Structure of cortical microtubule arrays in plant cells

Genetic evidence that cellulose synthase activity influences microtubule cortical array organization

To identify factors that influence cytoskeletal organization we screened for Arabidopsis (Arabidopsis thaliana) mutants that show hypersensitivity to the microtubule destabilizing drug oryzalin. We cloned the genes corresponding to two of the 131 mutant lines obtained. The genes encoded mutant alleles of PROCUSTE1 and KORRIGAN, which both encode proteins that have previously been implicated in cellulose synthesis. Analysis of microtubules in the mutants revealed that both mutants have altered orientation of root cortical microtubules. Similarly, isoxaben, an inhibitor of cellulose synthesis, also altered the orientation of cortical microtubules while exogenous cellulose degradation did not. Thus, our results substantiate that proteins involved in cell wall biosynthesis influence cytoskeletal organization and indicate that this influence on cortical microtubule stability and orientation is correlated with cellulose synthesis rather than the integrity of the cell wall.

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.

"Second extrinsic organizational mechanism" for orienting cellulose: modeling a role for the plasmalemmal reticulum

Oriented deposition of cellulose fibers by cellulose-synthesizing complexes typically occurs across the plasma membrane from microtubule bundles and is guided by them. However, aligned movement of the complexes can be shown even after applied oryzalin has depolymerized microtubules. Further, there is a claim that when (1) microtubules are depolymerized with oryzalin, (2) a microtubule-orienting stimulus is applied temporarily, and (3) oryzalin is washed out, the newly forming cellulose fibers are oriented with respect to the stimulus. With this in mind, the present paper gathers evidence from a diverse literature to suggest that the plasmalemmal reticulum, a major and structurally important form of cytoskeleton which connects cortical cytoplasm with wall, is a candidate to both independently and cooperatively participate in orienting microtubules and routing movements of cellulose-synthesizing complexes. Critical to this proposed function, the adhesion sites of the plasmalemmal reticulum have some morphological and molecular similarities to animal cell adhesion sites, known to play numerous integrative roles. The reticulum itself may be the morphological manifestation of the so-called lipid raft, previously known only on the basis of biochemical properties. According to the working model, the trusses interconnecting the adhesion sites shape the reticulum into apparently situation-dependent geometries. For example, in nongrowing or nonpolarized cells in which cellulose is deposited in brushy meshes, they form a nonpolar or weakly polar net; however, in elongating cells with oblique or otherwise polarized microtubules and newly forming cellulose fibers, there is suggestive evidence that net formation is dominated by trusses organized with correspondingly biased orientation. Consideration of such geometries and roles of the reticulum suggests several tests that could affirm, deny, or replace key aspects of this proposal to expand the theory of the peripheral cytoskeleton.

Effect of steady torque twisting on the orientation of cortical microtubules in the epidermis of the sunflower hypocotyl

Orientation of cortical microtubules (cMTs) is suggested to be affected by mechanical stress existing in cell walls. However, in mutants exhibiting helical (chiral) growth, there is a correlation between orientation of cMTs in outer tissues and helical growth direction. The aim of this research was to examine the effect of a chiral mechanical stimulation on cMTs. For this purpose, the orientation of cMTs was investigated in hypocotyls subjected to either a right- or a left-handed twist, resulting from a steady torque. cMTs were visualised in fixed material using the immunofluorescence method. The cMTs in untouched control hypocotyls were mostly transverse with respect to the cell long axis. In immobilised, but not twisted control hypocotyls, the transverse orientation was also most frequent, while applied twisting resulted in a change in cMT orientation from transverse to oblique. The data provide additional evidence that changes in tissue stress can be reorganized by cortical microtubules.

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.

Demarcation of the cortical division zone in dividing plant cells

Somatic cytokinesis in higher plants involves, besides the actual construction of a new cell wall, also the determination of a division zone. Several proteins have been shown to play a part in the mechanism that somatic plant cells use to control the positioning of the new cell wall. Plant cells determine the division zone at an early stage of cell division and use a transient microtubular structure, the preprophase band (PPB), during this process. The PPB is formed at the division zone, leaving behind a mark that during cytokinesis is utilized by the phragmoplast to guide the expanding cell plate toward the correct cortical insertion site. This review discusses old and new observations with regard to mechanisms implicated in the orientation of cell division and determination of a cortical division zone.

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.

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

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

n-Butanol induces depolymerization of microtubules in vivo and in vitro

The effects of butanol on microtubules (MTs) were examined by immunofluorescence microscopy. Fragmentation of cortical MTs was induced by n-butanol, but not by s- and t-butanols, in cultured tobacco BY-2 cells. Taxol prevented n-butanol-induced MT fragmentation. Fragmented cortical MTs were still attached to the inner face of the plasma membrane when n-butanol-treated protoplasts were ruptured on the slide glass. Moreover, MTs were depolymerized in the presence of n-butanol in vitro. Therefore, n-butanol is not only an activator of phospholipase D but also an effective MT-depolymerizing agent.

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.

Establishment of polarity during organization of the acentrosomal plant cortical microtubule array

The plant cortical microtubule array is a unique acentrosomal array that is essential for plant morphogenesis. To understand how this array is organized, we exploited the microtubule (+)-end tracking activity of two Arabidopsis EB1 proteins in combination with FRAP (fluorescence recovery after photobleaching) experiments of GFP-tubulin to examine the relationship between cortical microtubule array organization and polarity. Significantly, our observations show that the majority of cortical microtubules in ordered arrays, within a particular cell, face the same direction in both Arabidopsis plants and cultured tobacco cells. We determined that this polar microtubule coalignment is at least partially due to a selective stabilization of microtubules, and not due to a change in microtubule polymerization rates. Finally, we show that polar microtubule coalignment occurs in conjunction with parallel grouping of cortical microtubules and that cortical array polarity is progressively enhanced during array organization. These observations reveal a novel aspect of plant cortical microtubule array organization and suggest that selective stabilization of dynamic cortical microtubules plays a predominant role in the self-organization of cortical arrays.

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.

In vivo dynamics and differential microtubule-binding activities of MAP65 proteins

Plant cells produce different microtubule arrays that are essential for cell division and morphogenesis without equivalent in other eukaryotes. Microtubule-associated proteins influence the behavior of microtubules that is presumed to culminate into transitions from one array to another. We analyzed the microtubule-binding properties of three Arabidopsis (Arabidopsis thaliana) members, AtMAP65-1, AtMAP65-4, and AtMAP65-5, in live cells using laser scanning confocal microscopy. Depending on the overall organization of the cortical array, AtMAP65-1-GFP (green fluorescent protein) and AtMAP65-5-GFP associated with a subset of microtubules. In cells containing both coaligned and oblique microtubules, AtMAP65-1-GFP and AtMAP65-5-GFP tended to be associated with the coaligned microtubules. Cortical microtubules labeled with AtMAP65-1-GFP and AtMAP65-5-GFP appeared as thick bundles and showed more resistance to microtubule-destabilizing drugs. The polymerization rates of AtMAP65-1-GFP and AtMAP65-5-GFP microtubules were similar to those of tubulin-GFP marked microtubules but were different from AtEB1a-GFP, a microtubule plus-end-binding EB1-like protein that stimulated polymerization. By contrast, depolymerization rates of AtMAP65-1-GFP- and AtMAP65-5-GFP-labeled microtubules were reduced. AtMAP65-1-GFP associated with polymerizing microtubules within a bundle, and with fixed microtubule termini, suggesting that AtMAP65-1's function is to bundle and stabilize adjacent microtubules of the cortex. Polymerization within a bundle took place in either direction so that bundling occurred between parallel or antiparallel aligned microtubules. AtMAP65-4-GFP did not label cortical microtubules or the preprophase band, despite continuous expression driven by the 35S promoter, and its subcellular localization was restricted to microtubules that rearranged to form a spindle and the polar sides of the spindle proper. The expression of AtMAP65-4 peaked at mitosis, in agreement with a function related to spindle formation, whereas AtMAP65-1 and AtMAP65-5 were expressed throughout the cell cycle.

Encounters between dynamic cortical microtubules promote ordering of the cortical array through angle-dependent modifications of microtubule behavior

Ordered cortical microtubule arrays are essential for normal plant morphogenesis, but how these arrays form is unclear. The dynamics of individual cortical microtubules are stochastic and cannot fully account for the observed order; however, using tobacco (Nicotiana tabacum) cells expressing either the MBD-DsRed (microtubule binding domain of the mammalian MAP4 fused to the Discosoma sp red fluorescent protein) or YFP-TUA6 (yellow fluorescent protein fused to the Arabidopsis alpha-tubulin 6 isoform) microtubule markers, we identified intermicrotubule interactions that modify their stochastic behaviors. The intermicrotubule interactions occur when the growing plus-ends of cortical microtubules encounter previously existing cortical microtubules. Importantly, the outcome of such encounters depends on the angle at which they occur: steep-angle collisions are characterized by approximately sevenfold shorter microtubule contact times compared with shallow-angle encounters, and steep-angle collisions are twice as likely to result in microtubule depolymerization. Hence, steep-angle collisions promote microtubule destabilization, whereas shallow-angle encounters promote both microtubule stabilization and coalignment. Monte Carlo modeling of the behavior of simulated microtubules, according to the observed behavior of transverse and longitudinally oriented cortical microtubules in cells, reveals that these simple rules for intermicrotubule interactions are necessary and sufficient to facilitate the self-organization of dynamic microtubules into a parallel configuration.

Molecular dissection of plant cytokinesis and phragmoplast structure: a survey of GFP-tagged proteins

To identify molecular players implicated in cytokinesis and division plane determination, the Arabidopsis thaliana genome was explored for potential cytokinesis genes. More than 100 open reading frames were selected based on similarity to yeast and animal cytokinesis genes, cytoskeleton and polarity genes, and Nicotiana tabacum genes showing cell cycle-controlled expression. The subcellular localization of these proteins was determined by means of GFP tagging in tobacco Bright Yellow-2 cells and Arabidopsis plants. Detailed confocal microscopy identified 15 proteins targeted to distinct regions of the phragmoplast and the cell plate. EB1- and MAP65-like proteins were associated with the plus-end, the minus-end, or along the entire length of microtubules. The actin-binding protein myosin, the kinase Aurora, and a novel cell cycle protein designated T22, accumulated preferentially at the midline. EB1 and Aurora, in addition to other regulatory proteins (homologs of Mob1, Sid1, and Sid2), were targeted to the nucleus, suggesting that this organelle operates as a coordinating hub for cytokinesis.

Microtubules become more dynamic but not shorter during preprophase band formation: a possible "search-and-capture" mechanism for microtubule translocation

The dynamic behavior of the microtubule cytoskeleton plays a crucial role in cellular organization, but the physical mechanisms underlying microtubule (re)organization in plant cells are poorly understood. We investigated microtubule dynamics in tobacco BY-2 suspension cells during interphase and during the formation of the preprophase band (PPB), the cytoskeletal structure that defines the site of cytokinesis. Here we show that after 2 h of microtubule accumulation in the PPB and concurrent disappearance elsewhere in the cortex, the PPB is completed and starts to breakdown exponentially already 20 min before the onset of prometaphase. During formation of the PPB, the dynamic instability, i.e., the stochastic alternating between growing and shrinking phases, of the cortical microtubules outside the PPB increases significantly, but the microtubules do not become shorter. Based on this, as well as on the cross-linking of microtubules in the PPB and the lack of evidence for motor involvement, we propose a "search-and-capture" mechanism for PPB formation, in which the regulation of dynamic instability causes the cortical microtubules to become more dynamic and possibly longer, while the microtubule cross-linking activity of the developing PPB preferentially stabilizes these "searching" microtubules. Thus, microtubules gradually disappear from the cortex outside the PPB and aggregate to the forming PPB.

The EF-hand Ca2+-binding protein p22 plays a role in microtubule and endoplasmic reticulum organization and dynamics with distinct Ca2+-binding requirements

We have reported that p22, an N-myristoylated EF-hand Ca(2+)-binding protein, associates with microtubules and plays a role in membrane trafficking. Here, we show that p22 also associates with membranes of the early secretory pathway membranes, in particular endoplasmic reticulum (ER). On binding of Ca(2+), p22's ability to associate with membranes increases in an N-myristoylation-dependent manner, which is suggestive of a nonclassical Ca(2+)-myristoyl switch mechanism. To address the intracellular functions of p22, a digitonin-based "bulk microinjection" assay was developed to load cells with anti-p22, wild-type, or mutant p22 proteins. Antibodies against a p22 peptide induce microtubule depolymerization and ER fragmentation; this antibody-mediated effect is overcome by preincubation with the respective p22 peptide. In contrast, N-myristoylated p22 induces the formation of microtubule bundles, the accumulation of ER structures along the bundles as well as an increase in ER network formation. An N-myristoylated Ca(2+)-binding p22 mutant, which is unable to undergo Ca(2+)-mediated conformational changes, induces microtubule bundling and accumulation of ER structures along the bundles but does not increase ER network formation. Together, these data strongly suggest that p22 modulates the organization and dynamics of microtubule cytoskeleton in a Ca(2+)-independent manner and affects ER network assembly in a Ca(2+)-dependent manner.

Phospholipase d activation correlates with microtubule reorganization in living plant cells

A phospholipase D (PLD) was shown recently to decorate microtubules in plant cells. Therefore, we used tobacco BY-2 cells expressing the microtubule reporter GFP-MAP4 to test whether PLD activation affects the organization of plant microtubules. Within 30 min of adding n-butanol, a potent activator of PLD, cortical microtubules were released from the plasma membrane and partially depolymerized, as visualized with four-dimensional confocal imaging. The isomers sec- and tert-butanol, which did not activate PLD, did not affect microtubule organization. The effect of treatment on PLD activation was monitored by the in vivo formation of phosphatidylbutanol, a specific reporter of PLD activity. Tobacco cells also were treated with mastoparan, xylanase, NaCl, and hypoosmotic stress as reported activators of PLD. We confirmed the reports and found that all treatments induced microtubule reorganization and PLD activation within the same time frame. PLD still was activated in microtubule-stabilized (taxol) and microtubule-depolymerized (oryzalin) situations, suggesting that PLD activation triggers microtubular reorganization and not vice versa. Exogenously applied water-soluble synthetic phosphatidic acid did not affect the microtubular cytoskeleton. Cell cycle studies revealed that n-butanol influenced not just interphase cortical microtubules but also those in the preprophase band and phragmoplast, but not those in the spindle structure. Cell growth and division were inhibited in the presence of n-butanol, whereas sec- and tert-butanol had no such effects. Using these novel insights, we propose a model for the mechanism by which PLD activation triggers microtubule reorganization in plant cells.

Timing of neuronal and glial ultrastructure disruption during brain slice preparation and recovery in vitro

Hippocampal slices often have more synapses than perfusion-fixed hippocampus, but the cause of this synaptogenesis is unclear. Ultrastructural evidence for synaptogenic triggers during slice preparation was investigated in 21-day-old rats. Slices chopped under warm or chilled conditions and fixed after 0, 5, 25, 60, or 180 minutes of incubation in an interface chamber were compared with hippocampi fixed by perfusion or by immersion of the whole hippocampus. There was no significant synaptogenesis in these slices compared with perfusion-fixed hippocampus, but there were other structural changes during slice preparation and recovery in vitro. Whole hippocampus and slices prepared under warm conditions exhibited an increase in axonal coated vesicles, suggesting widespread neurotransmitter release. Glycogen granules were depleted from astrocytes and neurons in 0-min slices, began to reappear by 1 hour, and had fully recovered by 3 hours. Dendritic microtubules were initially disassembled in slices, but reassembled into normal axial arrays after 5 minutes. Microtubules were short at 5 minutes (12.3 +/- 1.1 microm) but had recovered normal lengths by 3 hours (84.6 +/- 20.0 microm) compared with perfusion-fixed hippocampus (91 +/- 22 microm). Microtubules appeared transiently in 15 +/- 3% and 9 +/- 4% of dendritic spines 5 and 25 minutes after incubation, respectively. Spine microtubules were absent from perfusion-fixed hippocampus and 3-hour slices. Ice-cold dissection and vibratomy in media that blocked activity initially produced less glycogen loss, coated vesicles, and microtubule disassembly. Submersing these slices in normal oxygenated media at 34 degrees C led to glycogen depletion, as well as increased coated vesicles and microtubule disassembly within 1 minute.

Sustained microtubule treadmilling in Arabidopsis cortical arrays

Plant cells create highly structured microtubule arrays at the cell cortex without a central organizing center to anchor the microtubule ends. In vivo imaging of individual microtubules in Arabidopsis plants revealed that new microtubules are initiated at the cell cortex and exhibit dynamics at both ends. Polymerization-biased dynamic instability at one end and slow depolymerization at the other end result in sustained microtubule migration across the cell cortex by a hybrid treadmilling mechanism. This motility causes widespread microtubule repositioning and contributes to changes in array organization through microtubule reorientation and bundling.

Alteration of microtubule dynamic instability during preprophase band formation revealed by yellow fluorescent protein-CLIP170 microtubule plus-end labeling

At the onset of mitosis, plant cells form a microtubular preprophase band that defines the plane of cell division, but the mechanism of its formation remains a mystery. Here, we describe the use of mammalian yellow fluorescent protein-tagged CLIP170 to visualize the dynamic plus ends of plant microtubules in transfected cowpea protoplasts and in stably transformed and dividing tobacco Bright Yellow 2 cells. Using plus-end labeling, we observed dynamic instability in different microtubular conformations in live plant cells. The interphase plant microtubules grow at 5 micro m/min, shrink at 20 micro m/min, and display catastrophe and rescue frequencies of 0.02 and 0.08 events/s, respectively, exhibiting faster turnover than their mammalian counterparts. Strikingly, during preprophase band formation, the growth rate and catastrophe frequency of plant microtubules double, whereas the shrinkage rate and rescue frequency remain unchanged, making microtubules shorter and more dynamic. Using these novel insights and four-dimensional time-lapse imaging data, we propose a model that can explain the mechanism by which changes in microtubule dynamic instability drive the dramatic rearrangements of microtubules during preprophase band and spindle formation in plant cells.

A kinesin-like protein is essential for oriented deposition of cellulose microfibrils and cell wall strength

Cortical microtubules have long been hypothesized to regulate the oriented deposition of cellulose microfibrils. However, the molecular mechanisms of how microtubules direct the orientation of cellulose microfibril deposition are not known. We have used fibers in the inflorescence stems of Arabidopsis to study secondary wall deposition and cell wall strength and found a fragile fiber (fra1) mutant with a dramatic reduction in the mechanical strength of fibers. The fra1 mutation did not cause any defects in cell wall composition, secondary wall thickening, or cortical microtubule organization in fiber cells. An apparent alteration was found in the orientation of cellulose microfibrils in fra1 fiber walls, indicating that the reduced mechanical strength of fra1 fibers probably was attributable to altered cellulose microfibril deposition. The FRA1 gene was cloned and found to encode a kinesin-like protein with an N-terminal microtubule binding motor domain. The FRA1 protein was shown to be concentrated around the periphery of the cytoplasm but absent in the nucleus. Based on these findings, we propose that the FRA1 kinesin-like protein is involved in the microtubule control of cellulose microfibril order.

Microtubule organization in the green kingdom: chaos or self-order?

Plant microtubule arrays differ fundamentally from their animal, fungal and protistan counterparts. These differences largely reflect the requirements of plant composite polymer cell walls and probably also relate to the acquisition of chloroplasts. Plant microtubules are usually dispersed and lack conspicuous organizing centres. The key to understanding this dispersed nature is the identification of proteins that interact with and regulate the spatial and dynamic properties of microtubules. Over the past decade, a number of these proteins have been uncovered, including numerous kinesin-related proteins and a 65 kDa class of structural microtubule-associated proteins that appear to be unique to plants. Mutational analysis has identified MOR1, a probable stabilizer of microtubules that is a homologue of the TOGp-XMAP215 class of high-molecular-weight microtubule-associated proteins, and a katanin p60 subunit homologue implicated in the severing of microtubules. The identification of these two proteins provides new insights into the mechanisms controlling microtubule assembly and dynamics, particularly in the dispersed cortical array found in highly polarized plant cells.

2',3'-Cyclic nucleotide 3'-phosphodiesterase: a membrane-bound, microtubule-associated protein and membrane anchor for tubulin

2',3'-Cyclic nucleotide-3'-phosphodiesterase (CNP) is firmly associated with tubulin from brain tissue and FRTL-5 thyroid cells as demonstrated by copolymerization with microtubules through several warm/cold cycles, the presence of CNP activity in purified tubulin preparations, and identical behavior during various extraction procedures. CNP acts as a microtubule-associated protein in promoting microtubule assembly at low mole ratios. This activity resides in the C terminus of the enzyme, which, by itself, promotes microtubule assembly at higher mole ratios. Phosphorylation of CNP interferes with its assembly-promoting activity, as does deletion of the C terminus, which leads to abnormal microtubule distribution in the cell. Submembranous colocalization of the proteins and CNP-dependent microtubule organization suggest that CNP is a membrane-bound microtubule-associated protein that can link tubulin to membranes and may regulate cytoplasmic microtubule distribution.

Plasma membrane localization of palmitoylated tubulin

PC12 pheochromocytoma cells incorporate [(3)H]palmitic acid into tubulin in a time- and cell-density-dependent manner. The plasma membrane-enriched fraction contains most of the radioactivity of the membrane pellet. While palmitoylated tubulin is found in both the cytoplasm and particulate fraction, the bulk of [(3)H]palmitic acid bound to tubulin is present in the crude membrane pellet and the tubulin extracted from the plasma membrane is more heavily palmitoylated than that extracted from endoplasmic reticulum. Detergent-extracted tubulin from plasma membrane is, to a large extent, polymerization competent; a substantial fraction, increasing as a function of labeling time, is not hydroxylamine-labile. The requirement for detergent extraction, the accompanying changes in tubulin properties and the present findings of preferential incorporation of labeled tubulin into plasma membranes, make it clear that direct incorporation of tubulin into the plasma membrane can occur.

Isolation of cortical MTs from tobacco BY-2 cells

We isolated the cortical microtubules (CMTs) from tobacco BY-2 cells to identify their components. By centrifugation of protoplasts homogenized in the presence of taxol, a MT-stabilizing reagent, in a density gradient of Percoll, we obtained membranous vesicles to which MTs forming a sheet-like bundle were attached. Rhodamine-conjugated Ricinus communis agglutinin I (RCA-I), a lectin that bound to the surface of protoplasts, stained these vesicles, indicating that they were plasma membrane (PM) vesicles that retained CMTs. CMTs were released by solubilization of PM vesicles with Triton X-100. A sheet-like array of CMTs was retained even after solubilization of PM vesicles. Immunoblot analysis of the isolated CMTs demonstrated the presence of tubulin, actin, the 65 kDa microtubule-associated protein (MAP) and a 130 kDa RCA-I binding protein. Purification of the isolated CMTs by the temperature dependent disassembly-reassembly cycling method revealed four polypeptides, 190, 120, 85 and 65 kDa, co-assembling with CMTs.

In vitro assembled plant microtubules exhibit a high state of dynamic instability

Higher plants possess four distinct microtubule arrays. One of these, the cortical array, is involved in orienting the deposition of cellulose microfibrils. This plant interphase array is also notable because it contains exceptionally dynamic microtubules. Although the primary sequence of plant and animal tubulin is similar (79-87% amino acid identity overall) there are some regions of divergence. Thus, one possible explanation for the high state of polymer assembly and turnover that is observed in plant interphase arrays is that the tubulins have evolved differently and possess a higher intrinsic dynamic character than their animal counterparts. This hypothesis was tested using highly purified plant tubulin assembled in vitro. Using high-resolution DIC video-enhanced microscopy, we quantified the four characteristic parameters of dynamic instability of plant microtubules and compared them with animal microtubules. The elongation velocities between plant and animal microtubules are similar, but plant microtubules undergo catastrophes more frequently, do not exhibit any rescues, and have an average shortening velocity of 195 microm/min (compared with 21 microm/min for animal microtubules). These data support the hypothesis that plant tubulin forms microtubules that are intrinsically more dynamic than those of animals.

CYTOSKELETAL PERSPECTIVES ON ROOT GROWTH AND MORPHOGENESIS

Growth and development of all plant cells and organs relies on a fully functional cytoskeleton comprised principally of microtubules and microfilaments. These two polymeric macromolecules, because of their location within the cell, confer structure upon, and convey information to, the peripheral regions of the cytoplasm where much of cellular growth is controlled and the formation of cellular identity takes place. Other ancillary molecules, such as motor proteins, are also important in assisting the cytoskeleton to participate in this front-line work of cellular development. Roots provide not only a ready source of cells for fundamental analyses of the cytoskeleton, but the formative zone at their apices also provides a locale whereby experimental studies can be made of how the cytoskeleton permits cells to communicate between themselves and to cooperate with growth-regulating information supplied from the apoplasm.

The 65-kDa carrot microtubule-associated protein forms regularly arranged filamentous cross-bridges between microtubules

In plants, cortical microtubules (MTs) occur in characteristically parallel groups maintained up to one microtubule diameter apart by fine filamentous cross-bridges. However, none of the plant microtubule-associated proteins (MAPs) so far purified accounts for the observed separation between MTs in cells. We previously isolated from carrot cytoskeletons a MAP fraction including 120- and 65-kDa MAPs and have now separated the 65-kDa carrot MAP by sucrose density centrifugation. MAP65 does not induce tubulin polymerization but induces the formation of bundles of parallel MTs in a nucleotide-insensitive manner. The bundling effect is inhibited by porcine MAP2, but, unlike MAP2, MAP65 is heat-labile. In the electron microscope, MAP65 appears as filamentous cross-bridges, maintaining an intermicrotubule spacing of 25-30 nm. Microdensitometer-computer correlation analysis reveals that the cross-bridges are regularly spaced, showing a regular axial spacing that is compatible with a symmetrical helical superlattice for 13 protofilament MTs. Because MAP65 maintains in vitro the inter-MT spacing observed in plants and is shown to decorate cortical MTs, it is proposed that this MAP is important for the organization of the cortical array in vivo.

How I learned to love carrots: the role of the cytoskeleton in shaping plant cells

The carrot cell suspension was originally used because it provided a model system for studying directional cell expansion - a key process in plant morphogenesis. Early immunofluorescence studies of plant microtubules, using these cells, provided hints that the cortical array of microtubules was dynamic and this was later confirmed by microinjection studies on plant epidermal cells. A nonfixation approach for detecting F-actin was then developed on these cells and showed that, unlike animal cells, actin filaments remained associated with the nucleus throughout division and could have a role in aligning the plane of cell division. Currently, we are using detergent-extracted carrot cytoskeletons for isolating microtubule-associated proteins (MAPs). I discuss how MAPs may be involved in the oriented deposition of cellulose in the cell wall.

In situ synthesis of beta-glucan microfibrils on tobacco plasma membrane sheets

A major concern in plant morphogenesis is whether cortical microtubules are responsible for the arrangement and action of beta-glucan synthases in the plasma membrane. We prepared isolated plasma membrane sheets with cortical microtubules attached and tested whether beta-glucan synthases penetrated through the membrane to form microfibrils and whether these synthases moved in the fluid membrane along the cortical microtubules. This technique enabled us to examine synthesis of beta-glucan as a fiber with a two-dimensional structure. The synthesis of beta-glucan microfibrils was directed in arrays by cortical microtubules at many loci on the membrane sheets. The microfibrils were mainly arranged along the microtubules, but the distribution of microfibrils was not always parallel to that of the microtubules. The rate of beta-glucan elongation as determined directly on the exoplasmic surface was 620 nm per min. When the assembly of microtubules was disrupted by treatment with propyzamide, the beta-glucans were not deposited in arrays but in masses. This finding shows that the arrayed cortical microtubules are not required for beta-glucan synthesis but are required for the formation of arranged microfibrils on the membrane sheet.

Microtubule-organizing centers and nucleating sites in land plants

Microtubule-organizing centers (MTOCs) are morphologically diverse cellular sites involved in the nucleation and organization of microtubules (MTs). These structures are synonymous with the centrosome in mammalian cells. In most land plant cells, however, no such structures are observed and some have argued that plant cells may not have MTOCs. This review summarizes a number of experimental approaches toward the elucidation of those subcellular sites involved in microtubule nucleation and organization. In lower land plants, structurally well-defined MTOCs are present, such as the blepharoplast, multilayered structure, and polar organizer. In higher plants, much of the nucleation and organization of MTs occurs on the nuclear envelope or other endomembranes, such as the plasmalemma and smooth (tubular) endoplasmic reticulum. In some instances, one endomembrane may serve as a site of nucleation whereas others serve as the site of organization. Structural and motor microtubule-associated proteins also appear to be involved in MT nucleation and organization. Immunochemical evidence indicates that at least several of the proteins found in mammalian centrosomes, gamma-tubulin, centrin, pericentrin, and polypeptides recognized by the monoclonal antibodies MPM-2, 6C6, and C9 also recognize putative lower land plant MTOCs, indicating shared mechanisms of nucleation/organization in plants and animals. The most recent data from tubulin incorporation in vivo, mutants with altered MT organization, and molecular studies indicate the potential of these research tools in investigation of MTOCs in plants.

The characterization of plasma membrane-bound tubulin of cauliflower using Triton X-114 fractionation

The cortical microtubules determine how cellulose microfibrils are deposited in the plant cell wall and are thus important for the control of cell expansion. To understand how microtubules can control microfibril deposition, the components that link the microtubules to the plasma membrane (PM) of plant cells must be isolated. To obtain information on the properties of the tubulin-membrane associations, cauliflower (Brassica oleracea) PM was subjected to Triton X-114 fractionation, and the distribution of alpha- and beta-tubulin was analyzed using immunoblotting. Approximately one-half of the PM-associated tubulin was solubilized by Triton X-114 and 10 to 15% of both alpha- and beta-tubulin was recovered in the detergent phase (indicative of hydrophobic properties) and 30 to 40% was recovered in the aqueous phase. The hydrophobic tubulin could be released from the membrane by high pH extraction with preserved hydrophobicity. A large part of the PM-associated tubulin was found in the Triton-insoluble fraction. When this insoluble material was extracted a second time, a substantial amount of hydrophobic tubulin was released if the salt concentration was increased, suggesting that the hydrophobic tubulin was linked to a high-salt-sensitive protein aggregate that probably includes other components of the cytoskeleton. The hydrophobicity of a fraction of PM-associated tubulin could reflect a direct or indirect interaction of this tubulin with the lipid bilayer or with an integral membrane protein and may represent the anchoring of the cortical microtubules to the PM, a key element in the regulation of cell expansion.

Cell model systems in plant cytoskeleton studies

Cytoskeletons play an essential role in cellular functions in both animal and plant cells. In studies of the molecular mechanisms of their functions, a variety of cell model systems, mainly of animal cells, have yielded much information. With plant cells, cell model systems have mostly been restricted to studies on the mechanism of cytoplasmic streaming. Recently, however, there have been several reports of studies employing plant cell model systems to investigate plant cytoskeletons that have revealed new concepts about their structure and functions. To promote and support a general understanding of cell model systems, this review attempts to categorize them, present currently known information on the structure and function of plant cytoskeletons, and offer a possible role of cell model systems in future studies of plant cytoskeletons.

γ-Tubulin and microtubule organization in plants

In animal cells, microtubule assembly is usually initiated at one specialized structure, the centrosome. By contrast, in plant cells, microtubule assembly begins at a variety of locations within the cell. A member of the tubulin gene family, gamma-tubulin, is localized to the centrosome in animal cells and is important in the assembly of microtubules in vivo. Recent reports have identified gamma-tubulin genes in plants and have described the complex intracellular distribution of the encoded polypeptides. Here, Harish Joshi and Barry Palevitz comment upon how this information may help elucidate the organizing principles of the complex arrays of microtubules in plant cells.

Plant cell growth responds to external forces and the response requires intact microtubules

Microfibril deposition in most plant cells is influenced by cortical microtubules. Thus, cortical microtubules are templates that provide spatial information to the cell wall. How cortical microtubules acquire their spatial information and are positioned is unknown. There are indications that plant cells respond to mechanical stresses by using microtubules as sensing elements. Regenerating protoplasts from tobacco (Nicotiana tabacum) were used to determine whether cells can be induced to expand in a preferential direction in response to an externally applied unidirectional force. Additionally, an anti-microtubule herbicide was used to investigate the role of microtubules in the response to this force. Protoplasts were embedded in agarose, briefly centrifuged at 28 to 34g, and either cultured or immediately prepared for immunolocalization of their microtubules. The microtubules within many centrifuged protoplasts were found to be oriented parallel to the centrifugal force vector. Most protoplasts elongated with a preferential axis that was oriented 60 to 90 degrees to the applied force vector. Protoplasts treated transiently with the reversible microtubule-disrupting agent amiprophos-methyl (applied before and during centrifugation) elongated but without a preferential growth axis. These results indicate that brief biophysical forces may influence the alignment of cortical microtubules and that microtubules themselves act as biophysical responding elements.

Elucidating the mechanism of cortical microtubule reorientation in plant cells

Reorientation of the cortical microtubule array is an essential component of cellular development in plants. However, mechanistic details of this process are unknown. The cortical microtubule array of freshly isolated protoplasts (obtained from Nicotiana tabacum BY-2 suspension culture) is relatively random, but upon culturing the cell wall regenerates and the microtubules begin to reorganize. Because cortical microtubules are highly dynamic, we postulated that their reorganization is accomplished solely by the depolymerization of disordered microtubules, followed by repolymerization into an ordered array. This hypothesis was tested on freshly isolated protoplasts using drugs that alter the dynamic status of microtubules by either hyperstabilizing the polymer (taxol); or preventing the addition of subunits to the microtubules (amiprophosmethyl; APM). Microtubule arrays that were hyperstabilized with 10 microM taxol not only reordered, but did so more quickly than untreated cells. Moreover, protoplasts treated with taxol and 20 microM APM also showed accelerated reorganization. Control experiments, performed in vivo and in vitro, confirmed that subunit addition was hindered by APM. Thus, microtubules appear capable of reorienting as relatively intact units. Sodium azide (1 mM) and sodium cyanide (1 mM) can prevent reorientation, indicating that cellular energy is required for this event but this energy is not used by the actin-myosin system because the microfilament-disrupting drug cytochalasin D (50 microM) did not affect reorientation. These results indicate that cortical microtubule array reorganization is a complex process that can involve polymer movement.

Experimental manipulation of gamma-tubulin distribution in Arabidopsis using anti-microtubule drugs

gamma-Tubulin-specific antibodies stain the microtubule (Mt) arrays of Arabidopsis suspension cells in a punctate or patchy manner. During division, staining of kinetochore fibers and the phragmoplast is extensive, except in the vicinity of the plus ends at the metaphase plate and cell plate. gamma-Tubulin localization responds to low levels of colchicine, with staining receding farther toward the minus (pole) ends of kinetochore fibers. At higher drug concentrations, gamma-tubulin also associates with abnormal Mt foci as well as with the surface of the daughter nuclei facing the phragmoplast. During UV-induced recovery from colchicine, gamma-tubulin increases along the presumptive minus ends of mitotic Mts as well as the phragmoplast near the daughter nuclei. With CIPC, immunostaining is concentrated around the centers of focal Mt arrays in multipolar spindles. In the presence of taxol, Mts are more prominent but the mitotic apparatus and phragmoplast are abnormal. As with CIPC, gamma-tubulin is concentrated at focal arrays. Increased punctate staining is also present in interphase arrays, with fluorescent dots often located at the ends of Mts. These results support a preferential association between gamma-tubulin and Mt minus ends, but are also consistent with more general binding along the walls of Mts. Thus, minus ends (and Mt nucleation sites) may be present throughout plant Mt arrays, but gamma-tubulin may also serve another function, such as in structural stabilization.

Improvements in immunostaining samples embedded in methacrylate: localization of microtubules and other antigens throughout developing organs in plants of diverse taxa

Microtubules are important in plant growth and development. Localizing microtubules in sectioned material is advantageous because it allows any tissue of interest to be studied and it permits the positional relations of the cells within the organ to be known. We describe here a method that uses semi-thin (0.5-2 μm) sections of material embedded in butyl-methylmethacrylate, to which 10 mM dithiothreitol was added. After removing the embedding material and using indirect immunofluorescence staining, we obtain clear images of microtubules, actin microfilaments, callose and pulse-fed bromodeoxyuridine. This method works on the root tissues of Arabidopsis thaliana(L.) Heynh, Pinus radiataD. Don, Zamia furfuraceaAit., Azolla pinnataR. Br. and on sporophytic tissues of Funaria hygrometricaHedw. In general, most of the cells in the organs studied are successfully stained. Using this method, we find that interphase meristematic cells in all of these species have microtubules not only in the usual cortical array but also throughout their cytoplasm. The presence of the calcium chelator ethylene glycol-bis(β-aminoethyl ether)N,N,N',N'-tetraacetic acid EGTA in fixation buffers led to some tissue damage, and did not enhance the preservation of microtubules. The common assumption that EGTA-containing buffers stabilize plant microtubules during fixation appears unwarranted.

The biochemistry of compounds with anti-microtubule activity in plant cells

The experimental use of anti-microtubule compounds has revealed essential functions of microtubules in plant cytoskeletal arrays, including the pre-prophase band, the mitotic and meiotic spindles, the phragmoplast, and the cortical array. The most commonly used plant microtubule depolymerization compounds are colchicine, and several synthetic herbicides belonging to three different chemical classes, the dinitroanilines, phosphoric amides, and N-phenyl carbamates. Taxol, a secondary plant product, is the only drug found to promote the polymerization of plant microtubules. This paper summarizes our current understanding of the biochemical interactions of colchicine, anti-microtubule herbicides, and taxol with plant tubulin and microtubules.