Ultrastructural observations of maize root tips following exposure to monensin

Summary Epidermal and outer rootcap cells of maize root tips were treated with the sodium selective ionophore, monensin, and the ultrastructural changes were studied. In the presence of 10^–5 to 10^–3 M monensin, dictyosomes became distorted, cisternae separated from the stack, and secretory vesicles were released. Released secretory vesicles disappeard from the cytoplasm suggesting that their transport to, and fusion with, the plasma membrane was unaffected. Monensin did not inhibit cytoplasmic streaming of the outer rootcap cells. No new secretory vesicles were formed on the remaining dictyosomes or dictyosome fragments. In contrast to results with animal cells, swelling of plant dictyosome cisternae was observed only after fixation in glutaraldehyde-osmium tetroxide and not after fixation in potassium permanganate. Other cell components were not altered structurally by monensin. The effects of monensin on the Golgi apparatus were reversible, and dictyosomes were either repaired or new dictyosomes were formed after the root tips were removed from the monensin.Dictyosomes in epidermal cells reacted in the same manner as those in the rootcap except that numerous secretory vesicles remained in the cytoplasm, mostly in association with dictyosome fragments. Some secretory vesicles increased in size but no evidence of vesicle-vesicle fusion was noted. Cell plate formation was partially inhibited or blocked by monensin.Mention of a commercial or proprietary product in this paper does not constitute an endorsement of this product by the USDA.     

The transmitting tract in Trimezia fosteriana (Iridaceae).

Observations of the transmitting tract cells in Trimezia fosteriana were made from the pre-secretory stage until anthesis. Secretory products appear about 14 days before anthesis in all parts of the pistil. Simultaneously starch disappears from the plastids and the dictyosomes are surrounded by more and larger vesicles than before. In the beginning of the secretory stage multivesicular bodies may be in contact with ER profiles and an ER origin is therefore presumed. Later during the secretory stage the multivesicular bodies are larger and more abundant. Their envelope is often partly fused with the plasma membrane and vesicle swarms are common on the outside of it. Close to flower opening many organelles change in appearence. RER becomes more abundant and the mitochondrial matrix highly electron dense and the cristae expanded. Starch grains reappear and large ones are common in the stigma and ovary plastids. The dictyosomes are numerous and surrounded by the largest vesicles observed during the secretory stage. Osmiophilic granules are common in the dictyo-some vesicles and under the cell walls abutting the transmitting tract. In the ovary they are present one week before anthesis. In the stigma and style corresponding granules of high electron density appears at the day of flower opening. The extracellular secretory product contains fibrillar and granular substances consisting of carbohydrates, including pectic substances, and proteins.     

Unusual transfer cells in the epithelium of the nectaries ofAsclepias curassavica L.

Summary The secretory cells of the nectaries ofAsclepias curassavica form a glandular epithelium in the inner parts of the stigmatic chambers. They resemble transfer cells in having many infoldings of the plasmalemma. The wall protuberances, however, are poorly developed and often lacking. The plasmalemma is highly convoluted and forms, in places, external compound membranes where the extracytoplasmic space is collapsed completely. Active glands contain dilated cisternae of the ER and large vesicles which are mainly associated with the cis face of the dictyosomes. In addition, small vesicles are observed in high number. It is discussed whether the secretion is granulocrine or eccrine and whether the enlargement of the plasmalemma is the cause or the consequence of the high secretory activity. After the secretory phase the outer peripheral part of the cytoplasm disintegrates. The remaining part of the protoplast is covered by a new plasmalemma.     

Developmental and comparative ultrastructure of glandular hairs in the two Urticaceae. I. Urtica dioica

Secretion produced by glandular hairs is deposited mainly in the periplasmic space of the head cells. It stains intensely for both proteins and polysaccharides. The ultrastructure of meristematic, differentiating, mature and senescent head cells as well as the stalk and basal cells has been described in comparison to that in other cell types of the leaf. The specific features of the head cells are the proliferation of the granular endoplasmic reticulum as well as the multiplication of the dictyosomes and mitochondria during transition to the secretion stage. However, the frequency of dictyosomes varies among secreting hairs. The ER produces neither secretory nor transition vesicles and does not anastomose with the plasmalemma. In the absence of transition vesicles, the transport of secretory proteins and enzymes of polysaccharide synthesis from the ER to dictyosomes apparently includes the cytosolic step. Dictyosomes, though not appearing hypersecretory, produce two types of smooth secretory vesicles generated by the trans Golgi reticulum. The vectorial transfer of prosecretion and membranes across the dictyosome stack proceeds via the transport (shuttle) vesicles. It is, therefore, concluded that exocytosis of smooth secretory Golgi vesicles is the sole mechanism of release of both proteins and polysaccharides. Coated vesicles occasionally seen near the plasmalemma are likely to be involved in the endocytotic membrane retrieval. The secretion product disappears during senescence of the hairs and the secretory cells undergo vacuolation by means of local autophagy.     

Membrane traffic from the endoplasmic reticulum to the Golgi apparatus is disturbed by an inhibitor of the Ca2+-ATPase in the ER

Summary An electron microscopic study of cress (Lepidium sativum L.) roots treated with cyclopiazonic acid (CPA), an inhibitor of the Ca²⁺-ATPase in the endoplasmic reticulum (ER) has been carried out. Drastic changes in the endomembrane system of the secretory root cap cells were observed. After treatment with CPA dense spherical or elliptoidal aggregates of ER (diameter 2–4 m) were formed in addition to the randomly distributed ER cisternae characteristic for control cells. The formation of ER aggregates indicates that in spite of an inhibition of the Ca²⁺ -ATPase in the ER by CPA, membrane synthesis in the ER continued. The ER aggregates are interpreted as a reservoir of ER membrane material newly synthesized during the 2 h CPA-treatment. Hypertrophied Golgi cisternae and secretory vesicles, which are characteristic for secretory cells under control conditions, were completely absent. Additionally the shape of the Golgi stacks was flat and the diameter of the cisternae was shortened by about one third. These phenomena are indicative of an inactive state of the Golgi apparatus. The cellular organization of both other cell types of the root cap, meristematic cells and statocytes, was not visibly affected by CPA, both having a relatively low secretory activity. The formation of ER aggregates as well as the reduction of Golgi compartments are indications for the existence of a unidirectional transport of membrane material from the ER to the Golgi. It is suggested that the membrane traffic from the ER to the Golgi apparatus is regulated by the cytosolic and/or luminal calcium concentration in secretory cells of the root cap.Abbreviations CPA cyclopiazonic acid - ER endoplasmic reticulum     

Cellular compartments in mitotic cells: ultrahistochemical identification of Golgi elements in PtK-1 cells

This investigation focuses on the identification, distribution, and transport of intracellular membrane systems during mitosis. The membranes of the Golgi apparatus can be identified cytochemically by staining for acid phosphatase (acPase) and thiamine pyrophosphatase (TPPase) activity. Using this approach we are able to study the disintegration of the Golgi apparatus during mitosis and to follow the dislocation as well as the organized reappearance of Golgi elements after the completion of mitosis. We are able to demonstrate that during mitosis the activity of both enzymes is strong enough to react with the substrate applied during the staining procedure. Furthermore, we observe a characteristic pattern of membrane distribution in mitotic cells. During interphase the TPPase reaction is characteristically limited to one or two cisternae of a dictyosomal stack. The acPase reaction stains the membranes of the total stack, of the GERL, of some vesicles and cisternae near the dictyosomes and lysosomes. After the mitotic breakdown of the dictyosomal stacks the forming vesicles still stain positively and are distributed over the entire cytoplasm. At late anaphase and early telophase the enzyme activity occurs not only in the reconstituting dictyosomes but also in the nuclear envelope and in some ER cisternae. The extended spectrum of membrane structures indicating Golgi enzyme activity becomes obvious. This phenomenon favors the idea that at least some functions of the Golgi apparatus persist during mitosis.     

Post Golgi apparatus structures and membrane removal in plants

Summary In nongrowing secretory cells of plants, large quantities of membrane are transferred from the Golgi apparatus to the plasma membrane without a corresponding increase in cell surface area or accumulation of internal membranes. Movement and/or redistribution of membrane occurs also in trans Golgi apparatus cisternae which disappear after being sloughed from the dictyosome, and in secretory vesicles which lose much of their membrane in transit to the cell surface. These processes have been visualized in freeze-substituted corn rootcap cells and a structural basis for membrane loss during trafficking is seen. It involves three forms of coated membranes associated with the trans parts of the Golgi apparatus, with cisternae and secretory vesicles, and with plasma membranes. The coated regions of the plasma membrane were predominantly located at sites of recent fusion of secretory vesicles suggesting a vesicular mechanism of membrane removal. The two other forms of coated vesicles were associated with the trans cisternae, with secretory vesicles, and with a post Golgi apparatus tubular/vesicular network not unlike the TGN of animal cells. However, the trans Golgi network in plants, unlike that in animals, appears to derive directly from the trans cisternae and then vesiculate. The magnitude of the coated membrane-mediated contribution of the endocytic pathway to the formation of the TGN in rootcap cells is unknown. Continued formation of new Golgi apparatus cisternae would be required to maintain the relatively constant form of the Golgi apparatus and TGN, as is observed during periods of active secretion.     

AN ULTRASTRUCTURAL BASIS FOR HYPHAL TIP GROWTH IN PYTHIUM ULTIMUM

Hyphae of the fungus Pythium ultimum extend by tip growth. The use of surface markers demonstrates that cell expansion is limited to the curved portion of the hyphal apex. Growing and non-growing regions are reflected in internal organization as detected by light and electron microscopy. The young hypha consists of three regions: an apical zone, a subapical zone and a zone of vacuolation. The apical zone is characterized by an accumulation of cytoplasmic vesicles, often to the exclusion of other organelles and ribosomes. Vesicle membranes are occasionally continuous with plasma membrane. The subapical zone is non-vacuolate and rich in a variety of protoplasmic components. Dictyosomes are positioned adjacent to endoplasmic reticulum or nuclear envelope, and vesicles occur at the peripheries of dictyosomes. A pattern of secretory vesicle formation by dictyosomes is described which accounts for the formation of hyphal tip vesicles. Farther from the hyphal apex the subapical zone merges into the zone of vacuolation. As hyphae age vacuolation increases, lipid accumulations appear, and the proportional volume of cytoplasm is reduced accordingly. The findings are integrated into a general hypothesis to explain the genesis and participation of cell components involved directly in hyphal tip growth: Membrane material from the endoplasmic reticulum is transferred to dictyosome cisternae by blebbing; cisternal membranes are transformed from ER-like to plasma membrane-like during cisternal maturation; secretory vesicles released from dictyosomes migrate to the hyphal apex, fuse with the plasma membrane, and liberate their contents into the wall region. This allows a plasma membrane increase at the hyphal apex equal to the membrane surface of the incorporated vesicles as well as a contribution of the vesicle contents to surface expansion.     

Structural differentiation of membranes involved in the secretion of polysaccharide slime by root cap cells of cress (Lepidium sativum L.)

The peripheral secretion tissue of the root cap of Lepidium sativum L. was investigated by electronmicroscopy and freeze-fracturing in order to study structural changes of membranes involved in the secretion process of polysaccharide slime. Exocytosis of slime-transporting vesicles occurs chiefly in the distal region of the anticlinal cell walls. The protoplasmic fracture face (PF) of the plasmalemma of this region is characterized by a high number of homogenously distributed intramembranous particles (IMPs) interrupted by areas nearly free of IMPs. Near such areas slime-transporting vesicles are found to be underlying the plasma membrane. It can be concluded that areas poor in particles are prospective sites for membrane fusion. During the formation of slime-transporting vesicles, the number of IMPs undergoes a striking change in the PF of dictyosome membranes and their derivatives. It is high in dictyosome cisternae and remarkably lower in the budding region at the periphery of the cisternae. Slime-transporting vesicles are as poor in IMPs as the areas of the plasmalemma. Microvesicles rich in IMPs are observed in the surroundings of dictyosomes. The results indicate that in the plasmalemma and in membranes of the Golgi apparatus special classes of proteins — recognizable as IMPs — are displaced laterally into adjacent membrane regions. Since the exoplasmic fracture face (EF) of these membranes is principally poor in particles, it can be concluded that membrane fusion occurs in areas characterized by a high quantity of lipid molecules. It is obvious that the Golgi apparatus regulates the molecular composition of the plasma membrane by selection of specific membrane components. The drastic membrane transformation during the formation of slime-transporting vesicles in the Golgi apparatus causes the enrichment of dictyosome membranes by IMPs, whereas the plasma membrane probably is enriched by lipids. The structural differentiations in both the plasma membrane and in Golgi membranes are discussed in relation to membrane transformation, membrane flow, membrane fusion, and recycling of membrane constituents.Abbreviations PF protoplasmic fracture face - EF exoplasmic fracture face - IMP intramembranous particle     

The Root Cap as a Test System for the Evaluation of Golgi Inhibitors: I. STRUCTURE AND DYNAMICS OF THE SECRETORY SYSTEM AND RESPONSE TO SOLVENTS

Maize (Zea mays) seedlings were grown under standard conditionsand the time for reformation of a slime droplet of standardsize was determined. The structure of the secretory cells wasexamined by light and electron microscopy. Osmium-zinc iodide(OZI) impregnation was used to provide contrast enhancementof the dictyosome forming-face cisternae and the endoplasmicreticulum (ER). The three-dimensional structure and relationshipsof these membrane systems were established from examinationof 300 nm thick sections. The presence of a marginal cisternalnetwork at the forming-face was confirmed, but no specific orientationof the ER to this face, or connection of ER to any part of thedictyosome could be demonstrated. Neither observation of reforming slime droplets, nor quantificationof secretory vesicle numbers gave any support to the suggestionthat slime formation is a phasic, or cyclic, process. Dilute solutions of solvents (1% ethanol, 1% and 0.1% DMSO)commonly used to solubilize potential inhibitors of Golgi activitywere found to delay significantly the time taken for reformationof a slime droplet of standard size. These solutions also hadslight effects on the numerical density of secretory vesicles. Cytochalasin D-induced accumulation of secretory vesicles wasused to determine rates of vesicle formation (0.39 dictyosome^–1min^–1) and hence turnover times for cisternal (6.5 min)and total dictyosome (26–39 min) membranes and the plasmamembrane (9.7 min). The volume contribution of the secretory vesicles to the slimedroplet was shown to be exceedingly small (0–1%), throwingdoubt on the value of using droplet reformation rates as anindicator of secretory activity. Key words: Maize, Root cap secretion, Dictyosome activity     

Fine structure of the wet,detached cell stigma of the orchid Dendrobium speciosum Sm.

Summary The stigmatic surface of the orchid Dendrobium speciosum is a cup containing detached cells suspended in a mainly carbohydrate mucilage. The fine structure of the detached cells and their organelles is indicative of secretory cells. The cells contain numerous mitochondria with well-developed cristae, dictyosomes containing extensive cisternae, an extensive network of rough and smooth endoplasmic reticulum and free polysomes throughout. There are many amyloplasts in the vicinity of the nucleus. Vesicles are seen arising from the dictyosomes and endoplasmic reticulum. The plasmalemma is undulating, and vesicles can be seen in its vicinity, giving the typical appearance of a granulocrine secretory system. Cetylpyridinium chloride (CPC) fixation to immobilise acidic carbohydrates detected a highly electron-opaque layer surrounding each cell and globules dispersed through the cell wall. The walls of the detached cells show irregular surface projections which are the remains of pitfields. Biochemical analysis showed that carbohydrates and arabinogalactan proteins are major components of the mucilage.     

Membranes in the spindle of Iris pollen mother cells during the second division of meiosis

Summary Spindle membrane in the second division of meiosis inIris comprises endoplasmic reticulum (ER) and small spherical vesicles (40–80 nm diameter). The vesicles are in all respects similar to those of dictyosomes and it is proposed that they originate in the Golgi system. ER on the other hand is generated from the nuclear envelope (NE). Invaginations and evaginations of the inner and outer membranes of the NE are present at interphase and they enlarge during prophase II and break to form spindle ER elements at the beginning of prometaphase II. The area enclosed by this ER continues to increase until the entire spindle is filled with irregular profiles at mid-prometaphase II. During late-prometaphase II the quantity of ER decreases and, concurrent with an increase in the number of microtubules (MTs), the cisternae align alongside MT bundles and accumulate at the poles. It is suggested that ER increases in quantity by growth of NE membranes. Furthermore, the alignment of ER along the interpolar axis and its transport polewardsprior to anaphase suggest the action of a force on each ER element.     

Golgi apparatus and slime secretion in plants: the early implications and recent models of membrane traffic

Summary The function of the Golgi apparatus in the secretion of plant slimes is reviewed. It is shown how the research on slime secretion has increased the knowledge on the structure and dynamics of dictyosomes. Current models on intradictyosomal membrane traffic-anterograde progression of complete cisternae or anterograde movement of lateral vesicles with stationary cisternae-are discussed in the light of old and new results on slime secreting plant cells.Dedicated to Hilton H. Mollenhauer on the occasion of his retirement     

Identification of secretory vesicles in homogenates of pea stem segments

In homogenates of stem sections from etiolated pea (Pisum sativum L.) seedlings, secretory vesicles can be separated from Golgi-apparatus cisternae by rate-zonal centrifugation in renografin gradients. Optically, two bands of turbidity are observed, the uppermost containing the secretory vesicles and the lower one the Golgi-apparatus cisternae. The absence of glutaraldehyde in the homogenizing medium has allowed the effective characterization of marker-enzyme activities. Golgi-apparatus cisternae have been recognized by the presence of inosine-diphosphatase and glucan-synthase I activities as well as by electron microscopy. In contrast, although secretory vesicles also bear inosine diphosphatase they do not appear to possess glucan-synthase activity. Three plasma-membrane markers, NPA-binding, glucan synthase II, and KCl,Mg²⁺-adenosine triphosphatase (pH 6.5), were not detected in secretory vesicles. Pulse-chase experiments with [³H]glucose support our designation of secretory vesicles and Golgi-cisternal fractions.Abbreviations ER endoplasmic reticulum - GSI, GSII glucan, synthase I, II, respectively - IDPase inosine diphosphatase - PM plasma membrane - SV(s) secretory vesicle(s)     

Dictyosome polarity and membrane differentiation in outer cap cells of the maize root tip

Outer rootcap cells of maize produce large numbers of secretory vesicles that ultimately fuse with the plasma membrane to discharge their product from the cell. As a result of the fusion, these vesicles contribute large quantities of membrane to the cell surface. In the present study, this phenomenon has been investigated using sections stained with phosphotungstic acid at low pH (PACP), a procedure in plant cells that specifically stains the plasma membrane. In the maize root tip, the PACP also stains the membranes of the secretory vesicles derived from Golgi apparatus to about the same density that it stains the plasma membrane. Additionally, the membranes of the secretory vesicles acquire the staining characteristic while still attached to the Golgi apparatus. The staining progresses across the dictyosome from the forming to the maturing pole, thus confirming the marked polarity of these dictyosomes. Interestingly, the PACP staining of Golgi apparatus is confined to the membranes of the secretory vesicles. It is largely absent from the central plates or peripheral tubules and provides an unambiguous example of lateral differentiation of membranes orthogonal to the major polarity axis. In the cytoplasm we could find no vesicles other than secretory vesicles bearing polysaccharide that were PACP positive. Even the occasional coated vesicle seen in the vicinity of the Golgi apparatus did not stain. Thus, if exocytotic vesicles are present in the maize root cap cell, they are formed in a manner where the PACP-staining constituent is not retained by the internalized membrane. The findings confirm dictyosome polarity in the maize root cap, provide evidence for membrane differentiation both across and at right angles to the major polarity axis, and suggest that endocytotic vesicles, if present, exclude the PACP-staining component.     

Pentachlorophenol affects mitochondria and induces formation of golgi apparatus-endoplasmic reticulum hybrids in tobacco pollen tubes

Summary The toxic effect of pentachlorophenol (PCP) on the growth and ultrastructure of tobacco pollen tubes was tested using a semivivo technique of tube culture. In this technique the pollen tubes were allowed to grow in the pistilin situ for 24 hr before they protruded from the cut end of the style and came into contact with the medium containing PCP. The inhibitory effect of different PCP concentrations was determined by measuring the length of tube bundles. The intracellular action of PCP was analysed by electron microscopy. This biocide caused four obvious alterations in the pollen tube ultrastructure: (1) swelling of the mitochondrial saccules; (2) enlargement of the dictyosomes by the increase of the cisternal diameter and the number of cisternae per stack; (3) formation of cup-shaped Golgi apparatus-endoplasmic reticulum hybrid structures (GER hybrids) showing continuities of ER and Golgi cisternae; (4) formation of stacked and/or concentric arrangements of rough ER cisternae. It is suggested that swelling of saccules was directly due to the uncoupling of oxidative phosphorylation whereas the changes of the endomembrane system were caused by energy depletion due to the inhibition of ATP synthesis. These changes are consistant with dynamic concepts of dictyosome and ER function when membrane formation exceeds membrane use in the production of secretory and transition vesicles. Thus, the enlargement of the dictyosomes and the formation of GER hybrids are thought to result from inhibition of budding of vesicles from the Golgi apparatus or from both the ER and the Golgi apparatus, respectively.     

Actin-dependent deposition of putative endosomes and endoplasmic reticulum during early stages of wound healing in characean internodal cells

We investigated the behaviour of organelles stained with FM1-43 (putative endosomes) and/or LysoTracker Red (LTred; acidic compartments) and of the endoplasmic reticulum (ER) during healing of puncture and UV-induced wounds in internodal cells of Nitella flexilis and Chara corallina. Immediately after puncture, wounds were passively sealed with a plug of solid vacuolar inclusions, onto which a bipartite wound wall was actively deposited. The outer, callose-containing amorphous layer consisted of remnants of FM1-43- and LTred-labelled organelles, ER cisternae and polysaccharide-containing secretory vesicles, which became deposited in the absence of membrane retrieval (compound exocytosis). During formation of the inner cellulosic layer, exocytosis of secretory vesicles with the newly formed plasma membrane is coupled to endocytosis via coated vesicles. Migration of FM1-43- and LTred-stained organelles, ER and secretory vesicles towards the cell cortex and deposition of a bipartite wound wall could also be induced by spot-like irradiation with ultraviolet light. Cytochalasin D reversibly inhibited the accumulation and deposition of organelles. Our study indicates that active actin-dependent deposition of putative recycling endosomes is required for wound healing (plasma membrane repair) and supports the hypothesis that deposition of ER cisternae helps to restore wounding-disturbed Ca(2+) metabolism.     

Structural organization of ultrarapidly frozen barley aleurone cells actively involved in protein secretion

The ultrastructural organization of actively secreting barley (Hordeum vulgare L. cv. Himalaya) aleurone cells was examined using ultrarapid-freezing (<-10 000°C s^-1) followed by freeze-fracture and freeze-substitution. Our analysis indicates that much of the evidence supporting a direct pathway from the endoplasmic reticulum (ER) to the plasma membrane (i.e. bypassing the Golgi apparatus) for the secretion of -amylase (EC 3.2.1.1) may not be valid. Cryofixed ER cisternae show no sign of vesiculation during active -amylase secretion in gibberellic acid (GA₃)-treated cells. At the same time, Golgi complexes are abundant and numerous small vesicles are associated with the edges of the cisternae. Vesicles appear to be involved in the delivery of secretory products to the plasma membrane since depressions containing excess membrane material appear there. Treatment with GA₃ also induces changes in the composition of Golgi membranes; most notably, the density of intramembrane particles increases from 2700 m^-2 to 3800 m^-2 because of an increase of particles in the 3–8.5-nm size range. A slight decrease in 9–11-nm particles also occurs. These changes in membrane structure appear to occur as the Golgi complex becomes committed to the processing and packaging of secretory proteins. We suggest that secretory proteins in this tissue are synthesized in the abundant rough ER, packaged in the Golgi apparatus, and transported to the plasma membrane via Golgi-derived secretory vesicles. Mobilization of reserves is also accompanied by dynamic membrane events. Our micrographs show that the surface monolayer of the lipid bodies fuses with the outer leaflet of the bilayer of protein-body membranes during the mobilization of lipid reserves. Following the breakdown of the protein reserves, the protein bodies assume a variety of configurations.Abbreviations ER endoplasmic reticulum - GA₃ gibberellic acid - P protoplasmic - E exoplasmic     

The Root Cap as a Test System for the Evaluation of Golgi Inhibitors: II. EFFECT OF POTENTIAL INHIBITORS ON SLIME DROPLET FORMATION AND STRUCTURE OF THE SECRETORY SYSTEM

The effects of four potential inhibitors of dictyosome activityon the root cap secretory system were monitored by visual estimationof slime droplet reformation rates and by quantitative microscopyof the secretory cells. Only monensin was found to affect bothdroplet reformation and cell structure. While some of our structuralobservations on the effects of this drug, such as swelling ofvesicles and dictyosome cisternae, agreed with those made previously,others did not. We are able to confirm a real increase in vesiclenumber, in addition to the numerical increase in vesicle profilesthat follows from an increase in vesicle size. Formation ofcup-shaped dictyosomes and separation of cisternae were foundto be just as prevalent in the normal and in the solvent controls,especially when fixed with permanganate. Scopoletin, tunicamycin and 2, 6-dichlorobenzonitrile all affecteddroplet formation but had no significant effect on cell structure.It is suggested that these chemicals were affecting water flowinto the slime droplet, rather than directly inhibiting Golgi-activityor release of carbohydrates by the secretory vesicles. The problems of using the root cap system for the identificationof specific Golgi inhibitors are discussed. Key words: Maize, Root cap secretion, Golgi activity inhibitors, Dichlorobenzonitrile, Monensin, Scopoletin, Tunicamycin     

Structure,differentiation, and multiplication of Golgi apparatus in fungal hyphae

Summary Golgi apparatus in subapical regions of hyphae consist of paranuclear dictyosomes with 4–5 cisternae each. Transverse and tangential sections provide ultrastructural evidence for a three-dimensional architectural model of the Golgi apparatus and a stepwise mechanism for dictyosome multiplication. The dictyosomes are polarized, with progressive morphological and developmental differentiation of cisternae from the cis to the trans pole. Small membrane blebs and transition vesicles provide developmental continuity between the nuclear envelope and the adjacent dictyosome cisterna at the cis face. Cisternae are formed as fenestrated plates with extended tubular peripheries. The morphology of each cisterna depends on its position in the stack, consistent with a developmental gradient of progressive maturation and turnover of cisternae. Mature cisternae at the trans face are dissociated to produce spheroid and tubular vesicles. Evidence in support of a schematic sequence for increasing the numbers of dictyosomes comes from images of distinctive and unusual forms of Golgi apparatus in hyphal regions where nuclei and dictyosomes multiply, as follows: (a) The area of the nuclear envelope exhibiting forming-face activity next to a dictyosome expands, which in turn increases the size of cisternae subsequently assembled at the cis face of the dictyosome. (b) As subsequent large cisternae are formed and mature as they pass through the dictyosome, an entire dictyosome about twice normal size is built up. The number of cisternae per stack remains the same because of continuing turnover and loss of cisternae at the trans face, (c) This enlarged dictyosome becomes separated into two by a small region of the nuclear envelope next to the cis face that acquires polyribosomes and no longer generates transition vesicles, (d) As a consequence, assembly of new dictyosomes is physically separated into two adjacent regions, (e) As.the enlarged cisternae are lost to vesiculation at the trans pole, they are replaced by two separate stacks of cisternae with typical normal diameters, (f) The net result is two adjacent dictyosomes where one existed previously. Dictyosome multiplication is thus accomplished as part of the normal developmental turnover of cisternae, without interrupting the functioning of the Golgi apparatus as it continues to produce new secretory vesicles from mature cisternae at the trans face. Coordination of Golgi apparatus multiplication with nuclear division ensures that each daughter nucleus receives a complement of paranuclear dictyosomes.