THE FINE STRUCTURE OF DIFFERENTIATING XYLEM ELEMENTS

Differentiating xylem elements of Avena coleoptiles have been examined by light and electron microscopy. Fixation in 2 per cent phosphate-buffered osmium tetroxide and in 6 per cent glutaraldehyde, followed by 2 per cent osmium tetroxide, revealed details of the cell wall and cytoplasmic fine structure. The localized secondary wall thickening identified the xylem elements and indicated their state of differentiation. These differentiating xylem elements have dense cytoplasmic contents in which the dictyosomes and elements of rough endoplasmic reticulum are especially numerous. Vesicles are associated with the dictyosomes and are found throughout the cytoplasm. In many cases, these vesicles have electron-opaque contents. "Microtubules" are abundant in the peripheral cytoplasm and are always associated with the secondary wall thickenings. These microtubules are oriented in a direction parallel to the microfibrillar direction of the thickenings. Other tubules are frequently found between the cell wall and the plasma membrane. Our results support the view that the morphological association of the "microtubules" with developing cell wall thickenings may have a functional significance, especially with respect to the orientation of the microfibrils. Dictyosomes and endoplasmic reticulum may have a function in some way connected with the synthetic mechanism of cell wall deposition.

The nuclear pore complex: structure, function, and dynamics

A full understanding of nucleocytoplasmic transport depends on knowledge of nuclear pore complex (NPC) structure, the functional roles of NPC components, their interactions during transport and dynamics during the cell cycle. NPC structure is conserved, flexible, and is not simply a tunnel between the nucleus and cytoplasm but appears to be actively involved in the transport process by a series of structural modifications. Transport through the NPC begins in either of its asymmetrical peripheral compartments that are both structurally reorganized during transport in different ways. The central compartment is composed of two symmetrical halves, and functions as a system of transiently open, discrete gates that is not believed to play a role in determining direction. Each NPC subunit has a specific morphology that corresponds to the functional role it plays. A complicated system of vertical and horizontal connections may allow one part of the NPC to transmit a signal to other parts, leading to an ordered series of conformational changes that drive translocation. High-resolution scanning electron microscopy has identified sequential stages of NPC assembly in vitro and revealed how the individual NPC components are assembled into a mature NPC. This review focuses on structural events during transport and on possible mechanisms of NPC assembly.

A comparison of the responses of dispersed steroidogenic cells derived from embryonic adrenal tissue from the domestic chicken (Gallus domesticus), the domestic Pekin duck and the wild mallard duck (Anas platyrhynchos), and the domestic muscovy duck (Cairina moschata)

The steroidogenic responsiveness of adrenal cell suspensions prepared from domestic chicken adrenal tissue at the end of embryogenesis was compared to the responses of similar preparations derived from the wild and domesticated mallard duck (Anas platyrhynchos), and the domesticated muscovy duck (Cairina moschata). In all cases, the masses of corticosterone (B), aldosterone (Aldo), and deoxycorticosterone (DOC) released from cells incubated in medium containing 1-24 ACTH exceeded the estimated hormone content of the freshly dispersed cells; the induced rates of corticosteroid release were, therefore, presumed to reflect de novo hormone synthesis. When chicken cells were incubated in medium containing 1-24 ACTH, there were progressive, dose-dependent increases in B and DOC synthesis over a range of concentrations spanning two orders of magnitude; only small, non-dose-related, albeit significant, increases in Aldo release were observed. The 1-24 ACTH-induced increases in B and Aldo synthesis by the mallard and Pekin duck cells exposed to the same range of concentrations were up to 40 and 60 times greater than the corresponding responses of the chicken cells. The rates of 1-24 ACTH-induced B and DOC release from muscovy duck cells were similar to those from the Pekin duck cells; compared with the mallard duck cells, however, the muscovy duck cells were less sensitive and the maximum inducible rate of B release was significantly lower. The pattern of 1-24 ACTH-induced Aldo release from the muscovy duck cells was indistinguishable from that of the mallard duck cells, although the maximum inducible increase occurred at a lower concentration. Angiotensin II (AII) induced very small, but significant increased in B, Aldo, and DOC release from the chicken cells but in a nondose-related fashion. In contrast, mallard, Pekin, and muscovy duck cells all responded in a dose-dependent manner when incubated in medium containing AII. In each instance the maximum rate of Aldo synthesis induced by AII was about one-tenth of the corresponding rate induced by 1-24 ACTH. The maximum rates of B synthesis induced by AII, however, were extremely low compared with the rates induced by 1-24 ACTH. Thus, when maximally stimulated with AII the B:Aldo output ratio ranged from 1.0 to 1.9, whereas the corresponding ratio was between 9.0 and 17.0 when the cells were maximally stimulated with 1-24 ACTH.

Functional and morphological changes associated with the ageing of primary cultures of embryonic adrenal gland cells derived from the Pekin duck

The morphological and functional changes associated with ageing were studied in adrenal steroidogenic cells derived from duck embryos. Cells grown for not more than three days had structural characteristics similar to their counterparts in vivo; they contained numerous lipid droplets and mitochondria, an abundant smooth endoplasmic reticulum, an even network of microtubules, and microfilaments that formed extensive and elaborate systems of parallel stress fibers. After the 3rd day of growth in culture, many of the cells started to decrease in size and become elongated; the older cells showed less well-defined actin filaments and contained elongated mitochondria, fewer lipid droplets, less smooth endoplasmic reticulum, and swollen cisternae of rough endoplasmic reticulum. The proliferative capacity of the cells was the same when they were cultured in either the presence or the absence of 1-24 ACTH. After the first day of growth in culture, the steroidogenic capacity of the cells declined and the addition of 1-24 ACTH to the growth medium did not prevent changes in their structure and function. The decline in steroidogenic capacity occurred both in terms of the amount of hormone released into the culture medium and in the ability of the cells to respond when incubated in buffer containing 1-24 ACTH. Since the basal unstimulated rates of corticosteroid production also declined as the cells aged, it is probable that the steroidogenic deficiency occurs at a site distal to the corticotropin receptor; this is also consistent with the ultrastructural observations that suggest a relationship between the morphological changes and the decline in steroidogenic capacity as the cells age.

Cytoskeletal changes accompanying ACTH-induced steroidogenesis in cultured embryonic adrenal gland cells from the Pekin duck

Cells derived from the adrenal glands of duck embryos immediately prior to hatching were grown in culture and used to study the morphological and cytoskeletal changes and steroidogenic responses induced by 1-24 ACTH. Changes in the cytoskeletal components were observed by rhodamine-phalloidin staining for actin and by staining the tubulin immunoreactive components with FITC. The cultures were comprised of a small population of chromaffin cells and a larger population of steroidogenic cells. The chromaffin cells were distinguished by their tyrosine hydroxylase immunoreactivity. The steroidogenic cells were characterized by the presence of sudanophilic lipid droplets, numerous mitochondria, abundant smooth endoplasmic reticulum, microtubules distributed as a fairly even network throughout the cytoplasm, and microfilaments that formed an extensive and elaborate system of stress fibers with many parallel arrays. The cells readily responded to stimulation with ACTH by releasing corticosterone, aldosterone and deoxycorticosterone. Stimulation with ACTH also induced changes in both the cell morphology and the cytoskeleton. Exposure of the cells to Krebs-Henseleit buffer containing 1-24 ACTH caused them to form numerous fine filopodia, to lose their stress fibers, and to form a thick ring of actin at the periphery of the cell. In addition, many cells became extremely arborized with many long branched dendritic processes. The morphological changes appeared to be related to a redistribution of the actin components, and may be explained only in part by the rounding up or retraction of the cytoplasm. The results strongly suggest an involvement of the actin components of the cytoskeleton in the steroidogenic response to corticotropic stimulation.

Pre-natal development of the adrenal gland in the mallard duck (Anas platyrhynchos)

The differentiating nephrotome in the 10-day-old mallard duck embryo is able to synthesize corticosterone, aldosterone and deoxycorticosterone even though an adrenal anlage cannot be identified histologically until the 12th day of incubation. At this time, sudanophilic cells containing much smooth endoplasmic reticulum and numerous mitochondria with tubular cristae are located adjacent to the developing mesonephros. Chromaffin cells appear in this region on about the 14th day of embryogenesis. A discrete glandular structure containing measurable quantities of corticosteroids can be identified on the 15th day, and during the next 2 days the tissue becomes encapsulated. Concomitantly, the ACTH-inducible rates of corticosteroid hormone synthesis increase several fold. The corticotropic responsiveness of the developing adrenal steroidogenic tissue increases progressively during the remainder of embryogenesis.

Changes in the adrenal steroidogenic responsiveness of the mallard duck (Anas platyrhynchos) during early post-natal development

1. Plasma concentrations of corticosterone (B), aldosterone (Aldo) and deoxycorticosterone (DOC) were measured in mallard ducklings immediately before and after exposure to acute immobilization stress. 2. Except for transient declines in B and DOC between the 4th and 14th days after hatching, the resting concentration of each hormone did not change significantly during post-natal development. 3. The stress-induced in Aldo was maximal at hatching while maximal increases in B and DOC did not occur until one day later. 4. Thereafter the magnitude of the stress-induced increases in the concentrations of all of the hormones decreased steadily and on the 21st and 28th days after hatching only B increased significantly in response to stress.

Effects of short-term changes in electrolyte intake on the adrenal steroidogenic responses of juvenile mallard ducks (Anas platyrhynchos)

1. Adjusting the Na+ and K+ intake of juvenile mallard ducks caused the plasma concentrations of corticosterone (B) and aldosterone (Aldo) to increase independently of one another, but none of these changes in electrolyte intake had a significant effect on the deoxycorticosterone (DOC) concentration. 2. With the exception of DOC in birds consuming the control diet, the plasma concentration of each hormone, regardless of diet, increased significantly following exposure to stress. 3. Stress-induced increases in Aldo concentration were greatest in birds given diets containing low concentrations of Na+. 4. Unlike the mammal and some other species of birds, Na+ may be the primary secretagogue responsible for the regulation of both corticosterone and aldosterone synthesis in the mallard duck.

Adrenal gland: some evidence for the structural and functional zonation of the steroidogenic tissues

There are two regions of steroidogenic tissue in the bird adrenal gland: a subcapsular zone (SCZ) 40-60 cells thick consisting of cells with irregularly shaped nuclei, relatively little smooth endoplasmic reticulum, and mitochondria with shelflike cristae that surrounds an inner zone (IZ) of tissue comprised of smaller cells with rounded nuclei, a more abundant endoplasmic reticulum, and mitochondria with tubular cristae. The cristae in the mitochondria of IZ cells, but not the SCZ cells, seem to be quite labile and only assume the tubular configuration when subjected to corticotropic stimulation. The properties of the steroidogenic cells in the two zones are quantitatively distinct, the cells of the SCZ producing relatively more aldosterone and relatively less corticosterone than the cells of the IZ. The corticotropic responsiveness of the IZ cells is dependent on the synthesis of protein in the cytoplasm and a normal pattern of microfilament assembly. The dose responsiveness of both the IZ and the SCZ cells in vitro is a complex quadratic function of the corticotropin concentration to which they are exposed; the semilogarithmic linear response to corticotropin is restricted to a narrow midrange of concentrations, and at high concentrations cells from both zones of the gland respond submaximally. Throughout the dose-response range, however, the steroidogenic cells of the IZ are more sensitive and more responsive to corticotropic stimulation than are the cells of the SCZ.

The effects of colchicine, vinblastine and cytochalasins on the corticotropic responsiveness and ultrastructure of inner zone adrenocortical tissue in the Pekin duck

Tissues slices superfused with medium containing no ACTH released only traces of corticosterone. Addition of ACTH to the medium caused the rate of corticosterone release to increase to a maximum about 45 min after the addition of ACTH, after which time it either remained constant or started to wane slightly. The rate of release was affected by tissue thickness; the maximum rate of corticosterone release occurred when the tissue slices were 200 microns. Stimulated adrenocortical cells had large spherical nuclei, numerous mitochondria with tubular cristae, numerous lipid droplets, and a large amount of smooth endoplasmic reticulum. Many cells had an extensive network of microfilaments adjacent to the plasma membrane and some microtubules. Prolonged superfusion caused degenerative changes in some of the cells. Both cytochalasin B and cytochalasin D, dissolved in DMSO before addition to the superfusion medium, inhibited the corticotropic responsiveness in a dose-dependent manner. Control tissue samples superfused with medium containing DMSO, but no ACTH and no cytochalasin, released significantly more corticosterone than corresponding unstimulated samples. Few or no microfilaments were observed in adrenocortical cells after treatment with cytochalasin. Neither colchicine nor vinblastine had any discernible effect on the corticotropic responsiveness. After treatment with colchicine, adrenocortical cells had an ultrastructure characteristic of inner zone stimulated cells except that they were mainly devoid of microtubules.

ATPase in mature and differentiating phloem and xylem

A cytochemical study using a lead precipitation technique has been made of the distribution of adenosine triphosphatase (ATPase) in mature and differentiating phloem and xylem cells of Nicotiana tabacum and Pisum sativum. The sites of ATPase localization in tobacco phloem were the plasma membrane, endoplasmic reticulum, mitochondria, dictyosomes, plasmodesmata, and the dispersed P proteins of mature sieve elements. In pea phloem sieve elements ATPase was localized in the endoplasmic reticulum, but was not associated with the P proteins or plasma membranes at any stage of their differentiation. In pea transfer cells ATPase activity was associated with the endoplasmic reticulum at all stages of their differentiation and with the plasma membrane of transfer cells that had formed wall ingrowths. In xylem cells of both tobacco and pea the patterns of ATPase activity was similar. At early stages of differentiation ATPase activity was associated with the plasma membrane and the endoplasmic reticulum. At intermediate stages of differentiation ATPase activity continued to be associated with the endoplasmic reticulum, but was no longer associated with the plasma membrane. At later stages of xylem element differentiation ATPase activity was associated with disintegrating organelles and with the hydrolyzing cell walls.

Functional significance of interrenal cell zonation in the adrenal gland of the duck (Anas platyrhynchos)

Slices of whole adrenal gland tissue, incubated in vitro in the presence of ACTH for 1 h and 2 h produced corticosterone and aldosterone in constant ratio (16:1). Tangential slices taken from the region immediately below the connective tissue capsule and slices taken from deeper regions of the gland consisted primarily of cells conforming to the distinct structural characteristics of the subcapsular zone (SCZ) and inner zone (IZ) tissues respectively. When samples were incubated in the presence of ACTH for 1 h and 2 h, the interrenal cells of the SCZ produced relatively more aldosterone than cells taken from the IZ of the gland. The corticosterone: aldosterone ratio for the IZ after 1 h (68:1) and after 2 h (102:1) were ten times greater than the ratios for the SCZ after 1 h (7:1) and after 2 h (10:1). The SCZ slices were not more than 60 cells thick and consisted of cells arranged in cords. These cells contained irregular nuclei, mitochondria with shelf-like cristae and a moderate abundance of smooth endoplasmic reticulum. In contrast, the production of large amounts of corticosterone by the cells of the IZ was associated with tissue containing more vascular space than the SCZ and the cells contained large round nuclei surrounded by an abundance of smooth endoplasmic reticulum and the mitochondria had tubular rather than shelf-like cristae.

Structural changes occurring in internal tissue of the duck (Anas platyrhynchos) following adenohypophysectomy and treatment in vivo and in vitro with corticotropin

Adrenal glands from ACTH-treated intact ducks and chronically adenohypophysectomized ducks showed clear zonation into a subcapsular zone (SCZ) and an inner zone (IZ). Adenohypophysectomy caused ultrastructural changes in the IZ but not in the SCZ cells. These included increases in lipid droplets, changes in mitochondrial cristae from tubular to shelf-like, and changes in the shape of the nuclei from spherical to crenated. These changes were reversed by treatment with ACTH. Also, cells of the IZ, but not the SCZ, of adrenals from intact birds given ACTH showed more SER, more dense bodies, fewer lipid droplets and more prominent Golgi complexes. IZ cells incubated in buffer containing no ACTH developed mitochondria with shelf-like cristae and numerous opaque granules in the matrix. Exposure to buffer containing ACTH caused the mitochondrial cristae to become tubular and the matrix granules either decreased in number or disappeared. The granules could be extracted by incubating sections with chelating agents. The mitochondria in SCZ cells did not respond structurally to the presence of ACTH in the incubation medium but the matrix granules, like those in IZ cells, responded to the presence of chelating agents.

The effects of ingested petroleum on oviposition and some aspects of reproduction in experimental colonies of mallard ducks (Anas platyrhynchos)

Compared to unmated mallard ducks fed an uncontaminated diet, unmated birds given food contaminated with 3 ml South Louisiana crude oil per 100 g dry weight showed an 84% decline in the daily rate of oviposition, a 33% decrease in egg-shell thickness and at autopsy more than 82% of the ovarian mass consisted of atretic follicles. Similar studies on groups of mated females showed that although the addition of 1 ml South Louisiana crude oil/100 g dry food had no effect on the daily rate of oviposition, none of the eggs had been fertilized while a concentration of 3 ml South Louisiana crude oil/100 g dry food suppressed the daily rate of oviposition significantly. Less than 25% of these eggs had been fertilized and only 40% of the fertilized eggs yielded viable ducklings. In both of these groups of mated birds, normal patterns of oviposition, fertilization and hatchability were restored after removal of petroleum from the diet. The addition of 1 ml Kuwait crude oil/100 g dry food had no effect on the rate of oviposition, the incidence of fertility or the hatchability of the fertilized eggs. The addition of 3 ml oil/100 g dry food completely abolished oviposition, but a normal rate of oviposition was restored when the concentration of the crude oil was reduced from 3 to 1 ml/100 g dry food. However, the incidence of fertilization remained low and none of the fertilized eggs gave rise to viable ducklings. Kuwait crude oil had no effect on shell thickness.

Evidence for the zonation of interrenal tissue in the adrenal gland of the duck (Anas platyrhynchos)

Although interrenal tissue from the intact duck does not show a clear zonation when examined by light microscopy, the tissue does develop a well defined zonation following exposure to high and low levels of corticotropic stimulation. Under these conditions clear ultrastructural differences are seen between cells of the subcapsular zone (SCZ) and the inner zone (IZ). Based on these observations, the ultrastructure of tissue from intact birds was examined retrospectively and in addition, cell sizes and the relative volumes and areas of intracellular components were measured morphometrically. These analyses reveal morphological and quantitative differences between cells from the IZ and the SCZ. Cells of the IZ have small rounded nuclei, extensive smooth endoplasmic reticulum (SER) and mitochondria with tubular cristae, whereas cells from the SCZ contain pleomorphic nuclei, less SER and mitochondria with shelf-like cristae. The mean cell volume in the IZ is significantly less than that in the SCZ. In the SCZ cells the volume densities (volume per unit volume cytoplasm) of mitochondria and lipid droplets and the surface densities (area per unit volume cytoplasm) of the outer mitochondrial membranes are significantly greater than those in IZ cells. Conversely, in the cells of the IZ the volume densities of the nuclei and dense bodies and the surface density of the SER are greater than the corresponding values estimated for the cells of the SCZ. Although the mitochondria comprise a smaller fraction of the mean volume of IZ cells than SCZ cells, the total surface area of the cristae is approximately the same in the cells of both zones.

Cytochemical localization of adenosine triphosphatase in the phloem of Pisum sativum and its relation to the function of transfer cells

The cytochemical localization of ATPase in differentiating and mature phloem cells of Pisum sativum L. has been studied using a lead precipitation technique. Phloem transfer cells at early stages of differentiation exhibit strong enzyme activity in the endoplasmic reticulum (ER) and some reaction product is deposited on the vacuolar and plasma membranes. As the phloem transfer cells mature and develop their characteristic wall structures, strong enzyme activity can be observed in association with the plasma membranes and nuclear envelopes. Mature phloem transfer cells with elaborate cell-wall ingrowths show ATPase activity evenly distributed on plasma-membrane surfaces. Differentiating sieve elements show little or no enzyme activity. When sieve elements are fully mature they have reaction product in the parietal and stacked cisternae of the ER. There is no ATPase activity associated with P-protein at any stage of sieve-element differentiation or with the sieve-element plasma membranes. It is suggested that the intensive ATPase activity on the plasma membranes of the transfer cells is evidence for a transport system involved in the active movement of photosynthetic products through these cells.

The fine structure of the interrenal cells of the duck (Anas platyrhynchos) with evidence for possible exocytotic release of steroids

The duck interrenal cell possesses ultrastructural characteristics common to other steroid-secreting cells. Lipid droplets and mitochondria are abundant and lie principally at the apical end of the cell. Lipid droplets are not membrane-limited. Cisternae of smooth endoplasmic reticulum that are occasionally continuous with the less abundant rough endoplasmic reticulum are a prominent feature of the interrenal cell. Tubular profiles of rough endoplasmic reticulum often lie tangentially to mitochondria and ribosomes are either free, grouped in polyribosomal clusters, or bound to the endoplasmic reticulum. Mitochondria possess tubular cristae in the inner regions of the gland and frequently contain a paracrystalline array of small 10nm (o.d.) tubules and less frequently a hexagonal array of 40 nm trilaminar rings. Other cytoplasmic components include dense bodies, residual bodies, microtubules, microfilaments and specialized single membrane-bound vesicles. Gap junctions, intermediate junctions and interdigitating processes constitute the main intercellular associations. No tight junctions were identified. The single membrane-bound vesicles which are occasionally filled with a low electron-dense, lipid-like material form septate-like "junctions" with the plasma membrane. The septa bridge an intracellular gap of 15-17 nm. The vesicles are usually located near the subendothelial space at the basal and basilateral regions of the cell. Occasionally, vesicles fuse with the plasma membrane. It is suggested that these vesicles represent morphological evidence for the exocytotic release of steroid hormones.

Biochemistry and cytochemical localization of acid phosphatase in the phloem of Nicotiana tabacum

A biochemical and cytochemical study has been made of the distribution of β-glycerophosphatase (EC 3.1.3.2) activity in mature and differentiating phloem cells of Nicotiana tabacum L. and the pH dependence and kinetics of β-glycerophosphate hydrolysis of homogenates of fresh leaf midveins and midveins fixed in formaldehyde-gluteraldehyde. β-glycerophosphatase showed two peaks of activity at pH 5.5 and 6.2. Enzyme saturation kinetics were exhibited by both fresh and fixed tissue homogenates. At a substrate concentration of 2 mM, 65% of the enzyme activity survived fixation. Specimens for cytochemical localization were incubated with 2 mM β-glycerophosphate at pH 5.5 and 6.2. Specimens showed consistent patterns of reaction product deposition. Little or no reaction product was deposited in controls incubated without substrate or with substrate plus 0.01 M fluoride. β-glycerophosphatase activity in the phloem and xylem is considerably higher than in surrounding tissue. Dense localization of reaction product was demonstrated on the vacuolar membranes, the inner membranes of mitochondria, and the dictysomes of phloem parenchyma and companion cells. The plasma membrane and endoplasmic reticulum cisternae of these cells were usually free of reaction products. Enzyme activity in mature sieve elements was associated with the parietal and stacked systems of endoplasmic reticulum and with the P-protein. There was inconsistency of staining of P-protein in mature sieve elements although the association of reaction products with the P-protein appeared to show a correlation with maturity and dispersal. The P-protein bodies of differentiating sieve elements showed no reaction product deposition. The distribution of β-glycerophosphatase activity has been compared with that previously recorded for ATPase activity in the phloem of Nicotiana tabacum.

A biochemical and cytochemical study of adenosine triphosphatase activity in the phloem of Nicotiana tabacum

A biochemical and cytochemical study has been made of the distribution of ATPase in mature and differentiating phloem cells of Nicotiana tabacum and of the substrate specificity and effects of fixation on enzyme activity. Homogenates of unfixed leaf midveins and midveins fixed in formaldehyde-glutaraldehyde were assayed for enzyme activity by determining the amount of P(i), liberated per milligram of protein from various substrates in a 30 min period at pH 7.2. In fresh homogenates, hydrolysis of ATP was not significantly different from that of ITP, CTP, and UTP. Hydrolysis of GTP was slightly higher than that of ATP. ATP hydrolysis by fresh homogenates was 17% more extensive than that of ADP, 76% more extensive than that of 5'-AMP, and was inhibited by fluoride and p-chloromercuribenzoate (PCMB). There was little or no hydrolysis of the competitive inhibitors 2'- and 3'-AMP nor with the alternate substrates p-nitrophenylphosphate (PNP) or beta-glycerophosphate (beta-GP). In homogenates of material fixed in formaldehyde-glutaraldehyde for 1(1/4) h, ATPase activity was 13% preserved. Hydrolysis of ATP by fixed homogenates was not significantly different from that of ADP, 5'-AMP, ITP, CTP, and GTP. Hydrolysis of UTP was lower. Fluoride and PCMB inhibited fixed ATPase activity. The results of cytochemical localization experiments using a lead phosphate precipitation technique were in agreement with the biochemical results. Similar localization patterns were obtained with the nucleoside triphosphates ATP, CTP, GTP, ITP, and UTP. Activity was also localized with ADP and 5'-AMP but not with the competitive inhibitors 2'- and 3'-AMP, nor with PNP or beta-GP. Little or no reaction product was deposited in other controls incubated without substrate or with substrate plus fluoride, PCMB, or N-ethylmaleimide. ATPase activity was demonstrated chiefly at the plasma membrane of mature and differentiating phloem cells and was associated with the P-protein of mature sieve elements. It is suggested that the phloem transport system derives its energy from the demonstrated nucleoside triphosphatase activity.

Adenosine triphosphatase in the phloem of Cucurbita

The distribution of adenosine triphosphatase (ATPase) activity in the phloem of petioles and minor veins of Cucurbita maxima has been studied using a lead phosphate precipitation procedure. ATPase activity was localized in sieve elements, companion cells and parenchyma cells. Activity was found at the cell surfaces, associated with the dispersed P-protein of mature sieve elements, in mitochondria, sieve-element reticulum, and at specific regions of the cell walls. It is suggested that the ATPase at the phloem cell surfaces may function in intercellular transport of assimilates or ions, and that the ATPase activity associated with the P-protein may function in the translocation process or in callose deposition.

Fine structure of P-protein filaments from Ricinus communis

Exudate from the phloem of Ricinus communis L. was negatively stained, examined in the electron microscope, and the filamentous components compared with those in fixed, sectioned material. In the exudate, two main fibrillar components were observed. One component has a diameter of 20±0.35 (standard error) nm, the other of 14.1±0.34 nm. This second compoent has projections along its length measuring 5 by 14 nm and spaced at intervals of 6.5-10 nm. Fibrils have been found possessing characteristics of both fibril types, suggesting some structural relationship between the two, possibly an interconvertibility. Several other types of fibrils occurred less frequently in the exudate. The exudate also contains torus-shaped structures measuring 13.5-15 nm in diameter. Sections of mature sieve elements of Ricinus and Acer rubrum L. contain fibrils structurally similar to the 14-nm fibrils from the exudate of Ricinus. Ricinus exudate was also fixed and pelleted in the ultracentrifuge. Thin sections of the pellet afforded cross-sectional views of the 20-nm fibrils, and showed that these fibrils apparently have a solid core. Possible models for the structure of the 20-nm filaments are described.

Sieve-plate pores in tobacco and bean

Tobacco and bean plants were wilted and then fixed as whole plants with formaldehyde-glutaraldehyde for electron microscopy. In some tobacco plants the sieve-plate pores were large, with little callose. Light slime plugs were present, but there was no compaction of P-protein in the pores. Some pores in wilted bean plants were also unplugged. In other plants of both tobacco and bean the sieve-plate pores were plugged. The pores in unwilted control plants of both tobacco and bean were invariably plugged. Tobacco plants were also cut into thin slices and then immediately fixed. In specimens prepared in this way there was little callose in the pores, and many of the pores were not plugged with P-protein. These observations provide additional evidence that sieve-plate pores may be unplugged in vivo.

P protein in the phloem of Cucurbita. II. The P protein of mature sieve elements

During maturation of sieve elements in Cucurbita maxima Duchesne, the P-protein bodies (slime bodies) usually disperse in the tonoplast-free cell. In some sieve elements the P-protein bodies fail to disperse. The occurrence of dispersal or nondispersal of P-protein bodies can be related to the position of the sieve elements in the stem or petiole. In the sieve elements within the vascular bundle the bodies normally disperse; in the extrafascicular sieve elements the bodies often fail to disperse. Extrafascicular sieve elements showing partial dispersal also occur. The appearance of the sieve plate in fixed material is related to the degree of dispersal or nondispersal of the P-protein bodies. In sieve elements in which complete dispersal occurs the sieve plate usually has a substantial deposit of callose, and the sieve-plate pores are filled with P protein. In sieve elements containing nondispersing P-protein bodies the sieve plate bears little or no callose, and its pores usually are essentially "open." The dispersed P-protein components may aggregate into loosely organized "strands," which sometimes extend vertically through the cell and continue through the sieve-plate pores; but they may be oriented otherwise in the cell, even transversely.

P protein in the phloem of Cucurbita. I. The development of P-protein bodies

Light and electron microscopical observations of the cells of the phloem of Cucurbita maxima have shown that two distinct types of P-protein bodies are formed: a larger type which arises as fine fibrils and a smaller type which apparently arises as groups of tubules. The tubules of the smaller type of body measure 242 +/- 3.6 (SE) A (n = 48) and appear morphologically identical with the P1-protein tubules of Nicotiana tabacum L. In some of these P1-protein bodies the tubules are arranged in a regular manner with a center-to-center distance of 295 A. The P protein of the larger type of P-protein body is first apparent in the cytoplasm as small aggregates of fine fibrils. This P-protein component has been designated P3 protein. As the P3 protein accumulates it is organized into large bodies. Some of these bodies contain only P3 protein, others a tubular form of protein, and still others a combination of P3 protein and a tubular form. This variability indicates that there is a developmental sequence of the formation of tubules from the P3-protein fibrils. These tubules measure 179 +/- 8.2 (SE) A (n = 31) and have been designated P4 protein.

Tubular and fibrillar components of mature and differentiating sieve elements

An ontogenetic study of the sieve element protoplast of Nicotiana tabacum L. by light and electron microscopy has shown that the P-protein component (slime) arises as small groups of tubules in the cytoplasm. These subsequently enlarge to form comparatively large compact masses of 231 +/- 2.5 (SE)A (n = 121) tubules, the P-protein bodies. During subsequent differentiation of the sieve element, the P-protein body disaggregates and the tubules become dispersed throughout the cell. This disaggregation occurs at about the same stage of differentiation of the sieve elements as the breakdown of the tonoplast and nucleus. Later, the tubules of P-protein are reorganized into smaller striated 149 +/- 4.5 (SE)A (n = 43) fibrils which are characteristic of the mature sieve elements. The tubular P-protein component has been designated P1-protein and the striated fibrillar component P2-protein. In fixed material, the sieve-plate pores of mature sieve elements are filled with proteinaceous material which frays out into the cytoplasm as striated fibrils of P2-protein. Our observations are compatible with the view that the contents of contiguous mature sieve elements, including the P-protein, are continuous through the sieve-plate pores and that fixing solutions denature the proteins in the pores. They are converted into the electron-opaque material filling the pores.

Relation of tobacco mosaic virus to the host cells

The relation of tobacco mosaic virus (TMV) to host cells was studied in leaves of Nicotiana tabacum L. systemically infected with the virus. The typical TMV inclusions, striate or crystalline material and ameboid or X-bodies, which are discernible with the light microscope, and/or particles of virus, which are identifiable with the electron microscope, were observed in epidermal cells, mesophyll cells, parenchyma cells of the vascular bundles, differentiating and mature tracheary elements, and immature and mature sieve elements. Virus particles were observed in the nuclei and the chloroplasts of parenchyma cells as well as in the ground cytoplasm, the vacuole, and between the plasma membrane and the cell wall. The nature of the conformations of the particle aggregates in the chloroplasts was compatible with the concept that some virus particles may be assembled in these organelles. The virus particles in the nuclei appeared to be complete particles. Under the electron microscope the X-body constitutes a membraneless assemblage of endoplasmic reticulum, ribosomes, virus particles, and of virus-related material in the form of wide filaments indistinctly resolvable as bundles of tubules. Some parenchyma cells contained aggregates of discrete tubules in parallel arrangement. These groups of tubules were relatively free from components of host protoplasts.

Relation of beet yellows virus to the phloem and to movement in the sieve tube

In minor veins of leaves of Beta vulgaris L. (sugar beet) yellows virus particles were found both in parenchyma cells and in mature sieve elements. In parenchyma cells the particles were usually confined to the cytoplasm, that is, they were absent from the vacuoles. In the sieve elements, which at maturity have no vacuoles, the particles were scattered throughout the cell. In dense aggregations the particles tended to assume an orderly arrangement in both parenchyma cells and sieve elements. Most of the sieve elements containing virus particles had mitochondria, plastids, endoplasmic reticulum, and plasma membrane normal for mature sieve elements. Some sieve elements, however, showed evidence of degeneration. Virus particles were present also in the pores of the sieve plates, the plasmodesmata connecting the sieve elements with parenchyma cells, and the plasmodesmata between parenchyma cells. The distribution of the virus particles in the phloem of Beta is compatible with the concept that plant viruses move through the phloem in the sieve tubes and that this movement is a passive transport by mass flow. The observations also indicate that the beet yellows virus moves from cell to cell and in the sieve tube in the form of complete particles, and that this movement may occur through sieve-plate pores in the sieve tube and through plasmodesmata elsewhere.

Ultrastructural features of Beta leaves infected with beet yellows virus

A cytochemical and electron microscope study has been made of leaves of sugar beet infected with beet yellows virus. Inclusions of particles, which agree in size with beet yellows virus particles isolated by other investigators, have been localized in the ground cytoplasm, in the chloroplasts, and in the nuclei. These particles are circa 100 A in diameter and have an electron-transparent core of 30 to 40 A. Use of acridine orange, azure B, and pyronine Y has revealed that the cytoplasmic inclusion bodies, which consist wholly of the elongate particles, have a strong RNA reaction removable by RNase pretreatment. Particles observed in the chloroplasts may or may not be associated with lipid spheres. If they are, the particles are confined to the periphery of the spheres. In this position the particles are arranged tangentially and are further arranged parallel into groups which lie at various angles to one another. Within the groups the particles are regularly spaced in a three dimensional lattice. Particles located free in the stromal regions are often arranged regularly in curved rows which lie parallel to one another so that a three dimensional lattice is formed. The dispersed and compact forms of virus inclusions are described and related to the condition of the associated cytoplasm. The ground cytoplasm of cells associated with the sieve elements contains numerous ribosomes. A decrease in the number of ribosomes is concomitant with the increase in size of virus aggregations in a cell. Vesiculation of some component of the cytoplasm occurs during the period of virus replication. The vesicles are approximately 100 mmicro in diameter and could be derived from the dictyosomes. At later stages of infection these vesicles collapse and convoluted membranous material appears.

Organization of beet yellows-virus inclusions in leaf cells of beta

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Tracheid differentiation in tobacco pith cultures

Sterile pith cultures of Nicotiana tabacum have been induced to form localized regions of differentiating tracheids. These localized regions have been examined by phase, fluorescence, and electron microscopy, and polarization optics. Fixation for electron microscopy was with glutaraldehyde-osmium. The differentiating tracheids develop characteristic thick cell walls which are eventually lignified. The lignifications appear to be uniform throughout the secondary wall and little or no lignin appears to be deposited in the primary walls or intercellular layer. At all stages of secondary wall deposition, the peripheral cytoplasm contains a system of microtubules which form a pattern similar to that of the developing thickenings. Within this system the microtubules are oriented, the direction of orientation mirroring that of the fibrils in the most recently deposited parts of the wall. The observations support the view that the microtubules are somehow involved in microfibril orientation. The microtubules appear to be attached to the plasma membrane which has a triple layered structure. The two electron dense layers of the plasma membrane have a particulate structure. In the differentiating tracheids at regions where secondary wall thickening has not yet been deposited numerous invaginations of the plasma membrane are observed which contain loosely organized fibrillar material. It is suggested that these are areas of localized activity of the plasma membrane and that the enzymes concerned with the final organization of the cellulose microfibrils are situated at the surface of the plasma membrane. Dictyosomes in the differentiation cells give rise to vesicles which contain fibrous material and the contents are incorporated into the cell wall. Numerous profiles characteristic of plasmodesmata are evident in sections of the secondary thickenings.