Aspects of sieve element ultrastructure in Primula obconica

Summary At maturity, the enucleate sieve element of Primula obconica is lined with a parietal layer of cytoplasm consisting of plasmalemma, one or more cisterna-like layers of endoplasmic reticulum, numerous mitochondria and plastids, and a membrane which apparently separates these cytoplasmic components from a large central cavity. The central cavity contains numerous longitudinally oriented slime tubules. We believe these tubules normally form strands which run the length of the cell and traverse consecutive cells through the sieve-plate pores. Developmental aspects are discussed.This research has been supported by NSF Grant GB 3193.     

Another View of the Ultrastructure of Cucurbita Phloem

The primary phloem of young internodes of Cucurbita maxima wasstudied with the electron microscope. Phloem parenchyma cellsare highly vacuolated and contain nuclei, endoplasmic reticulum,ribosomes, mitochondria, chloro-plasts, and occasional dictyosomes.As compared with parenchyma cells, the most distinctive featuresof companion cells are their extremely dense cytoplasm, lowdegree of vacuolation, lack of chloroplasts, and numerous sieve-elementconnexions. Companion cells contain plastids with few internalmembranes. At maturity the enucleate sieve element is linedby a plasmalemma, one or more cistema-like layers of endoplasmicreticulum, and a membrane which apparently delimits the parietallayer of cytoplasm from a large central cavity. In OsO_4–-andglutaraldehyde-fixed elements, the central cavity is traversedby numerous strands, which run from cell to cell through thepores of sieve plates and lateral sieve areas, and which arederived ontogenetically from the slime bodies of immature cells.Numerous normal-appearing mitochondria are present in the parietallayer of cytoplasm. The pores of sieve plates and lateral sieveareas are lined with cytoplasm. The ultrastructural detailsof young sieve elements differ little from those of other youngnucleate cells. During sieve-element development, the sieveelement increases in vacuolation. At the same time, slime bodiesdevelop in the cytoplasm. With glutaraldehyde fixation, thesebodies often exhibit a double-layered limiting membrane. Asthe sieve element continues to differentiate, the slime bodiesincrease in size and the parietal layer of cytoplasm becomesvery narrow. Presently, the slime bodies begin to disperse andtheir contents fuse. This phenomenon occurs in the parietallayer of cytoplasm, while the latter is still delimited fromthe large central vacuole by a distinct tonoplast. The initiationof slime-body dispersal more or less coincides with perforationof the pore sites, and many pores are traversed by slime earlyin their development. Before slime-body dispersal, all dictyosomesand associated vesicles disappear from the cytoplasm. Eventually,the tonoplast diappears and the slime becomes distributed throughoutthe central cavity in the form of strands. Nuclei and ribosomesdisappear before breakdown of the tonoplast. Sieve elementsare connected with companion cells and parenchyma cells by plasmodesmata.     

LIGHT MICROSCOPE INVESTIGATION OF SIEVE-ELEMENT ONTOGENY AND STRUCTURE IN ULMUS AMERICANA

At maturity the sieve elements of Ulmus americana L. contain a parietal network of very fine strands of slime which is continuous from one sieve element to the next through the sieve-plate pores. Upon injury this parietal network, which is derived from the slime bodies of immature sieve elements, sometimes becomes distorted into longitudinally oriented strands. Some of these strands frequently extend the length of the cells and often are continuous from one sieve element to the next through the sieve-plate pores. At times past such strands have erroneously been interpreted as normal constituents of the mature sieve-element protoplast. Many mature sieve elements of U. americana contain nuclei, which apparently persist for the life of the sieve elements. In addition, some evidence has been found in mature sieve elements for the presence of a membrane which delimits the parietal layer of cytoplasm, including its network of slime strands, from the vacuolar region of the cell.     

ULTRASTRUCTURE OF THE SECONDARY PHLOEM OF TILIA AMERICANA

Summer and winter (July and January) samples of secondary phloem of Tilia americana were studied with the electron microscope. Parenchyma cells contain: nuclei, endoplasmic reticulum, ribosomes, plastids, mitochondria and occasional dictyosomes. Well-defined tonoplasts separate vacuoles from cytoplasmic ground substance. Vacuoles often contain tannins. Lipid droplets are common in cytoplasm. Endoplasmic reticulum–connected plasmodesmata are aggregated in primary pit fields. Companion cells differ from parenchyma cells in having numerous sieve-element connections, possibly slime, and in lacking plastids. Mature, enucleate sieve elements possess 1–4 extruded nucleoli. Numerous vesicles occupy a mostly parietal position in association with plasmalemma. The mature sieve element lacks endoplasmic reticulum, organelles (except for few mitochondria) and tonoplast. In OsO_4– and glutaraldehyde-fixed elements, slime has a fine, fibrillar appearance. Normally, these fine fibrils are organized into coarser ones which form strands that traverse the cell and the plasmalemma-lined pores of sieve plates and lateral sieve areas.     

DEVELOPMENT AND STRUCTURE OF PHLOEM IN THE PETIOLE OF LUFFA CYLINDRICA

Differentiation of external phloem is earlier than that of internal phloem in the young petiole of Luffa cylindrical. For a single sieve-tube element, one to six companion cells are present. The young sieve element shows many globular slime bodies which fuse longitudinally and disperse into the cytoplasm. Simultaneously the nucleus loses its stainable contents and later disorganizes. The contents of the sieve element are in the form of plugs, strands or a granular mass. Undispersed slime in the form of discrete bodies along the lateral walls is also observed. During one stage, at least, the dispersed slime and other contents of a mature sieve element lie at the periphery around a central cavity. A special type of phloem-parenchyma cell shows disorganizing chloroplasts, an extruded nucleolus, and callose on primary pit fields.     

THE FINE STRUCTURE AND DEVELOPMENT OF THE COMPANION CELL OF THE PHLOEM OF ACER PSEUDOPLATANUS

At maturity the companion cell of the phloem of the sycamore Acer pseudoplatanus has a large nucleus, simple plastids closely sheathed with rough endoplasmic reticulum, and numerous mitochondria. The cytoplasm contains numerous ribosomes, resulting in a very electron-opaque cytoplasm after permanganate fixation. Bodies similar to the spherosomes of Frey-Wyssling et al. (4) are collected in clusters and these also contain bodies of an unidentified nature similar to those found by Buttrose (1) in the aleurone cells of the wheat grain. The pores through the wall between the companion cell and sieve tube are complex and develop from a single plasmodesma. Eight to fifteen plasmodesmata on the companion cell side communicate individually with a cavity in the centre of the wall which is linked to the sieve tube by a single pore about twice the diameter of an individual plasmodesma. This pore is lined with material of an electron opacity equivalent to that of material bounding the sieve plate pores. The development of the cell organelles, the possible role played in the phloem tissue by the companion cell, and the function of the complex pores contained in its wall are discussed.     

ULTRASTRUCTURE OF THE NACREOUS LEPTOIDS (SIEVE ELEMENTS) IN THE POLYTRICHACEOUS MOSS ATRICHUM UNDULATUM

The physiological phloem equivalents, leptoids, of the polytrichaceous moss Atrichum undulatum appear to be similar to the nacreous sieve elements that occur in many higher plants. These leptoids are elongated cells with nacreous thickenings on their radial and tangential walls. Their oblique end walls, which lack such thickenings, are traversed by numerous pores through which the plasmalemma, endoplasmic reticulum, and cytoplasm are continuous between adjacent leptoids of a longitudinal file. These end walls closely resemble the simple sieve areas of the sieve elements found in Polypodium vulgare. The leptoid sieve pores have a median expanded area and frequently are occluded by small amorphous protein plugs at each end. Also, callose was observed as electron-luscent areas both on the faces of the end walls and as a thin cylinder surrounding the lateral area of each pore. Amorphous and granular cytoplasmic contents of the leptoids appear to be morphologically similar to the slime (P-protein) found in the sieve-tube elements of many angiosperms. Differentiating leptoids are characterized by the formation of numerous membrane-bound protein bodies in close association with polysomes and endoplasmic reticulum. As the leptoid matures, the contents of the protein bodies become dispersed in the cytoplasm. Ultrastructurally and ontogenetically the leptoids in the gametophores of A. undulatum appear almost identical to the sieve elements of P. vulgare and therefore should be considered sieve elements rather than phloem-like equivalents.     

Structure and development of sieve areas in leaf veins ofWelwitschia

Summary The sieve areas inWelwitschia are essentially similar to those of coniferous sieve cells, consisting of groups of plasmalemma-lined pores, which are joined in the middle of the wall by a median cavity. The median cavity contains membranes which apparently are connected with aggregates of endoplasmic reticulum bordering the sieve areas. The median cavity is formed through union of smaller median enlargements, the median nodules, each initially associated with a plasmodesma, during perforation of the young sieve area. Callose platelets are not associated with developing pores. All fully-developed pores were lined with callose. The sieve cells are connected with only one other cell type, the albuminous cell. On the sieve-cell side of the wall these connections are similar to sievearea pores, on the albuminous-cell side to plasmodesmata. These connections are also characterized by median enlargements.This work was supported in part by a grant from the South African Council for Scientific and Industrial Research and in part by the U.S. National Science Foundation (GB 31417).     

ONTOGENY AND STRUCTURE OF CONIFEROUS SIEVE CELLS

Studies of the secondary phloem of 6 species of conifers revealed that mature sieve-cell protoplasts contain internal strands which are derived ontogenetically from slime bodies of immature cells. These strands, each measuring about 0.3 μ in diameter, traverse the cell and run from cell to cell through sieve-area pores. Coniferous sieve cells have much in common—both ontogenetically and structurally—with dicotyledonous sieve-tube members.     

Development and Structure of Phloem in the Petiole of Lagenaria siceraria (Mol.) Standl. and Momordica charantia L.

Light microscopic studies of the petioles of Lagenaria sicerariareveal that the external phloem of each bicollateral vascularbundle develops earlier than the internal phloem, and that thesieve elements of the external phloem are arranged in the outerand inner zones. Each sieve element of L. siceraria and Momordicacharantia is vertically associated with a maximum of six andtwo companion cells respectively. Discrete granular bodies seenin the cytoplasm of young sieve elements develop into globular,oval, or elongated slime bodies. Enlargement and fusion of slimebodies, and the subsequent dispersal of slime occur in the parietalcytoplasm. The dispersal of slime coincides with degradationof the nucleus and perforation of the pore sites. Before nucleardisorganization, the sieve-element nucleolus is extruded. Slimeafter its immediate dispersal appears amorphous and uniformlydistributed in the sieve elements. Plugs exhibit varying degreesof condensation of slime near the sieve plates. Certain maturesieve elements in the external phloem of L. siceraria have ovalbodies which we consider reaggregated or undispersed slime.Evidence has been obtained that a central cavity occurs in afew, almost mature, sieve elements wherein the cytoplasm includingthe slime is peripheral.     

Endoplasmic reticulum and crystalline fibrils in the root protophloem of Nymphoides peltata

Endoplasmic reticulum in the root protophloem of Nymphoides peltata (S.G. Gmel.) O. Kuntze changes form as sieve elements differentiate. In immature sieve elements the individual endoplasmic reticulum (ER) cisternae form large irregular aggregates in the cytoplasm. In older immature sieve elements the ER aggregates are more ordered and membranes in them are convoluted. Although convoluted ER predominates in immature sieve elements the ER of the mature sieve elements consists mainly of flattened stacks of ER cisternae. Some of these stacks of ER may be derived from the existing convoluted ER. Crystalline fibrils first appear in the cytoplasm of the sieve element when the ER starts to aggregate. The crystalline fibrils move to the parietal layer of the sieve element along with the aggregates of ER. A possible ontogenetic relationship between ER and crystalline fibrils is discussed.Abbreviation ER endoplasmic reticulum     

Ultrastructural Observations of Papaver rhoeas Mature Pollen Grains

The mature pollen grain of Papaver rhoeas is bicellular. The vegetative cell contains numerous mitochondria; endoplasmic reticulum is not very extensive and there are few ribosomes and plastids. Golgi bodies are in a very active state. The generative cell is lobed and spindle-shaped. The cytoplasm contains many, generally longitudinally arranged, bundles of microtubules. Other organelles are few in number, and include mitochondria, Golgi bodies and short cisternae of endoplasmic reticulum.     

ULTRASTRUCTURE OF DEVELOPING SIEVE ELEMENTS IN THLASPI ARVENSE L. II. MATURATION

Developing sieve elements of pennycress (Thlaspi arvense L.) were studied with the electron microscope. The maturation of sieve elements involved loss of ribosomes from cytoplasm; degeneration of nulcei; modification of endoplasmic reticulum (ER); loss of tonoplast; and disappearance of dictyosomes and dictyosomes vesicles, coated vesicles, microtubules, and microbodies. Such changes produce a mature, presumably conducting cell that contains no nucleus or central vacuole but which retains a thin layer of peripheral cytoplasm with plastids, mitochondria, and smooth ER. Some similar changes have been described in a variety of developing sieve elements of angiosperms, but coated vesicles and microbodies previously have not been followed through sieve-element maturation. Likewise, few developmental studies have been made of plant sieve elements that exhibit two types of P-protein, the tubular type and the granular P-protein body.     

Electron microscope investigation of sieve-element ontogeny and structure inUlmus americana

Summary During advanced stages of sieve-element differentiation inUlmus americana L., dispersal of the P-protein (slime) bodies results in formation of a peripheral network of strands consisting of aggregates of P-protein components having a striated, fibrillar appearance. The tonoplast is present throughout the period of P-protein body dispersal. Perforation of the sieve plates is initiated during early stages of P-protein body dispersal.Small P-protein bodies consist of tubular components, most of which measure about 180 Å in diameter. With increase in size of the P-protein bodies narrower components appear. At the time of initiation of P-protein body dispersal, most of the components comprising the bodies are of relatively narrow diameters (most 130–140 Å) and have a striated, fibrillar appearance. Both wide and narrow P-protein components are present throughout the period of sieve-element differentiation and in the mature cell as well, and a complete intergradation in size and appearance exists between the two extremes. Both extremes of P-protein component have a similar substructure: an electron-transparent lumen and an electronopaque wall composed of subunits, apparently in helical arrangement. The distribution of P protein in mature sieve elements was quite variable.The parietal layer of cytoplasm in matureUlmus sieve elements consists of plasmalemma, endoplasmic reticulum cisternae in two forms (as a complex network closely applied to the plasmalemma and in stacks along the wall), mitochondria, and plastids.     

THE FINE STRUCTURE OF SIEVE ELEMENTS OF NEREOCYSTIS LÜTKEANA

The sieve elements of Nereocystis from the base of phylloids contain numerous small vesicles, cytoplasm, ribosomes, and the usual organelles and membrane systems, including nuclei, plastids, mitochondria, dictyosomes, and endoplasmic reticulum. They have a thick secondary wall layer which is deposited along the longitudinal walls and at the sieve plate excluding the sieve pores. The sieve pores range in diameter from 100 to 400 nm and are lined by plasmalemma. The sieve elements from the hollow basal parts of the pneumatocyst show essentially the same features but have larger and fewer vesicles, relatively little cytoplasm, larger sieve pores, 400–900 nm in diameter, and may lack a nucleus. In old sieve elements there are large deposits of callose on the sieve plate and along the longitudinal wall; the vesicles seem to break down, and the protoplast appears necrotic. It is concluded that the trumpet hyphae and sieve tubes are basically the same type of cell, and that the trumpet-shape of the sieve elements is due to their passive stretching during extension growth of the organ in which they occur. There are minor but significant differences among the sieve elements from different regions of the thallus which may reflect possible levels of structural specialization of the sieve elements within the same plant.     

Polysomes and intracisternal accumulations in enucleate sieve elements of rice (Oryza sativa L.)

Polysomes in sieve elements of rice (Oryza sativa L.) were studied with the electron microscope. The polysomes were found on the rough endoplasmic reticulum (ER) present in immature sieve elements and also on the cisternae of aggregated ER in the parietal layer of mature, enucleate sieve elements. In the immature sieve elements the ER cisternae existed as narrow profiles while in the mature sieve elements the ER cisternae were considerably dilated and contained a fibrillar material and, occasionally, electron-opaque inclusions. In addition to the aggregated ER, single profiles of ER were found applied to the lateral walls and also the sieve plates. These cisternae also bore ribosomes and were separated from the plasmalemma by a narrow, dense space. In the mature sieve elements much of the surface of the ER membranes was covered with polysomes. The dimensions of the polysomes are described and the possibility that they contribute to the formation of the fibrillar material in the intracisternal space is discussed.Abbreviations ER endoplasmic reticulum     

ONTOGENY AND STRUCTURE OF THE SECONDARY PHLOEM IN EPHEDRA

The secondary phloem in Ephedra is atypical of the gymnosperms in general and exhibits several angiosperm-like characteristics. The ray system of the conducting phloem consists of parenchymatous, multiseriate rays. The axial system contains parenchyma cells, sieve cells, and unusual albuminous cells reminiscent of the specialized parenchyma cells found in some angiosperms. These cell types may intergrade with each other. P-protein in the developing sieve element appears early in the form of a single, ovoid slime body. Later, smaller slime bodies appear and quickly disperse. In the mature sieve element the single, ovoid slime body is lost, and P-protein is then evident in the form of a parietal cylinder, thread-like strands, amorphose globules, or a slime plug. Necrotic-appearing nuclei are commonly found in mature sieve cells.     

TUBULAR AND FIBRILLAR COMPONENTS OF MATURE AND DIFFERENTIATING SIEVE ELEMENTS

An ontogenetic study of the sieve element protoplast of Nicotiana tabacum L. by light and electron microscopy has shown that the P-protein component (slime) arises as small groups of tubules in the cytoplasm. These subsequently enlarge to form comparatively large compact masses of 231 ± 2.5 (SE)A (n = 121) tubules, the P-protein bodies. During subsequent differentiation of the sieve element, the P-protein body disaggregates and the tubules become dispersed throughout the cell. This disaggregation occurs at about the same stage of differentiation of the sieve elements as the breakdown of the tonoplast and nucleus. Later, the tubules of P-protein are reorganized into smaller striated 149 ± 4.5 (SE)A (n = 43) fibrils which are characteristic of the mature sieve elements. The tubular P-protein component has been designated P1-protein and the striated fibrillar component P2-protein. In fixed material, the sieve-plate pores of mature sieve elements are filled with proteinaceous material which frays out into the cytoplasm as striated fibrils of P2-protein. Our observations are compatible with the view that the contents of contiguous mature sieve elements, including the P-protein, are continuous through the sieve-plate pores and that fixing solutions denature the proteins in the pores. They are converted into the electron-opaque material filling the pores.     

STRUCTURE AND DEVELOPMENT OF THE SIEVE ELEMENTS IN PSILOTUM NUDUM

Shoot tissue of Psilotum nudum (L.) Griseb. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. Young sieve elements can be distinguished from contiguous parenchyma cells by their distinctive plastids, the presence of refractive spherules, and the overall dense appearance of their protoplast. The refractive spherules apparently originate in the intracisternal spaces of the endoplasmic reticulum (ER). With increasing age the sieve-element wall undergoes a marked increase in thickness. Concomitantly, a marked increase occurs in the production of dictyosome vesicles, many of which can be seen in varying degrees of fusion with the plasmalemma. Other fibril- and vesicle-containing vacuoles also are found in the cytoplasm. In many instances the delimiting membrane of these vacuoles was continuous with the plasmalemma. Vesicles and fibrillar materials similar to those of the vacuoles were found in the younger portions of the wall. At maturity the plasmalemma-lined sieve element contains a parietal network of ER, plastids, mitochondria, and remnants of nuclei. The protoplasts of contiguous sieve elements are connected by solitary pores on lateral walls and pores aggregated into sieve areas on end walls. All pores are lined by the plasmalemma and filled with numerous ER membranes which arise selectively at developing pore sites, independently of the ER elsewhere in the cell. P-protein and callose are lacking at all stages of development.     

Dorsal lingual epithelium of Platemys pallidipectoris (Pleurodira,Chelidae)

Scanning electron microscopy reveals that the flat tongue of Platemys pallidipectoris has shallow grooves and no lingual papillae. The surface of the tongue is covered with dome-shaped bulges, each corresponding to a single cell. Short microvilli are distributed over the cell surface. Light microscopy shows a stratified cuboidal epithelium with an underlying strong connective tissue. Transmission electron microscopy indicates four layers. The basal cells of the epithelium are electron-translucent and have a large central nucleus and a cytoplasm with keratin tonofilaments. Plasma cells with abundant rough endoplasmic reticulum and mitochondria occur in the basal layer. Production of secretory granules begins in the more electron-dense intermediate layers and increases as the cells move toward the surface. The membranes of the cells of the deep intermediate layer form processes that project into relatively wide intercellular spaces. In the superficial intermediate layer, the cytoplasm of the cells contains numerous fine granules; these increase in number but not in size in more distal layers. The cells of the surface layer are electron-translucent with a round nucleus. Contents of their fine granules are secreted into the oral cavity. © 1995 Wiley-Liss, Inc.