Rapid wound responses ofSaprolegnia ferax hyphae depend upon actin and Ca2+-involving deposition of callose plugs

Summary Growing hyphae of the oomyceteSaprolegnia ferax wounded by impalement with a ca. 0.2 m diameter glass microelectrode normally respond within seconds with an apically directed cytoplasmic contraction followed by production of a plug which encases the electrode and occludes its recording of transmembrane potentials. This plug contains callose and Ca²⁺-associated membranes. To characterize the rapid wounding response, we disrupted specific filamentous (F) actin populations and Ca²⁺ regulation. Plug formation is inhibited by disruption of F-actin populations and low exogenous Ca²⁺ but not by inhibition of stretch-activated Ca²⁺ channels with Gd³⁺. Therefore, stretch-activated channels are not the immediate sensor. Instead, sensing may involve strain on the actin cytoskeleton which triggers the occlusion response. This wound response is qualitatively similar to the production of septa which isolate developing sporangia and seal severed hyphae, indicating the use of a normal basic cellular developmental system as a protective mechanism against environmental damage. The wound response is essential, since an inability to seal sites of mechanical damage is potentially catastrophic in acellular coenocytic organisms.Abbreviations APW artificial pond water - BAPTA 1,2-bis(orthzo-aminophenoxy)ethane-N,N,N,N-tetrapotassium acetate - CTC chlortetracycline - DIC Nomarski differential interference contrast microscopy - F-actin filamentous actin - LatB latrunculin B - PM plasma membrane - RP rhodamine-labeled phalloidin - SA channels stretch-activated channels     

Biomechanical properties and holdfast morphology of coenocytic algae (Halimedales, Chlorophyta) in Bocas del Toro, Panama

For attached marine organisms, specific biomechanical properties may result in detachment or in tissue loss, when sufficient tensile force is applied. Algae experience such forces through water movement, which may thus act to limit size, abundance, and species composition, of populations of algae.Coenocytic construction is uncommon in the algae, but it occurs relatively more frequently in green algae found in shallow subtidal sediments associated with coral reefs, e.g., at our study site of Isla Colon, Bocas del Toro, Panama. We studied the biomechanical properties of some tropical coenocytic algae (Udotea flabellum (Ellis et Solander) Lamouroux, Penicillus capitatus Lamarck, P. pyriformis A. and E.S. Gepp, and Halimeda gracilis Harvey) anchored in sediments. We compare our results with published data on other coenocytic algae, as well as with multicellular algae. Our results show that properties of sand-dwelling coenocytes, such as mean force to dislodge (4.9-12.7 N), mean force to break (6.6-22.1 N), and mean strength (1.0-7.0 MN m^− 2), are all within the range reported for temperate, multicellular, algae. In contrast, the coenocytes differed markedly from the temperate non-coenocytes in the consequences of applied tensile force: coenocytes were removed whole, while most temperate algae attached to rocks break within the thallus. Some multicellular algae can regrow from tissue left on the substratum; three of the four coenocytic species we examined had rhizoids connecting closely adjacent (0.1-0.15 m) individuals, and these rhizoids may serve to regrow a new individual. While our experiments indicated that sufficient tensile force results in dislodgment, calculations using the experimentally determined variables led us to conclude that water velocities sufficient to dislodge individuals are unlikely to occur. Since dislodgment is usually fatal for algae, the role of the holdfast is a critical one. All of the species we investigated had similar holdfast morphology, a mass of rhizoids which entrained sand, the entire unit forming a hemispherical to cylindrical mass. Despite the consistency in holdfast form, and the initial prediction that this was an optimal form for anchoring these algae, our data suggest that this is not the case.     

Actin-based chloroplast rearrangements in the cortex of the giant coenocytic green algaCaulerpa

Summary The giant coenocytic green algaCaulerpa is well known for its large scale amyloplast transport. The majority of chloroplasts, however, is immobilized in the cortex of the cell. By applying UV-irradiation to localized areas of the cortex chloroplasts can be induced to slowly move towards and aggregate around the irradiated spot. Chloroplast movement is blocked by cytochalasin D, but not by colchicine or the microtubule herbicide cremart. The dynein inhibitor erythro-9-[3-(2-hydroxynonyl)] adenine (EHNA) also has no effect on chloroplast movement. However, both microtubule- and dynein-specific inhibitors block movement of amyloplasts. Using the previously developed technique of microdissection followed by immunofluorescence microscopy it can be shown that, concomitant with changes in motile behavior of chloroplasts upon irradiation, actin filaments form and rearrange around the irradiation spot. It is concluded that in contrast to amyloplast movement, immobilization and movement of chloroplasts are dependent on actin but not on microtubules. Therefore, two individual motile mechanisms appear to have evolved for independent positioning and motility of the two populations of plastids in the giant coenocyteCaulerpa.Abbreviations EHNA erythro-9-[3-(2-hydroxynonyl)] adenine - DMSO dimethylsulfoxide - MT microtubule - NEM N-ethylmaleimide     

When is a cell not a cell? A theory relating coenocytic structure to the unusual electrophysiology of Ventricaria ventricosa (Valonia ventricosa)

Summary. Ventricaria ventricosa and its relatives have intrigued cell biologists and electrophysiologists for over a hundred years. Historically, electrophysiologists have regarded V. ventricosa as a large single plant cell with unusual characteristics including a small and positive vacuole-to-outside membrane potential difference. However, V. ventricosa has a coenocytic construction, with an alveolate cytoplasm interpenetrated by a complex vacuole containing sulphated polysaccharides. We present a theory relating the coenocytic structure to the unusual electrophysiology of V. ventricosa. The alveolate cytoplasm of V. ventricosa consists of a collective of uninucleate cytoplasmic domains interconnected by fine cytoplasmic strands containing microtubules. The cytoplasm is capable of disassociating into single cytoplasmic domains or aggregations of domains that can regenerate new coenocytes. The cytoplasmic domains are enclosed by outer (apical) and inner (basolateral) faces of a communal membrane with polarised K⁺-transporting functions, stabilised by microtubules and resembling a tissue such as a polarised epithelium. There is evidence for membrane trafficking through endocytosis and exocytosis and so plasmalemma and tonoplast do not have fixed identities. Intra- and extracellular polysaccharide mucilage has effects on electrophysiology through reducing the activity of water and through ion exchange. The vacuole-to-outside potential difference, at which the cell membrane conductance is maximal, reverses its sign from positive under hypertonic conditions to negative under hypotonic conditions. The marked mirror symmetry of the characteristics of current as a function of voltage and conductance as a function of voltage is interpreted as a feature of the communal membrane with polarised K⁺ transport. The complex inhomogeneous structure of the cytoplasm places in doubt previous measurements of cytoplasm-to-outside potential difference.Correspondence and reprints: UNESCO Centre for Membrane Science and Technology, Department of Biophysics, School of Physics, University of New South Wales, Sydney, NSW 2052, Australia.     

Mitosis and mitotic wave propagation in the coenocytic alga,<Emphasis Type="Italic"> Vaucheria terrestris</Emphasis> sensu Goetz

Mitosis and cytoplasmic microtubule (MT) dynamics were observed for the first time in Vaucheria terrestris sensu Goetz. Mitosis could occasionally be seen in part of the cylindrical coenocytic cell. The frequency of encountering cells with dividing nuclei was highest (ca 12%) 4 h after the onset of light in 12 h light/12 h dark regimes; it decreased thereafter and approached zero during the dark period. From the anterior end of every interphase nucleus a unique, long MT bundle extended. Differential-interference optics reveals that there is a filamentous structure in front of the moving nucleus. In prophase, the interphase bundle disappeared and shorter MT bundles emanated from both ends of the nucleus. In metaphase, the cytoplasmic MTs completely disappeared, probably being recycled to spindles. Continuous MTs elongated in anaphase and developed into an interzonal spindle in telophase; this elongated up to as much as 10 m. The daughter nuclei were pushed away from each other by the interzonal spindle. Mitosis started synchronously in a relatively narrow region, and the mitotic stage propagated as a mitotic wave to adjacent regions, most frequently from tip to base. The role of the mitotic wave in tip growth and morphogenesis of a coenocytic cell is discussed.This paper is dedicated to the memory of Dr. Eiji Kamitsubo who passed away on 25 April 2003.