Subapical Cells Research Articles

Rhizogénèse adventive dans l'épicotyle du Pois : initiation et structuration de la racine

The ability of the first internode of Pisum sativum epicotylary axis to produce adventitious roots was examined with respect to dark and light growing conditions and depth position of the soaked seeds in vermiculite. Root initiation began on the 7th day and moved acropetally until the 16th day, with no formation of new roots between older ones. DNA synthesis in the diploid interfascicular cells followed the first mitoses of root initiation. The cell layer that would normally have differentiated into an endodermis reacted more slowly and built a primordial root cap, protecting the root as it grew through the cortex. When the new root emerged a new root cap – protoderm complex was rebuilt from the subapical cells of the root meristem. The increase in cell number was highest during the first 24 h following root initiation, and the cell cycle was extremely short with a very reduced G1 phase. Only the elongation zone of the mature adventitious root showed polyploidy.

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.

Cytoplasmic microtubules and fungal morphogenesis: ultrastructural effects of methyl benzimidazole-2-ylcarbamate determined by freeze-substitution of hyphal tip cells.

The effects of methyl benzimidazole-2-ylcarbamate (MBC), one of only a few agents that are active against microtubules of fungi, were analyzed at the ultrastructural level in freeze-substituted hyphal tip cells of Fusarium acuminatum. Nontreated and control cells had numerous microtubules throughout. After just 10 min of exposure to MBC, almost no cytoplasmic microtubules were present, except near spindle pole bodies. After 45 min of exposure to MBC, no microtubules were present in hyphal tip cells, but they were present in the relatively quiescent subapical cells. These observations suggested that there are different rates of turnover for cytoplasmic microtubules in apical and subapical cells and for microtubules near spindle pole bodies and that MBC acts by inhibiting microtubules assembly. A statistical analysis of the distribution of intracytoplasmic vesicles in thick sections of cells treated with MBC, D2O or MBC + D2O was obtained by use of a high-voltage electron microscope. More than 50% of the vesicles in the apical 30 micrometers of control cells were found to lie within 2 micrometers of the tip cell apex. MBC treatment caused this vesicle distribution to become uniform, resulting in a substantial increase in the number of vesicles in subapical regions. The reduction in the number of cytoplasmic microtubules, induced by MBC, apparently inhibited intracellular transport of these vesicles and rendered random the longitudinal orientation of mitochondria. In most cases, D2O appeared capable of preventing these MBC-effects through stabilization of microtubules. These observations support the "vesicle hypothesis" of tip growth and establish a transport role for cytoplasmic microtubules in fungal morphogenesis.

Studies onMetamastophora(Corallinaceae, Rhodophyta). I.M. flabellata(Sonder) Setchell: Morphology and anatomy

A morphological-anatomical study of Australian populations of Metamastophora flabellata (Sonder) Setchell, the type species of Metamastophora (Corallinaceae, Rhodophyta), has revealed that the primarily erect or ascending non-geniculate thallus possesses a dorsi-ventral organization of tissues. All conceptacles are uniporate and arise dorsally. Two distinct vegetative meristems occur: an apical primary meristem from which hypothallial cells are produced basipetally and a sub-epithallial secondary meristem which generates perithallial cells basipetally and secondary epithallial cells acropetally. Primary epithallial cells arise from divisions of subapical hypothallial cells. In younger parts, tissues are produced only dorsal to the hypothallium; in veins and stipes, tissue production occurs both dorsal and ventral to the hypothallium. Mature tetrasporic conceptacles contain peripheral tetrasporangia with zonately divided contents and a central sterile columella. Gametic conceptacles produce fertile tissue across the entire conceptacle chamber floor. After fertilization, the zygotic nucleus or a derivative is transferred (presumably) to an auxiliary cell through cells of the carpogonial branch; no tubular transfer siphon develops. Mature fusion cells are composed of the amalgamated supporting cells of carpogonial branches and are initiated from a single supporting cell which functions as an auxiliary cell. Unbranched 3–4 celled gonimoblast filaments arise from the fusion cell, do not become connected to other cells, and produce terminal carposporangia. Results from this study have led to a redefinition of hypothallium and perithallium in relation to meristems rather than substrate. In addition, carposporophyte ontogeny in the Corallinaceae is considered in terms of the presumed mode of transfer of the zygotic nucleus to the fusion cell, the extent of fusion cell development, and gonimoblast filament production in relation to auxiliary cells and fusion cells.

Patterns of cell elongation in some Audouinella spp. (Acrochaetiaceae: Rhodophyta)

Patterns of cell elongation and differentiation were examined in a number of Audouinella spp. In all species examined, cells reached their maximum length within a few cells of the apex after which the length (and diameter) of cells became stable. No size differentiation of cells associated with lateral branches was observed, although the basal cells of lateral branches were usually smaller than those in the main axis (except in A. purpurea (Lightf.) Woelkerling and A. parvula (Kylin) Dixon). The position of the lateral branches on branch bearing cells showed little variation and could be used to develop quantitative characteristics for species delimitation. Early stages of development in A. secundata (Lyngbye) Dixon showed an increase in size of apical and subapical cells with plant height, with cell length decreasing toward the base. These developmental features confirm the simple morphology of Audouinella spp, and show that models of growth and differentiation proposed for the Ceramiaceae generally do not apply to the Acrochaetiaceae.

Development of cells and hyphae of Geotrichum candidum in chemostat and batch culture

Apical cells of hyphae of Geotrichum candidum Link ex Pers. increase in width and volume as specific growth rate increases during glucose-limited chemostat culture and culture on solid media. Subapical-cell length is constant over a wide range of initial glucose concentrations on solid media but decreases with increasing specific growth rate in chemostat culture. In chemostat culture the mycelial fragments display the following trends as specific growth rate increases: the main hypha of each fragment decreases in length; the mean extension rate of the apical cells increases; more of the non-apical cells of the main hyphae form lateral branches and, ultimately, some of the subapical cells of the main hyphae form lateral branches as do some of the non-apical cells of the primary lateral branches. The length of the hyphal growth unit is constant for a range of sizes of mycelial fragments at a given specific growth rate and temperature but decreases markedly with increasing specific growth rate. The volume of the hyphal growth unit is constant over a wide range of specific growth rates at 25°C and at 30°. A model is derived for growth of G. candidum in chemostat culture at the specific growth rates and temperatures studied. The development and differentiation of somatic hyphae is discussed in relation to different culture conditions.

The oxiditative enzyme pattern in the regenerating tail of the scincid lizard,Mabuya striata

Summary The histochemical distribution of iso-citrate dehydrogenase, succinate dehydrogenase, cytochrome oxidase and reduced di-and tri-phosphopyridine nucleotide diaphorases in various tissues of the normal and regenerating tail of the scincid lizard, Mabuya striata was studied. Differences in localization and intensity of enzyme activity were observed in epidermal cells and skeletal muscles of the normal tail; and wound epithelial cells, sub-apical cells, blastema cells and differentiating myofibres of the regenerating tail. The significance of these enzymes in correlation with the regenerative processes in this reptile is discussed.

EXPERIMENTAL STUDIES OF CYTOPLASMIC ORGANIZATION IN ZONARIA APICAL CELLS

Rapidly growing thalli of Zonaria have been treated with colchicine (10,000, 5,000, and 1,250 μg/ml) and phenobarbital (10,000 and 5,000 μg/ml). Responses of the meristems to drug treatment have been described at the ultrastructural, cellular, and organismic levels. With colchicine treatment microtubule disruption, nuclear envelope abnormalities, nuclear lobing, and increased nuclear sizes were observed. Apical cells ceased dividing but continued to elongate until cessation of treatment when abnormal cytokineses, irregular cell shapes, and cell deaths were observed. The coordinated growth of the meristem margin was disrupted, regions recovering left a zone line on the thalli, and cell deaths resulted in thallus branching. With phenobarbital treatment ER elaboration, inactive‐appearing dictyosomes, pyrenoid‐like structures, and deeply cup‐shaped mitochondria aggregated with ER were observed. Apical cell elongation ceased and the mitotic cycle appeared to be arrested just after cytokinesis. Recovery from 10,000 μg/ml phenobarbital was low, with subapical cells and occasional apical cells starting new thalli. Recovery from 5,000 μg/ml was good, with occasional cell death dividing the meristem and zone lines resulting from the temporary synchrony. Correlations of responses at the ultrastructural, cellular, and organismic levels are discussed.

Experimental Studies of Cytoplasmic Organization in Zonaria Apical Cells

Rapidly growing thalli of Zonaria have been treated with colchicine (10,000, 5,000, and 1,250 μg/ml) and phenobarbital (10,000 and 5,000 μg/ml). Responses of the meristems to drug treatment have been described at the ultrastructural, cellular, and organismic levels. With colchicine treatment microtubule disruption, nuclear envelope abnormalities, nuclear lobing, and increased nuclear sizes were observed. Apical cells ceased dividing but continued to elongate until cessation of treatment when abnormal cytokineses, irregular cell shapes, and cell deaths were observed. The coordinated growth of the meristem margin was disrupted, regions recovering left a zone line on the thalli, and cell deaths resulted in thallus branching. With phenobarbital treatment ER elaboration, inactive-appearing dictyosomes, pyrenoid-like structures, and deeply cup-shaped mitochondria aggregated with ER were observed. Apical cell elongation ceased and the mitotic cycle appeared to be arrested just after cytokinesis. Recovery from 10,000 μg/ml phenobarbital was low, with subapical cells and occasional apical cells starting new thalli. Recovery from 5,000 μg/ml was good, with occasional cell death dividing the meristem and zone lines resulting from the temporary synchrony. Correlations of responses at the ultrastructural, cellular, and organismic levels are discussed.

Cytochemical and Subcellular Organization of the Shoot Apical Meristem of Dry and Germinating Jack Pine Embryos

Water added to dry seeds of jack pine evoked changes at the shoot apex which reflected the mobilization and consumption of food reserves for the synthesis of new protoplasm and for the formation of the prospective shoot. In shoot apices the distal zone contained large cells having nuclei with low affinity for stains. Flank and subapical zones had smaller cells with densely stained nuclei. These features did not alter during imbibition nor after the first wave of mitosis at 96 h when radicles emerged from seeds. Cells in the distal zone divided rarely. Mitosis, most abundant in subapices, decreased acropetally to the distal zone.In dry seeds, protein bodies and amyloplasts increased in size and frequency per cell from the distal zone in the basipetal direction towards the flanks and subapical zone. During imbibition most proteinaceous reserves, rich in arginine N, disappeared from all cells of the shoot apex. Lipid bodies in distal cells were small, numerous, and disappeared slowly as germination proceeded. Proplastids, mitochondria, and unidentified pleomorphic bodies were distributed in close proximity to the nucleus particularly in apical cells.Products of imbibed tritiated water (nonexchangeable tritium), and tritiated D-glucose, thymine, thymidine, and uracil were localized at the shoot apex and revealed assimilatory patterns consistent with cytochemical zonation. The covalent incorporation of tritium from water, while spread through most cells, was greatest in nuclei of flank and subapical cells. The radioactivity from glucose was found mainly in cell walls. Labelling by uracil, thymine, and thymidine by salvage mechanisms was especially abundant in nuclei of the flanks. These events coincided with dense RNA and DNA staining during germination. Collectively, observations indicated that at the flanks, water and seed reserves were concentrated at specific binding sites for the synthesis and dispersion of macromolecules before the visual production of leaf primordia.

Red Light Enhancement of the Phototropic Response of Etiolated Pea Stems

In the subapical third internode of 7-day-old etiolated pea seedlings, the magnitude of phototropic curvature in response to continuous unilateral blue illumination is increased when seedlings are pre-exposed to brief red light. The effect of red light on blue light-induced phototropism becomes manifest maximally 4 or more hours after red illumination, and closely parallels the promotive action of red light on the elongation of the subapical cells. Ethylene inhibits phototropic curvature by an inhibitory action on cell elongation without affecting the lateral transport of auxin. Pretreatment of seedlings with gibberellic acid causes increased phototropic curvature, but experiments using (14)C-gibberellic acid indicate that gibberellic acid itself is not laterally transported under phototropic stimuli. Neither red light nor gibberellic acid treatment has any promotive effect on blue light-induced lateral transport of (3)H-indoleacetic acid. Under conditions where phototropic curvature is increased by red light treatment, low concentrations of indoleacetic acid applied in lanolin paste to the apical cut end of the seedling cause an increased elongation response in subapical tissue. This could explain increased phototropic curvature caused by red light treatment.

The ultrastructure of Geotrichum candidum hyphae.

Complete septa divide the hyphae of Geotrichum candidum into many compartments or cells. Two cell types are readily recognizable, (i) the apical cell, delimited by one septum and the surrounding cell wall, and (ii) the sub- or non-apical cell, delimited by two septa and the surrounding cell wall. Vacuolation of the apical cells is slight compared with that of subapical cells. Apical cells contain many vesicles, some of which are elongated and branched, possibly forming an interconnecting tubular network; other vesicles were observed distributed about the apical zone or aggregated to form an apical body (the Spitzenkörper). Vesicles are also evident in subapical cells, but only in association with developing septa. Golgi bodies were not observed in any cells, their function in vesicle production possibly being taken by a modification of part of the endoplasmic reticulum. Both cell types contained mitochondria with contrasting electron-staining properties. Some stages of mitosis were observed. The nucleus appears to retain its envelope throughout division and exhibits intranuclear microtubules attached to spindle plaques. Septa were formed by a centripetal deposition of wall material, plasmodesmata developing during this process. Another deposition of wall material occurs after the centripetal deposition thus allowing a secondary thickening of the septum to take place.

Unusual Nucleolar Behaviour of the Rice Shoot Apex

SUMMARYAfter histological examination of young seedlings of rice shoot apex, a group of prominent nucleoli, localized in a few sub-apical cells, was detected. Three different staining techniques for cytochemical localization of nucleic acids, including enzymatic digestion, revealed a presence of small nucleolar bodies, which were pepsin resistant, and an amorphous susbstance, which was pepsin susceptible.The pair of nucleolar bodies adopted frequently either a parallel or perpendicular position with respect to the surface of shoot apex. The possible dependence of nucleolar orientation upon the nuclear rotation, as a consequence of deeply penetrating nucleolar chromatin, is admitted.The apparent resemblance with other examples of unusual nucleolar behaviour (in both plant and animal gametogenesis and in some cases of induced structural alterations) is discussed.

Intracellular Distribution of Mitochondria after Geotropic Stimulation of the Oat Coleoptile

The number of mitochondria is greater in the bottom than in the top of cells in geotropically stimulated oat (Avena sativa) coleoptiles. In the avascular tip and outer epidermis of subapical regions this difference occurs only in the lower tissues. These inequalities are found both in the KMnO(4) (-) and in the glutaraldehyde-fixed tissues; however, they are significant only in the former. Also, the number of mitochondria scored is consistently lower when the tissues were fixed in KMnO(4). These results suggest that mitochondria undergo a small degree of sedimentation after geostimulation, a redistribution reduced by the slower fixation with glutaraldehyde. Differences in mitochondrial number begin later than those in the amyloplast and the Golgi apparatus after geotropic stimulation. The cells in the avascular-tip region (a region having an important role in geotropism) have two to three times more mitochondria than the subapical cells.

Morphogenesis in the red alga, Griffithsia pacifica: Regeneration from single cells

The regeneration of plants of the red alga Griffithsia pacifica from single, isolated cells is described. Regeneration can start from any cell and is triggered by the removal of an abutting cell. An isolated, single shoot cell forms a shoot and a rhizoidal cell within one day. The shoot then adds new cells by apical division at the rate of 1-2 cells/day; branches are formed at predictable but not fixed locations by budding of subapical cells. Each shoot cell enlarges for 6-8 days. The resulting plant consists of uniseriate, pseudodichotomously-branched shoot filaments with multicellular rhizoidal filaments at their base. The predictability and rapidity of this development combined with the large size of these cells (1.0×0.2 mm) facilitate developmental studies on this organism.

INITIATION AND ONTOGENY OF THE MICROSPORANGIATE CONE IN CUPRESSUS ARIZONICA IN RESPONSE TO GIBBERELLIN

Cupressus arizonica, a member of the Cupressaceae, was induced to produce pollen cones in response to gibberellin treatment. All apices remained vegetative during the first 17 days of treatment. At this time many lateral vegetative apices began to undergo a transition to the reproductive state. The transition was marked by changes in apical zonation characterized by increased mitotic activity primarily in the subapical mother‐cell and peripheral zones. Also, precocious initiation of branch meristems occurred much higher on the shoot apex than before the transition period. About 22 days after the initial treatment, most apices became distinctly reproductive. The reproductive apex has a zonation pattern similar to the branching apex but is shorter and wider and quite distinct from the vegetative apex. The small subapical mother cells and cells of the peripheral zone form a continuous mantle of mitotically active cells with prominent nucleoli. This mantle encloses a very broad pith region which differentiates nearly to the summit of the apex. Microsporophyll and leaf initiation are similar and the protoderm of the apex remains discrete and does not contribute to deeper tissues. Sporangia do not originate from superficial cells of the microsporophyll. After all microsporophylls are initiated the reproductive apex becomes inactive. A discussion concerns the morphological implication in the origin of the foliar structures and of the similarity of C. arizonica to many angiosperms in the transition of the apex from vegetative to reproductive.

Effects of chronic gamma-irradiation on Pinus pinea and Pinus halepensis

A histological analysis has been made on shoot apices of Pinus pinea and Pinus halepensis collected from plants chronically irradiated in the gamma field ( 60Co source) at exposure rates of 18·8, 9·0, 4·30 and 2·64 R/day up to 1513 days, from plants exposed to 36·2, 20·6, 5·02 and 2·10 R/day and allowed to recover after receiving 1000 and 4000 R total exposures, and from plants exposed to 10 R/day and removed after total exposures of 200, 400, 500, 600, 700 and 800 R. The radiation response was evaluated by mitotic inhibition of the apical and subapical meristem cells in the shoot apices, necrosis of vegetative buds and plant death. A different radiosensitivity has been found among cells in the apical meristem and between apical and sub-apical meristems. The possible explanation of the different radiosensitivities is discussed in terms of duration of the cell cycle and DNA content of the nuclei. The very high radiosensitivity of the two species has been ascertained even at low exposures. The species P. halepensis, showing a higher radiosensitivity than P. pinea, has a DNA content per nucleus 17 per cent greater than the latter. The import role of the exposure rate has been clearly established for all the parameters studied. In fact, at all exposure rates, the higher the rate the lower is the total exposure needed for a given effect.

THE LASER AS A TOOL FOR ISOLATING SINGLE CELLS

A laser pulse of 15 millijoules was used to rupture the wall of the target cell of the protonema of the moss Bryum stenotrichum C.M. and cause the protoplast to be ejected. A technique for isolating cells from an organized system, such as a moss protonema, by the laser beam is described. The behavior of the isolated apical and subapical cells in culture has been compared.

Changes in Behavior of Mosses Treated with Ultrasound

Short, growing filaments of moss protonema [Physcomitrium turbinatum (Mx.) Bridel] develop readily on mineral agar into normal plants; they produce branch filaments by a regular pattern of lateral divisions of the subapical cells and extend themselves linearly by divisions of the apical cell. Divisions of intercalary cells that produce two in-line daughters in the main axis are quite rare, elongation of the axis being essentially restricted to the apical cell. Brief, low-amplitude, topically applied sonic vibration of subapical, individual cells in the filament frequently results in the division of the treated cell or one of its near or distant intercalary neighbors so as to produce in-line daughters. Persistent differences in growth pattern as compared to normal plants have been observed in several cultures of moss plants derived from single, treated, apical cells; they are described.

The Growth of Protonema Cells and Rhizoids in Bracken

The elongation of protonema cells and rhizoids of bracken was studied by the direct measurement of shift of marks which were made on wall surface with the micromanipulator. Those cells showed a typical tip growth. The apical cell of protonema elongates chiefly in its apical dome, very slightly in the region more than 10 microns from the apical end, and not at all in the region over 30 microns from the apical end. Some of the subapical cells elongate and make the tip growth, but their elongation is less than that of the apical cell. The third cell, less often than the subapical cell, grows slightly at its apical end. The fourth cell does not elongate. There exists the polarity, from apex to base, of the possibility and ability of elongation not only in each protonema cell but also in a protonema as a whole. The nearer to the apex the given part is, the larger the possibility and ability of elongation are, and vice versa. Rhizoids elongate at the part within 1 micron or less of their tip.