De-Esterified Homogalacturonan Enrichment of the Cell Wall Region Adjoining the Preprophase Cortical Cytoplasmic Zone in Some Protodermal Cell Types of Three Land Plants.

The distribution of highly de-esterified homogalacturonans (HGs) in dividing protodermal cells of the monocotyledon Zea mays, the dicotyledon Vigna sinensis, and the fern Asplenium nidus was investigated in order to examine whether the cell wall region adjoining the preprophase band (PPB) is locally diversified. Application of immunofluorescence revealed that de-esterified HGs were accumulated selectively in the cell wall adjacent to the PPB in: (a) symmetrically dividing cells of stomatal rows of Z. mays, (b) the asymmetrically dividing protodermal cells of Z. mays, (c) the symmetrically dividing guard cell mother cells (GMCs) of Z. mays and V. sinensis, and (d) the symmetrically dividing protodermal cells of A. nidus. A common feature of the above cell types is that the cell division plane is defined by extrinsic cues. The presented data suggest that the PPB cortical zone-plasmalemma and the adjacent cell wall region function in a coordinated fashion in the determination/accomplishment of the cell division plane, behaving as a continuum. The de-esterified HGs, among other possible functions, might be involved in the perception and the transduction of the extrinsic cues determining cell division plane in the examined cells.

Phosphorylation of a p38-like MAPK is involved in sensing cellular redox state and drives atypical tubulin polymer assembly in angiosperms.

Reactive oxygen species (ROS) imbalance is a stressful condition for plant cells accompanied by dramatic changes in tubulin cytoskeleton. Here, evidence is provided that alterations in ROS levels directly interfere with the phosphorylation state of a p38-like MAPK in the angiosperms Triticum turgidum and Arabidopsis thaliana. Both oxidative stress generators and chemicals inducing ROS scavenging or decreasing ROS production resulted in the accumulation of a phospho-p46 protein similar to p38-MAPK. Importantly, the rhd2 A. thaliana mutants exhibited a remarkable increase in levels of phospho-p46. The presence of the p38-MAPK inhibitor SB203580 attenuated the response to ROS disturbance, prevented microtubule disappearance and resulted in a dramatic decrease in the number of atypical tubulin polymers. Moreover, in roots treated simultaneously with substances inducing ROS overproduction and others resulting in low ROS levels, phospho-p46 levels and the organization of tubulin cytoskeleton were similar to controls. Collectively, our experimental data suggest, for the first time in plants, that p46 functions as a putative sensor of redox state, the activation of which initiates downstream signalling events leading to microtubule disruption and subsequent assembly of atypical tubulin polymers. Thus, p46 seems to participate in perception of ROS homeostasis disturbance as well as in cellular responses to redox imbalance.

Callose: a multifunctional (1, 3)-ß-D-glucan involved in morphogenesis and function of angiosperm stomata.

BACKGROUND: Although the cellulose microfibril organization in guard cell (GC) walls play a crucial role in the mechanism of the stomatal function, recent work showed that matrix cell wall materials are also involved. Especially in the kidney-shaped stomata of the fern Asplenium nidus, callose actively participates in the mechanism of opening and closure of the stomatal pore. SCOPE: The present review briefly presents and discusses recent findings concerning the distribution and role of callose in the kidney-shaped stomata of the dicotyledon Vigna sinensis as well as in the dumbbell-shaped stomata of the monocotyledon Zea mays. CONCLUSION: The discussed data support that, in both categories of angiosperm stomata, callose is implicated in the mechanism of stomatal pore formation and stomata function by locally affecting the mechanical properties of the GC cell walls.

Microtubule involvement in the deposition of radial fibrillar callose arrays in stomata of the fern Asplenium nidus L.

Aniline blue staining and callose immunolabeling revealed the deposition of significant callose quantities, in the form of fibrils, in the periclinal walls of guard cells (GCs) of stomata of the fern Asplenium nidus. The stomata that were at an early stage of differentiation displayed short callose fibrils at the junctions of the periclinal walls with the dorsal ones, which converged on the site of the future stomatal pore. In stomata being at an advanced stage of differentiation, callose fibrils were radially arranged around the stomatal pore, while in mature closed ones they were focused on the margins of the wall thickenings lining the stomatal pore. The pattern of the callose fibril organization resembled that of cellulose microfibrils in the same walls. Like the cellulose microfibrils, callose fibrils appeared coaligned with the underlying radial arrays of cortical microtubules (MTs). Moreover, the stomata treated with cellulose synthesis inhibitors (coumarin or dichlobenil) and those recovering from treatments with callose synthesis inhibitors (2-deoxy-D-glucose or tunicamycin) exhibited distinct radial callose fibril arrays. Cytochalasin B did not affect the organization of the radial callose fibril arrays. In contrast, oryzalin completely disturbed the pattern of callose deposition in the affected GCs. Therefore, the fibrillar callose orientation in the periclinal GC walls is probably controlled by MTs but not by actin filaments. The MTs seem to orient callose synthases in the plasmalemma, thus determining the fibrillar nature of callose deposits and their radial mode of arrangement. The cellulose microfibrils are not involved in the callose fibril alignment.

The involvement of phospholipases C and D in the asymmetric division of subsidiary cell mother cells of Zea mays.

In the present study, the involvement of phospholipase C and D (PLC and PLD) pathways in the asymmetric divisions that produce the stomatal complexes of Zea mays was investigated. In particular, the polar organization of microtubules (MTs) and actin filaments (AFs) and the process of asymmetric division were studied in subsidiary cell mother cells (SMCs) treated with PLC and PLD modulators. In SMCs treated with butanol-1 (but-1), which blocks phosphatidic acid (PA) production via PLDs, AF-patch formation laterally to the inducing guard cell mother cell (GMC) and the subsequent asymmetric division were inhibited. In these SMCs, cell division plane determination, as expressed by MT preprophase band (MT-PPB) formation, was not disturbed. Exogenously applied PA partially relieved the but-1 effects on SMCs. In contrast to SMCs, but-1 did not affect the symmetric GMC division. Inhibition of the PLC catalytic activity by neomycin or U73122 resulted in inhibition of asymmetric SMC division, while AF-patch and MT-PPB were organized as in control SMCs. These data show that the PLC and PLD signaling pathways are involved in the transduction and/or perception of the inductive stimulus that is emitted by the GMCs and induces the polar AF organization and asymmetric SMC division. In contrast, division plane determination in SMCs, as expressed by MT-PPB formation, does not depend on PLC and PLD signaling pathways.

Phospholipase C signaling involvement in macrotubule assembly and activation of the mechanism regulating protoplast volume in plasmolyzed root cells of Triticum turgidum.

The role of phosphoinositide-specific phospholipase C (PI-PLC) signaling in the macrotubule-dependent protoplast volume regulation in plasmolyzed root cells of Triticum turgidum was investigated. At the onset of hyperosmotic stress, PI-PLC activation was documented. Inhibition of PI-PLC activity by U73122 blocked tubulin macrotubule formation in plasmolyzed cells and their protoplast volume regulatory mechanism. In neomycin-treated plasmolyzed cells, macrotubule formation and protoplast volume regulation were not affected. In these cells the PI-PLC pathway is down-regulated as neomycin sequesters the PI-PLC substrate, 4,5-diphosphate-phosphatidyl inositol (PtdInsP(2)). These phenomena were unaffected by R59022, an inhibitor of phosphatidic acic (PA) production via the PLC pathway. Taxol, a microtubule (MT) stabilizer, inhibited the hyperosmotic activation of PI-PLC, but oryzalin, which disorganized MTs, triggered PI-PLC activity. Taxol prevented macrotubule formation and inhibited the mechanism regulating the volume of the plasmolyzed protoplast. Neomycin partly relieved some of the taxol effects. These data suggest that PtdInspP(2) turnover via PI-PLC assists macrotubule formation and activation of the mechanism regulating the plasmolyzed protoplast volume; and the massive disorganization of MTs that is carried out at the onset of hyperosmotic treatment triggers the activation of this mechanism.

ROS homeostasis as a prerequisite for the accomplishment of plant cytokinesis.

Reactive oxygen species (ROS) are emerging players in several biological processes. The present work investigates their potential involvement in plant cytokinesis by the application of reagents disturbing ROS homeostasis in root-tip cells of Triticum turgidum. In particular, the NADPH-oxidase inhibitor diphenylene iodonium, the ROS scavenger N-acetyl-cysteine, and menadione that leads to ROS overproduction were used. The effects on cytokinetic cells were examined using light, fluorescence, and transmission electron microscopy. ROS imbalance had a great impact on the cytokinetic process including the following: (a) formation of atypical "phragmoplasts" incapable of guiding vesicles to the equatorial plane, (b) inhibition of the dictyosomal and/or endosomal vesicle production that provides the developing cell plates with membranous and matrix polysaccharidic material, (c) disturbance of the fusion processes between vesicles arriving on the cell plate plane, (d) disruption of endocytic vesicle production that mediates the removal of the excess membrane material from the developing cell plate, and (e) the persistence of large callose depositions in treated cell plates. Consequently, either elevated or low ROS levels in cytokinetic root-tip cells resulted in a total inhibition of cell plate assembly or the formation of aberrant cell plates, depending on the stage of the affected cytokinetic cells. The latter failed to expand towards cell cortex and hence to give rise to complete daughter cell wall. These data revealed for the first time the necessity of ROS homeostasis for accomplishment of plant cytokinesis, since it seems to be a prerequisite for almost every aspect of this process.

Deliberate ROS production and auxin synergistically trigger the asymmetrical division generating the subsidiary cells in Zea mays stomatal complexes.

Subsidiary cell generation in Poaceae is an outstanding example of local intercellular stimulation. An inductive stimulus emanates from the guard cell mother cells (GMCs) towards their laterally adjacent subsidiary cell mother cells (SMCs) and triggers the asymmetrical division of the latter. Indole-3-acetic acid (IAA) immunolocalization in Zea mays protoderm confirmed that the GMCs function as local sources of auxin and revealed that auxin is polarly accumulated between GMCs and SMCs in a timely-dependent manner. Besides, staining techniques showed that reactive oxygen species (ROS) exhibit a closely similar, also time-dependent, pattern of appearance suggesting ROS implication in subsidiary cell formation. This phenomenon was further investigated by using the specific NADPH-oxidase inhibitor diphenylene iodonium, the ROS scavenger N-acetyl-cysteine, menadione which leads to ROS overproduction, and H2O2. Treatments with diphenylene iodonium, N-acetyl-cysteine, and menadione specifically blocked SMC polarization and asymmetrical division. In contrast, H2O2 promoted the establishment of SMC polarity and subsequently subsidiary cell formation in "younger" protodermal areas. Surprisingly, H2O2 favored the asymmetrical division of the intervening cells of the stomatal rows leading to the creation of extra apical subsidiary cells. Moreover, H2O2 altered IAA localization, whereas synthetic auxin analogue 1-napthaleneacetic acid enhanced ROS accumulation. Combined treatments with ROS modulators along with 1-napthaleneacetic acid or 2,3,5-triiodobenzoic acid, an auxin efflux inhibitor, confirmed the crosstalk between ROS and auxin functioning during subsidiary cell generation. Collectively, our results demonstrate that ROS are critical partners of auxin during development of Z. mays stomatal complexes. The interplay between auxin and ROS seems to be spatially and temporarily regulated.

Auxin as an inducer of asymmetrical division generating the subsidiary cells in stomatal complexes of Zea mays.

The data presented in this work revealed that in Zea mays the exogenously added auxins indole-3-acetic acid (IAA) and 1-napthaleneacetic acid (NAA), promoted the establishment of subsidiary cell mother cell (SMC) polarity and the subsequent subsidiary cell formation, while treatment with auxin transport inhibitors 2,3,5-triiodobenzoic acid (TIBA) and 1-napthoxyacetic acid (NOA) specifically blocked SMC polarization and asymmetrical division. Furthermore, in young guard cell mother cells (GMCs) the PIN1 auxin efflux carriers were mainly localized in the transverse GMC faces, while in the advanced GMCs they appeared both in the transverse and the lateral ones adjacent to SMCs. Considering that phosphatidyl-inositol-3-kinase (PI3K) is an active component of auxin signal transduction and that phospholipid signaling contributes in the establishment of polarity, treatments with the specific inhibitor of the PI3K LY294002 were carried out. The presence of LY294002 suppressed polarization of SMCs and prevented their asymmetrical division, whereas combined treatment with exogenously added NAA and LY294002 restricted the promotional auxin influence on subsidiary cell formation. These findings support the view that auxin is involved in Z. mays subsidiary cell formation, probably functioning as inducer of the asymmetrical SMC division. Collectively, the results obtained from treatments with auxin transport inhibitors and the appearance of PIN1 proteins in the lateral GMC faces indicate a local transfer of auxin from GMCs to SMCs. Moreover, auxin signal transduction seems to be mediated by the catalytic function of PI3K.

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

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

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

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

The interplay between ROS and tubulin cytoskeleton in plants.

Plants have to deal with reactive oxygen species (ROS) production, since it could potentially cause severe damages to different cellular components. On the other hand, ROS functioning as important second messengers are implicated in various developmental processes and are transiently produced during biotic or abiotic stresses. Furthermore, the microtubules (MTs) play a primary role in plant development and appear as potent players in sensing stressful situations and in the subsequent cellular responses. Emerging evidence suggests that ROS affect MTs in multiple ways. The cellular redox status seems to be tightly coupled with MTs. ROS signals regulate the organization of tubulin cytoskeleton and induce tubulin modifications. This review aims at summarizing the signaling mechanisms and the key operators orchestrating the crosstalk between ROS and tubulin cytoskeleton in plant cells. The contribution of several molecules, including microtubule associated proteins, oxidases, kinases, phospholipases, and transcription factors, is highlighted.

Plant cell division: ROS homeostasis is required.

Accumulated evidence indicates that ROS fluctuations play a critical role in cell division. Dividing plant cells rapidly respond to them. Experimental disturbance of ROS homeostasis affects: tubulin polymerization; PPB, mitotic spindle and phragmoplast assembly; nuclear envelope dynamics; chromosome separation and movement; cell plate formation. Dividing cells mainly accumulate at prophase and delay in passing through the successive cell division stages. Notably, many dividing root cells of the rhd2 Arabidopsis thaliana mutants, lacking the RHD2/AtRBOHC protein function, displayed aberrations, comparable to those induced by low ROS levels. Some protein molecules, playing key roles in signal transduction networks inducing ROS production, participate in cell division. NADPH oxidases and their regulators PLD, PI3K and ROP-GTPases, are involved in MT polymerization and organization. Cellular ROS oscillations function as messages rapidly transmitted through MAPK pathways inducing MAP activation, thus affecting MT dynamics and organization. RNS implication in cell division is also considered.

Disturbance of reactive oxygen species homeostasis induces atypical tubulin polymer formation and affects mitosis in root-tip cells of Triticum turgidum and Arabidopsis thaliana.

In this study, the effects of disturbance of the reactive oxygen species (ROS) homeostasis on the organization of tubulin cytoskeleton in interphase and mitotic root-tip cells of Triticum turgidum and Arabidopsis thaliana were investigated. Reduced ROS levels were obtained by treatment with diphenylene iodonium (DPI) and N-acetyl-cysteine, whereas menadione was applied to achieve ROS overproduction. Both increased and low ROS levels induced: (a) Macrotubule formation in cells with low ROS levels and tubulin paracrystals under oxidative stress. The protein MAP65-1 was detected in treated cells, exhibiting a conformation comparable to that of the atypical tubulin polymers. (b) Disappearance of microtubules (MTs). (c) Inhibition of preprophase band formation. (d) Delay of the nuclear envelope breakdown at prometaphase. (e) Prevention of perinuclear tubulin polymer assembly in prophase cells. (f) Loss of bipolarity of prophase, metaphase and anaphase spindles. Interestingly, examination of the A. thaliana rhd2/At respiratory burst oxidase homolog C (rbohc) NADPH oxidase mutant, lacking RHD2/AtRBOHC, gave comparable results. Similarly to DPI, the decreased ROS levels in rhd2 root-tip cells, interfered with MT organization and induced macrotubule assembly. These data indicate, for first time in plants, that ROS are definitely implicated in: (a) mechanisms controlling the assembly/disassembly of interphase, preprophase and mitotic MT systems and (b) mitotic spindle function. The probable mechanisms, by which ROS affect these processes, are discussed.

Formation of an endoplasmic reticulum ring associated with acetylated microtubules in the angiosperm preprophase band.

We investigated the organization of the cortical endoplasmic reticulum (ER) in prophase cells of the angiosperms Zea mays, Triticum turgidum, and Vigna sinensis. In both symmetrically and asymmetrically dividing protodermal leaf cells, cortical ER was enriched in the preprophase band and colocalized there with microtubules, forming a ring-like structure (ER ring). In contrast, ER ring was absent from prophase root-tip cells of the same plants, suggesting that ER ring formation in the preprophase band is organ specific. Immunolabeling of the protodermal leaf cells revealed the presence of acetylated microtubules, which are more stable than the nonacetylated ones. In contrast, neither this post-translational modification of tubulin nor an accumulation of ER in the preprophase band was detected in root-tip cells. Experimentally delaying the maturation/disassembly of the microtubule ring of the preprophase band by taxol or cyclopiazonic acid treatment led to the appearance of ER ring and acetylated microtubules in the preprophase band. Together, our data show that in dividing cells of angiosperms, an ER ring associated with acetylated microtubules forms in the preprophase band.

Actin filament-organized local cortical endoplasmic reticulum aggregations in developing stomatal complexes of grasses.

Endoplasmic reticulum (ER) immunolabeling in developing stomatal complexes and in the intervening cells of the stomatal rows (ICSRs) of Zea mays revealed that the cortical-ER forms distinct aggregations lining locally expanding wall regions. The polarized subsidiary cell mother cells (SMCs), displayed a cortical-ER-patch lining the wall region shared with the inducing guard cell mother cell (GMC), which disorganized during mitosis. In dividing SMCs, ER persisted in the preprophase band region and was unequally distributed in the mitotic spindle poles. The subsidiary cells (SCs) formed initially an ER-patch lining the common wall with the GMC or the young guard cells and afterwards an ER-ring in the junction of the SC wall with the neighboring ones. Distinct ER aggregations lined the ICSR wall regions shared with the SCs. The cortical-ER aggregations in stomatal cells of Z. mays were co-localized with actin filament (AF) arrays but both were absent from the respective cells of Triticum turgidum, which follow a different morphogenetic pattern. Experimental evidence showed that the interphase ER aggregations are organized by the respective AF arrays, while the mitotic ER aggregations by microtubules. These results revealed that AF and ER demarcated "cortical cytoplasmic domains" are activated below the locally expanding stomatal cell wall regions, probably via a mechanosensing mechanism triggered by the locally stressed plasmalemma/cell wall continuum. The probable role(s) of the local ER aggregations are discussed.

A new callose function: involvement in differentiation and function of fern stomatal complexes.

Callose in polypodiaceous ferns performs multiple roles during stomatal development and function. This highly dynamic (1→3)-ß-D-glucan, in cooperation with the cytoskeleton, is involved in: (a) stomatal pore formation, (b) deposition of local GC wall thickenings, and (c) the mechanism of stomatal pore opening and closure. This behavior of callose, among others, probably relies on the particular mechanical properties as well as on the ability to form and degrade rapidly, to create a scaffold or to serve as a matrix for deposition of other cell wall materials, and to produce fibrillar deposits in the periclinal GC walls, radially arranged around the stomatal pore. The local callose deposition in closing stomata is an immediate response of the external periclinal GC walls experiencing strong mechanical forces induced by the neighboring cells. The radial callose fibrils transiently co-exist with radial cellulose microfibrils and, like the latter, seem to be oriented via cortical MTs.

Cortical actin filament organization in developing and functioning stomatal complexes of Zea mays and Triticum turgidum.

Cortical actin filament (AF) organization was studied in detail in developing stomatal complexes of the grasses Zea mays and Triticum turgidum. AF arrays during the whole stomatal complex development are dynamic, partly following the pattern of cortical microtubule (MT) organization. They also exhibit particular patterns of organization, spatially and temporarily restricted. Among AF arrays, the radial ones that underlie young guard cell (GC) periclinal walls, those that line the bulbous GC ends and the AF ring at the junction between subsidiary cells (SCs) and GCs are described here for the first time. Although many similarities in cortical AF organization exist among the stomatal cells of both plants studied, considerable differences have also been observed between them. Our data reveal that the expanding areas of stomatal cell walls are lined by distinct cortical AF aggregations that probably protect the plasmalemma against mechanical stresses. Experimental AF disruption does not seem to affect detectably stomatal cell morphogenesis. Moreover, the structural and experimental data of this study revealed that, in contrast to the elliptical stomata, in the dumbbell-shaped ones the AFs and MTs seem not to be involved in the mechanism of opening and closing of the stomatal pore.

Cytoskeletal asymmetry in Zea mays subsidiary cell mother cells: a monopolar prophase microtubule half-spindle anchors the nucleus to its polar position.

Double labeling of microtubules and actin filaments revealed that in prophase subsidiary mother cells of Zea mays a monopolar prophase microtubule "half-spindle" is formed, which lines the nuclear hemisphere distal to the inducing guard mother cell. The nuclear hemisphere proximal to the guard mother cell is lined by an F-actin cap, consisting of a cortical F-actin patch and actin filaments originating from it. The microtubules of the "half-spindle" decline from the nuclear surface and terminate to the preprophase microtubule band. After disintegration of the latter, a bipolar metaphase spindle is organized. The polar F-actin cap persists during mitosis and early cytokinesis, extending to the chromosomes and the subsidiary cell daughter nucleus. In oryzalin treated subsidiary mother cells the prophase nuclei move away from the polar site. Cytochalasin B and latrunculin-B block the polar migration of subsidiary mother cell nuclei, but do not affect those already settled to the polar position. The prophase nuclei of latrunculin-B treated subsidiary mother cells are globally surrounded by microtubules, while the division plane of latrunculin-B treated subsidiary mother cells is misaligned. The prophase nuclei of brick 1 mutant Zea mays subsidiary mother cells without F-actin patch are also globally surrounded by microtubules. The presented data show that the prophase microtubule "half-spindle"-preprophase band complex anchors the subsidiary mother cell nucleus to the polar cell site, while the polar F-actin cap stabilizes the one metaphase spindle pole proximal to the inducing guard mother cell.

Cytoskeleton and morphogenesis in brown algae.

BACKGROUND: Morphogenesis on a cellular level includes processes in which cytoskeleton and cell wall expansion are strongly involved. In brown algal zygotes, microtubules (MTs) and actin filaments (AFs) participate in polarity axis fixation, cell division and tip growth. Brown algal vegetative cells lack a cortical MT cytoskeleton, and are characterized by centriole-bearing centrosomes, which function as microtubule organizing centres. SCOPE: Extensive electron microscope and immunofluorescence studies of MT organization in different types of brown algal cells have shown that MTs constitute a major cytoskeletal component, indispensable for cell morphogenesis. Apart from participating in mitosis and cytokinesis, they are also involved in the expression and maintenance of polarity of particular cell types. Disruption of MTs after Nocodazole treatment inhibits cell growth, causing bulging and/or bending of apical cells, thickening of the tip cell wall, and affecting the nuclear positioning. Staining of F-actin using Rhodamine-Phalloidin, revealed a rich network consisting of perinuclear, endoplasmic and cortical AFs. AFs participate in mitosis by the organization of an F-actin spindle and in cytokinesis by an F-actin disc. They are also involved in the maintenance of polarity of apical cells, as well as in lateral branch initiation. The cortical system of AFs was found related to the orientation of cellulose microfibrils (MFs), and therefore to cell wall morphogenesis. This is expressed by the coincidence in the orientation between cortical AFs and the depositing MFs. Treatment with cytochalasin B inhibits mitosis and cytokinesis, as well as tip growth of apical cells, and causes abnormal deposition of MFs. CONCLUSIONS: Both the cytoskeletal elements studied so far, i.e. MTs and AFs are implicated in brown algal cell morphogenesis, expressed in their relationship with cell wall morphogenesis, polarization, spindle organization and cytokinetic mechanism. The novelty is the role of AFs and their possible co-operation with MTs.