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.     

Ontogenesis in the Fucophyceae: case studies and comparison of fucoid zygotes and Sphacelaria apical cells

In multicellular eukaryotes, the zygote, a single cell, gives rise to the different cell types of the organism. The study of the mechanisms involved is a key point of developmental biology. Generally, the first stages are characterized by an orderly sequence of asymmetrical divisions resulting from an initial developmental polarity. The establishment of this initial polarity has been the subject of numerous studies in animals, but not in higher plants since the zygote is encased in several layers of tissues that prevent experimental approaches. Moreover, plant development is characterized by two successive ontogenetic steps: the construction of the embryonic apico-basal axis and the establishment of meristems in charge of organogenesis. Members of the Fucophyceae provide good models for the investigation of these processes. Any inferred homology of mechanisms must take into account the polyphyletic nature of the algae. This paper is a tentative review of two case studies: fucoid zygotes and Sphacelaria apical cells, and deals respectively with the two successive ontogenetic steps characteristic of higher plant development. The first part concerns development of the fucoid zygotes. Fucoid zygotes, including those of different species, are considered as model systems in plants for studying the establishment of the polarity axis because, at the moment of fertilization, they do not have any morphological or biochemical polarity. This report concerns progress in the identification of some cellular or molecular mechanisms involved in the settlement and/or stabilization of the polarity axis, and the consequence of this polar organisation for the control of asymmetrical divisions and the building of a functional embryo. The second part concerns the apical cell of Sphacelaria as a model for establishing and maintaining a meristematic cell. The apical cell exhibits a permanent polarized organisation throughout repetitive asymmetric divisions and can be comparatively analysed in situ and isolated as a protoplast. This allowed us to investigate the evolution of the cytoplasmic cytoskeleton, centrosomes and the mitotic apparatus during the cell cycle in relation to the cell polarity; particularly the interactions between the cytoskeleton and cell wall. For the two models, the results are compared with mechanisms involved in the development of other multicellular organisms, and their value in gaining an insight into higher plant ontogenesis is assessed.     

At the poles across kingdoms: phosphoinositides and polar tip growth

Phosphoinositides (PIs) are minor, but essential phospholipid constituents of eukaryotic membranes, and are involved in the regulation of various physiological processes. Recent genetic and cell biological advances indicate that PIs play important roles in the control of polar tip growth in plant cells. In root hairs and pollen tubes, PIs control directional membrane trafficking required for the delivery of cell wall material and membrane area to the growing tip. So far, the exact mechanisms by which PIs control polarity and tip growth are unresolved. However, data gained from the analysis of plant, fungal and animal systems implicate PIs in the control of cytoskeletal dynamics, ion channel activity as well as vesicle trafficking. The present review aims at giving an overview of PI roles in eukaryotic cells with a special focus on functions pertaining to the control of cell polarity. Comparative screening of plant and fungal genomes suggests diversification of the PI system with increasing organismic complexity. The evolutionary conservation of the PI system among eukaryotic cells suggests a role for PIs in tip growing cells in models where PIs so far have not been a focus of attention, such as fungal hyphae.     

Microtubules regulate tip growth and orientation in root hairs of Arabidopsis thaliana

The polarized growth of cells as diverse as fungal hyphae, pollen tubes, algal rhizoids and root hairs is characterized by a highly localized regulation of cell expansion confined to the growing tip. In apically growing plant cells, a tip-focused [Ca2+]c gradient and the cytoskeleton have been associated with growth. Although actin has been established to be essential for the maintenance of elongation, the role of microtubules remains unclear. To address whether the microtubule cytoskeleton is involved in root hair growth and orientation, we applied microtubule antagonists to root hairs of Arabidopsis. In this report, we show that depolymerizing or stabilizing the microtubule cytoskeleton of these apically growing root hairs led to a loss of directionality of growth and the formation of multiple, independent growth points in a single root hair. Each growing point contained a tip-focused gradient of [Ca2+]c. Experimental generation of a new [Ca2+]c gradient in root hairs pre-treated with microtubule antagonists, using the caged-calcium ionophore Br-A23187, was capable of inducing the formation of a new growth point at the site of elevated calcium influx. These data indicate a role for microtubules in regulating the directionality and stability of apical growth in root hairs. In addition, these results suggest that the action of the microtubules may be mediated through interactions with the cellular machinery that maintains the [Ca2+]c gradient at the tip.     

Establishment and expression of cellular polarity in fucoid zygotes.

Zygotes of fucoid algae have long been studied as a paradigm for cell polarity. Polarity is established early in the first cell cycle and is then expressed as localized growth and invariant cell division. The fertilized egg is a spherical cell and, by all accounts, bears little or no asymmetry. Polarity is acquired epigenetically a few hours later in the form of a rhizoid/thallus axis. The initial stage of polarization is axis selection, during which zygotes monitor environment gradients to determine the appropriate direction for rhizoid formation. In their natural setting in the intertidal zone, sunlight is probably the most important polarizing vector; rhizoids form away from the light. The mechanism by which zygotes perceive environmental gradients and transduce that information into an intracellular signal is unknown but may involve a phosphatidylinositol cycle. Once positional information has been recorded, the cytoplasm and membrane are reorganized in accordance with the vectorial information. The earliest detectable asymmetries in the polarizing zygote are localized secretion and generation of a transcellular electric current. Vesicle secretion and the inward limb of the current are localized at the presumptive rhizoid. The transcellular current may establish a cytoplasmic Ca2+ gradient constituting a morphogenetic field, but this remains controversial. Localized secretion and establishment of transcellular current are sensitive to treatment with cytochalasins, indicating that cytoplasmic reorganization is dependent on the actin cytoskeleton. The nascent axis at first is labile and susceptible to reorientation by subsequent environmental vectors but soon becomes irreversibly fixed in its orientation. Locking the axis in place requires both cell wall and F-actin and is postulated to involve an indirect transmembrane bridge linking cortical actin to cell wall. This bridge anchors relevant structures at the presumptive rhizoid and thereby stabilizes the axis. Approximately halfway through the first cell cycle, the latent polarity is expressed morphologically in the form of rhizoid growth. Elongation is by tip growth and does not appear to be fundamentally different from tip growth in other organisms. The zygote always divides perpendicular to the growth axis, and this is controlled by the microtubule cytoskeleton. Two microtubule-organizing centers on the nuclear envelope rotate such that they align with the growth axis. They then serve as spindle poles during mitosis. Cytokinesis bisects the axial spindle, resulting in a transverse crosswall. Although the chronology of cellular events associated with polarity is by now rather detailed, causal mechanisms remain obscure.     

肌动蛋白微丝与生长素浓度梯度分布

生长素参与植物生长发育的各个阶段,如胚胎发生、发育,营养器官发生与形态建成,极性与轴向的建立,维管组织分化,生殖器官的发育等。虽然生长素在植物的各组织器官和细胞中发挥着重要的作用,植物内源生长素的生物合成却是在特异的组织——细胞快速分裂的幼嫩组织中完成的,然后通过韧皮部或受严格控制的细胞—细胞运输系统运送至植物各个部分。生长素的极性运输导致其积累在某些局部组织和细胞内,形成特定梯度分布。生长素对植物生长发育众多方面的调节正是依赖于这一特性。该文综述了近年来有关植物生长发育过程中生长素浓度梯度的形成和相应的生理功能,以及细胞骨架中的微丝参与调控生长素极性运输的研究工作。     

ACTIN2 is essential for bulge site selection and tip growth during root hair development of Arabidopsis

Root hairs develop as long extensions from root epidermal cells. After the formation of an initial bulge at the distal end of the epidermal cell, the root hair structure elongates by tip growth. Because root hairs are not surrounded by other cells, root hair formation provides an excellent system for studying the highly complex process of plant cell growth. Pharmacological experiments with actin filament-interfering drugs have provided evidence that the actin cytoskeleton is an important factor in the establishment of cell polarity and in the maintenance of the tip growth machinery at the apex of the growing root hair. However, there has been no genetic evidence to directly support this assumption. We have isolated an Arabidopsis mutant, deformed root hairs 1 (der1), that is impaired in root hair development. The DER1 locus was cloned by map-based cloning and encodes ACTIN2 (ACT2), a major actin of the vegetative tissue. The three der1 alleles develop the mutant phenotype to different degrees and are all missense mutations, thus providing the means to study the effect of partially functional ACT2. The detailed characterization of the der1 phenotypes revealed that ACT2 is not only involved in root hair tip growth, but is also required for correct selection of the bulge site on the epidermal cell. Thus, the der1 mutants are useful tools to better understand the function of the actin cytoskeleton in the process of root hair formation.     

A S/M DNA replication checkpoint prevents nuclear and cytoplasmic events of cell division including centrosomal axis alignment and inhibits activation of cyclin-dependent kinase-like proteins in fucoid zygotes

S/M checkpoints prevent various aspects of cell division when DNA has not been replicated. Such checkpoints are stringent in yeast and animal somatic cells but are usually partial or not present in animal embryos. Because little is known about S/M checkpoints in plant cells and embryos, we have investigated the effect of aphidicolin, a specific inhibitor of DNA polymerases (alpha) and (delta), on cell division and morphogenesis in Fucus and Pelvetia zygotes. Both DNA replication and cell division were inhibited by aphidicolin, indicating the presence, in fucoid zygotes, of a S/M checkpoint. This checkpoint prevents chromatin condensation, spindle formation, centrosomal alignment with the growth axis and cytokinesis but has no effect on germination or rhizoid elongation. This S/M checkpoint also prevents tyrosine dephosphorylation of cyclin-dependent kinase-like proteins at the onset of mitosis. The kinase activity is restored in extracts upon incubation with cdc25A phosphatase. When added in S phase, olomoucine, a specific inhibitor of cyclin-dependent kinases, has similar effects as aphidicolin on cell division although alignment of the centrosomal axis still occurs. We propose a model involving the inactivation of CDK-like proteins to account for the S/M DNA replication checkpoint in fucoid zygotes and embryos.     

Apical cells of brown algae with particular reference to Sphacelariales, Dictyotales and Fucales

Brown algae show a significant diversity in thallus forms, giving a great number of model systems for the study of many important morphogenetic mechanisms. Thallus growth in brown algae is diffuse, intercalary or apical. The latter takes place by means of one or more apical cells. Among the brown algal groups, Sphacelariales, Dictyotales and Fucales give the best examples of apical growth, and have been repeatedly used for the study of the morphogenetic role of apical cells. In Sphacelariales the apical cells appear strongly polarized, the polarity expressed also on the organization of the microtubule cytoskeleton. These cells show a type of growth that can be compared with tip growth of root hairs, moss protonemata, pollen tubes and fungal hyphae, and is called ‘tip-like growth’. The thallus of Dictyotales grows by the activity of one or more apical cells showing variable degree of polarity. These cells do not exhibit any type of apical growth. In Fucales the vegetative thallus develops by means of an active apical meristem, which includes a large apical cell. This cell does not show polar organization or apical growth. However, in germinating zygotes of Fucales a polar axis is established and during the first stages of development they show a typical tip growth. In the present paper, the available information on the structure and division pattern of apical cells is presented. Their morphogenetic role is discussed, in relation to polarity, cytoskeleton organization, and apical dominance.     

Polarity and growth of caulonema tip cells of the moss Funaria hygrometrica

In the caulonema tip cells of Funaria hygrometrica, chloroplasts, mitochondria, and dictyosomes have differences in structure which are determined by cell polarity. In contrast to the slowly growing chloronema tip cells the apical cell of the caulonema contains a tip body. Colchicine stops tip growth; it causes the formation of subapical cell protrusions, redistribution of the plastids, and a loss of their polar differentiation. Cytochalasin B inhibits growth and affects the position of cell organelles. After treatment with ionophore A23 187, growth is slower and shorter and wider cells are formed. D₂O causes a transient reversion of organelle distribution but premitotic nuclei are not dislocated. In some tip cells the reversion of polarity persists; they continue to grow with a new tip at their base. During centrifugation, colchicine has only a slight influence on the stability of organelle anchorage. The former polar organization of most cells is restored within a few hours after centrifugation, and the cells resume normal growth. In premitotic cells the nucleus and other organelles cannot be retransported, they often continue to grow with reversed polarity. Colchicine retards the redistribution of organelles generally and increases the number of cells that form a basal outgrowth. The interrelationship between the peripheral cytoplasm and the nucleus and the role of microtubules in maintaining and reestablishing cell polarity are discussed.Abbreviations DMSO dimethylsulfoxide - CB cytochalasin B Dedicated to Prof. Dr. A. Pirson on the occasion of his 70. birthday     

RAC1 regulates actin arrays during polarity establishment in the brown alga, Silvetia compressa

Multicellular development has evolved independently on numerous occasions and there is great interest in the developmental mechanisms utilized by each of the divergent lineages. Fucoid algae, in the stramenopile lineage (distinct from metazoans, fungi and green plants) have long been used as a model for early development based on unique life cycle characteristics. The initially symmetric fucoid zygote generates a developmental axis that determines not only the site of growth, but also the orientation of the first cell division, whose products have distinct developmental fates. Establishment and maintenance of this growth axis is dependent on formation of a filamentous actin array that directs vesicular movement, depositing new membrane and wall material for development of the rhizoid. What is not well known, is how formation and placement of the actin array is regulated in fucoid algae. A candidate for this function is Rac1, a small GTPase of the highly conserved Rho family, which has been implicated in controlling the formation of actin arrays in diverse eukaryotes. We demonstrate that Rac1 is not only present during formation of the filamentous actin array, but that its localization overlaps with the array in polarizing zygotes. Pharmacologically inhibiting Rac1 activity was shown to impede formation and maintenance of the actin array, and ultimately polar growth. Evidence is provided that a requirement of Rac1 function is its ability to associate with membranes via a post-translationally added lipid tail. Taken together, the data indicate that Rac1 is a necessary participant in establishment of the growth pole, presumably by regulating the placement and formation of the actin array. A role for Rac1 and related proteins in regulating actin is shared by animals, plants, fungi and with this work, brown algae, thus a conserved mechanism for generating polarity is in operation in unique eukaryotic lineages.     

Emerging aspects of ER organization in root hair tip growth: Lessons from RHD3 and atlastin

Cell polarity is a fundamental aspect of eukaryotic cells. A central question for cell biologists is how the polarity of a cell is established and maintained. Root hairs are exceptionally polarized structures formed from specific root epidermal cells. The morphogenesis of root hairs is characterized by the localized cell growth in a small dome at the tip of the hair, a process called tip growth. Root hairs are thus an attractive model system to study the establishment and maintenance of cell polarity in eukaryotes. Research on Arabidopsis root hairs has identified a plethora of molecular and cellular components that are important for root hair tip growth. Recently, studies on RHD3 and Atlastin have revealed a surprising similarity with respect to the role of the tubular ER network in tip growth of root hairs in plants and the axonal outgrowth of corticospinal neurons in neurological disorders known as hereditary spastic paraplegia (HSP). In this mini-review, we highlight recent progress in understanding of the function and regulation of RHD3 in the generation of the tubular ER network and discussed ways in which RHD3 could be involved in the establishment and maintenance of root hair tip growth.     

Gene silencing in Fucus embryos: developmental consequences of RNAi‐mediated cytoskeletal disruption

Brown algae (Phaeophyceae) are an important algal class that play a range of key ecological roles. They are often important components of rocky shore communities. A number of members of the Fucales and Ectocarpales have provided models for the study of multicellular evolution, reproductive biology and polarized development. Indeed the fucoid algae exhibit the unusual feature of inducible embryo polarization, allowing many classical studies of polarity induction. The potential of further studies of brown algae in these important areas has been increasingly hindered by the absence of tools for manipulation of gene expression that would facilitate further mechanistic analysis and gene function studies at a molecular level. The aim of this study was to establish a method that would allow the analysis of gene function through RNAi‐mediated gene knockdown. We show that injection of double‐stranded RNA (dsRNA) corresponding to an α‐tubulin gene into Fucus serratus Linnaeus zygotes induces the loss of a large proportion of the microtubule cytoskeleton, leading to growth arrest and disruption of cell division. Injection of dsRNA targeting β‐actin led to reduced rhizoid growth, enlarged cells and the failure to develop apical hair cells. The silencing effect on actin expression was maintained for 3 months. These results indicate that the Fucus embryo possesses a functional RNA interference system that can be exploited to investigate gene function during embryogenesis.     

The Arp2/3 complex nucleates actin arrays during zygote polarity establishment and growth

Previous work has demonstrated that dynamic actin arrays are important for axis establishment and polar growth in the fucoid zygote, Silvetia compressa. Transitions between these arrays are mediated by depolymerization of an existing array and polymerization of a new array. To begin to understand how polymerization of new arrays might be regulated, we investigated the role of the highly conserved, actin-nucleating, Actin-related protein 2/3 (Arp2/3) complex. Arp2, a subunit of the complex, was cloned and peptide antibodies were raised to the C-terminal domain. In immunolocalization studies of polarizing zygotes, actin and Arp2 colocalized around the nucleus and in a patch at the rhizoid pole. In germinated zygotes, a cone of Arp2 and actin extended from the nucleus to the subapex. Within the rhizoid tip, three structural zones were observed in the majority of zygotes: the extreme apex was devoid of label, the subapex was enriched for Arp2, and further back both actin and Arp2 were present. This zonation suggests that actin nucleation occurs at the leading edge of the cone, in the Arp2-enriched region. In two sets of experiments, we showed that tip zonation is important for growth. First, pharmacological treatments that disrupted Arp2/actin zonation arrested tip growth. Second, changes in the direction of tip growth during negative phototropism were preceded by a reorientation of the zonation in accordance with the new growth direction. This work represents the first investigation of Arp2/3 complex localization in tip-growing algal cells.     

The Saccharomyces cerevisiae BEM1 homologue in Neurospora crassa promotes co‐ordinated cell behaviour resulting in cell fusion

Directed growth or movement is a common feature of microbial development and propagation. In polar growing filamentous fungi, directed growth requires the interaction of signal sensing machineries with factors controlling polarity and cell tip extension. In Neurospora crassa an unusual mode of cell–cell signalling mediates mutual attraction of germinating spores, which subsequently fuse. During directed growth of the two fusion partners, the cells co‐ordinately alternate between two physiological stages, probably associated with signal sending and receiving. Here, we show that the Saccharomyces cerevisiae BEM1 homologue in N. crassa is essential for the robust and efficient functioning of this MAP kinase‐based signalling system. BEM1 localizes to growing hyphal tips suggesting a conserved function as a polarity component. In the absence of BEM1, activation of MAK‐2, a MAP kinase essential for germling fusion, is strongly reduced and delayed. Germling interactions become highly instable and successful fusion is greatly reduced. In addition, BEM1 is actively recruited around the forming fusion pore, suggesting potential functions after cell–cell contact has been established. By genetically dissecting the contribution of BEM1 to additional various polarization events, we also obtained first hints that BEM1 might function in different protein complexes controlling polarity and growth direction.     

Establishing a growth axis in fucoid algae

Recent studies indicate that fucoid zygotes establish developmental polarity much earlier than previously thought. A growth axis is first set in place at fertilization, with the site of sperm entry defining the rhizoid pole of the axis. This initial axis is a default axis, which is only used as the final growth axis if the zygote fails to detect spatial cues (such as sunlight) in its intertidal environment. However, the zygote usually senses vectorial information; it then abandons the sperm-induced axis and assembles a new axis de novo in accordance with the perceived vector(s).     

Targeting and Regulation of Cell Wall Synthesis During Tip Growth in Plants

Root hairs and pollen tubes are formed through tip growth, a process requiring synthesis of new cell wall material and the precise targeting and integration of these components to a selected apical plasma membrane domain in the growing tips of these cells. Presence of a tip-focused calcium gradient, control of actin cytoskeleton dynamics, and formation and targeting of secretory vesicles are essential to tip growth. Similar to cells undergoing diffuse growth, cellulose, hemicelluloses, and pectins are also deposited in the growing apices of tip-growing cells. However, differences in the manner in which these cell wall components are targeted and inserted in the expanding portion of tip-growing cells is reflected by the identification of elements of the plant cell wall synthesis machinery which have been shown to play unique roles in tip-growing cells. In this review, we summarize our current understanding of the tip growth process, with a particular focus on the subcellular targeting of newly synthesized cell wall components, and their roles in this form of plant cell expansion.     

The Rac1 inhibitor,NSC23766, depolarizes adhesive secretion,endomembrane cycling,and tip growth in the fucoid alga, <Emphasis Type="Italic">Silvetia compressa</Emphasis>

Proper cell morphogenesis is dependent on the establishment and expression of cellular polarity. In the fucoid zygote, cell shape is critical for establishing the developmental pattern of the adult, and is achieved by guiding insertion of new membrane and wall to the rhizoid tip. Selection and growth of the appropriate tip site are accompanied by formation of dynamic actin arrays associated with the actin-nucleating Arp2/3 complex. In eukaryotes, a major pathway for activation of the Arp2/3 complex is via the Rho family GTPase, Rac1, which stimulates the Scar/WAVE complex. To determine whether Rac1 controls actin nucleation in Silvetia compressa (J. Agardh) E. Serrao, T. O. Cho, S. M. Boo et Brawley, we tested the effects of the Rac1-specific inhibitory compound, NSC23766, on actin dependent processes and on actin arrays. We found that NSC23766 disrupted polar secretion of adhesive, polarization of endomembranes, and tip-focused growth in the rhizoid. Similarly, NSC23766 altered actin and Arp2 localization in the growing rhizoid. In contrast, NSC23766 had no effect on selection of the growth site or on cytokinesis. These data suggest that Rac1 participates in nucleation of specific actin arrays in the developing zygote.     

Actin-Based Domains of the "Cell Periphery Complex" and their Associations with Polarized "Cell Bodies" in Higher Plants

Abstract: Nascent cellulosic cell wall microfibrils and transverse (with respect of cell growth axis) arrays of cortical microtubules (MTs) beneath the plasma membrane (PM) are two well established features of the periphery of higher plant cells. Together with transmembrane synthase complexes, they represent the most characteristic form of a “cell periphery complex” of higher plant cells which determines the orientation of the diffuse (intercalary) type of their cell growth. However, there are some plant cell types having distinct cell cortex domains which are depleted of cortical MTs. These particular cell cortex domains are, instead, typically enriched with components of the actin‐based cytoskeleton. In higher plants, this feature is prominent at extending apices of two cell types displaying tip growth ‐ pollen tubes and root hairs. In the latter cell type, highly dynamic F‐actin meshworks accumulate at extending tips, and they appear to be critical for the apparently motile character of these subcellular domains. Importantly, tip growth of both root hairs and pollen tubes is immediately stopped when the most dynamic F‐actin population is depolymerized with low levels of anti‐F‐actin drugs. Intriguingly, MTs of tip‐growing plant cells are organized in the form of longitudinal arrays, throughout the cytoplasm, which interconnect the extending tips with the subapical nuclei. This suggests that actin‐rich cell cortex domains polarize plant “cell bodies” represented by nucleus‐MTs complexes. A similar polarization of “cell bodies” is typical of mitotic and cytokinetic plant cells. A further type of MT‐depleted and actomyosin‐enriched plant cell cortex domain comprises the plasmodesmata. Primary plasmodesmata are formed during cytokinesis as part of the myosin VIII‐enriched callosic cell plates, representing “juvenile” forms of the plant “cell periphery complex”. In phylogenetic terms the association between F‐actin and the PM may be considered for a more “primitive” form of cellular organization than does the association of cortical MTs with the PM. We hypothesize that the actin cytoskeleton is a natural partner of the PM in all eukaryotic cells. In most plant cells, however, it was replaced by a tubulin‐based “cell periphery apparatus” which regulates, via still unknown mechanisms, the spatial deposition of nascent cellulosic microfibrils synthesized by PM‐associated synthase complexes.