206 research outputs found

    The organelle of differentiation in embryos: the cell state splitter

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    Differentiation of stomatal meristemoids and guard cell mother cells into guard-like cells in Vigna sinensis leaves after colchicine treatment - An ultrastructural and experimental approach

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    The temporary development of Vigna sinensis seedlings in the presence of colchicine results in the inhibition of stomata generation and the formation of numerous persistent stomatal meristemoids (P-SM) and guard cell mother cells (P-GMC). Before dividing differentially or becoming GMC, the untreated meristemoiidsundergo a 'preparatory' differentiation, during which a synthesis of new densely ribosomal cytoplasm, an increase of nuclear size, and a detectable proliferation of all the organelles are observed. The same process appears depressed and delayed in treated meristemoids; the cells have usually undergone only part of it when they reach the C mitosis. After the inhibition of their division, the bulged meristemoids II and GMC increase further in size, synthesize new nonribosomal cytoplasm, and start vacuolating slowly. The plastids also increase in size, change in shape, and become able to synthesize large quantities of starch. The cells retain a ribosomal cytoplasm, rough ER membranes, and active dictyosomes for a long time. At the advanced stages of differentiation, the microtubules reappear in the cells even when the plant remains under colchicine treatment. When mature, the P-GMC and P-SM are quite similar to the guard cells and possess considerably thickened periclinal walls, numerous mitochondria, and small vacuoles, while the nucleus, the plastids, and the cytoplasm occupy significant parts of the cell volume. In the epidermis displaying open stomata in light, significant K+ quantities are detectable in guard cells and P-GMC or P-SM, while they are almost absent from their surrounding cells. When the stomata close in darkness, K+ is accumulated primarily in the subsidiary or typical epidermal cells surrounding these idioblasts and only minimally inside them. Besides, the P-GMC and P-SM, like the guard cells, retain the starch for a long time and build up considerable starch quantities from exogenously supplied sugars. © 1977 Springer-Verlag


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    In primary leaves of Vigna sinensis Endl. seedlings growing in the presence of colchicine, some giant polyploid cells at the phloem pole of the vascular bundles differentiate into an extraordinary cell type acquiring most of the prominent features of the sieve elements. These aberrant sieve elements (a-SEs) show some exceptional features not found in normal sieve-tube elements. A heavy secondary thickening is deposited on their walls, which in surface view exhibits a reticulate or ‘pitted’ appearance. The thickenings reach the same depth into the cytoplasm and show a diffuse orientation of cellulose microfibrils. Amorphous callose is deposited around some fine cytoplasmic strands penetrating the secondary wall up to the primary one along the longitudinal and particularly the transverse walls. In the latter walls sieve pores are not formed. At first, the a-SEs undergo a partial protoplasmic degradation like normal sieve-tube elements but finally autolyse completely. During this process, the major portion of the unmasked primary and secondary wall is dissolved. The observations show that (a) a vascular bundle initiation and development proceeds in the affected primary leaves, in the absence of cell divisions. ‘Vascular bundles’ are formed consisting of a single row of cells, in which a-SE differentiation dominates. (b) The ‘sieve element differentiation’ is initiated without differential divisions. (c) Although the microtubules are absent, the cells determined to become a-SEs activate another morphogenetic mechanism to control a more or less patterned deposition of a secondary wall layer

    Microtubules and epithem-cell morphogenesis in hydathodes of Pilea cadierei

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    When cell divisions have ceased, the epithem of the hydathodes of Pilea cadierei Gagnep. et Guill. consists of small polyhedral cells exhibiting a meristematic appearance, and completely lacks intercellular spaces. The cortical microtubules in epithem cells exhibit a unique organization: they are not scattered along the whole wall surface but form groups lying at some distance from each other. In sections, from two to eight groups of microtubules can be observed, each lining a wall region averaging between 0.5 and 1.5 μm in length. These groups represent sections of microtubule bundles girdling a major part or the whole of the cell periphery. They are connected to one another by anastomoses, forming a microtubular reticulum. The assembly of microtubule bundles is followed by the appearance of distinct local thickenings in the adjacent wall areas. The cellulose microfibrils in the thickenings are deposited in parallel to the underlying microtubules. Gradually, the vacuolating epithem cells undergo swelling, except for the areas bounded by the wall thickenings. Since the latter, and actually their constituent bundles of cellulose microfibrils, cannot extend in length the differential cell growth results in schizogenous formation of intercellular spaces between contiguous cell walls at their thickened regions. The spaces then broaden and merge to become an extensive intercellular space system. As a result of the above processes, the epithem cells become constricted and finally deeply lobed. The observations show that (i) the cortical microtubules are intimately involved in the morphogenesis of the epithem cells and (ii) the initiation and development of the epithem intercellular spaces is a phenomenon directly related to cell morphogenesis and therefore to the cortical microtubule cytoskeleton. The sites of initiation of these spaces are highly predictable. © 1988 Springer-Verlag

    Studies on the development of the air pores and air chambers of Marchantia paleacea - IV. Cell plate arrangement in initial aperture cells

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    In Marchantia paleacea some of the superficial thallus cells are dividing at the same time as intercellular spaces (ISs) of the initial apertures (IAs) are being formed. The cell plate of these divisions exhibit a highly variable, but predictable to a significant extent, final arrangement. In many anticlinal and periclinal divisions, one or more cell plate edges following a curved path or continuously changing direction of growth, join(s) the parent wall regions delimiting the lower part of the IS(s) or that lining the surface cavity(ies) (SCs). The rest of the cell plate margins usually meet the parent wall region below the cortical cytoplasmic zone of the incomplete preprophase microtubule band (PMB) as well as at unpredictable wall positions which vary considerably along the older wall. The preferential final alignment of the cell plate probably mirrors an intimate interaction between its expanding margins and the cortical cytoplasmic site abutting on the wall facing the lower part of the growing IS(s) or that underlying the SC(s). These regions, as well as that of the incomplete PMB, appear able to control the direction of growth of the cell plate. In the former sites, prominent microtubule (MT) organizing centres (MTOCs) seem to operate during interphase and probably during preprophase-prophase. The mechanism of cell plate arrangement described above, successfully explains the formation of cell plates of highly abnormal shape laid down in IA cells of Marchantia paleacea. The phragmoplast-cell plate system responds to orientation mechanism(s) operating in different cytoplasmic sites, and seriously affecting cell plate morphogenesis. The MTOC cortical cytoplasmic sites, adjacent to the wall lining the lower part of the developing IS(s), behave like the PMB cortical zone. © 1985 Springer-Verlag

    Immunofluorescence and electron microscopic studies of microtubule organization during the cell cycle of Dictyota dichotoma (Phaeophyta, Dictyotales)

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    Interphase cells of Dictyota dichotoma (Hudson) Lamour. lack cortical microtubules (Mts) but display an impressive network of cytoplasmic microtubules (c-Mts). These are focussed on two opposed perinuclear centriolar sites where centrin or a centrin-homologue is localized. Some of the Mts surround the nucleus, but the majority traverse the cytoplasm as bundles variously directed towards the plasmalemma. In apical cells, and to a lesser extent in the square or slightly elongated meristematic cells, Mts are more or less evenly arranged. In elongated cells they form thick bundles longitudinally traversing the cytoplasm; a pattern maintained in differentiated cells. In early prophase the non-perinuclear Mts disappear but by late prophase a bi-astral arrangement of short Mts is observed. They enter polar nuclear depressions and attach to differentiated regions of the nuclear envelope where polar gaps open. By metaphase the spindle Mts converge on the centrioles at the polar gaps. At anaphase, interzonal Mts are evident and the asters start to reassemble. After telophase disruption of the interzonal Mts, the daughter nuclei approach each other, but move apart again before cytokinesis. The latter movement keeps pace with the development of two interdigitating Mt systems, ensheathing both daughter nuclei. The partition membrane "bisects" this Mt "cage". Between telophase and cytokinesis the centrosomes separate, finally occupying opposed perinuclear sites. New Mts arise at the new centrosomes, some terminating on the consolidating partition membrane. Our data show that D. dichotoma vegetative cells display a prominent cytoplasmic Mt cytoskeleton, which undergoes continual, but definite, change in organization during the cell cycle. © 1992 Springer-Verlag

    Patterns of microtubule reappearance in root cells of Vigna sinensis recovering from a colchicine treatment

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    The reticulum of paracrystalline tubulin strands, which is assembled in meristematic root cells of Vigna sinensis treated with a 0.08% colchicine solution, disaggregates and microtubules (Mts) reappear after a 10-14 h recovery of the seedlings from the drug. In recovering interphase cells, Mts reappear in the cortical cytoplasm. Initially, they are short and aligned in different directions but finally they elongate and usually become oriented transversely to the long cell axis. A single or a pair of preprophase Mt bands (PMBs) is organized in cells enclosing one or more nuclei. Simultaneously, Mts traverse the perinuclear cytoplasm. In recovering C-mitotic cells, Mt bundles emerge from the kinetochores. Initially, they exhibit diverse orientations. Afterwards, the C-chromosomes are aligned on ametaphase plate via kinetochore Mt bundles, which become parallel to one another. As time passes non-kinetochore Mts appear among the chromosomes and anaphase proceeds. In recovering cytokinetic cells, normal, abnormally curved or branched phragmoplasts are organized. The latter arise between the nuclei of multinucleate telophase cells or between the lobes of forming polyploid nuclei. In cells which were blocked at an advanced cytokinetic stage by colchicine, phragmoplasts return to the margins of the incomplete "cell walls". The observations presented here suggest that in recovering colchicine-treated root cells the Mts and the tubulin reticulum are interchangeable. Although Mts appear in cytoplasmic sites where they are expected to be nucleated, the pattern of Mt reformation differs from that operating in normal and to a smaller extent from that functioning in cells recovering from other anti-Mt drugs. © 1991 Springer-Verlag

    The role of the cytoskeleton in the morphogenesis and function of stomatal complexes

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    Microtubules (MTs) and actin filaments (AFs) form highly organized arrays in stomatal cells that play key roles in the morphogenesis of stomatal complexes. The cortical MTs controlling the orientation of the depositing cellulose microfibrils (CMs) and affecting the pattern of local wall thickenings define the mechanical properties of the walls of stomatal cells, thus regulating accurately their shape. Besides, they are involved in determination of the cell division plane. Substomatal cavity and stomatal pore formation are also MT-dependent processes. Among the cortical MT arrays, the radial ones lining the periclinal walls are of particular morphogenetic importance. Putative MT organizing centers (MTOCs) function at their focal regions, at least in guard cells (GCs), or alternatively, these regions either organize or nucleate cortical MTs. AFs are involved in cell polarization preceding asymmetrical divisions, in determination of the cell division plane and final cell plate alignment and probably in transduction of stimuli implicated in stomatal complex morphogenesis. Mature kidney-shaped GCs display radial AF arrays, undergoing definite organization cycles during stomatal movement. They are involved in stomatal movement, probably by controlling plasmalemma ion-channel activities. Radial MT arrays also persist in mature GCs, but a role in stomatal function cannot yet be attributed to them. © New Phytologist