Cell differentiation and the cytoskeleton in Acetabularia

Energide-cell body as smallest unit of eukaryotic life

Background

The evolutionary origin of the eukaryotic nucleus is obscure and controversial. Currently preferred are autogenic concepts; ideas of a symbiotic origin are mostly discarded and forgotten. Here we briefly discuss these issues and propose a new version of the symbiotic and archaeal origin of the eukaryotic nucleus.

Scope and Conclusions

The nucleus of eukaryotic cells forms via its perinuclear microtubules, the primary eukaryotic unit known also as the Energide-cell body. As for all other endosymbiotic organelles, new Energides are generated only from other Energides. While the Energide cannot be generated de novo, it can use its secretory apparatus to generate de novo the cell periphery apparatus. We suggest that Virchow's tenet Omnis cellula e cellula should be updated as Omnis Energide e Energide to reflect the status of the Energide as the primary unit of the eukaryotic cell, and life. In addition, the plasma membrane provides feedback to the Energide and renders it protection via the plasma membrane-derived endosomal network. New discoveries suggest archaeal origins of both the Energide and its host cell.

Darwin-Wallace Demons: survival of the fastest in populations of duckweeds and the evolutionary history of an enigmatic group of angiosperms

In evolutionary biology, the term 'Darwinian fitness' refers to the lifetime reproductive success of an individual within a population of conspecifics. The idea of a 'Darwinian Demon' emerged from this concept and is defined here as an organism that commences reproduction almost immediately after birth, has a maximum fitness, and lives forever. It has been argued that duckweeds (sub-family Lemnoideae, order Alismatales), a group containing five genera and 34 species of small aquatic monocotyledonous plants with a reduced body plan, can be interpreted as examples of 'Darwinian Demons'. Here we focus on the species Spirodela polyrhiza (Great duckweed) and show that these miniaturised aquatic angiosperms display features that fit the definition of the hypothetical organism that we will call a 'Darwin-Wallace Demon' in recognition of the duel proponents of evolution by natural selection. A quantitative analysis (log-log bivariate plot of annual growth in dry biomass versus standing dry body mass of various green algae and land plants) revealed that duckweeds are thus far the most rapidly growing angiosperms in proportion to their body mass. In light of this finding, we discuss the disposable soma and metabolic optimising theories, summarise evidence for and against the proposition that the Lemnoideae (family Araceae) reflect an example of reductive evolution, and argue that, under real-world conditions (environmental constraints and other limitations), 'Darwin-Wallace Demons' cannot exist, although the concept remains useful in much the same way that the Hardy-Weinberg law does.

Strasburger's legacy to mitosis and cytokinesis and its relevance for the Cell Theory

Eduard Strasburger was one of the most prominent biologists contributing to the development of the Cell Theory during the nineteenth century. His major contribution related to the characterization of mitosis and cytokinesis and especially to the discovery of the discrete stages of mitosis, which he termed prophase, metaphase and anaphase. Besides his observations on uninucleate plant and animal cells, he also investigated division processes in multinucleate cells. Here, he emphasised the independent nature of mitosis and cytokinesis. We discuss these issues from the perspective of new discoveries in the field of cell division and conclude that Strasburger's legacy will in the future lead to a reformulation of the Cell Theory and that this will accommodate the independent and primary nature of the nucleus, together with its complement of perinuclear microtubules, for the organisation of the eukaryotic cell.

Fast calcium waves

Calcium waves are propagated in five main speed ranges which cover a billion-fold range of speeds. We define the fast speed range as 3-30μm/s after correction to a standard temperature of 20°C. Only waves which are not fertilization waves are considered here. 181 such cases are listed here. These are through organisms in all major taxa from cyanobacteria through mammals including human beings except for those through other bacteria, higher plants and fungi. Nearly two-thirds of these speeds lie between 12 and 24μm/s. We argue that their common mechanism in eukaryotes is a reaction-diffusion one involving calcium-induced calcium release, in which calcium waves are propagated along the endoplasmic reticulum. We propose that the gliding movements of some cyanobacteria are driven by fast calcium waves which are propagated along their plasma membranes. Fast calcium waves may drive materials to one end of developing embryos by cellular peristalsis, help coordinate complex cell movements during development and underlie brain injury waves. Moreover, we continue to argue that such waves greatly increase the likelihood that chronic injuries will initiate tumors and cancers before genetic damage occurs. Finally we propose numerous further studies.

Evolution and cytological diversification of the green seaweeds (Ulvophyceae)

The Ulvophyceae, one of the four classes of the Chlorophyta, is of particular evolutionary interest because it features an unrivaled morphological and cytological diversity. Morphological types range from unicells and simple multicellular filaments to sheet-like and complex corticated thalli. Cytological layouts range from typical small cells containing a single nucleus and chloroplast to giant cells containing millions of nuclei and chloroplasts. In order to understand the evolution of these morphological and cytological types, the present paper aims to assess whether the Ulvophyceae are monophyletic and elucidate the ancient relationships among its orders. Our approach consists of phylogenetic analyses (maximum likelihood and Bayesian inference) of seven nuclear genes, small subunit nuclear ribosomal DNA and two plastid markers with carefully chosen partitioning strategies, and models of sequence evolution. We introduce a procedure for fast site removal (site stripping) targeted at improving phylogenetic signal in a particular epoch of interest and evaluate the specificity of fast site removal to retain signal about ancient relationships. From our phylogenetic analyses, we conclude that the ancestral ulvophyte likely was a unicellular uninucleate organism and that macroscopic growth was achieved independently in various lineages involving radically different mechanisms: either by evolving multicellularity with coupled mitosis and cytokinesis (Ulvales-Ulotrichales and Trentepohliales), by obtaining a multinucleate siphonocladous organization where every nucleus provides for its own cytoplasmic domain (Cladophorales and Blastophysa), or by developing a siphonous organization characterized by either one macronucleus or millions of small nuclei and cytoplasmic streaming (Bryopsidales and Dasycladales). We compare different evolutionary scenarios giving rise to siphonous and siphonocladous cytologies and argue that these did not necessarily evolve from a multicellular or even multinucleate state but instead could have evolved independently from a unicellular ancestor.

Dynamic morphology of plastids and stromules in angiosperm plants

Labelling of plastids with fluorescent proteins has revealed the diversity of their sizes and shapes in different tissues of vascular plants. Stromules, stroma-filled tubules comprising thin extensions of the stroma surrounded by the double envelope membrane, have been observed to emanate from all major types of plastid, though less common on chloroplasts. In some tissue types, stromules are highly dynamic, forming, shrinking, attaching, releasing and fragmenting. Stromule formation is negatively affected by treatment of tissue with cytoskeletal inhibitors. Plastids can be connected by stromules, through which green fluorescent protein (GFP) and fluorescently tagged chloroplast protein complexes have been observed to flow. Within the highly viscous stroma, proteins traffic by diffusion as well as by an active process of directional travel, whose mechanism is unknown. In addition to exchanging materials between plastids, stromules may also serve to increase the surface area of the envelope for import and export, reduce diffusion distance between plastids and other organelles for exchange of materials, and anchor the plastid onto attachment points for proper positioning with the plant cell. Future studies should reveal how these functions may affect plants in adapting to the challenges of a changing environment.

Morphogenesis in giant-celled algae

The giant-celled algae, which consist of cells reaching millimeters in size, some even centimeters, exhibit unique cell architecture and physiological characteristics. Their cells display a variety of morphogenetic phenomena, that is, growth, division, differentiation, and reproductive cell formation, as well as wound-healing responses. Studies using immunofluorescence microscopy and pharmacological approaches have shown that microtubules and/or actin filaments are involved in many of these events through the generation of intracellular movement of cell components or entire protoplasmic contents and the spatial control of cell activities in specific areas of the giant cells. A number of environmental factors including physical stimuli, such as light and gravity, invoke localized but also generalized cellular reactions. These have been extensively investigated to understand the regulation of morphogenesis, in particular addressing cytoskeletal and endomembrane dynamics, electrophysiological elements affecting ion fluxes, and the synthesis and mechanical properties of the cell wall. Some of the regulatory pathways involve signal transduction and hormonal control, as in other organisms. The giant unicellular green alga Acetabularia, which has proven its usefulness as an experimental model in early amputation/grafting experiments, will potentially once again serve as a useful model organism for studying the role of gene expression in orchestrating cellular morphogenesis.

Poly(A)+ RNA and cytoskeleton during cyst formation in the cap ray of Acetabularia peniculus

The configuration and distribution of polyadenylated RNA (poly(A)+ RNA) during cyst formation in the cap rays of Acetabularia peniculus were demonstrated by fluorescence in situ hybridization using oligo(dT) as a probe, and the spatial and functional relationships between poly(A)+ RNA and microtubules or actin filaments were examined by immunofluorescence microscopy and cytoskeletal inhibitor treatment. Poly(A)+ RNA striations were present in the cytoplasm of early cap rays and associated with longitudinal actin bundles. Cytochalasin D destroyed the actin filaments and caused a dispersal of the striations. Poly(A)+ RNA striations occurred in the cytoplasm of the cap rays up to the stage when secondary nuclei migrated into the cap rays, but they disappeared after the secondary nuclei were settled in their positions. At that time, a mass of poly(A)+ RNA was present around each of the secondary nuclei and accumulated rRNA. This mass colocalized with microtubules radiating from the surface of each secondary nucleus and disappeared when the microtubules were depolymerized by butamifos, which did not affect the configuration of actin filaments. These masses of poly(A)+ RNA continued to exist even after the cap ray cytoplasm divided into cyst domains. Thus two distinct forms of poly(A)+ RNA population, striations and masses, appear in turn at consecutive stages of cyst formation and are associated with distinct cytoskeletal elements, actin filaments and microtubules, respectively.

Development and organization of the central vacuole of Acetabularia acetabulum

* Here we analyzed the shape of the central vacuole of Acetabularia acetabulum by visualizing its development during diplophase (from juvenility through reproduction) and haplophase (from meiosis through mating). * Light microscopy and whole-organism applications of a pH-sensitive dye, neutral red, were used to visualize the anatomy of the central vacuole. We studied connectivity within the thallus by locally applying dye to morphologically distinct regions (rhizoid, stalk, apex, hairs) and observing dye movements. * In vegetative thalli most of the rhizoid, stalk and young hairs stained with dye. In reproductive structures (caps, gametangia) dye also stained the majority of the interiors. When applied to small areas, dye moved at different rates through each region of the thallus (e.g. within the stalk). Dye moved from younger hairs, but not from older hairs, into the stalk. Errors in incorporation of central vacuole into gametangia occurred at <10(-5). * These data indicate that the central vacuole of A. acetabulum is a ramified polar organelle with, potentially, a gel-like sap that actively remodels its morphology during development.

Differential messenger RNA gradients in the unicellular alga Acetabularia acetabulum. Role of the cytoskeleton

The unicellular green alga Acetabularia acetabulum has proven itself to be a superior model for studies of morphogenesis because of its large size and distinctive polar morphology. The giant cell forms an elongated tube (a stalk of up to 60 mm in length), which at its apical pole makes whorls of hairs, followed by one whorl of gametophores in the shape of a cap. At its basal pole, the cell extends into a rhizoid wherein the single nucleus is positioned. In this study, we have determined the level of specific messenger RNAs in the apical, middle, and basal regions using reverse transcriptase-PCR methodology. Four mRNA classes were distinguished: those that were uniformly distributed (small subunit of Rubisco, actin-1, ADP-glucose, centrin, and alpha- and beta-tubulin), those that expressed apical/basal (calmodulin-4) or basal/apical gradients (calmodulin-2 and a Ran-G protein), and those with development-specific patterns of distribution (mitogen-activated protein kinase, actin-2, and UDP-glucose-epimerase). Restoration of the apical/basal calmodulin-4 mRNA gradient after amputation of the apical region of the cell requires the nucleus and was abolished by cytochalasin D. Accumulation of actin-1 mRNA in the vicinity of the wound set by the amputation needs, likewise, the presence of the nucleus and was also inhibited by cytochalasin. This suggests that actin microfilaments of the cytoskeleton are involved in directed transport and/or anchoring of these mRNAs.

Aluminum toxicity studies in Vaucheria longicaulis var. macounii (Xanthophyta, Tribophyceae). II. Effects on the F-actin array

In this study, we test the hypothesis that exposure to environmentally significant concentrations of aluminum (Al, 80 µM) causes the microfilament array of Vaucheria longicaulis var. macounii vegetative filaments to become fragmented and disorganized. Changes in F-actin organization following treatment of vegetative filaments by Al are examined using vital staining with fluorescein phalloidin. In the cortical cytoplasm of the apical zone of pH 7.5 and pH 4.5 control cells, axially aligned bundles of F-actin lead to a region of diffuse, brightly stained material. Dimly stained focal masses are noted deeper in the cytoplasm of the apical zone whereas they are absent from the zone of vacuolation. The F-actin array is visualized in the cortical cytoplasm of the region of the cell, distal to the apical tip, which exhibits vigorous cytoplasmic streaming (zone of vacuolation) as long, axially aligned bundles with which chloroplasts and mitochondria associate. Thirty minutes following treatment with aluminum, and for the next 8-16 h, the F-actin array is progressively disorganized. The longitudinally aligned F-actin array becomes fragmented. Aggregates of F-actin, such as short rods, amorphous and stellate F-actin focal masses, curved F-actin bundles and F-actin rings replace the control array. Each of these structures may occur in association with chloroplasts or independently with no apparent association with organelles. Images are recorded which indicate that F-actin rings not associated with organelles may self-assemble by successive bundling of F-actin fragments. The fragmentation and bundling of F-actin in cells of V. longicaulis upon treatment with aluminum resembles those reported after diverse forms of cell disturbance and supports the hypothesis that aluminum-induced changes in the F-actin array may be a calcium-mediated response to stress.

Asymmetric subcellular mRNA distribution correlates with carbonic anhydrase activity in Acetabularia acetabulum

The unicellular green macroalga Acetabularia acetabulum L. Silva is an excellent system for studying regional differentiation within a single cell. In late adults, physiologically mediated extracellular alkalinity varies along the long axis of the alga with extracellular pH more alkaline along the apical and middle regions of the stalk than at and near the rhizoid. Respiration also varies with greater respiration at and near the rhizoid than along the stalk. We hypothesized that the apical and middle regions of the stalk require greater carbonic anhydrase (CA) activity to facilitate inorganic carbon uptake for photosynthesis. Treatment of algae with the CA inhibitors acetazolamide and ethoxyzolamide decreased photosynthetic oxygen evolution along the stalk but not at the rhizoid, indicating that CA facilitates inorganic carbon uptake in the apical portions of the alga. To examine the distribution of enzymatic activity within the alga, individuals were dissected into apical, middle, and basal tissue pools and assayed for both total and external CA activity. CA activity was greatest in the apical portions. We cloned two CA genes (AaCA1 and AaCA2). Northern analysis demonstrated that both genes are expressed throughout much of the life cycle of A. acetabulum. AaCA1 mRNA first appears in early adults. AaCA2 mRNA appears in juveniles. The AaCA1 and AaCA2 mRNAs are distributed asymmetrically in late adults with highest levels of each in the apical portion of the alga. mRNA localization and enzyme activity patterns correlate for AaCA1 and AaCA2, indicating that mRNA localization is one mechanism underlying regional differentiation in A. acetabulum.

Poly(A)+ RNA during vegetative development of Acetabularia peniculus

In the juvenile stage, the diploid giant-celled green algae Acetabularia spp. are differentiated into an upright stalk and an irregularly branched rhizoid. Early amputation and grafting experiments as well as biochemical and molecular analyses have shown that mRNA (as poly(A)+ RNA) is continuously supplied from the primary nucleus in the rhizoid and accumulates in the stalk apex. In the present study, localization of poly(A)+ RNA in the juvenile stage of the Acetabularia peniculus was investigated by fluorescent in situ hybridization using oligo(dT) as a probe. The signal was localized in the apical cytoplasm and, in addition, multiple longitudinal striations throughout the stalk and rhizoid cytoplasm. A large portion of the poly(A)+ RNA striations exhibited structural polarity, broadened at one end and gradually thinned toward the other end. Some of the striations in the rhizoid cytoplasm were continuous with a zone of signal in the area of the perinuclear rim. The poly(A)+ RNA striations were associated with thick bands of longitudinal actin bundles which run through the entire length of the stalk. Cytochalasin D caused fragmentation of the actin bundles and irregular distribution of the fluorescent signal. We suggest that the poly(A)+ RNA striations constitute a hitherto unknown form of packaged mRNA that is transported over large distances along the actin cytoskeleton to be stored and expressed in the growing apex.

Whorl morphogenesis in the dasycladalean algae: the pattern formation viewpoint

The dasycladalean algae produce diverse whorled structures, among which the best known are the vegetative and reproductive whorls of Acetabularia acetabulum. In this paper, we review the literature pertaining to the origin of these structures. The question is addressed in terms of the necessary pattern-forming events and the possible mechanisms involved, an outlook we call the pattern formation viewpoint. The pattern-forming events involved in the morphogenesis of the vegetative and reproductive whorls of Acetabularia have been used to define five and six morphogenetic stages, respectively. We discuss three published mechanisms which account, at least in part, for the pattern-forming events. The mechanisms are mechanical buckling of the cell wall, reaction-diffusion of morphogen molecules along the cell membrane, and mechanochemical interactions between Ca2+ ions and the cytoskeleton in the cytosol. The numerous differences between these mechanisms provide experimental grounds to test their validity. To date, the results of these experiments point towards reaction diffusion as the most likely patterning mechanism. Finally, we consider the evolutionary origin of the vegetative and reproductive whorls and provide mechanistic explanations for some of the major evolutionary advances.

Actin cytoskeleton in plants: from transport networks to signaling networks

The plant actin cytoskeleton is characterized by a high diversity in regard to gene families, isoforms, and degree of polymerization. In addition to the most abundant F-actin assemblies like filaments and their bundles, G-actin obviously assembles in the form of actin oligomers composed of a few actin molecules which can be extensively cross-linked into complex dynamic meshworks. The role of the actomyosin complex as a force generating system - based on principles operating as in muscle cells - is clearly established for long-range mass transport in large algal cells and specialized cell types of higher plants. Extended F-actin networks, mainly composed of F-actin bundles, are the structural basis for this cytoplasmic streaming of high velocities On the other hand, evidence is accumulating that delicate meshworks built of short F-actin oligomers are critical for events occurring at the plasma membrane, e.g., actin interventions into activities of ion channels and hormone carriers, signaling pathways based on phospholipids, and exo- and endocytotic processes. These unique F-actin arrays, constructed by polymerization-depolymerization processes propelled via synergistic actions of actin-binding proteins such as profilin and actin depolymerizing factor (ADF)/cofilin are supposed to be engaged in diverse aspects of plant morphogenesis. Finally, rapid rearrangements of F-actin meshworks interconnecting endocellular membranes turn out to be especially important for perception-signaling purposes of plant cells, e.g., in association with guard cell movements, mechano- and gravity-sensing, plant host-pathogen interactions, and wound-healing.

Statoliths motions in gravity-perceiving plant cells: does actomyosin counteract gravity?

Statocytes from plant root caps are characterized by a polar arrangement of cell organelles and sedimented statoliths. Cortical microtubules and actin microfilaments contribute to development and maintenance of this polarity, whereas the lack of endoplasmic microtubules and prominent bundles of actin microfilaments probably facilitates sedimentation of statoliths. High-resolution video microscopy shows permanent motion of statoliths even when sedimented. After immunofluorescence microscopy using antibodies against actin and myosin II the most prominent labeling was observed at and around sedimented statoliths. Experiments under microgravity have demonstrated that the positioning of statoliths depends on the external gravitational force and on internal forces, probably exerted by the actomyosin complex, and that transformation of the gravistimulus evidently occurs in close vicinity to the statoliths. These results suggest that graviperception occurs dynamically within the cytoplasm via small-distance sedimentation rather than statically at the lowermost site of sedimentation. It is hypothesized that root cap cells are comparing randomized motions with oriented motions of statoliths and thereby perceiving gravity.

The anti-proliferative agent jasplakinolide rearranges the actin cytoskeleton of plant cells

In the present study, we have characterized the action of the natural cyclodepsipeptide jasplakinolide (JAS) on the cytoplasmic architecture, actin-based cytoplasmic motility, and the organization of the actin cytoskeleton in selected examples of green algae (Acetabularia, Pseudobryopsis and Nitella) and higher plant cells (Allium bulb scale cells and Sinapis root hairs). JAS was capable of influencing the actin cytoskeleton and inhibiting cytoplasmic streaming in a differential, cell type-specific manner. With the exception of Nitella, two consecutive responses were observed upon incubation with 2.5 microM JAS: In the first phase cytoplasmic streaming increased transiently alongside with minor modifications of the actin cytoskeleton in the form of adventitious actin spots and spikes appearing throughout the cell cortex in addition to the normal actin bundle system typical for each cell type. In the second phase, cytoplasmic streaming stopped and the actin cytoskeleton became heavily reorganized into shorter, straight, more and more randomly oriented bundle segments. JAS exerted severe long-term effects on the actin cytoskeleton when treatments exceeded 30min at a concentration of 2.5 microM. An in situ competition assay using equimolar concentrations of JAS and FITC-phalloidin suggested that JAS has a phalloidin-like action. Effects of JAS were significantly different from those of cytochalasin D with respect to the resulting degree of perturbance of cytoplasmic organization, the distribution of actin filaments and the speed of reversibility.

ELABORATION OF BODY PLAN AND PHASE CHANGE DURING DEVELOPMENT OF ACETABULARIA: How Is the Complex Architecture of a Giant Unicell Built?

While uninucleate and unicellular, Acetabularia acetabulum establishes and maintains functionally and morphologically distinct body regions and executes phase changes like those in vascular plants. Centimeters tall at maturity, this species has allowed unusual experimental approaches. Amputations revealed fates of nucleate and enucleate portions from both wild type and mutants. Historically, graft chimeras between nucleate and enucleate portions suggested that morphological instructions were supplied by the nucleus but resided in the cytoplasm and could be expressed interspecifically. Recently, graft chimeras enabled rescue of mutants arrested in vegetative phase. Since the 1930s, when Acetabularia provided the first evidence for the existence of mRNAs, a dogma has arisen that it uses long-lived mRNAs to effect morphogenesis. While the evidence favors translational control, the postulated mRNAs have not been identified, and the mechanism of morphogenesis remains unknown. Amenable to biochemistry, physiology, and both classical and molecular genetics, Acetabularia may contribute yet new insights into plant development and morphogenesis.

Differentiation, growth and morphogenesis: Acetabularia as a model system

The aim of this paper is to review the present knowledge of the main aspects of differentiation of Acetabularia, a unicellular, eukaryotic organism, and to underline the multiple control pathways modulated by circadian rhythmicity. Growth and morphogenesis are sequentially programmed. Timing of cap differentiation is highly dependent on external conditions. The importance of the sequence of processes is shown by experimental disregulation. The alga is a highly polarized cell, both in morphology and in the relative concentrations of a number of the molecules it contains. Apical cap differentiation is regulated at the post-transcriptional level and could also depend in part on polyamines and on proteolytic activity. Acetabularia displays a number of circadian rhythms (CR). These rhythms form an elaborate biological time structure (also called temporal morphology, or morphology in time as opposed to morphology in space): the distribution in the 24 h cycle of the peaks and troughs of the oscillating functions. The oscillations display fixed relations both with the other functions and with external conditions (such as the transition from dark to light). Interestingly, the CR modulate Acetabularia's development, which is influenced by photoperiod; we present preliminary experiments suggesting that disruption of temporal morphology is deleterious to morphogenesis. Induction of growth and of morphogenesis are totally dependent on blue light. However, blue light receptors in plants arc probably multiple, but we present arguments suggesting that flavin-cytochrome b and the associated KHAM-sensitive molecule are present in Acetabularia plasma membrane and are involved in blue light perception. Agents interfering with different steps of signal perception and transduction show that at least some of these steps are temporally regulated. According to recent experiments from our laboratory, the existence of a redox signalling mechanism appears to be highly probable. The phytohormones (or plant regulators), auxin (indole acetic acid), abscisic acid and ethylene, exert cell-regulatory functions and are involved in Acetabularia differentiation. They also modulate at least some circadian rhythms. Finally, circadian rhythms intervene in differentiation and are proposed to have an integrative function. CONTENTS Summary 1 I. Introduction: the cell cycle and morphology of Acetabularia 2 II. Growth and cap morphogenesis: the developmental programme 3 III. Polarity 5 IV. Temporal morphology 6 V. Induction of growth and cap morphogenesis 9 VI. The plasma membrane 12 VII. Hormones: development and metabolic activity in Acetabularia 12 VIII. Phytohormones receptors and insulin receptors 15 IX. Other possible hormones 16 X. Fundamental role of CR: their intervention in modulating multiple steps in differentiation 16 XI. Conclusions and perspectives 17 Acknowledgements 17 References 17.

The role of the cytoskeleton in polarity and morphogenesis of algal cells

Selected algal species continue to serve as model organisms for the study of cell growth and cellular morphogenesis. Recent improvements in immunohistochemical and microinjection methods have helped to consolidate our views of the role of the cytoskeleton as a generator of spatial patterns in the cytoplasm before cellular morphogenesis. Progress has also been made in the discovery and characterization of molecular components of both the cytoskeleton and the extracellular matrix (ECM). Studies on the oocytes of fucoid brown algae have demonstrated that the ECM serves an active role in controlling cell shape and in defining the developmental fate of a cell. Actin, transmembrane proteins of the beta-integrin type, and vitronectin-like proteins in the ECM have been discussed as important elements in polar axis formation in the early steps of post-fertilization development. The mechanism of cell expansion has been investigated in the large coenocytic cells of the siphonoclad green algae. It was shown that the alignment of cell wall microfibrils in these cells depends on the degree of order in the cortical microtubule system. However, in contrast to earlier hypotheses, microtubules do not appear to function as physical boundaries guiding the paths of cellulose synthesizing terminal complexes in the plane of the plasma membrane. Recent work on the giant unicellular green alga Acetabularia has revealed dynamic reorganizations of the actin cytoskeleton during the course of apical morphogenesis. Actin has also been suggested to play a role, in more subtle ways, in the establishment of membrane prepatterns during cellular morphogenesis of the desmid green alga Micrasterias, prepatterns that predict regions of future surface expansion.