Cytochemical localization of cellulase activity in pollen mother cells of david lily during meiotic prophase I and its relation to secondary formation of plasmodesmata

Summary Cellulase activity was localized at the ultrastructural level in pollen mother cells (PMCs) of David lily [Lilium davidii var.willmottiae (Wilson) Roffill] at different stages of meiotic prophase I. The enzyme was observed to appear at the early leptotene stage and reached its highest level at the subsequent zygotene stage, and its subcellular distribution revealed by the presence of electron-dense deposits of reaction product was found to be restricted exclusively to the endoplasmic reticulum (ER), the vesicles derived from that, and the cell wall, especially at the sites of secondary plasmodesmata and cytoplasmic channels where the wall was being digested. Other cytoplasmic organelles, such as dictyosomes and Golgi vesicles, lacked such deposits of reaction product. After zygotene the enzyme activity decreased abruptly, and at the pachytene stage only very few deposits could be observed in the cell wall. Our results indicate that cellulase is synthesized on rough ER and secreted directly via the smooth ER and ER-derived vesicles into the cell wall by exocytosis, where it brings about local wall breakdown, leading to the secondary formation of plasmodesmata and cytoplasmic channels.     

Sequential actions of pectinases and cellulases during secretory cavity formation in Citrus fruits

Pectinase and cellulase activities are involved in a number of intercellular space-forming processes in plants. In this study, we combined cytochemistry with ultrastructural analysis to investigate the ontogeny of secretory cavity in fruits of Citrus medica L. var. sarcodactylis (Noot.) Swingle, Citrus reticulata Blanco and Citrus limon (L.) Burm. f. Pectinase activity was first detectable at the initial stage of cavity formation, peaked at the intercellular space-forming stage, and diminished at the following stages. In comparison, no cellulase activity was detected until the early lumen-expanding stage. The cellulase activity increased at the late lumen-expanding stage and culminated at the near-mature stage. In the fruit of C. medica var. sarcodactylis, the distribution of pectinase and cellulase reaction products was restricted to the endoplasmic reticulum (ER), the vesicles derived from ER and the cell wall. We also observed that multivesicular structure containing the pectinase reaction product at the initial stage of cavity formation. Our results suggest that pectinase and cellulase are synthesized on ER and secreted directly into the cell wall through exocytosis of ER-derived vesicles. Our observations are consistent with the notion that the secretory cavity in Citrus fruits is formed through a schizolysigenous process in which pectinase activity is involved in the degradation of the middle lamella, whereas cellulase activity is responsible for the degradation of the cell wall.     

Development of cycad ovules and seeds. 1. Implication of the ER in primary cellularisation of the megagametophyte in <Emphasis Type="Italic">Encephalartos natalensis</Emphasis> Dyer and Verdoorn

Very little is known about the pre- and post-shedding megagametophyte development and utilisation of accumulated reserves, respectively, in cycads (Zamiaceae). In the present study on developing ovules of the recalcitrant-seeded species, Encephalartos natalensis, cells of the megagametophyte were found to become progressively packed with starch and protein as the two main storage reserves, a limited number of discrete lipid bodies, and occasional mitochondria all of which appeared to be embedded in a homogeneous matrix. ER-derived vesicles (and not Golgi-derived vesicles) appeared to be the principal contributors of the primary cell wall components, pectin and xylan, during megagametophyte cellularisation. This was confirmed by the use of enzyme-gold localisation. High-pressure freezing (HPF) and freeze substitution (FS) of samples the following season showed that while the apparently featureless cytomatrix of the megaspore was an artefact of conventional fixation, there was still an insignificant occurrence of Golgi bodies during primary wall formation. When enzyme-gold localisation was employed on the HPF-FS material, label for pectin and xylan was found only in the regions of ER and vesicles and not in any of the few Golgi bodies or their associated vesicles. Immunocytochemistry revealed that pectin and xylan were restricted to the ER and not to any vesicles or to the occasional Golgi body that was found. This suggests that the ER exclusively, is involved in the deposition of these primary cell wall components during the cellularisation of the E. natalensis megagametophyte. While cellularisation took place over approximately 1–2 weeks, subsequent development of the megagametophyte cells involved the accumulation of storage reserves, this phase lasting approximately 8 months—after which the seeds were shed whether pollination/fertilisation had recently occurred, or not.     

Wall structure and bud formation in Rhodotorula glutinis

Summary The first stage in the formation of a bud in Rhodotorula glutinis is the production of a tapered plate of new wall material between the existing wall and the plasmalemma. The parent cell wall is lysed, allowing the bud to emerge enveloped in this new wall. Mucilage is synthesised to surround the developing bud. As the bud grows a septum forms centripetally dividing the two cells. When the daughter cell reaches maximum size the septum cleaves along its axis, producing the bud scar on the parent cell and the birth scar on the daughter cell. The birth scar is obliterated later as the wall of the young cell grows. A system of endoplasmic reticulum and vesicles is found in young buds and is thought to be responsible for the transport of wall material precursors.     

Cytology of budding and cleavage in tobacco mesophyll protoplasts cultivated in saline medium

Tobacco (Nicotiana tabacum L., cv. Maryland) mesophyll protoplasts cultivated in saline medium divide by bud formation, migration of one nucleus into the bud, and subsequent furrowing. This process was investigated light and electron microscopically. The cytoplasm of the growing bud is richer in dictyosomes, rough endoplasmic reticulum profiles, mitochondria, and small vacuoles than is the cytoplasm of the mother cell, but in early stages lacks plastids. Only patches of wall material are found; most of the cell surface appears naked. Oriented sections of the cleavage furrow do not reveal a contractile ring of microfilaments under the fixation conditions used. The furrow is flanked by numerous microtubules, and is rich in coated vesicles. Nuclear division appears normal, but the phragmoplast vesicles appear empty, and the phragmoplast seems to disintegrate again later. The nucleus migrating into the bud does not show any signs of associated contractile structures. The results demonstrate that, in principle, higher plant cells are capable of a mode of division usually said to be yeast-like. The events of karyokinesis and cell plate formation are not therefore obligatorily linked processes.     

CYTOCHALASIN B INFLUENCES DICTYOSOMAL VESICLE PRODUCTION AND MORPHOGENESIS IN THE DESMID EUASTRUM1

Cytochalasin B (CB) applied to young developing cells of the desmid Euastrum oblongum Ralfs ex Ralfs, at concentrations that do not entirely inhibit cytoplasmic streaming, retarded cell growth and caused malformations of cell shape. While the basic symmetry of the cell was maintained, only the first indentations were formed and the cell body appeared to be swollen. Electron microscopic investigations revealed that vesicle production at the dictyosomes was disturbed by cytochalasin. In contrast to untreated control cells, where vesicles with electron-dense contents (“dark vesicles”) were formed during primary wall formation, vesicles pinched off by the dictyosomes during CB treatment exhibited an “empty” appearance. These vesicles, which correspond to the “dark vesicles” in size, were accumulated around the dictyosomes without being transported to the plasma membrane and were frequently connected to the trans-cisternae of the Golgi bodies. We speculate that CB may influence the transfer of products from the endoplasmic reticulum (ER) to the dictyosomes via transition vesicles, which results in a disturbed vesicle production at the Golgi bodies. CB also causes a shift in ER and dictyosome distribution. Moreover, a cortical actin system appears to be involved in the cell shaping of Euastrum. The arrangement of microtubules around the nucleus is not affected by the drug.     

Brefeldin A effects on the trans-Golgi network and Golgi bodies inBotryococcus braunii are not uniform during the cell cycle

Summary Using cryo-fixation and freeze-substitution electron microscopy, the effects of brefeldin A (BFA) on the structure of the trans-Golgi network (TGN), the endoplasmic reticulum (ER), and Golgi bodies in the unicellular green algaBotryococcus braunii were examined at various stages of the cell cycle. In the presence of BFA, all the TGNs of interphase and dividing cells aggregated to form a single tubular mass. In contrast, the TGNs decomposed just after cell division and disappeared during cell wall formation. Throughout the cell cycle, the TGN produced at least six kinds of vesicles, of which two were not formed in the presence of BFA: vesicles with a diameter of 200 nm and fibrillar substances, which formed in interphase cells; and vesicles with a diameter of 180–240 nm, which may participate in septum formation. In addition, the number of clathrin-coated vesicles attaching to the TGN decreased. In interphase cells, BFA induced the disassembly of Golgi bodies and an increase in the smooth-ER cisternae at the cis-side of Golgi bodies. This result may suggest the existence of retrograde transport from the Golgi bodies to the ER in the presence of BFA. These drastic structural changes in the Golgi bodies and the ER of interphase cells were not observed in BFA-treated dividing cells.Abbreviations BFA brefeldin A - ER endoplasmic reticulum - TGN trans-Golgi network     

Disturbance of the secretory pathway inMicrasterias denticulata by tunicamycin and cyclopiazonic acid

Summary Both tunicamycin, an inhibitor of N-linked glycosylation of proteins, and cyclopiazonic acid, which inhibits the Ca²⁺-dependent ATPase in the ER, influence the secretory pathway at the ER level and lead to a cessation of cell growth inMicrasterias. Electron microscopical investigations reveal that the mode of action of the two inhibitors differs. While tunicamycin treatment results in a disintegration of the Golgi bodies into small vesicles, cyclopiazonic acid prevents products being supplied from the ER, resulting in the dilatation of ER cisternae and a reduction in the number of Golgi cisternae, combined with a loss of dictyosomal activity. The disturbed cell wall formation under tunicamycin indicates that N-linked glycosylation of proteins is required for normal cell growth inMicrasterias. Moreover, our studies reveal that changes in cytoplasmic free calcium concentration, as a consequence of ATPase inhibition in the ER by cyclopiazonic acid, may inhibit wall material secretion by interrupting the normal ER-dictyosome association.Abbreviations CPA cyclopiazonic acid - ER endoplasmic reticulum - TM tunicamycin     

Histogenesis of the transmitting tract in Strelitzia reginae

Organisation and development of the stigmatic, stylar and ovarian parts of the transmitting tract in Strelitzia reginae were evaluated. They were characterised by 1) cell shape, 2) appearance of distal cell wall, 3) type of plastid, 4) and vacuolar system. The long stigmatic trichomes have a secondary irregular wall layer separated from the primary wall. Cell structures include pleomorphic plastids with vesiculated thylakoids and frequently a crystal or a lipidic globule. In early stages of bud development the extensive endoplasmic reticulum (ER) is smooth, whereas it is mainly rough in older buds. Coated vesicles are frequent, as are dictyosomes. Prominent invaginations along the plasma membrane contain floccular deposits in the older flower buds. These deposits are similar in structural appearance to material in the large vacuoles. The basal part of the stigma has wedge-shaped cells with wall ingrowths. Three stages of stigmatic secretion during the development were characterised. The stylar canal is initially narrow but widens subsequently. The cuticle is detached and the apical cell walls show a fringed surface; from this wall inbuddings develop shortly before anthesis. At this stage sheets of rough ER are evident in the cell cortex. The plastids have a few vesiculated thylakoids, proteinaceous crystals and starch grains. The epithelial cells of young buds have numerous vacuoles, the volume of which decreases in more mature cells. The stylar canal is filled with a secretion at all stages of bud development. The face of the ovarian transmitting tract, lining the placenta, is smooth in young buds but lobed in older ones due to the division pattern of the epithelial cells. These cells are large, elongated and culumnar as young but narrow and wedge-shaped when more mature. Cell wall inbuddings are formed late during bud development.     

Biosynthetic protein transport through the early secretory pathway

 Newly synthesized proteins destined for delivery to the cell surface are inserted cotranslationally into the endoplasmic reticulum (ER) and, after their correct folding, are transported out of the ER. During their transport to the cell surface, cargo proteins pass through the various cisternae of the Golgi apparatus and, in the trans-most cisternae of the stack, are sorted into constitutive secretory vesicles that fuse with the plasma membrane. Simultaneously with anterograde protein transport, retrograde protein transport occurs within the Golgi complex as well as from the Golgi back to the ER. Vesicular transport within the early secretory pathway is mediated by two types of non-clathrin coated vesicles: COPI- and COPII-coated vesicles. The formation of these carrier vesicles depends on the recruitment of cytosolic coat proteins that are thought to act as a mechanical device to shape a flattened donor membrane into a spherical vesicle. A general molecular machinery that mediates targeting and fusion of carrier vesicles has been identified as well. Beside a general overview of the various coat structures known today, we will discuss issues specifically related to the biogenesis of COPI-coated vesicles: (1) a possible role of phospholipase D in the formation of COPI-coated vesicles; (2) a functional role of a novel family of transmembrane proteins, the p24 family, in the initiation of COPI assembly; and (3) the direction COPI-coated vesicles may take within the early secretory pathway. Moreover, we will consider two alternative mechanisms of protein transport through the Golgi stack: vesicular transport versus cisternal maturation. Accepted: 24 October 1997     

The structure of the Golgi apparatus: a sperm’s eye view in principal epithelial cells of the rat epididymis

 The Golgi apparatus of epididymal principal cells shares many structural features with other cell types. Saccular regions are arranged in a cis-Golgi network, eight flattened saccules, and several trans-Golgi networks (TGNs). Dilated tubules form intersaccular connecting regions which joint together saccules at the same or different levels between adjacent stacks. Wells exist as large perforations in register with the four cis-most saccules and serve as areas of vesicular interactions. TGNs are variable and can appear to peel off the stack or to be detached from it in the form of an anastomotic tubular network with pale dilated areas corresponding to prosecretory granules connected by short narrow bridges. Elongated or discoid dilated cisternae of endoplasmic reticulum (ER) (sparsely granulated) lie over the cis face of the stack, from which they are separated by an intermediate compartment filled with vesicles and tubules. The ER is also closely juxtaposed to the TGNs and the eighth saccule but interconnections are never seen between them. Vesicles of the COP variety reside at all levels of the stack and appear to bud off the cis-located ER and the edges of the saccules, while clathrin-coated vesicles appear mainly on the trans face of the stack and next to lysosomes. In the supranuclear cytoplasm, clusters of vesicles and tubules, at times budding off enveloping ER, appear to radiate toward the Golgi stacks where they fuse with cis Golgi elements. Taken together, these observations suggest dynamic functions and interactions for the various Golgi elements, associated vesicles, ER, and vesicular tubular clusters. Accepted: 29 January 1998     

Cotton fibre differentiation: Occurrence and distribution of coated and smooth vesicles during primary and secondary wall formation

Summary Coated vesicles occur in differentiating cotton fibres during primary and secondary wall formation. The coated vesicles are often associated with the plasmalemma, or with membranes at the secreting face of dictyosomes, corresponding positionally to GERL. During secondary wall formation the number of dictyosome-associated coated vesicles seems to be smaller than during primary wall formation. When sections are stained for periodateoxidizable polysaccharides (Thiéry reaction) the membrane of plasmalemma-associated coated vesicles is intensely stained. The membrane of dictyosome-associated coated vesicles is only weakly stained. On the basis of the present evidence it is not possible to clearly decide, whether the staining in plasmalemma-associated coated vesicles is due to obliquely cut membrane or to vesicle contents. The vesicle coat material is not stained. Possible functions of coated vesicles in differentiating cotton fibres are discussed.Vesicles with contents positively stained with the Thiéry reaction are observed only during primary wall formation. The membrane of these vesicles is smooth and seems to bud from the same cisternae, probably GERL, as do the coated vesicles. During secondary wall formation no vesicles containing periodate-oxidizable polysaccharides could be detected, even under conditions that result in a strong, specific reaction in the cellulosic secondary wall. In some instances polysaccharidic material, resembling secondary wall material, has been seen to adhere to the outside of the plasmalemma. These results are consistent with the hypothesis that, in higher plants, at least part of primary wall material may already be synthesized in dictyosome vesicles, whereas cellulose biosynthesis occurs at the cell surface.     

Studies on graft unions

Summary De novo formation of cytoplasmic cell connections are studied at the graft interface of 5 day old in vitro heterografts ofVicia faba onHelianthus annuus. Continuous and half plasmodesmata, both branched and unbranched, are described at various stages of development in non-division walls between unlike and like dedifferentiated callus cells. In apical portions of protruding callus cells and in the contact zone between opposing cells extremely thin wall parts with a striking ER/plasmalemma contact are observed. During subsequent thickening of the modified wall parts cytoplasmic strands enclosing constricted ER cisternae are entrapped within the newly deposited wall material. These cytoplasmic strands represent half plasmodesmata which—in case of fusion with corresponding structures of adjoining cells across the loosened wall matrix — form continuous cell connections. Golgi vesicles secreting wall material are involved in the process of forming half and continuous plasmodesmata, thus following the same mechanism of plasmodesmata development as described for isolated protoplasts in cell cultures. The findings suggest the existence of a unifying mechanism of secondary formation of plasmodesmata showing far-reaching similarities with the establishment of primary cell connections.     

ULTRASTRUCTURE OF TETRASPOROGENESIS IN PALMARIA PALMATA (RHODOPHYTA)1

The tetrasporangial initial in Palmaria palmata (L.) O. Kuntze (formerly Rhodymenia palmata (L.) Greville) arises from a cortex cell which enlarges and deposits a protein-rich wall layer. This cell undergoes mitosis to form a tetrasporocyte and a stalk cell. Synaptonemal complexes are formed in the sporocyte nucleus while in the cytoplasm floridean starch is deposited in association with ER or with particles presumed to be ribosomes. Microbody-like structures become numerous between the nuclear envelope and perinuclear ER, and clusters of non-membranous, spherical structures also are associated with the nucleus. Chromatin condensation is reversed following pachytene and a prolonged diffuse stage ensues, when dictyosomes and ER produce vesicles which deposit mucilage rich in sulfated and acidic polysaccharides around the tetrasporocyte. A conspicuous lenticular thickening of the mucilage sheath develops at the apical end of the sporangium. Dictyosomes are frequently associated with mitochondria which may be associated with chloroplasts. Following nuclear divisions the tetrasporocyte is cleaved into four spores by sequentially initiated, but simultaneously completed periclinal and anticlinal furrows. When mucilage deposition ceases, the dictyosomes begin to produce vesicles with glycoprotein-rich contents. These vesicles are abundant in released tetraspores, and they probably contain adhesive material aiding in the attachment of the liberated spores.     

SECRETIONS FROM THE PISTIL OF LILIUM LONGIFLORUM

The pistil of Lilium longiflorum secretes two forms of exudate, one from the stigma surface and the other from the canal cavity. Electrophoretic studies of these exudates have revealed quantitative and qualitative differences in protein profiles. The exudatic components which are transferred to the cell wall by endoplasmic reticulum and Golgi vesicles, are stored within the cell wall of the secretive tissues and secreted from the cell walls directly. The cell wall structure of these secretive tissues differs. The canal cell wall has thick characteristic ingrowths that are supplied mainly from Golgi vesicles, while the papilla cell wall of the stigma is thinner, lacks ingrowths, and is supplied from ER vesicles.     

ULTRASTRUCTURE OF TETRASPOROGENESIS IN THE PARASITIC RED ALGA LEVRINGIELLA GARDNERI (SETCHELL) KYLIN12

Ultrastructural studies on tetraspore formation in Levringiella gardneri revealed that 3 stages may be recognized during their formation. The youngest stage consists of a uninucleate tetraspore mother cell with synaptonemal complexes present during early prophase of meiosis I. Mitochondria are aggregated around the nucleus, dictyosome activity is low, and chloroplasts occur in the peripheral cytoplasm. A 4-nucleate tetraspore mother cell is formed prior to tetrahedral cell cleavage, and an increase in the number of chloroplasts and mitochondria occurs. Small straight-profiled dictyosomes secrete vesicles into larger fibrous vesicles or contribute material to the developing tetraspore wall. During the second stage of tetraspore formation, striated vesicles form within endoplasmic reticulum, semicircular profiled dictyosomes secrete vesicles for fibrous vesicles or wall material, and starch formation increases. The final stage is characterized by the disappearance of striated vesicles, presence of straight, large dictyosomes which secrete cored vesicles, and an abundance of starch grains. Cleavage is usually complete at this stage and the tetraspore wall consists of a narrow outer layer of fibrillar material and an inner, electron transparent layer. These spores are surrounded by a tetrasporangial wall which was the original wall surrounding the tetraspore mother cell.     

Actin filament-organized local cortical endoplasmic reticulum aggregations in developing stomatal complexes of grasses

Endoplasmic reticulum (ER) immunolabeling in developing stomatal complexes and in the intervening cells of the stomatal rows (ICSRs) of Zea mays revealed that the cortical-ER forms distinct aggregations lining locally expanding wall regions. The polarized subsidiary cell mother cells (SMCs), displayed a cortical-ER-patch lining the wall region shared with the inducing guard cell mother cell (GMC), which disorganized during mitosis. In dividing SMCs, ER persisted in the preprophase band region and was unequally distributed in the mitotic spindle poles. The subsidiary cells (SCs) formed initially an ER-patch lining the common wall with the GMC or the young guard cells and afterwards an ER-ring in the junction of the SC wall with the neighboring ones. Distinct ER aggregations lined the ICSR wall regions shared with the SCs. The cortical-ER aggregations in stomatal cells of Z. mays were co-localized with actin filament (AF) arrays but both were absent from the respective cells of Triticum turgidum, which follow a different morphogenetic pattern. Experimental evidence showed that the interphase ER aggregations are organized by the respective AF arrays, while the mitotic ER aggregations by microtubules. These results revealed that AF and ER demarcated “cortical cytoplasmic domains” are activated below the locally expanding stomatal cell wall regions, probably via a mechanosensing mechanism triggered by the locally stressed plasmalemma/cell wall continuum. The probable role(s) of the local ER aggregations are discussed.     

Cotton embryogenesis: The pollen cytoplasm

Summary The ultrastructure and composition of cotton (Gossypium hirsutum) pollen, exclusive of the wall, was examined immediately before and after germination. The pollen grain before germination consists of two parts: the outer layer and a central core. The outer layer contains large numbers of mitochondria and dictyosomes as well as endoplasmic reticulum (ER). The core contains units made of spherical pockets of ER which are lined with lipid droplets and filled with small vesicles; the ER is rich in protein and may contain carbohydrate while the vesicles are filled with carbohydrate. Starch-containing plastids are also present in the core as are small vacuoles. The cytoplasm of the pore regions contains many 0.5 spherical bodies containing carbohydrate. After germination the ER pockets open and the lipid droplets and small vesicles mix with the other portions of the cytoplasm. With germination the pore region becomes filled with mitochondria and small vesicles. The vegetative nucleus is large, extremely dense and contains invaginations filled with coils of ER. A greatly reduced nucleolus is present in the generative cell which is surrounded by a carbohydrate wall. The cytoplasm of the generative cell is dense and contains many ribosomes, a few dictyosomes and mitochondria, many vesicles of several sizes, and some ER. No plastids were identified. The generative nucleus is also dense with masses of DNA clumped near the nuclear membrane. An unusual tubular structure of unknown origin or function was observed in the generative cell.     

Isolation and characterization of prosthecae ofAsticcacaulis biprosthecum

The budding process of the yeast form of Mucor rouxii was examined by electron microscopy of thin sections with particular reference to wall ontogeny. In most instances the bud wall is seen as a continuation of the inner layers of the parent cell wall. As the bud emerges it ruptures the outer layers of the parent wall. The bud wall is much thinner than the parent wall and remains so while the bud grows into a sphere of about one half the diameter of the parent cell. Then a septum begins to form centripetally, at the neck, by invagination of the plasmalemma. Before the neck canal is completely occuluded, electron-dense wall material is deposited into the septum space. Two separate septum walls are deposited, one on the parent side and one on the bud side of the invaginating plasmalemma. Septum wall formation extends to the surrounding neck walls. In this manner, the parent and bud cytoplasms become fully separated and each is surrounded by a continuous wall. The two cells remain attached to each other by the original neck wall; eventually, the bud abscisses leaving a birth scar on the bud cell and a more pronounced bud scar on the parent cell. In general, the mechanism of budding in this zygomycetous fungus resembles that of an ordinary ascomycetous yeast such as Saccharomyces cerevisiae.     

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.