Polarity in higher plant dictyosomes

Summary Dictyosomes in three higher plant cell secretory systems,Zea root cap cells,Tradescantia pollen tubes and digestive glands ofDionaea fly traps, are shown to possess a structural polarity in terms of either the production of secretory vesicles, or the reaction of individual cisternae to the osmium-zinc-iodide impregnation procedure. These observations contradict previous claims that higher plant dictyosomes lack structural polarity. Doubts about the applicability of the endomembrane flow concept to higher plant cell dictyosomes are discussed in relation to the relative balance between carbohydrate and protein in secreted products. The lack of dictyosome-associated endoplasmic reticulum in some of these plant cell systems is confirmed.     

Cytochemical Localization of Phenolic Compounds in Columella Cells of the Root Cap in Seeds of Brassica napus--Changes in the Localization of Phenolic Compounds during Germination

Changes in the localization of phenolic compounds were investigatedin columella cells of embryonic roots in 1-year-old Brassicanapus seeds during 48 h of imbibition and germination. In dry,dormant seeds, phenolic compounds were located in the apoplasticcompartment between the cell wall and plasmalemma in the outermostlayer of the columella. These apoplastic phenolic deposits disappearedduring the activation processes associated with imbibition andgermination, but new deposits appeared successively in the nucleus,ER cisternae, protein bodies and on the outer surface of theroot cap. A large number of phenolic deposits were observedin the outermost part of the columella, becoming less frequenttowards the initial centre. Their appearance coincided withrestoration of the ER and immediately preceded cytological activation.After the primary root had emerged from the seed coat, depositsof phenolic compounds disappeared from the cytoplasm, but simultaneouslyappeared in the vacuoles. Copyright 1999 Annals of Botany Company Brassica napus var. oleifera, root columella, germination, phenolic compounds localization.     

Morphogenesis of the root cap in adventitious roots of Salix viminalis

The preformed root primordia in stems of Salk viminalis L. consist of undifferentiated cells. Forty-eight hours after activation of the primordia in cuttings a root cap meristem was initiated four to five cell tiers from the surface of the primordia. The cells distal to the meristem divided only in an anticlinal plane, while in the meristem they divided mostly periclinally but sometimes anticlinally. After 72 hours a columella was established and the amyloplasts began to sediment in response to gravity. Shortly after this stage the roots began to bend slightly downward, probably as a geo-tropic response. Six days after activation the root cap consisted of up to 15 tiers of cells. The ultrastucture of the cap cells just prior to emergence was studied in more detail. The plastids in the cells adjoining the root proper were typical proplastids. Distal to this cell tier starch accumulated in the plastids. In the fifth tier the amyloplasts were fully sedimented to the lowermost cell walls. The amount of ER increased with the distance from the initial cells and most of it was located at the distal periclinal cell wall. The nucleus and the vacuoles in the geo-sensitive cells occurred in the space above the sedimented amyloplasts. The cytoplasm was less electron opaque than in the initial cells and the mitochondria had more cristae. In the distal cells of the columella and the lateral root cap secretion of mucilage seemed to have started. Numerous large dictyosomes were associated with large vesicles containing a fibrillar or granular material. The plasmalemma lining the distal periclinal cell wall had separated from the wall. A fibrillar material was present between the plasmalemma and the wall and also in intercellular spaces outside the root cap.     

A COMPARISON OF ROOT CAP CELLS OF EPIPHYTIC,TERRESTRIAL AND AQUATIC PLANTS

Outer cells from the root cap of Cattleya orchids are characterized by their secretory activity. They are arranged in layers intercalated with layers of secretory product and form a protective mantle over the root tip. The ultrastructure of these cells is similar to those of terrestrial roots (for example Zea mays) in that they are characterized by copious quantities of endoplasmic reticulum and numerous dense-staining prevacuolar bodies. In contrast, most root cap cells of water hyacinth and duckweed are highly vacuolate with no dense-staining prevacuolar bodies. The endoplasmic reticulum is sparse and dictyosomes are small and without secretory activity.     

STUDIES IN THE QUIESCENT ROOT MERISTEM OF THE BULB OF ALLIUM CEPA L.: FINE STRUCTURE AND DNA SYNTHESIS *

Study of the quiescent root meristem of Allium cepa L. bulbs has revealed that its histological organization does not differ significantly from the growing meristem, except for the fact that the cells are all arrested in interphase. Ultrastructure of the quiescent tissue is, however, different in the organization of the nucleolus and in the absence of prominent endoplasmic reticulum, microtubules and golgi complexes. A variety of lomasome-like structures, plasma membrane modifications and vacuoles have been recorded. Most of the cells except for the ones in the root cap and quiescent center are highly vacuolated; vacuolation is maximum in the cortical zone of the meristem. The pattern of ³H thymidine incorporation during early stages of sprouting indicates that asynchrony of the mitotic cycle, which is the characteristic of the growing meristem, is maintained during quiescence by the arrest of nuclei at different subphases of interphase.     

ZOOSPORE STRUCTURE OF THE MYCOPARASITIC CHYTRID CAULOCHYTRIUM PROTOSTELIOIDES OLIVE

The subcellular organization of zoospores released from sessile, parasitic sporangia of Caulochytrium protostelioides was studied with light and electron microscopy. A single flagellum is posteriorly directed but laterally inserted into the cylindrical motile zoospore. A striated rhizoplast attaches the proximal end of the kinetosome to a specialized region of the nuclear envelope. A system of rough endoplasmic reticulum, smooth endoplasmic reticulum, dictyosomes and bristle-coated vesicles are associated with the one to several pulsating vacuoles typically located near the flagellar apparatus. The microbody-lipid globule complex (MLC) comprises one to many lipid globules. An extensive microbody branches around each lipid globule and encloses a portion of the rhizoplast. A reticulum of smooth surfaced cisternae interdigitates among the branches of the complex microbody, and cisternae are opposed to the surface of lipid globules opposite the microbodies. Mitochondria with predominantly circular profiles are scattered throughout the zoospore body, but several are always adjacent to the microbody, and hence, are also part of the MLC. Ribosomes are uniformly distributed throughout the zoospore, and one to several cisternae of rough endoplasmic reticulum are adjacent to the nuclear envelope. Zoospores of C. protostelioides are similar to several other chytrid zoospores, which also have the same type of microbody-lipid globule complex, but yet are structurally distinct from any other chytrid zoospore.     

STRATIFICATION AND SUBSEQUENT BEHAVIOR OF PLANT CELL ORGANELLES

Living excised roots of pea were centrifuged at 20,000 g for 24 hours, and the behavior of organelles was followed by electron microscopy at various intervals after centrifugation. With these forces, organelles are not perceptibly or irreversibly damaged, nor is the viability of the whole root destroyed. Organelles stratify generally in the order of lipid (centripetal pole), vacuoles, smooth endoplasmic reticulum and dictyosomes, proplastids (without starch), mitochondria, rough endoplasmic reticulum, proplastids with starch. The nucleus distends from the vacuolar region to the extreme centrifugal pole of the cell, while the chromatin and nucleolus seek the centrifugal pole of the nucleus. During the redistribution of organelles the rough endoplasmic reticulum is among the first to reorient, and possible explanations for this are discussed. Mitochondria can be stretched elastically many times their original length, but proplastids seem fairly rigid. Small vacuoles, forced together during centrifugation, apparently may fuse to form a large unit. Lipid droplets, on the other hand, tend to remain separate. Dictyosomes and smooth endoplasmic reticulum layer in the same region of the centrifuged cell, indicating a density similarity between these two organelles.     

Regions of lead uptake inLemna minor plants and localization of this metal within selected parts of the root

Investigations were carried out to determine the sites of lead uptake within the frond and the root ofLemna minor. With the sodium rhodizonate four regions favoured in lead uptake were distinguished: the frond region between the base and the node, the basal part of the root, and the regions at the proximal and distal ends of the root cap. For analysis in electron microscope only the root regions were chosen. The highest rate of lead uptake was found in the basal part of the root. Lead was present in the apoplast of this region after 5 min of exposure and was observed in the stelar cells after 30 min of incubation. Lead deposits were detected mostly in the cell walls adjacent to the plasma membrane and in the lumen of several endomembrane compartments - the endoplasmic reticulum (ER), dictyosomal vesicles, nuclear envelope and the vacuoles. Lead induced changes of cell ultrastructure; an increase in the number of membraneous structures, swelling of ER cisternae and distortion of the dictyosomal cisternae were observed after 2 to 6 h of exposure. We wish to thank Mrs. G. Winiecka for her technical assistance in preparing the photographs.     

THE QUIESCENT CENTER IN CULTURED ROOTS OF CONVOLVULUS ARVENSIS L.

Cultured roots of Convolvulus arvensis were incubated in 0.2–0.3 μc/ml methyl-³H-thymidine for different intervals of time. In roots supplied with tritiated thymidine for 12 hr, 14 hr, 48 hr, or 14 hr followed by transfer to fresh medium for 24 hr, autoradiographs prepared of serial, longitudinal sections of the root tips showed the presence of a subterminal quiescent center in the root proper at the distal poles of the central cylinder and cortex. In addition, a zone of unlabelled cells in the columella, distal to the root cap initials, was present. In roots supplied continuously with tritiated thymidine for 64 hr, 96 hr, and 120 hr, the quiescent center was either reduced in size or was not present.     

LIGHT AND ELECTRON MICROSCOPIC STUDIES OF THE FUSION CELL IN ASTEROCOLAX GARDNERI SETCH. (RHODOPHYTA,CERAMIALES)12

The fusion cell in Asterocolax gardneri Setch, is a large, multinucleate, irregularly-shaped cell resulting from cytoplasmic fusions of haploid and diploid cells. Subsequent enlargement takes place by incorporating adjacent gonimoblast cells. The resultant cell consists of two parts—a central portion of isolated cytoplasm, surrounded by an electron dense cytoplasmic barrier, and the main component of the fusion cell cytoplasm surrounding the isolated cytoplasm. The fusion cell contains many nuclei, large quantities of floridean starch, endoplasmic reticulum, and vesicles, but few mitochondria, plastids and dictyosomes. The endoplasmic reticulum forms vesicles that apparently secrete large quantities of extracellular mucilage which surrounds the entire carposporophyte. The isolated cytoplasm also is multinucleate but lacks starch and a plasma membrane. Few plastids, ribosomes and mitochondria are found in this cytoplasm. However, numerous endoplasmic reticulum cisternae occur near the cytoplasmic barrier and they appear to secrete material for the barrier. In mature carposporophytes, all organelles in the isolated cytoplasm have degenerated.     

Cotton embryogenesis: The tissues of the stigma and style and their relation to the pollen tube

Summary The stigma of cotton (Gossypium hirsutum) is covered by unicellular hairs. The cytoplasm of these hairs degenerates before the stigma becomes receptive. The vacuole remains intact, but the hair cytoplasm becomes a mass of dark, amorphous material with only a few organelles still being visible. The rest of the stigma consists of thin-walled parenchyma cells with large vacuoles and large amounts of starch. The cells of the style are differentiated into a uniseriate epidermis, vascular tissue, a cortex of thin-walled, vacuolate parenchyma cells, and the transmitting tissue. This latter tissue occupies the center of the style and consists of thick-walled cells with few vacuoles. The cells are rich in starch, ribosomes, endoplasmic reticulum and dictyosomes. They also contain deposits of calcium salts in the form of druses. The pollen germinates on the stigmatic hairs, grows down the outside of the hair and between the cells of the stigma to the transmitting tissue of the style. There the tubes grow between the walls of the cells but do not enter the cells themselves. Some transmitting cells adjacent to the pollen tube degenerate after the tip of the pollen tube has grown past them. However, not all degenerate, and those that do show no fixed spatial relationship to one another. The cells which do degenerate follow a characteristic pattern of breakdown. No ultrastructural evidence was found for the secretion of hydrolytic enzymes by the pollen tube.     

CYTOLOGY OF DIFFERENTIATING TRACHEARY ELEMENTS I. ORGANELLES AND MEMBRANE SYSTEMS

The distribution of organelles, membrane systems, and ribosomes is not at any time obviously related to the pattern of secondary wall in helically thickened tracheary elements in leaves of Beta vulgaris L. (sugar beet) and Cucurbita maxima Duchesne, fixed with potassium permanganate and osmium tetroxide. During the differentiation of the secondary wall, cisternae of the endoplasmic reticulum and dictyosomes are particularly conspicuous, and the dictyosomes are associated with numerous vesicles. Similar vesicles appear to be in various stages of fusion with the secondary wall thickenings. The tracheary elements contain plastids which may include starch granules. Ribosomes occur free in the cytoplasm and in association with endoplasmic membranes.     

Cytochemical Localization of Phenolic Compounds in Columella Cells of the Root Cap during Maturation of Seeds of Brassica napus L.

Abstract: Ultrastructural localization of phenolic compounds in the columella of the root cap of a canola embryo in Brassica napus L. during subsequent seed maturation stages was investigated. In bright green seeds the first signs of maturation and appearance of the first phenolic structures in the cell nuclei and cytoplasm in the area of the initial centre were found. Phenolics spread acropetally, gradually covering the whole columella in the dark green seeds. An increase in the number of phenolics occurred, together with an increase in the number of endoplasmic reticulum structures and in storage materials. In brown seeds, together with ultrastructural degradation and the end of production of storage substances, the number of phenolic deposits in the nucleus and cytoplasm decreases until their disappearance in black dormant seeds. Simultaneously, numerous phenolic deposits appear between the plasmalemma and the cell walls in the outer layer of the cap columella.     

Histochemical and ultrastructural changes in the haustorium of date (Phoenix dactylifera L.)

Summary During imbibition ofPhoenix dactylifera embryos, all cotyledon cells show the same changes: protein and lipid bodies degrade, smooth endoplasmic reticulum (ER) increases in amount, and dictyosomes appear. At germination, the distal portion of the cotyledon expands to form the haustorium. At this time, epithelial cells have a dense cytoplasm with many extremely small vacuoles. Many ribosomes are present along with ER, dictyosomes, and mitochondria. The parenchyma cells have large vacuoles and a small amount of peripheral cytoplasm. Between 2 and 6 weeks after germination, epithelial cells still retain the dense cytoplasm and many organelles appear: glyoxysomes, large lipid bodies, amyloplasts, large osmiophilic bodies, and abundant rough and smooth ER which appear to merge into the plasmalemma. A thin electron-transparent inner wall layer with many small internal projections is added to the cell walls. Starch grains appear first in the subsurface and internal parenchyma and subsequently in the epithelium. Lipid bodies, glyoxysomes, protein, and osmiophilic bodies occur in the epithelial and subepithelial cell layers but not in the internal parenchyma. At 8 weeks after germination, the cytoplasm becomes electron transparent, vacuolation occurs, lipid bodies and osmiophilic bodies degrade, and the endomembranes disassemble. After 10 weeks, the cells are empty. These data support the hypothesis that the major functions of the haustorium are absorption and storage.     

ULTRASTRUCTURE OF MAIZE ENDOSPERM SUSPENSION CULTURES

Suspension cultures derived from developing maize (Zea mays L.) endosperm were examined by electron microscopy, after both glutaraldehyde-OsO₄ and KMnO₄ fixation, and compared with intact endosperm. Tissue clumps consisted of interconnected cell clusters without any organization of the different cell types. The cultures were comprised of cells with dense cytoplasm and small vacuoles, large vacuolate cells, and cells in which storage products (starch, protein bodies, or lipid) accumulated. The endomembrane system of cultured cells was more highly developed than that of cells of the intact endosperm. In particular, arrays of smooth endoplasmic reticulum were seen only in the cultured cells. An abundance of endoplasmic reticulum, dictyosomes, and ribosomes is consistent with the recently reported extracellular secretion of enzymes by these cultures. Cell wall ingrowths, a characteristic of basal endosperm transfer cells, were observed occasionally in cultured cells, but cells with ingrowths had no histological organization. Some of the observed features may have resulted from perturbation of normal cellular events caused by the conditions of in vitro growth. These cultures are a useful tool for studying cellular mechanisms of protein secretion and storage product accumulation in developing maize endosperm.     

Root Cap Structure in Isoetes macrospora Dur

Root meristem cells of Isoetes macrosporausually have one plastidwhich is associated with the prominent nucleus, numerous ribosomesand mitochondria, and small vacuoles. During mitosis each plastidappears to replicate so that each daughter cell contains oneplastid. The cell walls of the meristem cells are traversedby numerous plasmodesmata. Central cells of the root cap lackdistally displaced plastids but have one or more amyloplastsassociated with the nucleus. These cells also contain largeprotein deposits. Peripheral root cap cells are characterizedby being vacuolated, and by possessing a few dictyosomes andprotein deposits. They appear to be sloughed infrequently. Isoetes macrospora Dur, root cap, protein bodies, ultrastructure     

Ultrastructure and movements of cell structures in normal pea and an ageotropic mutant

The pea mutant (Pisum sativum ageotropum) and the normal pea (P. sativum cv. Sabel) were compared in order to see if there were any differences in root anatomy or submorphology which could explain the presumed ageotropic behaviour of the mutant. In both types the root cap consists of a central core (columella) distinct from the peripheral part. The core contains five to six rows of columella cells, each consisting of 10 to 16 storeys of statocytes. The ultrastructure of the columella cells in the two types is very similar; the main difference is confined to the distribution of rough endoplasmic reticulum (ER), which in the mutant statocytes is evenly distributed throughout the cell, while in the normal pea statocytes it is mainly concentrated in the distal part at the “floor” of the cell. Using light micrographs, the movement of amyloplasts and nuclei have been followed in detail during a 40 min inversion period. The pattern of movement of the amyloplasts is apparently identical in the two types and the distances moved during the inversion period are 39 μm and 44 μm in the normal and mutant statocytes, respectively. The nucleus has not been observed to move in normal pea; a slight rearrangement of the nucleus position can be observed during the period 30 to 40 min after the start of inversion of the mutant. Based on magnified electron micrographs of the statocytes a morphometrical analysis was made of five cell structures – amyloplasts, nuclei, mitochondria, vacuoles and ER – which appeared to be freely movable or redistributable under the influence of the gravitational force.     

Inhibitory effect of κ/β-carrageenan from red alga <Emphasis Type="Italic">Tichocarpus crinitus</Emphasis> on the development of a potato virus X infection in leaves of <Emphasis Type="Italic">Datura stramonium</Emphasis> L.

The effect of κ/β-carrageenan from red alda Tichocarpus crinitus on the development of a potato virus X (PVX) infection in the leaves of Datura stramonium L. has been studied. The treatment of leaves with carrageenan stimulates a protein synthesis in the cells, causing an increase in the size of nucleoli and in the number of mitochondria and membranes of the rough endoplasmic reticulum. At the same time, such treatment slightly stimulates lytic processes, causing an increase in the number of smooth endoplasmic reticulum cisternae, dictyosomes, and cytoplasmic vacuoles and the formation of cytoplasmic electron-transparent zones. The carrageenan-induced stimulation of lytic processes results in the destruction of viral particles and can be considered as one of the defense mechanisms, preventing the intracellular accumulation of virus. The carrageenan-stimulated formation of PVX-specific laminar structures, able to bind viral particles and, therefore, prevent their intracellular translocation and reproduction, represents another carrageenan-induced mechanism of the antiviral defense in plant cells.     

The neural gland in tunicates: fine structure and intracellular distribution of phosphatases

Summary The cells comprising the neural gland in the ascidians Ciona, Styela, and Botryllus have been examined for their fine structural features and enzyme cytochemistry. The gland cells are either cuboidal or irregular in outline. They are full of small vesicles, of which some are pinocytotic, as well as larger vacuoles; they become increasingly vacuolated as their shape decreases in regularity. At the same time, glycogen deposits accumulate and the cisternae of the endoplasmic reticulum become distended. Some of the vacuoles contain an electron dense material or a fibrillar substance, but the cells contain no obvious electron opaque secretory granules associated with an extensive Golgi complex such as occur in the vertebrate adenohypophysis.Acid phosphatase is localized in some of the vesicles and vacuoles, indicating that they are a kind of lysosome, the latter possibly representing autophagic vacuoles. Thiamine pyrophosphatase is also found in many vacuoles as well as in the saccules of the Golgi apparatus which in these cells is in the form of dictyosomes.The results suggest a developmental cycle of increasing cytoplasmic vacuolation, ultimately leading to a breakdown and release of the vacuolar products. The significance of these observations is considered, particularly with respect to the hypothesis that the gland represents the ascidian equivalent of the vertebrate pituitary.I am grateful to Miss Yvonne R. Carter for technical assistance with the photography and to Mr. John Rodford for producing the diagram.     

Ultrastructure of endopolyploid antipodals inAconitum vulparia Rchb.

Summary The ultrastructure of the antipodals ofAconitum vulparia Rchb. was studied in mature embryo sacs. Antipodal cell wall thickness varies in different parts of the cells. The antipodals resemble transfer cells with distinctly marked wall ingrowths which are particularly well developed in the chalazal part and between the antipodals. A few plasmodesmata occur in the cell wall between the antipodals and the central cell. The cytoplasm is rich in ribosomes which occur free or bound to the membranes of the well developed endoplasmic reticulum. Only in the micropylar region of the cells are some larger vacuoles found. The antipodals contain numerous mitochondria, plastids and apparently active dictyosomes. Vesicles with electron dense contents, microbodies, multivesicular bodies as well as lipid droplets and small multiple concentric cisternae are also present in the cytoplasm. The giant endopolyploid nuclei have lobed outlines, especially at the chalazal side of the nuclei.Ultrastructural features, especially the occurrence of numerous free ribosomes and the development of extensive rough endoplasmic reticulum, suggest high metabolic activity in the growing and differentiating antipodals of this species.