Kathryn Platt-Aloia

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.

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.

Ultrastructural and lipid changes associated with the aging of citrus leaves

SummaryDuring maturation and senescence of leaves of navel orange (Citrus sinensis L.), total lipids per gram of leaf steadily decline. The decline is attributable to the galactolipid and phospholipid partners, while chlorophylls, carotonoids and tocopherols increase during maturation. The phospholipid/galactolipid ratio declines steadily during maturation and senescence but the monogalactosyldiglyceride/digalactosyldiglyceride ratio remains relatively constant. The phospholipid composition remains relatively constant even though the total phospolipid declines markedly.Ultrastructural changes concomittant with changes in lipid composition include the development of an extensive internal membrane system (grana-fretwork system), and several large-plastoglobuli in the chloroplasts of mature green leaves. With the conversions of chloroplasts to chromoplasts in the senescing leave the internal membrane system is reduced and numerous, large plastoglobuli appear.

An ultrastructural study of two forms of chilling-induced injury to the rind of grapefruit (Citrus paradisi, Macfed)

Abstract The ultrastructural manifestations of storage chilling-injury and concentric ring stipple indicate that the two types of injury are similar in some respects and different in others. The low temperature storage-induced injury involves the epidermal cells and several layers of epicarpal cells below. It is manifested as an increase in lipid material in the cytoplasm and vacuole and in eventual degradation and collapse of the cytoplasm. The field injury, concentric ring stipple, apparently involves primarily the epidermal cells which become extremely electron dense. The epicarp cells of the injured region do not show extensive damage but do exhibit increased lipid accumulation. This accumulation of lipid is a phenomenon common to both types of injury and may be indicative of an altered metabolism due to the low temperatures. The apparent loss of organization and compartmentation in the chilling injury is also suggestive of membrane degradation. The storage chilling-injury occurs after exposure to low temperatures for several weeks, whereas the concentric ring stipple is evidently induced by an exposure in the field for only a few hours. The ultrastructural differences observed in these two injuries, therefore, may be due to the different time- and stress- factors involved.

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

SummaryUnripe 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.

ULTRASTRUCTURAL LOCALIZATION OF LUCIFER YELLOW AND ENDOCYTOSIS IN PLANT CELLS

SummaryThe fluorescent dye Lucifer Yellow CH (LYCH) was localized at the ultrastructural level with a precipitation method using barium chloride. Applying this technique, endocytosis of LYCH was examined in the nutrient absorptive trichomes of a carnivorous bromeliad. After a two hour incubation, the electron dense reaction product was localized in the membrane compartments of the endocytotic system. These structures included coated regions of the plasma membrane, coated and smooth vesicles, dictyosomes, partially coated reticulum, and smooth endoplasmic reticulum. This procedure demonstrates for the first time at the ultrastructural level endocytosis in whole plant cells, using a non-toxic compound.

Ultrastructure and Development of the Trichomes of Larrea (Creosote Bush)

Ultrastructural observations indicate that, during development, the single-celled trichomes on the young leaves, petioles, and stipules become isolated from symplastic and apoplastic continuity with other leaf cells. This occurs with the development of a suberin layer internal to the primary wall of the trichome. With maturation, a massive, internal wall layer develops. Subsequently, the trichomes senesce and collapse, resulting in the extrusion of the degraded cellular material in the lumen of the trichomes through pores onto the surface of the leaves. Extrusion of material from the trichomes primarily occurs on young organs. Apparently no extrusion occurs from the trichomes on the older leaves since they are all senescent, collapsed, and have a much reduced luminal content. Changes in the ultrastructure of the fluid, sticky resin on the young organs, compared with that of the dry, hard resin on the older leaves, indicate that chemical modification of the resin occurs during and after extrusion. The extensive formation and transitions in rough and smooth endoplasmic reticulum, free and aggregated ribosomes, dictyosomes, and other cytoplasmic structures which occur with maturation of the trichomes are interpreted to be primarily involved in synthetic events that occur in development and elongation of the trichome and in the formation of the internal suberin and massive wall layers of the mature trichomes.

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

SummaryFreeze-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.