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.     

Effects of de- and rehydration on food-conducting cells in the moss Polytrichum formosum: a cytological study

BACKGROUND AND AIMS: Moss food-conducting cells (leptoids and specialized parenchyma cells) have a highly distinctive cytology characterized by a polarized cytoplasmic organization and longitudinal alignment of plastids, mitochondria, endoplasmic reticulum and vesicles along endoplasmic microtubules. Previous studies on the desiccation biology of mosses have focused almost exclusively on photosynthetic tissues; the effects of desiccation on food-conducting cells are unknown. Reported here is a cytological study of the effects of de- and rehydration on food-conducting cells in the desiccation-tolerant moss Polytrichum formosum aimed at exploring whether the remarkable subcellular organization of these cells is related to the ability of mosses to survive desiccation. METHODS: Shoots of Polytrichum formosum were dehydrated under natural conditions and prepared for transmission and scanning electron microscopy using both standard and anhydrous chemical fixation protocols. Replicate samples were then fixed at intervals over a 24-h period following rehydration in either water or in a 10 microM solution of the microtubule-disrupting drug oryzalin. KEY RESULTS: Desiccation causes dramatic changes; the endoplasmic microtubules disappear; the nucleus, mitochondria and plastids become rounded and the longitudinal alignment of the organelles is lost, though cytoplasmic polarity is in part retained. Prominent stacks of endoplasmic reticulum, typical of the hydrated condition, are replaced with membranous tubules arranged at right angles to the main cellular axis. The internal cytoplasm becomes filled with small vacuoles and the plasmalemma forms labyrinthine tubular extensions outlining newly deposited ingrowths of cell wall material. Whereas plasmodesmata in meristematic cells at the shoot apex and in stem parenchyma cells appear to be unaffected by dehydration, those in leptoids become plugged with electron-opaque material. Starch deposits in parenchyma cells adjoining leptoids are depleted in desiccated plants. Rehydration sees complete reestablishment over a 12- to 24-h period of the cytology seen in the control plants. Oryzalin effectively prevents leptoid recovery. CONCLUSIONS: The results point to a key role of the microtubular cytoskeleton in the rapid re-establishment of the elaborate cytoplasmic architecture of leptoids during rehydration. The reassembly of the endoplasmic microtubule system appears to dictate the time frame for the recovery process. The failure of leptoids to recover normal cytology in the presence of oryzalin further underlines the key role of the microtubules in the control of leptoid cytological organization.     

Plasmodesmatal frequency and radial translocation rates in ray cells of poplar (Populus x canadensis Moench ‘robusta’)

The minimum radial translocation rate of sugars has been determined from the starchaccumulation rate for the wood rays of Populus x canadensis Moench robusta, and related to ultrastructural peculiarities of the cell walls to be passed. The minimum radial flux or flow of sugars through the tangential walls, the pit fields, and per plasmodesma was 80.7 pmol · cm^-2 · s^-1, 400 to 800 pmol · cm^-2 · s^-1, and 1.0 to 1.7 · 10^-7 pmol · plasmodesma^-1 · s^-1, respectively. These values exclude a transmembrane flux mechanism and indicate that the radial translocation in this tissue must proceed via plasmodesmata. In the isolation cells of the ray center we found 39 plasmodesmata per m² of pit field, 8.0 per m² of tangential wall, and 1.98% of the wall occupied by plasmodesmata. Cells of the ray margins show plasmodesmata on only 1.16% of their tangential wall area and thus appear to be slightly inferior for radial translocation. As judged from both the observed plasmodesmatal frequencies and the translocation rates, the ray parenchyma cells are comparable to cells specialized in short-distance translocation.Abbreviations CCR contact-cell row - IC isolation cell - ICR isolation-cell row     

Parenchyma cells of secondary phloem in Pinus strobus

Summary Parenchyma cells of the secondary phloem in Pinus strobus have all the cellular organelles common in other plant cells. They have mitochondria, endoplasmic reticulum, ribosomes, dictyosomes, and plastids. Parenchyma cells are very conspicuous because of their organic inclusions, starch and lipids. Plasmodesmata in transverse and tangential walls of axial parenchyma cells and in end walls of ray parenchyma cells are regularly distributed and of uniform size, about 500 Å in diameter. In radial walls of axial parenchyma cells and horizontal walls of ray parenchyma cells plasmodesmata are located in primary pit-fields; there they are of variable size and often divided into several branches. The branches are confluent into a median nodule. Perforation of the transverse wall between two axial parenchyma cells and the resultant union of the cellular material of the two connected cells is reported.This research has been supported by NSF Grant GB 3193.     

ELECTRON MICROSCOPIC OBSERVATIONS OF CULTURED CELLS AND PROTOPLASTS OF AMMI VISNAGA

Protoplasts were prepared from cultured cells of Ammi visnaga (Umbelliferae) by enzymatic digestion of the cell walls and examined microscopically. Staining of fresh protoplasts with Calcofluor and silver hexamine demonstrated the apparent absence of wall material. Protoplasts contained more cell organelles than the whole cells, particularly endoplasmic reticulum and associated polysomes. The plasmalemma of most protoplasts appeared smooth; some protoplasts were connected by structures resembling plasmodesmata. Multinucleates resulting from fusion were frequently observed.     

Production of l-glutamate by immobilized protoplasts

Summary Protoplasts of Brevibacterium flavum cultured in a medium containing 50 g·l^-1 of biotin were prepared with lysozyme and immobilized in matrices of agar-acetylcellulose filters. The immobilized protoplasts were applied to l-glutamate production from glucose and urea in a batch system. The productivity of l-glutamate by the immobilized protoplasts was 2.5 times higher than that by immobilized whole cells under optimal conditions. Maximal productivity initially reached 1.5 mg·ml^-1. The immobilized protoplasts of B. flavum could be used six times for l-glutamate production with retention of about 70% of the initial productivity.     

Comparative Structure of Companion Cells and Phloem Parenchyma Cells in Mimosa pudica L.

The phloem of Mimosa pudica L. furnishes an example of definablediversification of the parenchymatic members of the tissue intocompanion cells and parenchyma cells. The companion cells havedense protoplasts which contain the typical organelles of plantcells, including chloroplasts and many ribosomes. The sieveelements and companion cells are interconnected by numerousbranched plasmodesmata. The companion cells degenerate whenthe associated sieve elements cease to function. The parenchymacells have less dense protoplasts than the companion cells.In many parenchyma cells the rough endoplasmic reticulum assumesa tubular form, and bundles of microfilaments are present. Thecytoplasmic ribosomes occur in groups apparently held togetherby fibrils. Chloroplasts, mitochondria (some are exceptionallylong), dictyosomes, microbodies, and microtubules are the othercell components. Whether the parenchyma cells are ontogeneticallyrelated to the sieve elements or not, they do not degeneratewhen the sieve element ceases to function.     

Elektronenmikroskopische Untersuchungen von Differenzierungsvorgängen bei Moosen

Summary In meristematic cells of the gemma of Riella helicophylla and in young bud cells from the protonema of Funaria hygrometrica the cell plate is formed by fusion of small vesicles originating from the Golgi apparatus. These spherical vesicles of about 0.1 m diameter have an electron dense centre, probably consisting of pectic substances or their precursors. The endoplasmic reticulum producing multivesicular bodies participate in cell plate formation too. Another cytoplasmic component forming the cell plate are coated vesicles, the origin of which is the Golgi apparatus and perhaps also the endoplasmic reticulum. In view of these observations the question of whether the endoplasmic reticulum or the Golgi apparatus forms the cell plate must be answered in this way: both endoplasmic reticulum and Golgi apparatus supply material for growth of the cell plate. Multivesicular bodies, coated vesicles and other small vesicles of unknown nature participate in the formation of the primary wall.

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Zum Teil finanziert mit Sondermitteln des Landes Niedersachsen an Prof. Dr. M. Bopp.     

The release of α-amylase through gibberellin-treated barley aleurone cell walls

Localisation of -amylase (EC 3.2.1.1.) in low-temperature-embedded isolated barley (Hordeum vulgare L.) aleurone has been achieved using rhodamine-labelled secondary antibodies and the protein A-gold technique. Treatment with gibberellic acid (GA₃) resulted in an increase of immunofluorescence in the cytoplasm of aleurone cells and also its appearance in specific regions of the cell walls. Cytoplasmic label was neither perinuclear nor associated specifically with aleurone grains as had been found in earlier work, but was present throughout the cytoplasm of all cells. A relatively high level of labelling occurred in hydrolysed wall regions. Label was also associated with plasmodesmata in both hydrolysed and unhydrolysed wall regions. The pattern of labelling indicates that -amylase is released from aleurone via digested wall channels and that, except for the inner wall layer, unhydrolysed regions are impermeable to the enzyme. It is suggested that the resistant wall tubes around plasmodesmata may facilitate enzyme release by providing a pathway for transfer, especially of wall hydrolases, into the more impermeable parts of the wall.Abbreviations ER endoplasmic reticulum - GA₃ gibberellic acid - RER rough endoplasmic reticulum     

Stereologic and fine-structural studies of prostatic acinar basal cells in the dog

Summary There are two distinct types of epithelial cells in the lining of the glandular acini of the prostate in adult male Beagle dogs, i.e., the columnar secretory epithelial cells and the basal cells. In contrast to the secretory epithelial cells, basal cells exhibit an abundance of micropinocytotic vesicles on their basal surface. Blood capillaries are often found in the stromal tissue in close proximity to these cells and their walls frequently display chains of fenestrations bridged by diaphragms.Stereological analysis shows that the volume density of the basal cells in the reference volume of acinar parenchyma is 0.056, and there are approximately 132.14 million cells per cm³ of prostatic tissue. An average basal cell has a volume of 373.5 m³, and the volume densities of its nucleus, rough endoplasmic reticulum, Golgi apparatus, mitochondria and micropinocytotic vesicles, are 0.49, 0.04, 0.04, 0.094 and 0.013, respectively. These data are distinctly different from those that have been reported for the prostatic secretory epithelial cells of the same animals.Visiting scientist to the Institut für Pathologie, Universität Basel (Prof. Dr. med. Hanspeter Rohr), and a recipient of the Aichi Medical University Grant for overseas investigations (1983)     

Inhibiteurs anti-trypsine et activités protéolytiques des Albumines de graine de Vigna unguiculata

Summary The extracellular space of tentacles of Drosera capensis L. is divided into two compartments by cuticular material between cells of an endodermoid layer and by the nonporous cuticle of the stalk and neck. The distal compartment includes the mucilaginous secretion as well as the free space of the secretory cap, since the cuticle covering the cap is perforated by numerous 0.05–0.3 m pores. The proximal compartment includes xylem and the intercellular space of the stalk. The existence of the endodermoid partition is consistent with the observation that action potentials recorded extracellularly from the head may be positive-going while those recorded extracellularly from the stalk are negative-going. The partitioning is also consistent with the hypothesis previously proposed to explain why the amplitude of action potentials recorded from the mucilage varies as a function of the amplitude of the receptor potential.The living cells are united by plasmodesmata. Unusually abundant plasmodesmata were observed in the walls between endodermoid cells and neck cells, between neck cells and the next row of outer stalk cells, in the end walls connecting the outer stalk cells, and the end walls connecting the inner stalk cells: these strategically located plasmodesmata presumably permit the electrotonic spread of receptor potentials and action potentials between cells.     

The localization of pentosans within the cell wall of aspen (Populus tremuloides Michx.) by high resolution autoradiography

Summary Radiochemical studies of Populus tremuloides xylem tissue administered l-[1-³H]arabinose, d-[1-³H]glucose, and d-[6-³H]glucose demonstrate that l-[1-³H]arabinose is an excellent precursor for pentosan in this tissue. Transverse sections of first-year xylem (from cambial zone to pith) were examined by light and electron microscope autoradiography. Relatively large amounts of labeled pentosan are found in parenchyma cell walls, including the protective layer of ray parenchyma. Computeraided analyses of grain distributions in electron micrographs of cell walls of individual fibers localized the labeled wall components after different periods of incubation by comparison to model behavior. These analyses indicate that pentosan is added to the secondary cell wall of developing fibers by an appositional mechanism.     

Studies on the leaf of Amaranthus retroflexus (Amaranthaceae): ultrastructure,plasmodesmatal frequency,and solute concentration in relation to phloem loading

Both the mesophyll and bundle-sheath cells associated with the minor veins in the leaf of Amaranthus retroflexus L. contain abundant tubular endoplasmic reticulum, which is continuous between the two cell types via numerous plasmodesmata in their common walls. In bundle-sheath cells, the tubular endoplasmic reticulum forms an extensive network that permeates the cytoplasm, and is closely associated, if not continuous, with the delimiting membranes of the chloroplasts, mitochondria, and microbodies. Both the number and frequency of plasmodesmata between various cell types decrease markedly from the bundle-sheath — vascular-parenchyma cell interface to the sicve-tube member — companion-cell interface. For plants taken directly from lighted growth chambers, a stronger mannitol solution (1.4 M) was required to plasmolyze the companion cells and sieve-tube members than that (0.6 M) necessary to plasmolyze the mesophyll, bundle-sheath, and vascular-parenchyma cells. Placing plants in the dark for 48 h reduced the solute concentration in all cell types. Judging from the frequency of plasmodesmata between the various cell types of the vascular bundles, and from the solute concentrations of the various cell types, it appears that assimilates are actively accumulated by the sieve-tube — companion-cell complex from the apoplast.     

On the ultrastructure of granulosa lutein cells in porcine corpus luteum

Summary In young corpora lutea the endoplasmic reticulum membranes are sparse. A marked increase of smooth membranes then follows up to the peak of dioestrus. Continuities between smooth and rough endoplasmic reticulum are obvious during the same period. These observations suggest that the agranular membranes develop from the granular ones.During the most intense development of the endoplasmic reticulum the membranes show a tendency to be arranged in whorls. Since these are numerous only during the period of high progesterone secretion, a multitude of whorls constitutes a useful morphologic sign of high functional activity in the porcine granulosa lutein cells.During the first half of the oestrous cycle the increase in endoplasmic reticulum in general also parallels the increase in progesterone secretion. However, this secretion as well as ⁵-3-hydroxysteroid dehydrogenase activity declines earlier and more rapidly than the endoplasmic reticulum regresses. Steroid hormone synthesis may therefore be lacking although the agranular membranes appear morphologically normal.The mechanisms of induction of the endoplasmic reticulum membranes and enzymes active in steroid synthesis are discussed and it is suggested that luteinizing hormone (LH) may act as a trigger by increasing transport across membranes.Read at the Meeting of the Swedish Society for Pathology in Umeå, September 25, 1965 (Bjersing, 1966).This investigation was supported by grants from the Swedish Medical Research Council (Projects No. 13 X-78-01, 12 X-78-02, and 12 X-78-03).     

Elektronenmikroskopische Analyse der Musterbildung im Antheridium derPolypodiaceae

An electron microscopical investigation of the cell walls in young antheridia ofPolypodium crassifolium andPlatycerium alcicorne confirms the classical developmental model as postulated byStrasburger and byKny. The structure of the basal cell walls, both of the funnel cell and of the operculum, and especially the evidence of plasmodesmata in those walls, disprove the widely accepted interpretation presented 1951 byDavie.

     

Primitive-like metaphloem sieve elements in the aerial stem ofEquisetum hyemale

Summary Internodal metaphloem sieve elements located near the nodes of aerial stems ofEquisetum hyemale contain very oblique end walls. During maturation, the connections, or plasmodesmata, in these walls undergo little or no structural modification. By contrast, the endwall connections uniting the protoplasts of mature sieve elements elsewhere in the aerial stem ofE. hyemale are pores.This work as supported by U.S. National Science Foundation grant GB 31417 to R. F.Evert.     

Ultrastructure,plasmodesmatal frequency,and solute concentration in green areas of variegated Coleus blumei Benth. leaves

The photosynthetic tissue of green portions of variegated Coleus blumei leaves consists primarily of palisade and spongy parenchyma cells as well as bundle-sheath cells. The moderate numbers of plasmodesmata connecting these cells may be sufficient to provide a symplastic pathway for assimilates moving toward the minor veins. The minor veins, however, are unusual in having two sets of phloem-loading cells which have little symplastic continuity with one another: one consisting of large, peripherally located intermediary cells, and a second set made up of smaller, usually more internal companion cells, both sets having their associated sieve-tube members. The intermediary cells are connected to vascular-parenchyma and bundle-sheath cells by unique branched plasmodesmata which are particularly abundant at the bundle-sheath interface. In addition, numerous plasmodesmata-pore connections occur between the intermediary cells and their associated sieve-tube members. Neither the intermediary cells nor their sieve-tube members plasmolyze when treated with 1.4 M mannitol, whereas mesophyll and vascular-parenchyma cells plasmolyze at 0.5 M and bundle-sheath cells at 0.6 M mannitol. By contrast, the companion cells and their associated sieve-tube members are symplastically isolated from the bundle-sheath cells and the sieve-tube-intermediary-cell complexes, and share few plasmodesmata with the vascular-parenchyma cells. Moreover, the companion cells plasmolyze at 1.1 M mannitol and their sieve tubes at 1.3 M. The intermediary-cell-sieve-tube complex thus appears to be structurally equipped to load assimilates entirely via the symplast, while the sieve-tube-companion-cell complex is probably loaded from the apoplast.Abbreviation ER endoplasmic reticulum     

The distribution of plasmodesmata in the phloem ofHevea brasiliensis in relation to laticifer loading

Summary Cell to cell connections, including plasmodesmata and perforations, were examined in the non-conducting secondary phloem ofHevea brasiliensis. Samples were taken from trunks of numerous trees, from several clones, and prepared for thin sectioning and transmission or scanning electron microscopy and as optical sections for fluorescence microscopy. Numerous plasmodesmata were found clustered in primary pit-fields between the ray and axial parenchyma cells. Between the laticifers and adjacent parenchyma sheath cells, structures corresponding to functional plasmodesmata were not observed. But some unusual structural features were occasionally seen in these walls. These observations are discussed in relation to the possible function of the cell types, and to the loss of latex on the tapping ofHevea. It is suggested that the loading of the laticifer might first require a symplastic pathway for the transport of metabolites, at the end of which the assimilates must enter the apoplast. A transmembrane active transport system then transfers the metabolites in the laticifer. The presumable role of parenchyma cells in the loading of laticifers is emphasized.     

Actin associated with plasmodesmata

Summary We have used several methods to localise actin associated with plasmodesmata. In meristematic plant material fixed in 0.1% glutaraldehyde/1% paraformaldehyde and embedded in LR White resin, actin was localised (in TEM using 5 nm gold-labelled secondary antibody to C₄ anti-actin primary antibody) in the neck region by the plasma membrane and endoplasmic reticulum, and also down the length of the plasmodesma, deep in the cell wall. When the chemical fixation was replaced by rapid freezing in liquid propane (without cryoprotectants) and substitution in acetone, the plasmodesmata were labelled in similar positions, but with less background label on sections. While only 8–20% of plasmodesmata were labelled, the label was 10 to 100 fold denser over plasmodesmata than over the surrounding wall indicating specific association with plasmodesmata. We presume the apparent extracellular location of some label was due to the size of the antibodies between the site of attachment and the observed position of the gold particle. Gold label was found in similar locations in material fixed in 3% paraformaldehyde, infiltrated with sucrose, frozen, sectioned (10–12 m thick), then labelled with antibodies before resin embedding. Furthermore, cell walls in epidermal peels stained with rhodamine-phalloidin showed localised patches of fluorescence, presumably at the site of plasmodesmata (or primary pit-fields), which were connected on either side to fluorescent strands of actin in the cytoplasm. Suspension cultured cells ofNicotiana plumbaginifolia similarly stained showed very faint, narrow fluorescent strands crossing the walls of sister cells, which may indicate actin associated with individual plasmodesmata, shown in TEM to be sparsely distributed in these walls. In addition, the neck regions of cytochalasin-treated plasmodesmata were greatly enlarged and lacked the normal extracellular ring of particles. We propose that actin associated with plasmodesmata stabilizes the neck region and possibly also the cytoplasmic sleeve, and may be actively involved in regulating cell-to-cell transport.Abbreviations BSA bovine serum albumin - EDTA ethylenediaminetetraacetic acid - EGTA ethyleneglycol bis-(-aminoethyl ether)-N,N,N,N-tetraacetic acid - PAGE polyacrylamide gel electrophoresis - PBS phosphate buffered saline - Pipes piperazine-N,N-bis(2-ethanesulphonic acid) - Mes 2(N-morpholino)ethanesulfonic acid - SDS sodium dodecyl sulphate - Tris tris-(hydroxymethyl)aminomethane     

Integument cell differentiation in dandelions (<Emphasis Type="Italic">Taraxacum</Emphasis>, Asteraceae,Lactuceae) with special attention paid to plasmodesmata

The aim of the paper is to determine what happens with plasmodesmata when mucilage is secreted into the periplasmic space in plant cells. Ultrastructural analysis of the periendothelial zone mucilage cells was performed on examples of the ovule tissues of several sexual and apomictic Taraxacum species. The cytoplasm of the periendothelial zone cells was dense, filled by numerous organelles and profiles of rough endoplasmic reticulum and active Golgi dictyosomes with vesicles that contained fibrillar material. At the beginning of the differentiation process of the periendothelial zone, the cells were connected by primary plasmodesmata. However, during the differentiation and the thickening of the cell walls (mucilage deposition), the plasmodesmata become elongated and associated with cytoplasmic bridges. The cytoplasmic bridges may connect the protoplast to the plasmodesmata through the mucilage layers in order to maintain cell-to-cell communication during the differentiation of the periendothelial zone cells.