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     

Pigment biogenesis in freshwater shrimp ventral nerve chord chromatophores

Summary The possible biogenesis of two pigment granule types present in the monochromatic, brown chromatosomes enveloping the ventral nerve chord of the freshwater palaemonid shrimps Macrobrachium acanthurus, M. heterochirus and M. olfersii is examined by transmission electron microscopy in thin section and freeze fracture replicas. Prominent, membrane limited granules are suggested to have their origin in a complex, juxtanuclear, smooth endoplasmic reticulum labyrinth, continuous with the nuclear envelope. Amembranous, lipocarotenoid granules possibly derive from the external surface of the smooth endoplasmic reticulum. Nuclear envelope and SER membranes contain numerous 11 nm diameter intramembranous particles while pigment granule membranes exhibit fewer particles. A dictyosomal origin for the lipocarotenoid granules is discounted. Granulogenesis is suggested to be a continuous process in crustacean chromatophores.     

Effects of Ga3 and Ca2+ on barley aleurone protoplasts: a freeze-fracture study

Summary Freeze-fracture electron microscopy was used to study changes in the endomembrane system of barley (Hordeum vulgare L. cv. Himalaya) aleurone protoplasts. Protoplasts were used for this study because their response to calcium and the plant hormone gibberellic acid (Ga₃) can be monitored prior to rapid freezing of cells for electron microscopy. Protoplasts incubated in Ga₃ plus Ca²⁺ secrete elevated levels of a-amylase relative to cells incubated in Ga₃ or Ca²⁺ alone. The endoplasmic reticulum (ER) and Golgi apparatus of protoplasts incubated in Ga₃ plus Ca²⁺ undergo changes that are well correlated with the synthesis and secretion of a-amylase. The ER, which appears as short, single sheets of membrane in Ca²⁺-and Ga₃-treated protoplasts, exists as a series of long fenestrated stacks of membranes following incubation in Ga₃ plus Ca²⁺. The Golgi apparatus is also more highly developed in protoplasts treated with Ga₃ plus Ca²⁺. This organelle is larger and has more vesicles associated with its periphery in protoplasts that actively secrete a-amylase. Evidence that the Golgi apparatus participates in a-amylase secretion is also provided by experiments with the ionophore monensin, which causes pronounced swelling of Golgi cisternae and inhibits the secretion of a-amylase. We interpret these observations as showing that the ER and Golgi apparatus of barley aleurone participate in the intracellular transport and secretion of a-amylase. The plasmalemma (PF face) of barley aleurone protoplasts shows a high density of intramembranous particles (IMPs) which, in general, are evenly distributed. Occasionally, ordered arrays of IMPs are observed, possibly resulting fro m osmotic stress. after 48 hours the plasmalemma of some Ga₃-treated protoplasts show particle-free areas considered to be indications of senescence.abbreviations ER endoplasmic reticulum - Ga₃ gibberellic acid - IEF isoelectric focusing - IMP intramembranous particle - PF protoplasmic fracture - PL plasmalemma     

Effects of amphotericin B and its methyl ester on plasma membranes of Candida albicans and erythrocytes as examined by freeze-fracture electron microscopy

Freeze-fracture electron microscopy of the plasma membranes of Candida albicans yeast cells and red blood cells treated with amphotericin methyl ester and amphotericin B showed that amphotericin B (50 micrograms ml-1) caused extreme aggregation of intramembranous particles on the protoplasmic fracture face of the C. albicans membrane, and a marked reduction of the density of intramembranous particles. On the other hand, the rearrangement of intramembranous particles induced by amphotericin methyl ester (50 micrograms ml-1) produced elevations of the particle-free membrane domains toward the outside of the cells, so that the particles were aggregated in linear furrows surrounding these elevations on the protoplasmic fracture face, and the corresponding ridges on the exoplasmic fracture face. The density of intramembranous particles was greatly reduced on the protoplasmic fracture face. Both polyenes produced only small changes in the erythrocyte membranes at the same concentration. These results suggest that amphotericin methyl ester affects the ergosterol-containing membranes more than amphotericin B, and that ergosterol has a higher sensitivity for these two polyene antibiotics than cholesterol.     

An electron-microscope and freeze-fracture study of the egg cortex of Brachydanio rerio

Summary We have examined the cortex of the teleost (Brachydanio rerio) egg before and during exocytosis of cortical granules by scanning, transmission, and freeze-fracture electron microscopy. In the unactivated egg, the P-face of the plasma membrane exhibits a random distribution of intramembranous particles, showing a density of 959/m² and an average diameter of 8 nm. Particles over P- and E-faces of the membranes of cortical granules are substantially larger and display a significantly lower density. An anastomosing cortical endoplasmic reticulum forms close associations with both the plasma membrane of the egg and the membranes of cortical granules. Exocytosis begins with cortical granules pushing up beneath the plasma membrane to form domeshaped swellings, coupled with an apparent clearing of particles from the site of contact between the apposed membranes. A depression in the particle-free plasma membrane appears to mark sites of fusion and pore formation between cortical granules and plasma membranes. Profiles of exocytotic vesicles undergo a predictable sequence of morphological change, but maintain their identity in the egg surface during this transformation. Coated vesicles form at sites of cortical granule breakdown. Differences in particle density between cortical granules and egg plasma membranes persist during transformation of the exocytotic profiles. This suggests that constituents of the 2 membrane domains remain segregated and do not intermix rapidly, lending support to the view that the process of membrane retrieval is selective (i.e., cortical granule membrane is removed).     

Ultrastructure of the Mesocarp of Mature Avocado Fruit and Changes Associated with Ripening

The mesocarp tissue of ripening avocado fruits was studied byfreeze fracture, thin section and scanning electron microscopy.Carbon dioxide and ethylene production by individual fruit weremonitored, and samples were analysed at several stages of theripening process. The tissue is composed primarily of large, isodiametric, lipid-containingparenchyma cells. At maturity these cells contain the normalcomplement of cell organelles, and all membranes appear intact.When ripening begins, several changes in the ultrastructureoccur. The most obvious changes are a loosening and eventualbreakdown of the cell wall, and swelling and vesiculation ofthe rough endoplasmic reticulum. In freeze fracture replicasa significant increase in the number of intramembranous particlesin the EF face of the plasmamembrane was observed at the climactericpeak. In post-climacteric, soft fruit the particle density of theEF face of the plasmamembrane decreased to the density observedin the membrane of pre-climacteric cells. All of the organellesand membranes appear whole and intact whether examined by thinsection, freeze fracture or scanning electron microscopy. However,the cell walls in post-climacteric fruit have almost completelydisappeared. These results indicated that the ripening process per se inavocados does not involve a complete loss of compartmentationnor a breakdown of organelle and membrane integrity. It may,however, lead to these or similar senescence changes as a resultof the loss of the cell walls. The variations in particle densityof the plasmamembrane during ripening may reflect one or moreof several structural, compositional, or functional membranephenomena, and this aspect of ripening warrants further study. Persea americana Mill., avocado pear, freeze fracture, fruit ripening, scanning electron microscopy, senescence, ultrastructure     

Yolk organelles and their membranes during vitellogenesis ofXenopus oocytes

Summary The yolk platelets ofXenopus laevis have been studied by thin-section and freeze-fracture electron microscopy to characterize the boundary membrane during yolk formation. Throughout vitellogenesis, large yolk platelets are in close contact with smaller nascent yolk organelles. Two types of primordial yolk platelets (I and II) have been discriminated. After membrane fusion these precursors can be completely incorporated into the main body of existing platelets, numerous yolk crystals then merge and form one uniformly stratified core. Lipid droplets are tightly attached to the membrane at all developmental stages of yolk platelets. A direct connection of endoplasmic reticulum to the membranes of yolk platelets was not observed. On freezeetching replicas, yolk-platelet membranes present fracture faces with intramembranous particles (IMP) of various sizes and a heterogeneous distribution of approximately 200–600 IMP/μm² at the E face, and 1200–2100 IMP/μm² at the P face. Again, this presentation of the membrane exhibits neither anastomoses to the endoplasmic reticulum, nor caveolae that exclude the uptake of yolk-containing vesicles into these yolk organelles. Proteinaceous yolk platelets tend to fracture along their periphery through the superficial layers.     

Quantitative analysis of intramembranous particles in the membranous system of rat liver cells at different stages of development and aging

The density of intramembranous protein particles was studied by freeze-fracture. Particle density on the fracture faces of the plasmalemma and the rough endoplasmic reticulum (RER), as well as the outer and inner membranes of the nucleus and the mitochondria in rat hepatocytes were quantified. Comparison among different age groups sampled days postcoitum (dpc), days postpartum (dpp), and months postpartum (mpp) shows age-related changes in particle density in each membrane system. With the exception of the RER, particle densities increased after the 16th dpc, reached a maximum at birth, and then decreased with increasing age. Simultaneously, the number nuclear pores shows a positive correlation with the particle density of the nuclear membranes. The particle density on the membranes of the RER shows a maximum on the 16th dpc, and on the 6th dpp. Thereafter, the density of the RER decreases slightly. In all membrane systems, the density of the particles on the external fracture faces is more variable than the density of the particles on the protoplasmic fracture faces.     

Freeze fracture evidence for lateral phase separations in the plasmalemma of chilling-injured avocado fruit

Summary Unripe avocado fruit (Persea americana Mill. cv Hass) were held at 6 °C either in air or in an atmosphere with 100 PPM ethylene and were assessed for chilling injury after one and two weeks. Injury did not occur in any fruit after one week. After two weeks, the fruit in air were still uninjured, but the fruit subjected to ethylene exhibited chilling injury. When the uninjured fruit (both air-treated for one and two weeks and ethylene-treated for one week) were allowed to warm to room temperature before freezing for freeze fracture electron microscopy, replicas revealed membranes with a randomly dispersed pattern of intramembranous particles (IMPs). However, when these uninjured fruit were frozen for freeze fracture without warming, particle-free domains were visible in the plasmalemma. The membranes of the ethylene-treated, chilling-injured (2 weeks) fruit, on the other hand, contained particle-depleted regions in the plasmalemma of fruit frozen not only from 6 °C but also in those allowed to warm to room temperature before freezing for freeze fracture. These particle depleted microdomains were not seen in fruit kept continuously at room temperature (20 °C), even in the presence of high levels of endogenous ethylene which is produced during normal ripening. We suggest these particle-depleted microdomains formed in the fruit frozen for freeze fracture from low temperatures and in the chilling-injured fruit to be due to lateral phase separations of the membrane components, possibly due to an increase in the viscosity of some membrane lipids, leading to the formation of microdomains of gel phase lipid in the plane of the membrane. These phase separations appear to be initially reversible by raising the temperature, however, this reversibility is apparently lost after injury has occurred. With regard to the cause of chilling injury in avocados, we suggest that some secondary effect is involved due to the long term presence of gel phase lipids in the membrane.     

Regular arrays of intramembranous particles in the plasmalemma of guard cell and mesophyll cell protoplasts of Vicia faba

Freeze fractures of the plasmalemma membranes of guard-cell and mesophyll protoplasts of Vicia faba demonstrate that the inner monolayer of the plasmalemma is compartmentalized into areas with distinct, highly organized structures. Between areas of intramembranous particles dispersed randomly on a relatively smooth fracture face, membrane domains showing an extremely regular planar, hexagonal array of particles are interspersed. The dimensions of these hexagonal lattices are about 0.5 m in diameter, the center-to-center spacing is about 22 nm, and the particle size is about 9 nm. The particle in the hexagonal arrays are accompanied by complementary pits in the opposite monolayer fracture of the plasmalemma membrane.The freeze-fracture preparation was performed by using an improved Leybold Bioetch device which provides a sufficiently high cooling rate and allows the omission of cryoprotectants, like glycerol.Presented by H. Schnabl on the Workshop on Plant Membrane Transport, Toronto, Canada, July 1979     

A freeze-etch study of the effects of extracellular freezing on cellular membranes of wheat

Seedlings of Triticum aestivum L. cv. Lennox were grown in different environments to obtain different hardiness. Pieces of laminae and leaf bases were slowly cooled to sub-zero temperatures and the damage caused was assessed by an ion-leakage method. Comparable pieces of tissue were slowly cooled to temperatures between 2° and-14°C and were then freeze-fixed and freeze-etched. Membranes generally retained their lamellar structures indicated by the abundance of typical membrane fracture faces in all treatments, and some membrane fracture faces had patches which lacked the usual scattering of intramembranous particles (IMP). These IMP-free areas were present in the plasma membrane of tissues given a damaging freezing treatment, but were absent from the plasma membrane of room-temperature controls, of supercooled tissues, and of tissues given a non-damaging freezing treatment. The frequency of IMP-free areas and the proportion of the plasma membrane affected increased with increasing damage. In the most damaged tissue (79% damage; leaf bases exposed to-8°C), 20% of the plasma membrane was IMP-free. The frequencies of IMP at a distance from the IMP-free areas were unaffected by freezing treatments. There was a patchy distribution of IMP in other membranes (nuclear envelope, tonoplast, thylakoids, chloroplast envelope), but only in the nuclear envelope did it appear possible that their occurrence coincided with damage. The IMP-free areas of several membranes were sometimes associated together in stacks. Such membranes lay both to the outside and inside of the plasma membrane, indicating that at least some of the adjacent membrane fragments arose as a result of membrane reorganization induced by the damaging treatment. Occasional views of folded IMP-free plasma membrane tended to confirm this conclusion. The following hypothesis is advanced to explain the damage induced by extracellular freezing. Areas of plasma membrane become free of IMP, probably as a result of the freezing-induced cellular dehydration. The lipids in these IMP-free patches may be in the fluid rather than the gel phase. The formation of these IMP-free patches, especially in the plasma membrane, initiates or involves proliferation and possibly fusion of membranes, and during or following this process, the cells become leaky.Abbreviations EF exoplasmatic fracture face - IMP intramembranous particles - PF protoplasmatic fracture face     

Immunogold-localization and synthesis of an oil-body membrane protein in developing soybean seeds

The synthesis of a major oil-body membrane brotein was studied in maturing soybean (Glycine max (L.) Merr.) cotyledons. The membrane contained four abundant proteins with apparent molecular mass (M_r) of 34000, 24000, 18000 and 17000. The M_r=24000 protein (mP 24) was selected for more detailed analysis. The protein was purified to apparent homogeneity by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and isolated from the gel by electroelution or chemical hydrolysis of gel crosslinks. It was then used to elicit rabbit antibodies which were judged to be specific when assayed by SDS-PAGE-immunoblot procedures. The mP 24 was localized in immature soybean cotyledon cells by indirect immunogold procedures on thin sections of Lowicryl- and LR-White-embedded tissue. Indirect labeling with the primary antiserum followed by colloidal gold-protein A showed specific labeling of the oil-body membrane and an absence of label on the other subcellular organelles including the endoplasmic reticulum (ER). Parallel tissue samples were studied by conventional transmission electron microscopy. Although segments of the ER were observed to be closely juxtaposed to the oil bodies, continuity between the two organelles was not observed. The synthesis of mP 24 was studied by in-vitro translation and in-vivo labeling with [³H]leucine followed by indirect immunoaffinity isolation of the labeled products. The SDS-PAGE fluorography results indicated that the primary translation product and the in-vivo synthesized protein have the same M_r, and this is also the same M_r as the protein in the mature membrane.Abbreviations and symbols DATD N N'-diallyltartardiamide - EM electron microscopy/scopic - ER endoplasmic reticulum - IgG immunoglobulin G - M_r apparent molecular mass - PBS phosphate-buffered saline - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis - TBS Trisbuffered saline     

Membranes and organelles of dehydratedSelaginella andTortula retain their normal configuration and structural integrity

Summary Dry (7–10% water content) leaves of the spikemossSelaginella lepidophylla (resurrection plant) and of the desiccationtolerant moss,Tortula ruralis were examined by freeze fracture electron microscopy. As has been described for dry seeds, the cells of these dehydrated leaves were shrunken, with highly convoluted walls and membranes. The membranes of all samples had a lipid bilayer organization with dispersed intramembranous particles (IMPs). Lipid droplets were very closely associated with the plasmamembrane. Chloroplasts were surrounded by a double membrane envelope and contained well-organized grana. Mitochondria were irregular in outline, and endoplasmic reticulum and cytoplasmic vesicles were present.Abbreviations ABA abscisic acid - EF exoplasmic fracture - FTIR Fourier transform infrared analysis - H_II hexagonal II - IMPs intramembranous particles - MGDG monogalactosyl diacylglycerol - NMR nuclear magnetic resonance - PE phosphatidylethanolamine - PF protoplasmic fracture - PS I photosystem I - PS II photosystem II     

Fine structure of Blastocrithidia culicis as seen in thin sections and freeze-fracture replicas

The fine structure of epimastigotes of Blastocrithidia culicis was studied by transmission electron microscopy of thin sections and freeze-fracture replicas. This parasite presents a well developed endoplasmic reticulum and Golgi complex systems. Differences in the density and organization of the intramembranous particles were observed between the membranes which enclose the cell body and the flagellum. Ridge-like elevations, visualized in freeze-fracture replicas, were observed in sites where the mitochondrial branches touched the plasma membrane. A special array of membrane particles was observed on both faces of the flagellar and the cell body membranes at the region where the flagellum adheres to the cell body. It appeared as strands made of two rows of membrane particles. Filipin-treated cells were used for the localization of membrane sterols in freeze-fracture replicas. The number of filipin-sterol complexes varied from cell to cell. In some cells, rows of filipin-sterol complexes were seen. No complexes were observed in the region of the attachment of the flagellum to the cell body.     

Micromorphological Aspects of the Development of Rubella Virus in BHK-21 Cells

Sequential effects of rubella virus infection in BHK-21 cells were studied by electron microscopy of thin sections of control and infected cells, 2 to 7 days after infection. Vacuolization of cytoplasm in Golgi areas apparently preceded budding of virions from vacuole membranes and involvement of the endoplasmic reticulum. Newly formed endoplasmic reticulum cisternae encircled and segregated virionforming vacuoles together with other cellular elements. Large vacuolar complexes with numerous virus particles developed, and virus release from these areas occurred with disruption at the cell periphery. The viral particles, with a mean diameter of about 56 nm, consisted of an electron-dense core surrounded by a less dense capsid, enveloped by a typical unit membrane derived from the vacuole membrane.     

Freeze-fracture of membrane fusions during exocytosis in pancreatic B- cells

To examine the freeze-fracture appearance of membrane alterations at sites of exocytosis in mammalian cells, we studied the secretory granule and plasma membrane of rat pancreatic B-cells during glucose-stimulated insulin secretion. Constant features observed were the scarcity of particles in secretory-granule P-fracture faces and the almost total clearance of intramembranous particles in P-and E fracture faces of the plasma membrane in areas of close apposition of these two membranes preceding fusion; also observed was the temporary persistence of particle-cleared regions after the fusion was completed. Our observations thus support the concept that membranes fuse at sites of closely apposed, particle-free regions and that the physiologically created clear areas found in freeze-fracture replicas of the plasma membrane are the hallmarks of incipient or recent membrane fusion.     

Formation of oleosomes (storage lipid bodies) during embryogenesis and their breakdown during seedling development in cotyledons of Sinapis alba L.

Electron microscopic and biochemical investigations of developing embryonic mustard cotyledons provided no evidence for the widely accepted hypothesis that oleosomes of fat-storing tissues originate from the endoplasmic reticulum and are surrounded by a unit- or half-unit membrane. In contrast, it was found that the first lipid droplets appear (about 12–14 d after pollination) in the ground cytoplasm near the surface of plastids. Subsequently these nascent lipid droplets, which lack any detectable boundary structure at this stage, become encircled by a cisterna of rough endoplasmic reticulum. At the same time an osmiophilic coat of about 3 nm thickness becomes detectable at the lipid/water interface. In the cotyledon cells of germinating seedlings a centrifugally moving front of fat degradation moves from the central vacuoles(s) towards the cell periphery, leaving behind collapsed coats of oleosomes which are depleted of their lipid contents (saccules). Although saccules appear tripartite in cross section, they are structurally different from endoplasmic reticulum membranes. The oleosome coats can be isolated from oleosome preparations by extracting lipids with organic solvents. The coat material is insoluble in detergents like Triton X-100 or deoxycholate and shows a tripartite, lamellar structure (similar to collapsed saccules) under the electron microscope. Upon dissolution with dodecylsulfate, polyacrylamide gel electrophoresis revealed a polypeptide composition (9 major bands) which is qualitatively different from that of the endoplasmic reticulum membrane. Also the buoyant densities of defatted oleosome coats and defatted endoplasmic reticulum membranes are very different. It is concluded that oleosome lipids accumulate in the ground cytoplasm and are bounded by a lamellar structure originating de novo from proteinaceous elements synthesized by specific regions of the endoplasmic reticulum.Abbreviation ER endoplasmic reticulum     

Membrane contacts of the endoplasmic reticulum and their possible functions in the plant cell

Close contacts of the endoplasmic reticulum membrane and plasmalemma have been visualized inside plant cells by means of electron microscopy. The qualitative similarity of these contacts to high-permeable intercellular contacts in animals has been shown. New data confirming the hypothesis of the identity of stromules, i.e., dynamic tubular protuberances of the plastid membrane of the plant cell, and tubular elements of the endoplasmic reticulum have been presented. New possible functions of the contacts of the endoplasmic reticulum membrane with other membranes inside the cell have been discussed on the basis of this hypothesis.     

The Fine Structure of Conidia of Botryodiplodia ricinicola with Observations on Effects of Chilling

Conidia of Botryodiplodia ricinicola (Saccardo) Petrak havebeen studied, principally by freeze-etch electron microscopy.Freshly harvested conidia have a thin scaly surface layer, freeof rodlets, which covers an otherwise homogeneous-looking wallwhich is continuous with the single centrally-perforate septum.The contours of the plasmalemma are usually smooth. Nuclei andsmall vacuoles are numerous. Hydrophobic fracture faces of theplasmalemma, tonoplasts and nuclear membranes variously revealintra-membrane particles or corresponding depressions or both.Lipid inclusions are small and numerous. Compact orderly stacksof membranes are present, sometimes one in each locule of theconidium. Conidia of a strain insensitive to chilling were seento differ only in respect of the distribution of intra-membraneparticles on fracture faces of tonoplasts. Chilled and chilled-and-soakedconidia of the wild type showed fine-structural differencesfrom untreated conidia, most obviously in respect of the greatersize of some of the lipid inclusions, but also in respect offeatures of the plasmalemma which after chilling contained plasmalemmasomesand, after subsequent immersion for 15 min, showed annular depressions.Also, intra-membrane particles in some membrane systems showedaltered distribution between the two hydrophobic fracture faces.It is concluded that cell lipids and cytoplasmic membrane systemsmay be involved in the previously demonstrated chilling sensitivityof conidia of this species. Botryodiplodia ricinicola, conidia, ultrastructure, chilling effects     

Structure of amyloplasts and endoplasmic reticulum in the root caps of Lepidium sativum and Zea mays observed after selective membrane staining and by high-voltage electron microscopy

The structure of plastids in the root cap of cress and maize was studied by low- and high-voltage electron microscopy after staining their membranes with a mixture of zinc iodide and osmium tetroxide. In plastids of both species electron-opaque membranes were found in the plastid interior while membranes of lesser electron-opacity comprised the outer envelope and vesicles and cisternae underlying it. Electron-opaque tubules, often in groups attached to the inner membrane of the amyloplast envelope, were found in cress but not in maize. The internal, less-opaque membranes were often found associated with the starch grains. No specific association could be seen between amyloplasts and endoplasmic reticulum (ER); their surfaces showed no regular contact or connexion, though the amyloplasts clearly indented the underlying ER. The ER in statocytes was predominantly tubular in cress but predominantly cisternal in maize.Abbreviations ER endoplasmic reticulum - ZIO zinc iodideosmium tetroxide