Kazuo Okuda

Structure and phylogeny of cell coverings

Abstract.Various cell coverings in structure and composition occur in algae. They include intracellular cell coverings, scaly cell coverings, and cell walls. Chemical components of cell coverings vary depending on phylogenetic groups and generations. In this paper, the evolution of cell coverings is discussed.

Cellulose microfibril assembly in Erythrocladia subintegra Rosenv.: an ideal system for understanding the relationship between synthesizing complexes (TCs) and microfibril crystallization

SummaryThe marine red algaErythrocladia subintegra synthesizes cellulose microfibrils as determined by CBH I-gold labelling, X-ray and electron diffraction analyses. The cellulose microfibrils are quite thin, ribbon-like structures, 1–1.5 nm in thickness (constant), and 10–33 nm in width (variable). Several laterally associated minicrystal components contribute to the variation in microfibrillar width. Electron diffraction analysis suggested a uniplanar orientation of the microfibrils with their (101) lattice planes parallel to the plasma membrane surface of the cell. The linear particle arrays bound in the plasma membrane and associated with microfibril impressions recently demonstrated inErythrocladia have been shown in this study to be the cellulose-synthesizing terminal complexes (TCs). The TCs appear to be organized by a repetition of transverse rows consisting of four TC subunits, rather than by four rows of longitudinallyarranged TC subunits. The number of transverse rows varied between 8–26, corresponding with variation in the length of the TCs and the width of the microfibrils. The spacings between the neighboring transverse rows are almost constant being 10.5–11.5 nm. Based on the knowledge thatAcetobacter, Vaucheria, andErythrocladia synthesize similar thin, ribbon-like cellulose microfibrils, the structural characteristics common to the organization of distinctive TCs occurring in these three organisms has been discussed, so that the mode of cellulose microfibril assembly patterns may be deciphered.

SEASONAL CHANGE IN THE DISTRIBUTION OF CELL SIZE OF COCCONEIS SCUTELLUM VAR. ORNATA (BACILLARIOPHYCEAE) IN RELATION TO GROWTH AND SEXUAL REPRODUCTION1

Growth and sexual reproduction of the marine littoral diatom Cocconeis scutellum Ehrenb. var. ornata Grun. were investigated at 30 different combinations of temperature (5, 10, 14, 18, 22° C), irradiance (20, 60, 100 μE·m−2·s−1) and daylength (14:10 and 10:14 h LD cycle). Growth occurred at all combinations. The optimal growth was observed at 14–18° C, long daylength and highest‐to‐moderate irradiance, and at 18° C, short daylength and highest irradiance. Sexual reproduction on the other hand occurred between 5 and 18° C, and the optimal condition was 10–14° C and short daylength.

Schizocladia ischiensis: a new filamentous marine chromophyte belonging to a new class, Schizocladiophyceae.

A new marine filamentous chromophyte Schizocladia ischiensis sp. nov. is described from Naples, Italy, and a new class, Schizocladiophyceae, is proposed to accommodate the species based on morphology, photosynthetic pigment analysis, and rbcL and 18S rRNA gene sequences. The vegetative thallus is composed of branched filaments, 3-7 microm in diameter, containing one to two light brown parietal plastids. Cell walls are composed of layered fibers containing alginates, but lacking cellulose. Plastids are of the typical chromophyte type, containing chlorophylls a and c, and abundant fucoxanthin. Zoospores are formed by direct transformation of vegetative cells or through a process including a multinucleated cell stage. Zoospores are teardrop-shaped with a longer anterior flagellum with tubular mastigonemes and a shorter smooth posterior flagellum with a basal swelling. Flagella have a single basal plate and multi-gyred transitional helix distal to the basal plate. Each zoospore has an eyespot. Phylogenetic analyses using rbcL and 18S rDNA sequences suggest the closest phylogenetic relationship with Phaeophyceae, and then with Xanthophyceae and Phaeothamniophyceae. Nevertheless, Schizocladia differs from Phaeophyceae in some essential features (i.e. cell wall lacking cellulose and plasmodesmata, presence of flagellar transitional helix). Therefore, an independent class Schizocladiophyceae is proposed to accommodate this new taxon.

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.

Immunogold-labeling analysis of alginate distributions in the cell walls of chromophyte algae

The immunogold electron microscopy technique was employed to detect the presence of alginates in the cell walls of selected chromophyte species. Anti‐alginate antiserum labeled the cell walls of Sphacelaria and Scytosiphon (Phaeophyceae), Tribonema, Vaucheria, Botrydium, Botrydiopsis (Xanthophyceae) and an‘un‐described filamentous species’ (incertae cedis), but it did not label those of Giraudyopsis, Phaeosaccion (Chrysomeridales), Antithamnion (Rhodophyceae) and Bryopsis (Ulvophyceae). This is the first report of the occurrence of alginates in the chromophyte outside Phaeophyceae. The absence of alginates in Chrysomeridales, which has an unclear phylogenetic position, implies a rather distant phylogenetic relationship of the order Chrysomeridales from Phaeophyceae/Xanthophyceae.

Ultrastructural study of plasmodesmata in the brown alga Dictyota dichotoma (Dictyotales, Phaeophyceae).

Plasmodesmata are intercellular bridges that directly connect the cytoplasm of neighboring cells and play a crucial role in cell-to-cell communication and cell development in multicellular plants. Although brown algae (Phaeophyceae, Heterokontophyta) are phylogenetically distant to land plants, they nevertheless possess a complex multicellular organization that includes plasmodesmata. In this study, the ultrastructure and formation of plasmodesmata in the brown alga Dictyota dichotoma were studied using transmission electron microscopy and electron tomography with rapid freezing and freeze substitution. D. dichotoma possesses plasma membrane-lined, simple plasmodesmata without internal endoplasmic reticulum (desmotubule). This structure differs from those in land plants. Plasmodesmata were clustered in regions with thin cell walls and formed pit fields. Fine proteinaceous “internal bridges” were observed in the cavity. Ultrastructural observations of cytokinesis in D. dichotoma showed that plasmodesmata formation began at an early stage of cell division with the formation of tubular pre-plasmodesmata within membranous sacs of the cytokinetic diaphragm. Clusters of pre-plasmodesmata formed the future pit field. As cytokinesis proceeded, electron-dense material extended from the outer surface of the mid region of the pre-plasmodesmata and finally formed the nascent cell wall. From these results, we suggest that pre-plasmodesmata are associated with cell wall development during cytokinesis in D. dichotoma.

Membrane fusion process and assembly of cell wall during cytokinesis in the brown alga, Silvetia babingtonii (Fucales, Phaeophyceae)

During cytokinesis in brown algal cells, Golgi-derived vesicles (GVs) and flat cisternae (FCs) are involved in building the new cell partition membrane. In this study, we followed the membrane fusion process in Silvetia babingtonii zygotes using electron microscopy together with rapid freezing and freeze substitution. After mitosis, many FCs were formed around endoplasmic reticulum clusters and these then spread toward the future cytokinetic plane. Actin depolymerization using latrunculin B prevented the appearance of the FCs. Fusion of GVs to FCs resulted in structures that were thicker and more elongated (EFCs; expanded flat cisternae). Some complicated membranous structures (MN; membranous network) were formed by interconnection of EFCs and following the arrival of additional GVs. The MN grew into membranous sacs (MSs) as gaps between the MNs disappeared. The MSs were observed in patches along the cytokinetic plane. Neighboring MSs were united to form the new cell partition membrane. An immunocytochemical analysis indicated that fucoidan was synthesized in Golgi bodies and transported by vesicles to the future cytokinetic plane, where the vesicles fused with the FCs. Alginate was not detected until the MS phase. Incubation of sections with cellulase-gold showed that the cellulose content of the new cross wall was not comparable to that of the parent cell wall.

Gamete discharge by Bryopsis plumosa (Codiales, Chlorophyta) induced by blue and UV-A light

Gamete discharge by the coenocytic green alga Bryopsis plumosa (Hudson) C. Agardh is induced by light. The mature male gametangia consist of a mass of quiescent male gametes and a large central vacuole. Within a few minutes after the onset of irradiation, breakdown of the tonoplast of the central vacuole and initiation of gamete motility occur simultaneously. This is followed by a forced discharge of moving gametes through a hole ruptured at the subapical region of the gametangium. The action spectrum for the light‐induced gamete discharge was determined from a series of fluence‐response curves. This action spectrum, having two major maxima at 370 and 450 nm, indicates the involvement of a blue light/UV‐A‐absorbing photoreceptor previously described as ‘cryptochrome’.

Cellulose‐synthesizing terminal complexes and microfibril structure in the brown alga Sphacelaria rigidula (Sphacelariales, Phaeophyceae)*

The brown alga Sphacelaria rigidula Kützing synthesizes cellulose microfibrils as determined by CBH I‐gold labeling. The cellulose microfibrils are thin, ribbon‐like structures with a uniform thickness of about 2.6 nm and a variable width in the range of 2.6‐30 nm. Some striations appear along the longitudinal axis of the microfibrils. The developed cell wall in Sphacelaria is composed of three to four layers, and cellulose micro‐fibrils are deposited in the third layer from the outside of the wall. A freeze fracture investigation of this alga revealed cellulose‐synthesizing terminal complexes (TCs), which are associated with the tip of microfibril impressions in the plasmatic fracture face of the plasma membrane. The TCs consist of subunits arranged in a single linear row. The average diameter of the sub‐units is about 6 nm, and the intervals between the neighboring subunits, about 9 nm, are relatively constant. The number of subunits constituting the TC varies between 10 and 100, so that the length of the whole TC varies widely. A model that has been proposed for the assembly of thin, ribbon‐like microfibrils was applied to microfibril assembly in Sphacelaria.