Dictyosome-derived Vesicles Research Articles

Astreptonema gammari: An Eccrinid with Appendaged Spores

Summary. Astreptonema gammari (L. Leger & Duboscq) Manier (Trichomycetes, Eccrinaceae) grows obligately attached to the cuticle lining the hindgut of Gammarus pulex L. (Crustacea, Gammaridae) found in freshwater. The primary infestation sporangiospores possess polar mucilage-like appendages that are extensions of the outer layer of the sporangiospore wall. Appendage formation from dictyosome-derived vesicles is described for the first time and compared with spore appendage formation in other species of the Trichomycetes, particularly the Harpellales. orders, the Harpellales, Eccrinales and Asellariales, and is assigned to the Zygomycota owing to asexual reproduction by sporangiospores and, in some species of the Harpellales, the formation of zygospores (Moss & Lichtwardt 1977). A fourth order, the Amoebidiales, characterised by the production of amoeboid sporangiospores, has been traditionally placed in the Trichomycetes but is probably better assigned to the Rhizopoda of the Protoctista (Moss 1998). Species of Trichomycetes are characterised by several features that adapt them to their ecological niche; these include a holdfast, determinate or restricted growth, and in many species inhabiting aquatic hosts, appendaged spores. Many fungal species that live in aquatic environments are characterised by appendaged spores; the appendages impart either a dispersal advantage and/or enhance the attachment of the spore to the substratum. Appendaged spores occur in a wide range of taxonomically diverse fungi, from the appendaged basidiospores of the marine gasteromycete Nia vibrissa Moore & Meyers (Moss 1990) to the freshwater stellate secondary "conidia" of the entomopathogen Erynia conica Nowak. (Descals et al. 1981). The form of the appendages ranges from hyphal branches in some of the tetraradiate conidia of aquatic Hyphomycetes (Ingold 1975) to ascospore wall elaboration and cell exudations of many marine Ascomycota (Jones & Moss 1987; Jones 1995). The development of appendages by diverse taxonomic and ecological groups of fungi

Structure and Function in the Grass Ligule: the Endomembrane System of the Adaxial Epidermis of the Membranous Ligule of Lolium temulentum L. (Poaceae)

The endomembrane system of the adaxial epidermal cells of the membranous ligule of the grass Lolium temulentum L. has been studied by conventional transmission electron microscopy, and by using zinc iodide-osmium tetroxide impregnation with examination of thick sections at 100 kV. The components of the endomembrane system are the nuclear membranes, rough endoplasmic reticulum (present in both cisternal and tubular forms), dictyosomes, dictyosome-derived vesicles, coated vesicles, plasma membrane and tonoplast. Direct continuity between tubular endoplasmic reticulum and the cis face of dictyosomes is demonstrated and the applicability of the endomembrane concept to this putative secretory tissue is discussed. The presence of vesicle-like profiles apparently within the outer tangential walls is reported.

Pollen Wall Development in Vigna vexillata. II. Ultrastructural Studies

Ultrastructural studies to determine the role of cytoplasmic organelles in pollen wall development in Vigna vexillata L. (Fabaceae) showed that plastids, mitochondria, and vacuoles do not directly participate in pollen wall morphogenesis, as inferred from their cytoplasmic distribution during microsporogenesis and pollen development. Spatial and temporal correlations between elements of the endomembrane system and pollen wall development suggest that dictyosomes, the endoplasmic reticulum, and multimembrane inclusions are involved in synthesis, processing, and/or transport of precursors for the various pollen wall layers. Large numbers of dictyosome-derived vesicles appear during development of the primexine matrix, the patterned primexine, and the apertural Zwischenkörper, suggesting that dictyosomes are involved in development of these layers. Tubules of smooth endoplasmic reticulum, preferentially located under developing primexine ridges and extending to the vicinity of the plasma membrane, support the hypothesis of endoplasmic reticulum involvement in pollen wall pattern formation. Multimembrane inclusions, abundant in the cortical cytoplasm of developing microspores, may be involved in development of the lamellate endexine.

Membrane flow during zoosporogenesis in the coenocytic green algaDictyochloris fragrans Vischer ex Starr (Chlorophyceae, Chlorellales)

Cytokinesis in the coenocytic coccoid green algaDictyochloris fragrans Vischer ex Starr was studied with transmission electron microscopy. In mature cells, the nuclei with associated basal bodies and dictyosomes are arranged in a distinct layer against the inner margin of the parietal reticulate chloroplast, enclosing a cytoplasmic centre in which an extensive system of ER has developed. Centrifugal cleavage of the multinucleate protoplast results in the formation of numerous wall-less, biflagellate zoospores. Cytochemical analysis using OsFeCN and PTA revealed that following sub-division of the ER into smaller fragments incipient cleavage furrows are formed in the centre of the cell by coalescing small, most probably dictyosome-derived vesicles. The initial fissures thereupon grow rapidly, apparently fed by large dictyosome-derived vesicles. Cleavage furrows are apparently guided by phycoplast MTs. The final partitioning into uninucleate daughter protoplasts occurs after a drastic rearrangement of cell organelles to ensure an equal distribution among the zoospores. A comparison with other multinucleate or true coenocytic chlorophycean species is presented.

A quantitative stereological analysis of the effect of indoleacetic acid on the dictyosomes in pea stem epidermal cells

A quantitative analysis of electron micrographs showed that IAA treatment caused an initial rapid increase in the amount of dictyosome material in pea stem epidermal cells. The increase was detected within 15 minutes of auxin presentation and reached a maximum around 30 minutes. This was followed by a decrease, presumably due to an increased utilization of the organelle. The decrease involved a fall in the amount of dictyosome-derived vesicles and in the actual number of dictyosomes. The results are discussed in relation to similar observations on expanding cells of monocotyledonous plants.

The distribution of calcium in the grass pollen tube

The distribution of calcium in the terminal regions of actively extending pollen tubes of two Gramineae,Zea maysandPennisetum americanum, has been investigated by chlorotetracycline (CTC) fluorescence and by energy-dispersive X-ray analysis. Neither method reveals a concentration gradient declining from the tip towards the older parts of the tube comparable with that reported from the pollen tubes ofLilium longiflorum. The difference evidently arises from dissimilarities in pollen-tube growth physiology and the apical organization of the tube. Growth is achieved by the insertion of dictyosome-derived vesicles carrying wallprecursor materials at the tube tip. InL.longiflorumthese are produced in a subapical zone of the tube, and a characteristic zonation of cytoplasmic organelles in the distal region develops during growth. In the grasses, large numbers of the wall precursor bodies (‘P-particles ’) are produced before pollen dispersal, and are stored in the grain; they are distributed throughout the tube during early growth, and the organelle zonation in the apex is less pronounced. CTC-induced fluorescence is strongly associated with mitochondria, membranes and P-particles, suggesting that the observed distribution of calcium may reflect mainly the distribution of the element held in organelles and membranes rather than mobile Ca2+in the cytosol.

Ultrastructure of Dormant, Converted, and Germinating Oospores of Pythium ultimum

The ultrastructural aspects of the conversion of constitutively dormant oospores of Pythium ultimum to thin-walled germinable oospores and their subsequent germination were studied by electron microscopy. During conversion, the thick inner oospore wall dissolves leaving the outer oospore wall as a loose but intact layer surrounding the spore. A thin germination wall, which eventually extends as the germ tube wall, becomes apparent around the protoplast. The protoplasm of the dormant oospore is predominantly reserve cytoplasm and includes a cortical layer of lipid bodies and a dense central ooplast. Converted oospores contain a more functional protoplasm including nuclei, mitochondria, endoplasmic reticulum, dictyosomes, and unattached ribosomes. The cortical lipids disperse within the cytoplasm while the ooplast breaks into fragments which are enveloped in membranes to form the densebody or “fingerprint” vacuoles characteristic of the oomycetous fungi. After sucrose is added, germinable oospores produce single germ tubes which rupture the outer oospore and oogonial walls. Germ tube tips are laden with dictyosome-derived vesicles.

Rice protein-body formation: all types are initiated by dilation of the endoplasmic reticulum

The ultrastructure of protein deposition in the starchy endosperm of developing rice (Oryza sativa L.) grains was examined in conventionally fixed (glutaraldehyde and osmium tetroxide) tissues and also in thick sections (0.3 μm) of zinc iodide-osmium tetroxide post-fixed tissue. Three types of previously characterised protein body were observed and it was shown that each type was initiated by dilations of the endoplasmic reticulum. Crystalline type protein bodies were initiated by a ribosome-free dilation from rough cisternal endoplasmic reticulum and developed by inclusion of protein from dictyosome-derived vesicles. The large spherical and small spherical protein bodies developed within the cisternae of the rough endoplasmic reticulum.

The glycosome of Chara: Ultrastructure, development, and composition

Certain membrane-bound inclusions which we have called glycosomes are a common feature of both young and mature cells of Chara corallina . Glycosomes are spherical in form, bound by a unit membrane, and contain a fine, granular, electron-opaque material. The glycosomes are derived from dictyosomes, initially as large vesicles containing a granular material. They apparently reach their full size some time after release from the dictyosome cisternae. Addition of material through fusion of smaller dictyosome-derived vesicles to the young glycosome is indicated by TEM observations. The silver methenamine staining technique for polysaccharides gives positive proof of the polysaccharide nature of the material within the glycosome. The contents of the glycosome stained intensely for polysaccharides, as did the contents of some of the associated dictyosome vesicles. Staining did not occur in sections of nonosmicated tissues which were not oxidized or in oxidized sections treated with an aldehyde blocking reagent (dimedone) prior to staining.

Electron microscopy of early gametangial interaction in Phytophthora megasperma var. sojae

The ultrastructure of gametangial development and oospore formation was studied in Phytophthora megasperma var. sojae, a species with the paragynous antheridium. Except for the early gametangial interaction, the developmental stages of oogonial expansion, fertilization tube formation, oosphere and oospore formation were very similar to those reported for amphigynous species. The initial contact differs in the two types of gametangial interaction in that in amphigyny the oogonial hypha contacts and penetrates the antheridial initial from the side, whereas in paragyny the antheridial hypha touches and fuses with the oogonial initial from the side without penetrating it. In both cases the end result seems to be the same: the establishment of a firm adhesion zone between the gametangia. Dictyosome-derived vesicles were active in the formation of the contact zone, during oogonial expansion and wall thickening, and during extension of the fertilization tube. An extensive conversion of cytoplasm occurred during differentiation within the oogonium from a largely 'functional' cytoplasm rich in mitochondria, ribosomes, rough endoplasmic reticulum, and dictyosomes to a largely 'storage' cytoplasm within the oospore consisting of an ooplast and surrounding lipid layer.

The amounts and rates of export of polysaccharides found within the membrane system of maize root cells.

1. Maize seedling roots were incubated in vivo with d-[U-(14)C]glucose for 2, 5, 10, 15, 30 and 45min. The total incorporation of radioactivity into polysaccharide components in isolated fractions was investigated, and the pattern of incorporation into different polysaccharide components within the rough endoplasmic reticulum, Golgi apparatus and exported material was analysed. 2. The membrane compartments reached a saturation value of radioactivity in polysaccharide components by 30min incubation. Radioactivity in exported polysaccharide continued to increase after that time. The latter was formed and maintained by a steady-state turnover of polysaccharide synthesis and transport from the membrane system. 3. If the only access of the slime polysaccharide to the cell surface is via dictyosome-derived vesicles, the amount of slime components in the Golgi apparatus would have to be displaced every 0.3min in order to maintain the observed rates of increase in slime. This is in contrast with a displacement time of about 2.5min that is necessary for polysaccharide components in the Golgi apparatus to produce the observed increase in cell-wall material. The activity of the membrane system in the production of maize root slime is 8 times as great as that of the membrane system involved in cell-wall synthesis. 4. If the amount of polysaccharide material in the Golgi apparatus is maintained only by inflow of polymeric material from the rough endoplasmic reticulum the total amount of slime components in the rough endoplasmic reticulum would have to be displaced every 7min to maintain a constant amount in the Golgi apparatus. If the endoplasmic reticulum contributed directly to the cell surface in the synthesis of cell-wall material, displacement times necessary to maintain the observed rate of polymer production would be very slow.

The ultrastructure and ontogeny of pollen in Helleborus foetidus l. iv. pollen grain maturation

ABSTRACT The final stages of Helleborus pollen-grain ontogeny, which culminate in maturation and germination of the grain, have been investigated at the ultrastructural level. Following the deposition of primary and secondary exine, and during the early stages of intine formation, the microspore passes through a vacuolate phase, in which the cytoplasm appears devoid of most organelles other than the prominent nucleus. The formation of the vacuole results in the displacement of the nucleus to one side of the pollen grain. The vacuole quickly disappears and a number of organelles reappear in the cytoplasm, in particular the dictyosomes and strands of endoplasmic reticulum, with associated grey bodies. Following mitotic division of the pollen grain, the first signs of the generative cell wall appear as a pair of tightly appressed unit membranes in the narrow strand of cytoplasm separating the two newly formed generative and vegetative nuclei. As development proceeds, the space between the two membranes gradually fills with an electron-transparent material similar to the substance found in the numerous dictyosome-derived vesicles which, together with the endoplasmic reticulum, are both closely associated with the developing cell wall. The generative cell wall fuses with the cellulosic intine, which has gradually increased in amount during these stages, and the cell division is complete. The smaller generative cell contains a prominent nucleus and a small amount of cytoplasm devoid of plastids and most other organelles. The larger vegetative cell also contains a prominent nucleus and a large amount of cytoplasm containing amyloplasts, mitochondria, dictyosomes and endoplasmic reticulum, and abundant ribosomes, many of which are in a polysome configuration. The final stages in development are characterized by a progressive decrease in the amount of starch in the vegetative cell and an increase in the size of grey bodies, many of which are invested in multilayered shrouds of endoplasmic reticulum. The generative cell wall disappears and a multivesicular/granular body gradually appears at the periphery of the pollen grain. The granular-vesicular material, which is formed from the dictyosomes and/or the degenerating plastids, is thought to represent metabolic reserves necessary for pollen-tube formation. One or more pollen tubes emerge from the apertural sectors of the pollen grain, and maturation of the grain is complete.

Ultrastructural aspects of encystation and cyst-germination in Phytophthora parasitica.

ABSTRACT Encystation in Phytophthora parasitica can be divided into 3 stages. In the first, the zoospores line their peripheries with flattened vesicles and nbrillar vacuoles in preparation for encystation. In the second stage, as the zoospores round up and shed their flagella, an initial wall is produced which takes the form of the mature cyst wall in thickness, but not in density. The participation of the flattened vesicles and fibrillar vacuoles in the formation of this initial wall is suggested by the disappearance of these organelles concomitant with wall formation. The third stage involves the maturation of the cyst wall and occurs only after dictyosomes produce vesicles which move to the cyst periphery and fuse to the plasmalemma. Germ tubes are formed in direct and indirect germination and involve the evagination of the plasmalemma and cyst wall proximal to an accumulation of dictyosome-derived vesicles. These vesicles remain at the germ-tube tip as it extends. In indirect germination the germ tube stops after having attained an average length of 6 μm and the vesicles appear to fuse at the hyphal apex, thus forming a cap. Lomasomes do not appear to be cell organelles with a specific function such as wall synthesis, but rather seem to represent aggregations of excess membranous material that have formed as a result of the discharge of vesicles at the cell periphery during wall formation. When dictyosome vesicles are inhibited from forming and moving toward the cell periphery, lomasomes are not formed.

Ultrastructural Changes in Directly Germinating Sporangia of Phytophthor parasitica

Flagella and cleavage vesicles form during the initial stages of direct germination in the same manner as they do in indirect germination. Later, however, the flagella degenerate and cleavage of the cytoplasm is not complete. Instead, a new wall layer is deposited onto the existing sporangial wall, and this germination wall extends with the germ tube and forms the hyphal wall. Material for the germination wall first appears around vesicular aggregates concentrated at the periphery of the sporangial cytoplasm. As the wall forms the vesicular aggregates become encased in it, and the wall eventually consists of irregular deposits of wall material interspersed with the vesicular aggregates. These are clearly cytoplasmic in nature and different from lomasomes, since they are associated with ribosomes and other small elements of the cytoplasm. Likely to contribute to the wall formation are the endoplasmic reticulum, microbodies, dictyosome-derived vesicles, and possibly polyvesicular bodies and larger vacuoles with fibrillar contents. The basal plug of the sporangium forms the same way as the germination wall and also contains numerous vesicular inclusions. Fewer vesicular inclusions are found in the sporangial wall, and none have been observed in the walls of the growing hyphae. This difference in wall structure can be explained on the basis of their different growth patterns. Penetration of the sporangial plug is concomitant with prominent lomasomal activity and the cytoplasm at the hyphal tip is characterized by the presence of numerous vesicles, which are probably derived from dictyosomes.

Vacuolation: Origin and development of the lysosomal apparatus in root-tip cells

The morphology of vacuolation has been investigated in root tip cells of corn using the freeze-etching technique. The genesis of vacuoles involves the following processes: a) Formation of small, endoplasmic-reticulum (ER)-derived vesicles (provacuoles); b) fusion of provacuoles resulting in the formation of small vacuoles, and followed by fusion and expansion of vacuoles; c) incorporation of large, dictyosome-derived vesicles into vacuoles by invagination of the tonoplast; d) invagination of the tonoplast resulting in the incorporation of cytoplasmic material into vacuoles. The morphological findings are correlated with biochemical data obtained from isolated vacuoles (lysosomes). Provacuoles (ER-derived vesicles) are shown to be primary lysosomes; their hydrolases arise from the ER. Vacuoles represent secondary lysosomes (digestive vacuoles) of the higher-plant cell. The metabolic role of lytic processes proceeding in the lysosomal apparatus is discussed.