Content uploaded by José Antonio González-Reyes
Author content
All content in this area was uploaded by José Antonio González-Reyes on Jun 18, 2016
Content may be subject to copyright.
A preview of the PDF is not available
Changes of dictyosome ultrastructure during the cell cycle in onion root meristematic cells. A morphometric and stereological study
Abstract and Figures
Dictyosome ultrastructure changes during the cell cycle in onion root meristematic cells. Changes were mainly related to cisternae and intercisternal spaces morphology. Taking each dictyosome to be composed of three different regions (CIS, medial, and TRANS), several quantitative changes were detected in some of the compartments. Many of the planimetric parameters evaluated showed higher values in medical cisternae, while CIS and TRANS remained nearly constant. We have also found an increased activity of dictyosomes, as indicated by increase in the volume fraction of vesicular attached structures. This reaches maximum values at ana-telophase in coincidence with the onset and progression of cytokinesis.
Figures - uploaded by José Antonio González-Reyes
Author content
All figure content in this area was uploaded by José Antonio González-Reyes
Content may be subject to copyright.
Two different types of Golgi vesicles involved in wall formation can be visualized during lobe growth inMicrasterias when using high-pressure freeze fixation followed by freeze substitution. One type that corresponds to the dark vesicles (DV) described in literature seems to arise by a developmental process occurring at the Golgi bodies with the single vesicles being forwarded from one cisterna to the next. The other vesicle type appears to be produced at thetrans Golgi network without any visible precursors at the Golgi cisternae. A third type of vesicle, produced by median andtrans cisternae, contains slime; these are considerably larger than those previously mentioned and they do not participate in wall formation. The distribution of the two types of cell wall vesicles at the cell periphery and their fusion with the plasma membrane are shown for the first time, since chemical fixation is too slow to preserve a sufficient number of vesicles in the cortical cytoplasm. The results indicate that fusions of both types of vesicles with the plasma membrane are possible all over the entire surface of the growing half cell. However, the DVs are much more concentrated at the growing lobes, where they form queues several vesicles deep behind zones on the plasma membrane thought to be specific fusion sites. The structural observations reveal that the regions of enhanced vesicle fusion correspond in general to the sites of calcium accumulation determined in earlier studies. By virtue of the absence of the DVs in the region of cell wall indentations the second type of wall forming vesicle appears prominent; they too fuse with the plasma membrane and discharge their contents to the wall.
In onion root meristems, the number of dictyosomes per cell shows a kinetics of growth strongly related to the cell cycle. During the interphase of steady-state proliferative cells, the volume density and numerical density of the Golgi apparatus decrease to reach minimum values in late-interphase cells, characterized by their greatest length. This pattern is also found in the total volume occupied by Golgi apparatus. Once in mitosis, the above-mentioned parameters begin to increase reaching maximum mean values in telophase. After the experimental uncoupling of chromosome and growth cycles by presynchronization with hydroxyurea, we found a similar behaviour pattern in the Golgi apparatus: decreasing values during interphase and a triggering of Golgi-apparatus growth in prophase independently of the bigger cell sizes reached in mitosis as an effect of pretreatment with hydroxyurea. These results indicate a cyclic kinetics of this subcellular component in higher-plant meristems, coupled with early mitotic events.
Using monoclonal antibodies and electron microscopy, we have localized N-acetylglucosamine transferase I within the Golgi apparatus. This enzyme initiates the conversion of asparagine-linked oligosaccharides to the complex type. We have found that the enzyme is concentrated in the central (or medial ) cisternae of the Golgi stack. Cisternae at the cis and trans ends of the Golgi complex appear to lack this protein. These experiments establish a function for the medial portion of the Golgi and imply that the Golgi is partitioned into at least three biochemically and morphologically distinct cisternal compartments.
Epidermal cells of maize roots were studied to determine the distribution of Golgi apparatus-derived secretory vesicles in various stages of cell division. The following conclusions were reached: 1) The pattern of Golgi apparatus secretion varies with the cell cycle. 2) Large numbers of secretory vesicles are incorporated into the cell plate. 3) Secretory vesicles from the Golgi apparatus are incorporated primarily in walls undergoing expansion. 4) Secretory vesicles are smaller during mitosis and the first part of cytokinesis than they are during interphase. 5) Secretory vesicles account for at least 12-23% of cell-plate plasma membrane and an estimated 25% of cell-plate volume.
The slime-producing dictyosomes in the placentary papillae of Aptenia cordifolia (L.f.) Schwant. show some structural peculiarities: (1) the number of their cisternae is conspicuously large in comparison with those of other cormophyta; (2) the spaces between the extremely flat vesicle-producing cisternae of the maturing face are considerably higher than those between the other cisternae; (3) the intercisternal elements show a pearl-string form rather than a fibrillar form-especially on tangential sections. Based on personal and on cited findings, the following hypothesis is developed: The intercisternal elements effect the compression of the central region of the secretory cisternae. This causes the production of vesicles to remain restricted to the marginal region of the cisternae, even if these cisternae contain hypertonic or soaking substances.
Generative cells from mature pollen grains of Haemanthus katherinae Baker (African blood lily) were isolated by means of a simple squash method and observed by differential interference contrast (DIC), fluorescence and polarizing microscopy. The isolated cells appeared structurally similar to those observed in vivo and gave no evidence of a typical cell wall. Their viability was confirmed using the fluorescein diacetate test. The cell shape changed rapidly as the sucrose concentration of the medium was varied. The squash method of isolating generative cells holds promise for the direct and experimental study of these cells, especially in the living state.
The association of endoplasmic reticulum (ER) with the developing cell plate has been analyzed in lettuce roots fixed in glutaraldehyde and post-fixed in a mixture of osmium tetroxide-potassium ferricyanide (OsFeCN). Electron microscopic observations show that elements of ER, which are selectively stained by the OsFeCN reagent, become loosely associated with aggregating dictyosome vesicles at the onset of plate formation. Subsequently the ER, in a tubular reticulate network, surrounds the vesicular aggregates creating a three dimensional membrane matrix. It is suggested that the ER (1) provides a structural framework that holds the vesicles in position and directs their fusion within the plane of the plate and/or (2) regulates the local release of calcium ions required for vesicle fusion.
OsFeCN post-fixation also provides new information about the cell plate vesicles themselves. The results demonstrate that vesicles derived from dictyosomes undergo an abrupt increase in staining as they fuse at the plate.
Finally the ER associated with developing and mature plasmodesmata has been examined. Electron micrographs reveal that the OsFeCN staining, seen traversing the cell plate in early stages, later becomes restricted from that portion of the ER extending through the plasmodesmatal canal. These structural observations support the idea that during formation of the plasmodesma a tubular element of ER is tightly furled upon itself and that its inner leaflet is compressed into a rod. The ER cisternal space appears occluded and thus it is argued that intercellular transport occurs through the cytoplasmic annulus of the plasmodesmata.