco BY-2 cells. Fluorescence accumulation was highest during prophase and decreased after nuclear envelope breakdown. A conserved D-BOX motif was identified through alignment of AtKINU homologous sequences. Mutagenesis work with D-BOX revealed that conserved residues were necessary for the observed degradation pattern of ATKINUA:GFP, as well as the targeted accumulation at the PPB. Overall results suggest that AtKINUa is necessary for normal plant growth and/or development and is likely involved with PPB organization through microtubule association and specific cell cycle regulation. The D-BOX motif may function to bridge microtubule organization with changes that occur during progression through mitosis and may represent a novel regulatory motif in plant microtubule motor proteins.

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.

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.

Arabidopsis kinetochore fiber-associated MAP65-4 cross-links microtubules and promotes microtubule bundle elongation

The acentrosomal plant mitotic spindle is uniquely structured in that it lacks opposing centrosomes at its poles and is equipped with a connective preprophase band that regulates the spatial framework for spindle orientation and mobility. These features are supported by specialized microtubule-associated proteins and motors. Here, we show that Arabidopsis thaliana MAP65-4, a non-motor microtubule associated protein (MAP) that belongs to the evolutionarily conserved MAP65 family, specifically associates with the forming mitotic spindle during prophase and with the kinetochore fibers from prometaphase to the end of anaphase. In vitro, MAP65-4 induces microtubule (MT) bundling through the formation of cross-bridges between adjacent MTs both in polar and antipolar orientations. The association of MAP65-4 with an MT bundle is concomitant with its elongation. Furthermore, MAP65-4 modulates the MT dynamic instability parameters of individual MTs within a bundle, mainly by decreasing the frequency of catastrophes and increasing the frequency of rescue events, and thereby supports the progressive lengthening of MT bundles over time. These properties are in line with its role of initiating kinetochore fibers during prospindle formation.