ZONATION IN THE APICAL CELL OF ZONARIA

Light and electron microscopic techniques were used to study the row of apical cells that form the meristem of the dictyotalean brown alga, Zonaria farlowii. A distinctly polar pattern occurs in the cells. Four ultrastructurally different cytoplasmic zones have been seen: an apical zone, a nuclear zone, a compound vacuolar zone, and a vegetative zone. The apical cell of Zonaria is a differentiating cytoplasmic unit where striking intracellular gradients occur.     

CELL DIVISION STUDIES OF THE SHOOT APEX OF DATURA STRAMONIUM DURING TRANSITION TO FLOWERING

Seedlings of Datura stramonium L., although not photoperiodically sensitive, are useful for floral transition studies when raised in a growth chamber at a constant temperature of 25 C with a photoperiod of 8 hr of light (1,600-2,000 ft-c) and 16 hr of darkness. A terminal flower is formed after the seventh or eighth leaf primordium is produced. A constant rate of leaf initiation up to the time of flowering enables specific apical stages to be obtained and studied. Changes in the mitotic index, substantiated with calculated rates of cell division (measured by the accumulation of metaphases following treatment with colchicine) were studied in shoot apical zones during transition to flowering. Fluctuations in the mitotic index of each zone in the vegetative and transition apex with respect to apical stage as well as time of day were not statistically significant. The mitotic index of the summit zone of the vegetative apex was significantly lower than in the other zones whose mitotic indices were not significantly different from one another. During floral transition the mitotic index of the summit zone as well as the central zone (just below the summit zone) significantly increased while no significant changes were detected in the flank zones. It was shown that the mitotic index could be considered representative of the rates of cell division in Datura.     

Aluminum toxicity studies in Vaucheria longicaulis var. macounii (Xanthophyta, Tribophyceae). II. Effects on the F-actin array

In this study, we test the hypothesis that exposure to environmentally significant concentrations of aluminum (Al, 80 μM) causes the microfilament array of Vaucheria longicaulis var. macounii vegetative filaments to become fragmented and disorganized. Changes in F-actin organization following treatment of vegetative filaments by Al are examined using vital staining with fluorescein phalloidin. In the cortical cytoplasm of the apical zone of pH 7.5 and pH 4.5 control cells, axially aligned bundles of F-actin lead to a region of diffuse, brightly stained material. Dimly stained focal masses are noted deeper in the cytoplasm of the apical zone whereas they are absent from the zone of vacuolation. The F-actin array is visualized in the cortical cytoplasm of the region of the cell, distal to the apical tip, which exhibits vigorous cytoplasmic streaming (zone of vacuolation) as long, axially aligned bundles with which chloroplasts and mitochondria associate. Thirty minutes following treatment with aluminum, and for the next 8-16 h, the F-actin array is progressively disorganized. The longitudinally aligned F-actin array becomes fragmented. Aggregates of F-actin, such as short rods, amorphous and stellate F-actin focal masses, curved F-actin bundles and F-actin rings replace the control array. Each of these structures may occur in association with chloroplasts or independently with no apparent association with organelles. Images are recorded which indicate that F-actin rings not associated with organelles may self-assemble by successive bundling of F-actin fragments. The fragmentation and bundling of F-actin in cells of V. longicaulis upon treatment with aluminum resembles those reported after diverse forms of cell disturbance and supports the hypothesis that aluminum-induced changes in the F-actin array may be a calcium-mediated response to stress.     

A MORPHOMETRIC STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). III. SUBAPICAL AND MATURE TISSUES

A stereological, morphometric study of the ultrastructure of subapical cells, xylem parenchyma, and cortical chlorenchyma of Echinocereus engelmannii shows that each of these cell types has a unique organellar composition. The cells of any of these tissues are unique not only in vacuolation (which is visible at the light microscope level), but also in the nucleus/cytoplasm ratio and the concentrations of mitochondria, chloroplasts, and dictyosomes. Furthermore, the differences between each of these cell types were large and statistically significant. It had previously been found that the cells of each zone of the shoot apical meristems of E. engelmannii are different from those of the other zones, but the present study suggests that, considering the large ranges of structure possible in the nonapical cells of this species, the apical meristem variation should be considered as only a minor difference and that the meristem zones are really quite similar to each other.     

Life Cycle of the pseudocolonial dinoflagellate Polykrikos kofoidii (Gymnodiniales,Dinoflagellata)

The athecate, pseudocolonial polykrikoid dinoflag‐ellates show a greater morphological complexity than many other dinoflagellate cells and contain not only elaborate extrusomes but sulci, cinguli, flagellar pairs, and nuclei in multiple copies. Among polykrikoids, Polykrikos kofoidii is a common species that plays an important role as a grazer of toxic planktonic algae but whose life cycle is poorly known. In this study, the main life cycle stages of P. kofoidii were examined and documented for the first time. The formation of gametes, 2‐zooid‐1‐nucleus stages very different from vegetative cells, was observed and the process of gamete fusion, isogamy, was recorded. Karyogamy followed shortly after completed plasmogamy. A complex reorganization of furrows (cinguli and sulci) and flagella followed zygote formation, resulting in a 4‐zooid zygote with one nucleus. The fate of zygotes under different nutritional conditions was also investigated; well‐fed zygotes were able to reenter the vegetative cycle via meiotic divisions as indicated by nuclear cyclosis. However, nuclear cyclosis was preceded by a presumably mitotic division of the primary zygote nucleus which by definition would imply that P. kofoidii has a diplohaplontic life cycle. Nuclear cyclosis in germlings hatched from spiny resting cysts indicate that these cysts are of zygote origin (hypnozygotes). Hypnozygote formation, cyst hatching, the morphology of the germling (a 1‐zooid cell), and its development into a normal pseudocolony are documented here for the first time. There is evidence that P. kofoidii has a system of complex heterothallism.     

AN ULTRASTRUCTURAL BASIS FOR HYPHAL TIP GROWTH IN PYTHIUM ULTIMUM

Hyphae of the fungus Pythium ultimum extend by tip growth. The use of surface markers demonstrates that cell expansion is limited to the curved portion of the hyphal apex. Growing and non-growing regions are reflected in internal organization as detected by light and electron microscopy. The young hypha consists of three regions: an apical zone, a subapical zone and a zone of vacuolation. The apical zone is characterized by an accumulation of cytoplasmic vesicles, often to the exclusion of other organelles and ribosomes. Vesicle membranes are occasionally continuous with plasma membrane. The subapical zone is non-vacuolate and rich in a variety of protoplasmic components. Dictyosomes are positioned adjacent to endoplasmic reticulum or nuclear envelope, and vesicles occur at the peripheries of dictyosomes. A pattern of secretory vesicle formation by dictyosomes is described which accounts for the formation of hyphal tip vesicles. Farther from the hyphal apex the subapical zone merges into the zone of vacuolation. As hyphae age vacuolation increases, lipid accumulations appear, and the proportional volume of cytoplasm is reduced accordingly. The findings are integrated into a general hypothesis to explain the genesis and participation of cell components involved directly in hyphal tip growth: Membrane material from the endoplasmic reticulum is transferred to dictyosome cisternae by blebbing; cisternal membranes are transformed from ER-like to plasma membrane-like during cisternal maturation; secretory vesicles released from dictyosomes migrate to the hyphal apex, fuse with the plasma membrane, and liberate their contents into the wall region. This allows a plasma membrane increase at the hyphal apex equal to the membrane surface of the incorporated vesicles as well as a contribution of the vesicle contents to surface expansion.     

Spatial organization of the three‐component lichen Peltigera aphthosa in functional terms

The cephalolichen Peltigera aphthosa (L.) Willd. is characterized by lateral heterogeneity, which manifests itself in the presence of three thallus zones, referred to as the apical, basal and medial zone. These zones differ in terms of interaction between lichen bionts and their physiological activity. The apical thallus zone is more efficient in establishing a contact with cyanobacteria, because of a higher lectin content and a larger overall thallus surface area due to the presence of numerous mycobiont hyphae. Cephalodia are formed in this zone. The interaction between the mycobiont and cyanobiont is more intense in the medial zone. However, the establishment of the contact with cyanobacteria in this zone less probable. The spatial distribution of lectins in the thallus was determined. To reveal the differences in photosynthetic activity in three thallus zones, transient analysis of chlorophyll a fluorescence and the assessment of non‐photochemical quenching of excited chlorophyll states were performed. Assimilation of absorbed light energy was more effective in the medial zone. The basal zone was characterized by decreased photosynthetic activity, lichen dissociation and thallus death.     

THE EFFECTS ON CHRONIC GAMMA RADIATION ON THE INTERNAL APICAL CONFIGURATIONS OF THE VEGETATIVE SHOOT APEX OF COLEUS BLUMEI

Clonal plants of a bushy variety ‘Trailing Queen’ of Coleus blumei Benth. were exposed to 600–700 R per day for 28 days. Shoot apices of control and irradiated plants were studied microscopically. The normal vegetative shoot apex has two tunica layers and a corpus which is partly layered and partly composed of randomly divided cells. Five cytohistological zones were observed, including a cup-shaped zone, imposed upon the tunica-corpus relationships. Response to gamma radiation resulted in abnormal internal apical configurations. All irradiated shoots was found to have the same internal configurations. Only a single tunica layer broken by occasional periclinal divisions was observed. The cytohistological organization of irradiated apices was different from that seen in the normal shoot tips in that two zones were found. Evidence of a radiosensitivity gradient within the vegetative shoot apex was also observed. The possible significance of the results is discussed.     

Apical assemblies of intermediate filament‐like protein FilP are highly dynamic and affect polar growth determinant DivIVA in Streptomyces venezuelae

Streptomyces spp. grow as branching hyphae, building the cell wall in restricted zones at hyphal tips. The organization of this mode of polar growth involves three coiled‐coil proteins: DivIVA and Scy, which form apical protein complexes referred to as polarisomes; and the intermediate filament‐like protein FilP, which influences cell shape and interacts with both Scy and DivIVA. Here, we use live cell imaging of Streptomyces venezuelae to clarify the subcellular localization and dynamics of FilP and its effect on hyphal morphology. By monitoring a FilP‐mCherry fusion protein, we show that FilP accumulates in gradient‐like zones behind the hyphal tips. The apical gradient pattern of FilP localization is dependent on hyphal tip extension and immediately dissipates upon growth arrest. Fluorescence recovery after photobleaching experiments show that FilP gradients are dynamic and subject to subunit exchange during vegetative growth. Further, the localization of FilP at hyphal tips is not directly dependent on scy, even though the strongly perturbed morphology of most scy mutant hyphae is associated with mislocalization of FilP. Finally, we find that filP has an effect on the size and position of the foci of key polar growth determinant DivIVA. This effect likely contributes to the phenotype of filP mutants.     

Study of the behavior of the vegetative meristem of alfa (Stipa tenacissima L.). Cytological and histological approaches

The biological and cytological studies of the vegetative meristem of Stipa tenacissima L. gave clear indication about its structure. It was similar to what was previously described in several species. This meristem showed an axial apical zone constituted by sommital cells of both tunica and corpus, a sub-apical lateral zone, very chromophilous, representing the initial ring and a medullar meristem. The cytofluorimetric determination of DNA in interphasic nuclei of these three zones revealed that the nuclei of the apical and lateral zones were in S phase, announcing the beginning of mitosis and meaning that these zones were the centers of the foliar initiation. The medullar meristem was in dormancy: all the nuclei were in G1 phase.     

CHANGES IN GLYCERALDEHYDE 3-PHOSPHATE DEHYDROGENASE ACTIVITY IN SHOOT APICAL MERISTEMS OF BRASSICA CAMPESTRIS DURING TRANSITION TO FLOWERING

An assay system has been developed for the histochemical determination of glyceraldehyde 3-phosphate dehydrogenase (G3PD) activity to indicate glycolytic pathway capacity during evocation in the shoot apical meristem of Brassica campestris L. G3PD activity was differentially distributed in a zonate pattern within the meristems at the vegetative, the transition, and the floral stages. The activity of G3PD changed in all apical zones of evoked apices, but especially in the central and the peripheral zones of apices at the prefloral stage. In the prefloral stage of development heavy enzyme activity was localized in island-like areas within the peripheral zone. These results indicate that 1) the capacity of glycolysis fluctuates during evocation, 2) during floral evocation the capacity of the glycolytic pathway parallels the capacity of the citric acid cycle and the electron transport system only at the prefloral stage, and 3) G3PD activity marks incipient floral primordia. It is proposed that the enzymic marking of an incipient floral primordium indicates the end of evocation in Brassica.     

Further evidence of a role for abscisic acid in conversion of somatic embryos ofDaucus carota

Summary Somatic embryogenesis has been shown to be an imperfect recapitulation of stages involved to form embryos from vegetative tissues. Although abscisic acid has been implicated in normalizing development, studies that specifically investigate conversion (vegetative leaf initiation) in somatic embryos are lacking. This report documents a follow-up of a study that implicated abscisic acid as a vital factor in allowing embryos ofDaucus carota to progress to the plantlet stage. Abscisic acid was determined to enhance conversion at doses ranging from 1 to 50 µM. Younger embryo stages were more responsive to abscisic acid application with regards to plantlet recovery. Pulses of abscisic acid were shown to elicit more rapid response with younger embryo stages, indicating more plastic development. Fluridone, an abscisic acid synthesis inhibitor, was shown to dramatically reduce conversion, even at low doses (<5µM). When abscisic acid was applied concurrently with fluridone, partial restoration of conversion was observed. Histologically, fluridone was seen to cause pronounced vacuolation in the shoot apical notch which resulted in the loss of meristematic cells, negating conversion capacity. Quantitation of total cytoplasmic area showed that abscisic acid reduced vacuolar intrusion into the apical notch, while fluridone caused a significant increase in vacuolation of cells in this region. This report documents further evidence of a role for abscisic acid in plantlet establishment from somatic embryos ofDaucus carota.     

The inhibition and stimulation of DNA synthesis in shoot apices ofChenopodium rubrum L. during photoperiodic induction of flowering

Three short-day inductive cycles bring about inhibition followed by transitional enhancement of growth, not only in roots and leaves but also in different zones of shoot apical meristem, as shown by measurement of DNA synthesis using³H-thymidine autoradiography. The first inductive cycle resulted in marked inhibition of the cells of the central zone (CZ), rib meristem (RM), and peripheral zone (PZ). Subsequent enhancement of DNA synthesis occurs in RM during the second inductive cycle, but in CZ only in the third cycle. The growth activation in PZ is counteracted by decrease in apical dominance which results in further inhibition of leaf primordia and increases in bud primordia. In plants induced only by one cycle, which later reverse the vegetative pattern of growth and differentiation, increased DNA synthesis in RM and CZ was not observed. The significance of inhibitory and stimulatory processes in particular zones of the shoot apex is discussed considering flower morphogenesis.     

A QUANTITATIVE STUDY OF CELLULAR EVENTS IN THE SHOOT APICAL MERISTEM OF BRASSICA CAMPESTRIS (CRUCIFERAE) DURING TRANSITION FROM VEGETATIVE TO REPRODUCTIVE CONDITION

Vegetative plants were induced to flower by 16-hr-long days. Apical buds were collected at intervals during several developmental phases up to 63 hr. A stereologic analysis and mitotic index study was conducted on median longitudinal sections of shoot apical meristems. A rise in the mitotic index occurred between 12 and 24 hr within central, peripheral and pithrib meristem zones. Preceding the floral stage a second increase in the mitotic index was observed in peripheral and central zones, but not in the pith-rib meristem zone. A significant rise in apical volume, cell number, height, and width began in the transitional stage and continued to the floral stage. Significant correlation coefficients were observed between these apical parameters. Relative volume and cell population of each zone remained constant from the vegetative to the reproductive stage. Volume fraction occupied by the nucleus and nucleolus remained constant within each zone during the same time period. In each zone the volume of the nucleus was significantly correlated to volume of the nucleolus. It appears a pre-inflorescence apex, while larger, is structurally similar to a vegetative apex.     

Les critères nucléaires d'une dormance vraie et totale dans le point végétatif du Fraxinus excelsior L.

Summary The cytohistological criteria for the vegetative shoot apex dormancy in Fraxinus excelsior L. have been quantitatively established with the aid of 3 techniques: historadiography after incorporation of [³H]thymidine, mitotic index and nuclear cytophotometry by the two wavelength method. Nuclear DNA content, mitotic activity and DNA synthesis were compared in 3 different zones(apical zone, lateralzone, rib meristem) of the dormant and non-dormant apices. The periodical break in morphogenetic activity, in contrast to the vegetative period (April to July), is characterized by the absence of zonation and by the fact that meristematic cells remain in the G1 phase of the mitotic cycle. In Fraxinus excelsior L., the meristem dormancy is complete (no DNA synthesis, no mitotic activity and no DNA content greater than 2C).

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Abréviations employées ZA zone apicale axiale du point végétatif - ZL zone latérale ou anneau initial - mm meristème médullaire - UA unité arbitraire de quantité de DNA     

INFLORESCENCE DEVELOPMENT IN BRASSICA CAMPESTRIS L.

Vegetative seedlings of the Ceres strain Brassica campestris L., a quantitative, long-day plant, were induced to flower by exposure to a 16-hr, long-day cycle. Cytohistological and cytohistochemical changes associated with inflorescence development were examined. Developing shoot apices were classified in vegetative, transitional, and reproductive stages. The vegetative apex possessed a biseriate tunica, central zone, peripheral zone and pith-rib meristem. The transitional stage at 48 hr was marked by an increase in size and by a stratification of the upper cell layers of the shoot apex with a concurrent decrease of apical cytohistochemical zonation. The reproductive stage was initiated at 58 hr by periclinal cell divisions in the 3rd and 4th cell layers of the flank region. Cytohistochemical zonation in the vegetative apical meristem was restored in the floral apex. An “intermediate developmental” phase was not observed between the vegetative and reproductive stage.     

Hyphal tip organization inAllomyces arbuscula

Summary Light and electron microscopic observations on vegetative hyphae ofAllomyces arbuscula revealed the specialized organization of the tip. There were some minor differences related to culture conditions, but the main ultrastructural features common to all hyphal tips disclosed a special type of organization distinct from that of other fungi. A crescent-shaped apical zone consisted of vesicles and membrane cisternae embedded in a granular matrix. Vesicles fused with the apical plasmalemma and presumably contributed to its expansion and to wall growth. The apical zone contained few ribosomes and generally no other organelles. Mitochondria were concentrated in the immediate subapical zone and scattered through the remainder of the hyphae, as were microbodies. Microtubules formed an asterlike structure with its center in the apical zone. Proximally of the apex, microtubules were axially oriented. Nuclei occurred only a certain distance from the tip. The elements of the apex may maintain the polarity of the hyphae via a gradient and hold it in a state of vegetative growth.     

Immunolocalization of myosin in rhizoids ofChara globularis Thuill

Summary Myosin-related proteins have been localized immunocytochemically in gravity-sensing rhizoids of the green algaChara globularis using a monoclonal antibody against the heavy chain of myosin from mouse 3T3 cells and a polyclonal antibody to bovine skeletal and smooth muscle myosin. In the basal zone of the rhizoids which contain a large vacuole, streaming endoplasm and stationary cortical cytoplasm, the monoclonal antibody stained myosin-related proteins as diffusely fluorescing endoplasmic strands. This pattern is similar to the arrangement of subcortical actin filament bundles. In the apical zone which contains an aggregation of ER membranes and secretory vesicles for tip growth, diffuse immunofluorescence was detected; the intensity of the signal increasing towards the apical cell wall. The most prominent myosin-staining was associated with the surface of statoliths in the apical zone. The polyclonal antibody produced a punctate staining pattern in the basal zone, caused by myosin-related proteins associated with the surface of drganelles in the streaming endoplasm and the periphery of the nucleus. In the apical zone, this antibody revealed myosin-immunofluorescence on the surface of statoliths in methacrylate-embedded rhizoids. Neither antibody revealed myosin-immunofluorescence on the surface of organelles and vesicles in the relatively stationary cytoplasm of the subapical zone. These results indicate (i) that different classes of myosin are involved in the various transport processes inChara rhizoids; (ii) that cytoplasmic streaming in rhizoids is driven by actomyosin, corresponding to the findings onChara internodal cells; (iii) that actindependent control of statolith position and active movement is mediated by myosin-related proteins associated with the statolith surfaces; and (iv) that myosin-related proteins are involved in the process of tip growth.     

Scanning Electron Microscopy of the Adoral Ciliary Zone of Cycloposthium Bundle (Ciliophora,Entodiniomorphida)1

The adoral ciliary zone of Cycloposthium spp., inhabiting the large intestine of the horse, was studied by scanning electron microscopy. It could be divided into four parts: outer, inner, left, and right zones. The outer zone, extending on the anterior periphery of the apical cone of the body, had 20 tuft-like syncilia arranged radially around the longitudinal axis. Each ciliary tuft consisted of about 170 cilia, and in cross section it had a rectangular shape. The cilia of the inner zone, situated at the top of the apical cone, were aggregated irregularly to form shorter bundles than the tufts of the outer zone. The innermost cilia of this zone were shorter than the outermost. There was a distinct non-ciliated border between the outer and inner zones. A horseshoe-like operculum having no cilia was present at the center of the adoral ciliary zone, and the opening of the vestibulum was situated as a cleft crossing from the center to the right periphery of this zone. No cilia extended onto the vestibular wall. The left ciliary zone was situated beneath the outer zone and consisted of five short rows of barren kinetosomes of which only the central row possessed very short cilia. The right ciliary zone, consisting of a few rows of cilia situated at the bottom of the inner adoral lip, was also easily distinguished from the other ciliary zones. This zone was interpreted as an extension of the outer adoral zone passing along the right side of the apical cone. These ciliary zones were considered to be highly differentiated and useful for both movement of and ingestion of food.