William W. Thomson

The influence of membranes on the temperature-induced changes in the kinetics of some respiratory enzymes of mitochondria.

Abstract Disruption and fragmentation of mitochondria from rat liver and sweet potato tissue by sonication, hypotonic swelling, and freezing and thawing is shown to have little or no effect on the discontinuity in the Arrhenius plot exhibited by the mitochondrial respiratory enzymes of these tissues. After disruption of the mitochondrial membranes with detergent, however, the succinate oxidase system, succinate dehydrogenase and cytochrome c oxidase of both tissues each exhibits a uniform activation energy over the temperature range of 1 to 36 °. The results suggest that the temperature-induced change in activation energy of the membrane-bound enzymes is associated with a phase change in the lipid component of the membranes which induces a configurational change in the enzyme proteins.

The ultrastructure of the salt gland of Spartina foliosa.

SummaryThe salt gland in Spartina foliosa is composed of two cells, a large basal cell and a smaller, dome-shaped cap cell which is located on a neck-like protrusion of the basal cell. There is no cuticular layer separating the salt gland from the mesophyll tissue. The basal cell has dense cytoplasm which contains numerous mitochondria, rod-like wall protuberances, and infoldings of the plasmalemma which extend into the basal cell and partition the basal cell cytoplasm. The protuberances originate on the wall between the basal and the cap cells and are isolated from the basal-cell cytoplasm by the infoldings of the plasmalemma. While the cap cell has no partitioning membrane system or wall protuberances, it resembles the basal cell by having dense cytoplasm and numerous mitochondria.The basal cell seems to be designed for efficient movement of ions toward the cap cell. The long, dead-end extracellular channels in the basal cell of Spartina appear comparable to surface specializations seen in the secreting epithelium of animal cells which carry out solute-linked water transport. The number of mitochondria and their close association with the plasmalemma extensions suggest that they have an important role in the transfer of ions through the basal cell.The accumulated ions would move into the extracellular spaces along an osomotic gradient where the accompanying passive flow of water would move the ions into the cap-cell wall and from there the solution would pass out through the pores in the cuticle.

Ultrastructural features of the salt gland of Tamarix aphylla L.

SummaryThe salt gland in Tamarix is a complex of eight cells composed of two inner, vacuolate, collecting cells and six outer, densely cytoplasmic, secretory cells. The secretory cells are completely enclosed by a cuticular layer except along part of the walls between the collecting cells and the inner secretory cell. This non-cuticularized wall region is termed the transfusion are (Ruhland, 1915) and numerous plasmodesmata connect the inner secretory cells with the collecting cells in this area. Plasmodesmata also connect the collecting cells with the adjacent mesophyll cells.There are numerous mitochondria in the secretory cells and in different glands they show wide variation in form. In some glands wall protuberances extend into the secretory cells forming a labyrinth-like structure; however, in other glands the protuberances are not extensively developed. Numerous small vacuoles are found in some glands and these generally are distributed around the periphery of the secretory cells in association with the wall protuberances. Further, an unusual structure or interfacial apparatus is located along the anticlinal walls of the inner secretory cells. The general structure of the gland including the cuticular encasement, connecting plasmodesmata, interfacial apparatus, and variations in mitochondria, vacuoles, and wall structures are discussed in relation to general glandular function.

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

SummaryDry (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.

Changes in Plastid Ultrastructure During Iron Nutrition-Mediated Chloroplast Development

SummaryWhen grown in iron-free media, the youngest leaves of healthy green sugar beet plants became completely yellow after 6 to 8 days. This chlorosis was quickly reversed by resupplying iron. A study of the ultrastructure of the iron -stressed leaves revealed apparently normal subcellular organization except for the plastids which were small and undeveloped, contained a rudimentary, disorganized grana-fretwork and clusters of vesicles in the periphery. Twelve to 16 hours after resupply of iron, aggregates of phytoferritin were observed in the stroma, and the granal fretwork underwent further development. There was an increased orientation of the membranes along the long axis of the plastids and an increase in the length of the individual grana stacks. By 48 hours, leaf chlorophyll content was about 40% of the control. At the ultrastructural level, parallel alignment of membrane orientation was complete and the grana stacks began to increase in the number of thylakoids per stack.

Ultrastructure of Oil Gland Development in the Leaf of Citrus sinensis L.

The young oil glands of Citrus sinensis L. consist of a central group of polyhedral cells encircled by layers of radially flattened cells The oil chamber forms schizogenously through a separation of the walls of the central cells, which are pushed outward and flattened with the expansion and filling of the oil chamber. With further expansion of the chamber, the surrounding cells become vacuolate and fragile, and there are indications of wall degradation suggestive of lysigeny of the cells at this stage. The extensive endoplasmic reticulum in the central cells of the young glands is suggested as the primary site of synthesis of the essential oil.

ULTRASTRUCTURAL CHANGES IN THE WALLS OF RIPENING AVOCADOS: TRANSMISSION, SCANNING, AND FREEZE FRACTURE MICROSCOPY

Avocado fruit at several documented stages of ripening was prepared for transmission, scanning, and freeze fracture electron microscopy. Changes in the ultrastructural organization of the cell wall were studied by each technique and correlated with changes in the activity of wall-hydrolytic enzymes. Initial wall breakdown apparently involves degradation of pectins in the matrix and in the middle lamella, corresponding to the reported increase in polygalacturonase activity in the tissue. In later stages of ripening, there is a loss of the organization and density of the wall striations accompanied by an increase in fruit softening. The role of cellulase, which becomes highly active during ripening of avocados and several other fruits, is still somewhat questionable. However, both thin sections and freeze fracture replicas of ripening avocados indicate a loss of fibrillar components of the wall during ripening and, therefore, indicate a possible role for cellulase in fruit softening. No correlation between localized wall degradation and the presence of plasmodesmata could be found.

Ultrastructural Study of Lipid Accumulation in Tapetal Cells of Brassica napus L. Cv. Westar during Microsporogenesis

Ultrastructural features of Brassica napus tapetal cells during microsporogenesis from early microspore development through late maturation are described Emphasis is placed on the two major lipid-containing components (plastids and lipid bodies) of the tapetal cells, particularly the little-studied lipid bodies. By the early microspore stage, the walls of the tapetal cells are mostly dissolved, and a lipoid layer has been deposited on the tapetal side of the middle lamella of the outer tangential wall between the tapetal cells and the anther wall cells An electron-dense layer of presumed sporopollenin is subsequently deposited on the tapetal side of the lipoid layer, thus forming a continuous peritapetal layer, occluding the plasmodesmata, and isolating the anther locule. A prominent feature of the young tapetal cells is an abundance of ribosomes and endoplasmic reticulum (ER); the vacuoles are small; plastids are undifferentiated; and only a few, small cytoplasmic lipid droplets are present. As maturation continues, the ER becomes associated with the developing lipid bodies; the plastids enlarge and accumulate plastoglobuli, forming elaioplasts The lipid bodies differentiate into complex structures composed of a mixture of lipid and apparent membranous components. We propose a structural model for the biogenesis of the lipid bodies. As the microspores reach maturity, the lipid bodies, plastids, and other tapetal organelles are released from lysed tapetal cells, and the remnants of these organelles are deposited on the surface of the maturing pollen, forming the tryphine.

THE VACUOLAR-TUBULAR CONTINUUM IN LIVING TRICHOMES OF CHICKPEA (CICER ARIETINUM) PROVIDES A RAPID MEANS OF SOLUTE DELIVERY FROM BASE TO TIP

SummaryA vacuolar continuum exists from base to tip in the secretory trichomes of chickpea (Cicer arietinum). This continuum is seen in living trichomes which have been labeled with Lucifer yellow CH and examined with confocal microscopy. It encompasses the large vacuole of the lower stalk cell, the vacuoles and tubules of the central stalk cell, the thin tubules of the upper stalk cell, and the tubules and vacuoles of the secretory head cells. The vacuolar-tubular system is structurally distinct within each cell, forming a gradient of large vacuoles in the lower stalk cell, thick tubules in the central stalk cell, and thin anastamozing tubules in the upper stalk cell. This membrane system appears to be continuous between trichome cells, as thin tubules emanate from plasmodesmata between stalk cells and between the upper stalk and lower head cell. In the upper stalk cell, the thin tubules of this continuum are streaming up and down the long axis of the cell at 0.67 μm/s. The larger vacuolar-tubular system in the central and lower stalk cells is also slowly moving, with apparent peristalsis occurring in the central cell. The vacuolar-tubular system of the secretory head cells is completely labeled with Lucifer yellow when the dye has only partly diffused up the long walls of the trichome, indicating that the streaming tubular system delivers solute through the stalk cells to the secretory head cells faster than diffusion through the trichome walls. In the lower head cells, tubules emanate from the plasmodesmata connecting to the upper stalk cell, and these tubules are continuous with the head cell vacuoles. In addition, another layer of thin tubules forms along the edges of the secretory head cells, at the site of exocytotic secretion. We propose that the continuous vacuolar-tubular system in these trichomes functions to rapidly deliver solute from the base of the trichome to the secretory head cells. This system provides a pathway for the transport of secretory material.

The ultrastructure of the plasmodesmata of the salt glands ofTamarix as revealed by transmission and freeze-fracture electron microscopy

SummaryNumerous plasmodesmata occur in the walls between the secretory cells ofTamarix salt glands. The plasmalemma bounds the plasmodesmata and is continuous from cell to cell. In freeze-fracture, the e-face of the plasmalemma within the plasmodesmata is virtually devoid of intramembranous particles while, in contrast, the p-face is decidedly enriched with particles. The axial components appear to be a tightly curved membrane bilayer, as judged from measurements and their appearance in freeze-fracture, and the e-face of this membrane is also devoid of particles. Observations from both thin sections and freeze-fracture replicas indicate the presence of a circular cluster of six particles around the axial component near the cytoplasmic termini of the plasmodesmata. These particles extend from the p-face of the axial component to the p-face of the plasmalemma. These observations are summarized in a model.