Topology of microtubules and actin in the life cycle of Xanthophyllomyces dendrorhous (Phaffia rhodozyma)

The morphology of budding and conjugating cells and associated changes in microtubules and actin distribution were studied in the yeast Xanthophyllomyces dendrorhous (Phaffia rhodozyma) by phase-contrast and fluorescence microscopy. The non-budding interphase cell showed a nucleus situated in the central position and bundles of cytoplasmic microtubules either stretching parallel to the longitudinal cell axis or randomly distributed in the cell; none of these, however, had a character of astral microtubules. During mitosis, the nucleus divided in the daughter cell, cytoplasmic microtubules disappeared and were replaced by a spindle. The cytoplasmic microtubules reappeared after mitosis had finished. Actin patches were present both in the bud and the mother cell. Cells were induced to mate by transfer to ribitol- containing medium without nitrogen. Partner cells fused by conjugation projections where actin patches had been accumulated. Cell fusion resulted in a zygote that produced a basidium with parallel bundles of microtubules extended along its axis and with actin patches concentrated at the apex. The fused nucleus moved towards the tip of the basidium. During this movement, nuclear division was taking place; the nuclei were eventually distributed to basidiospores. Mitochondria appeared as vesicles of various sizes; their large amounts were found, often lying adjacent to microtubules, in the subcortical cytoplasm of both vegetative cells and zygotes.     

Microtubules and actin cytoskeleton in Cryptococcus neoformans compared with ascomycetous budding and fission yeasts

Actin cytoskeleton and microtubules were studied in a human fungal pathogen, the basidiomycetous yeast Cryptococcus neoformans (haploid phase of Filobasidiella neoformans), during its asexual reproduction by budding using fluorescence and electron microscopy. Staining with rhodamine-conjugated phalloidin revealed an F-actin cytoskeleton consisting of cortical patches, cables and cytokinetic ring. F-actin patches accumulated at the regions of cell wall growth, i. e. in sterigma, bud and septum. In mother cells evenly distributed F-actin patches were joined to F-actin cables, which were directed to the growing sterigma and bud. Some F-actin cables were associated with the cell nucleus. The F-actin cytokinetic ring was located in the bud neck, where the septum originated. Antitubulin TAT1 antibody revealed a microtubular cytoskeleton consisting of cytoplasmic and spindle microtubules. In interphase cells cytoplasmic microtubules pointed to the growing sterigma and bud. As the nucleus was translocated to the bud for mitosis, the cytoplasmic microtubules disassembled and were replaced by a short intranuclear spindle. Astral microtubules then emanated from the spindle poles. Elongation of the mitotic spindle from bud to mother cell preceded nuclear division, followed by cytokinesis (septum formation in the bud neck). Electron microscopy of ultrathin sections of chemically fixed and freeze-substituted cells revealed filamentous bundles directed to the cell cortex. The bundles corresponded in width to the actin microfilament cables. At the bud neck numerous ribosomes accumulated before septum synthesis. We conclude: (i) the topology of F-actin patches, cables and rings in C. neoformans resembles ascomycetous budding yeast Saccharomyces, while the arrangement of interphase and mitotic microtubules resembles ascomycetous fission yeast Schizosaccharomyces. The organization of the cytoskeleton of the mitotic nucleus, however, is characteristic of basidiomycetous yeasts. (ii) A specific feature of C. neoformans was the formation of a cylindrical sterigma, characterized by invasion of F-actin cables and microtubules, followed by accumulation of F-actin patches around its terminal region resulting in development of an isodiametrical bud.     

Bud morphogenesis and the actin and microtubule cytoskeletons during budding in the corn smut fungus,Ustilago maydis

Ustilago maydis is a dimorphic Basidiomycete fungus with a yeast-like form and a hyphal form. Here we present a comprehensive analysis of bud formation and the actin and microtubule cytoskeletons of the yeast-like form during the cell cycle. We show that bud morphogenesis entails a series of shape changes, initially a tubular or conical structure, culminating in a cigar-shaped cell connected to the mother cell by a narrow neck. Labelling of cells with concanavalin A demonstrated that growth occurs at bud tip. Indirect immunofluorescence studies revealed that the actin cytoskeleton consists of patches and cables that polarize to the presumptive bud site and the bud tip and an actin ring that forms at the neck region. Because the bud tip corresponds to the site of active cell wall growth, we hypothesize that actin is involved in secretion of cell wall components. The microtubule cytoskeleton has recently been shown to consist of a cytoplasmic network during interphase that disassembles at mitosis when a spindle and astral microtubules are formed. We have carried out studies of U. maydis cells synchronized by the microtubule-depolymerizing drug thiabendazole which allow us to construct a temporal sequence of steps in spindle formation and spindle elongation during the cell cycle. These studies suggest that astral microtubules may be involved in early stages of spindle orientation and migration of the nucleus into the bud and that the spindle pole bodies may be involved in reestablishment of the cytoplasmic microtubule network.     

F-actin redistributions at the division site in livingTradescantia stomatal complexes as revealed by microinjection of rhodamine-phalloidin

Summary Microinjection of rhodamine-phalloidin into living cells of isolatedTradescantia leaf epidermis and visualisation by confocal microscopy has extended previous results on the distribution of actin in mitotic cells of higher plants and revealed new aspects of actin arrays in stomatal cells and their initials. Divisions in the stomatal guard mother cells and unspecialised epidermal cells are symmetrical. Asymmetrical divisions occur in guard mother precursor cells and subsidiary mother cells. Each asymmetrical division is preceded by migration of the nucleus and the subsequent accumulation of thick bundles of anticlinally oriented actin filaments localised to the area of the anticlinal wall closest to the polarised nucleus. During prophase, in all cell types, a subset of cortical actin filaments coaligns to form a band, which, like the preprophase band of microtubules, accurately delineates the site of insertion of the future cell wall. Following the breakdown of the nuclear envelope, F-actin in these bands disassembles but persists elsewhere in the cell cortex. Thus, cortical F-actin marks the division site throughout mitosis, firstly as an appropriately positioned band and then by its localised depletion from the same region of the cell cortex. This sequence has been detected in all classes of division inTradescantia leaf epidermis, irrespective of whether the division is asymmetrical or symmetrical, or whether the cell is vacuolate or densely cytoplasmic. Taken together with earlier observations on stamen hair cells and root tip cells it may therefore be a general cytoskeletal feature of division in cells of higher plants.Abbreviations GMC guard mother cell - MT microtubule - PPB preprophase band - Rh rhodamine - SMC subsidiary mother cell     

Cytoskeletal structures,ultrastructural characteristics and the capsule of the basidiomycetous yeast <Emphasis Type="Italic">Cryptococcus laurentii</Emphasis>

The cytoskeleton, capsule and cell ultrastructure were studied during the cell cycle of Cryptococcus laurentii. In an encapsulated strain, cytoplasmic microtubules and a mitotic spindle were detected. Mitosis was preceded by migration of the nucleus into the bud. F-actin failed to be visualised by rhodamine-phalloidin (RhPh) in encapsulated cells and therefore an acapsular strain was used. The following actin structures were found: actin dots, actin cables and cytokinetic ring. Ultrastructural studies showed the presence of a nucleus in the bud before mitosis. A collar-shaped structure was seen at the base of bud emergence. A lamellar cell wall and a rough outer surface of the cells were detected. Cytoskeletal structures found in C. laurentii are similar to those in Cryptococcus neoformans, which is a serious human pathogen.     

Behavior of mitochondria, microtubules, and actin in the triangular yeastTrigonopsis variabilis

Summary Dimorphic yeastTrigonopsis variabilis is a unique species that can form either an ellipsoidal or a triangular cell depending upon nutritional conditions. This fluorescence microscopic study was intended to correlate morphological changes of mitochondria in the triangular cells with the distribution of the cytoskeleton. In addition, unique features in the behavior of the cytoskeleton were also examined during triangular cell formation. In log-phase cells stained with 4,6-diamidino-2-phenylindole, mitochondrial nucleoids appeared as a string of beads throughout the vegetative growth. The profile of mitochondria stained by 3,3-dihexyloxacarbocyanine iodide showed a network corresponding to the fluorescence images of mitochondrial nucleoids in both mother and daughter cells. Cell-cycle-dependent fragmentation of mitochondria was not discerned. As the culture reached stationary phase, a network of mitochondria gradually changed to form unique rings that were located near the angles of triangular cells. When examined by immunofluorescence microscopy with anti-tubulin antibody, microtubules were found to be well developed along the sides of cells in the cytoplasm ofT. variabilis interphase cells. Although distributions of microtubules and mitochondria are different during cell cycle as a whole, cytoplasmic microtubules frequently extended along a part of the mitochondria in budded cells, suggesting correlation of microtubules and mitochondria. Rhodamine-phalloidin staining revealed both actin patches and cables. Actin cables elongated from mother cells into the buds and showed close proximity to mitochondria, although complete overlapping of both structures was rare. Moreover, actin patches localized on the mitochondrial network at a frequency of 65%. These results suggested that actin cables and patches, as well as microtubules, participated in the distribution of mitochondria. The localization of actin patches separated towards opposite ends at a bud tip when the bud grew to medium size. The unique localization of actin patches is responsible for bi-directional growth of the bud, forming triangular cells.     

Organization of the cytoskeleton in early Drosophila embryos

The cytoskeleton of early, non-cellularized Drosophila embryos has been examined by indirect immunofluorescence techniques, using whole mounts to visualize the cortical cytoplasm and sections to visualize the interior. Before the completion of outward nuclear migration at nuclear cycle 10, both actin filaments and microtubules are concentrated in a uniform surface layer a few micrometers deep, while a network of microtubules surrounds each of the nuclei in the embryo interior. These two filament-rich regions in the early embryo correspond to special regions of cytoplasm that tend to exclude cytoplasmic particles in light micrographs of histological sections. After the nuclei in the interior migrate to the cell surface and form the syncytial blastoderm, each nucleus is seen to be surrounded by its own domain of filament-rich cytoplasm, into which the cytoskeletal proteins of the original surface layer have presumably been incorporated. At interphase, the microtubules seem to be organized from the centrosome directly above each nucleus, extending to a depth of at least 40 microns throughout the cortical region of cytoplasm (the periplasm). During this stage of the cell cycle, there is also an actin "cap" underlying the plasma membrane immediately above each nucleus. As each nucleus enters mitosis, the centrosome splits and the microtubules are rearranged to form a mitotic spindle. The actin underlying the plasma membrane spreads out, and closely spaced adjacent spindles become separated by transient membrane furrows that are associated with a continuous actin filament-rich layer. Thus, each nucleus in the syncytial blastoderm is surrounded by its own individualized region of the cytoplasm, despite the fact that it shares a single cytoplasmic compartment with thousands of other nuclei.     

Rearrangement of the actin filament and microtubule cytoskeleton during induction of microspore embryogenesis inBrassica napus L. cv. Topas

Summary Changes in the actin filament and microtubule cytoskeleton were examined during heat- and cytochalasin D-induced embryogenesis in microspores ofBrassica napus cv. Topas by rhodamine phalloidin and immunofluorescence labelling respectively. The nucleus was displaced from its peripheral to a more central position in the cell, and perinuclear actin microfilaments and microtubules extended onto the cytoplasm. Heat treatment induced the formation of a preprophase band of microtubules in microspores; preprophase bands are not associated with the first pollen mitosis. Actin filament association with the preprophase band was not observed. The orientation and position of the mitotic spindle were altered, and it was surrounded with randomly oriented microfilaments. The phragmoplast contained microfilaments and microtubules, as in pollen mitosis I, but it assumed a more central position. Cytoskeletal reorganisation also occurred in microspores subjected to a short cytochalasin D treatment, in the absence of a heat treatment. Cytochalasin D treatment of microspores resulted in dislocated mitotic spindles, disrupted phragmoplasts, and symmetric divisions and led to embryogenesis, confirming that a normal actin cytoskeleton has a role in preventing the induction of embryogenesis.Abbreviations CD cytochalasin D - MF actin microfilament - MT microtubule - PPB preprophase band     

The role of microtubules in establishing nuclear spatial patterns in multinucleate green algae

Summary In uninucleate cells, cytokinesis follows karyokinesis, thereby reestablishing a specific nucleus-to-cytoplasm ratio. In multinucleate cells, cytokinesis is absent or infrequent; no plasmalemma boundary defines the cytoplasmic territory of an individual nucleus. Several genera of large multinucleate green algae were examined with epifluorescence light microscopy to determine whether the patterns of cytoplasmic organization establish nuclear cytoplasmic domains. Randomly spaced nuclei, singular mitotic events and cytoplasmic streaming characterize the organization of two genera,Derbesia andBryopsis (Caulerpales). The cells ofValonia, Valoniopsis, Boergesenia, Ventricaria (Siphonocladales), andHydrodictyon (Chlorococcales) display regularly spaced nuclei which undergo synchronous divisions in a stationary cytoplasm. In the cytoplasm of genera with regularly spaced nuclei, microtubules radiate from all nuclei in late telophase-early interphase. These internuclear microtubule arrays are not found in algal genera with randomly spaced nuclei. It is hypothesized that these microtubule arrays play a role in establishing the cytoplasmic domain of each nucleus in genera with regularly spaced nuclei. Loss of microtubule arrays during the events of mitosis correlated positively with the increasing randomization of nuclear patterns in algae grown in microtubule inhibitors. Cytoplasmic domains were maintained when cells were grown in the same media in the dark. This suggests that, after a round of division, regular nuclear spacing in certain multinucleate algae is reestablished by internuclear microtubule arrays, which are not, however, required to maintain spacing during interphase.Dedicated to the memory of Professor Oswald Kiermayer     

Cytoskeletal asymmetry in Zea mays subsidiary cell mother cells: a monopolar prophase microtubule half-spindle anchors the nucleus to its polar position

Double labeling of microtubules and actin filaments revealed that in prophase subsidiary mother cells of Zea mays a monopolar prophase microtubule "half-spindle" is formed, which lines the nuclear hemisphere distal to the inducing guard mother cell. The nuclear hemisphere proximal to the guard mother cell is lined by an F-actin cap, consisting of a cortical F-actin patch and actin filaments originating from it. The microtubules of the "half-spindle" decline from the nuclear surface and terminate to the preprophase microtubule band. After disintegration of the latter, a bipolar metaphase spindle is organized. The polar F-actin cap persists during mitosis and early cytokinesis, extending to the chromosomes and the subsidiary cell daughter nucleus. In oryzalin treated subsidiary mother cells the prophase nuclei move away from the polar site. Cytochalasin B and latrunculin-B block the polar migration of subsidiary mother cell nuclei, but do not affect those already settled to the polar position. The prophase nuclei of latrunculin-B treated subsidiary mother cells are globally surrounded by microtubules, while the division plane of latrunculin-B treated subsidiary mother cells is misaligned. The prophase nuclei of brick 1 mutant Zea mays subsidiary mother cells without F-actin patch are also globally surrounded by microtubules. The presented data show that the prophase microtubule "half-spindle"-preprophase band complex anchors the subsidiary mother cell nucleus to the polar cell site, while the polar F-actin cap stabilizes the one metaphase spindle pole proximal to the inducing guard mother cell.     

Microtubular and actin cytoskeletons and ultrastructural characteristics of the potentially pathogenic basidiomycetous yeast Malassezia pachydermatis

Microtubular and actin cytoskeletons were investigated in the lipophilic yeast Malassezia pachydermatis by fluorescence and electron microscopy. To detect microtubules by indirect immunofluorescence using monoclonal anti-tubulin antibody, a prolonged incubation with lysing enzymes was necessary due to its very thick cell wall. Cytoplasmic microtubules were detected in interphase and a spindle with astral microtubules was seen in M-phase. The disintegration of cytoplasmic microtubules and migration of the nucleus to the bud before mitosis were characteristic features of the basidiomycetous yeast Malassezia pachydermatis. The visualisation of F-actin structures (patches, cables and cytokinetic rings) by fluorescence microscopy using both monoclonal anti-actin antibody and rhodamine-phalloidin failed, but actin was detected by electron microscopy with immunogold labelling. Clusters of gold particles indicating actin structures were detected at the plasma membrane of cells with unique cortical ultrastructural features characteristic of the genus Malassezia. A possible association of these with the actin cytoskeleton is suggested.     

Embryogenesis in <Emphasis Type="Italic">Sedum acre</Emphasis> L.: structural and immunocytochemical aspects of suspensor development

The changes in the formation of both the actin and the microtubular cytoskeleton during the differentiation of the embryo-suspensor in Sedum acre were studied in comparison with the development of the embryo-proper. The presence and distribution of the cytoskeletal elements were examined ultrastructurally and with the light microscope using immunolabelling and rhodamine-phalloidin staining. At the globular stage of embryo development extensive array of actin filaments is present in the cytoplasm of basal cell, the microfilament bundles generally run parallel to the long axis of basal cell and pass in close to the nucleus. Microtubules form irregular bundles in the cytoplasm of the basal cell. A strongly fluorescent densely packed microtubules are present in the cytoplasmic layer adjacent to the wall separating the basal cell from the first layer of the chalazal suspensor cells. At the heart-stage of embryo development, in the basal cell, extremely dense arrays of actin materials are located near the micropylar and chalazal end of the cell. At this stage of basal cell formation, numerous actin filaments congregate around the nucleus. In the fully differentiated basal cell and micropylar haustorium, the tubulin cytoskeleton forms a dense prominent network composed of numerous cross-linked filaments. In the distal region of the basal cell, a distinct microtubular cytoskeleton with numerous microtubules is observed in the cytoplasmic layer adjacent to the wall, separating the basal cell from the first layer of the chalazal suspensor cells. The role of cytoskeleton during the development of the suspensor in S. acre is discussed.     

An electron microscopic study on the mating tube formation in the heterobasidiomycete Tremella mesenterica

Ultrastructure of the mating tube formed in yeast haplont of the heterobasidiomycete Tremella mesenterica was studied by electron microscopy. Cell wall of the mating tube emerged as evagination of the inner layers, rupturing outer layers of the mother cell wall. Comparison with budding cells suggested that the tube emergence place at bud scar and the process of tube emergence was the same as that of bud emergence. Electron transparent vesicles of 0.1 m diameter were scattered in the cytoplasm of the mating tube. Nucleus-associated organelle was located at one side of the nuclear envelope which extended towards the mating tube. A few microtubules were detected in the mating tube, but their association with a nucleus was not clear. The cytoplasmic structure of the mating tube was discussed in comparison with that of hyphae of the filamentous fungi.     

The cytoskeletal involvement in cellularization of theDrosophila melanogaster embryo

Summary The distribution and arrangement of cytoskeletal components in the early embryo ofDrosophila melanogaster were examined by thin-section electron microscopy to elucidate their involvement in the formation of the cellular blastoderm, a process called cellularization. During the final nuclear division in the cortex of the syncytial blastoderm bundles of astral microtubules were closely associated with the surface plasma membrane along the midline where a new gutter was initiated. Thus the new gutter together with the pre-formed ones compartmentalized the embryo surface to reflect underlying individual daughter nuclei. Subsequently such gutters became deeper by further invagination of the plasma membrane between adjacent nuclei to form so-called cleavage furrows. Nuclei simultaneously elongated in the direction perpendicular to the embryo surface and numerous microtubules from the centrosomes ran longitudinally between the nucleus and the cleavage furrow. Microtubules often appeared to be in close association with the nuclear envelope and the cleavage furrow membrane. The plasma membrane at the advancing tip of the furrow was always undercoated with an electron-dense layer, which could be shown to be mainly composed of 5–6 nm microfilaments. These microfilaments were decorated with H-meromyosin to be identified as actin filaments. As cleavage proceeded, each nucleus with its perikaryon became demarcated by the furrow membrane, which then extended laterally to constrict the cytoplasmic connection between each newly forming cell and the central yolk region. The cytoplasmic strand thus formed possessed a prominent circular bundle of microfilaments which were also decorated with H-meromyosin and bidirectionally arranged, similar in structure to the contractile ring in cytokinesis. These observations strongly suggest that both microtubules and actin filaments play a crucial role in cellularization ofDrosophila embryos.     

Microtubule interactions with the cell cortex causing nuclear movements in Saccharomyces cerevisiae

During mitosis in budding yeast the nucleus first moves to the mother-bud neck and then into the neck. Both movements depend on interactions of cytoplasmic microtubules with the cortex. We investigated the mechanism of these movements in living cells using video analysis of GFP-labeled microtubules in wild-type cells and in EB1 and Arp1 mutants, which are defective in the first and second steps, respectively. We found that nuclear movement to the neck is largely mediated by the capture of microtubule ends at one cortical region at the incipient bud site or bud tip, followed by microtubule depolymerization. Efficient microtubule interactions with the capture site require that microtubules be sufficiently long and dynamic to probe the cortex. In contrast, spindle movement into the neck is mediated by microtubule sliding along the bud cortex, which requires dynein and dynactin. Free microtubules can also slide along the cortex of both bud and mother. Capture/shrinkage of microtubule ends also contributes to nuclear movement into the neck and can serve as a backup mechanism to move the nucleus into the neck when microtubule sliding is impaired. Conversely, microtubule sliding can move the nucleus into the neck even when capture/shrinkage is impaired.     

Actin microfilament organization during pollen development ofBrassica napus cv. Topas

Summary The organization of actin microfilaments (MFs) was studied during pollen development ofBrassica napus cv. Topas. Cells were prepared using three techniques and double labelled for fluorescence microscopy with rhodamine-labelled phalloidin for MFs and Hoechst 33258 for DNA. Microfilaments are present at all stages of pollen development with the exception of tricellular pollen just prior to anthesis. Unicellular microspores contain MFs which radiate from the surface of the nuclear envelope into the cytoplasm. During mitosis MFs form a network partially surrounding the mitotic apparatus and extend into the cytoplasm. Both cytoplasmic and phragmoplast-associated MFs are present during cytokinesis. Nuclear associated-, cytoplasmic, and randomly oriented cortical MFs appear in the vegetative cell of the bicellular microspore. Cortical MFs in the vegetative cell organize into parallel MF bundles (MFBs) aligned transverse to the furrows. The MFBs disappear prior to microspore elongation. At anthesis MFs are restricted to the cortical areas subjacent to the furrows of the vegetative cell. The use of cytochalasin D to disrupt MF function resulted in: (1) displacement of the acentric nucleus in the unicellular microspore; (2) displacement of the spindle apparatus in the mitotic cell; (3) symmetrical growth of the bicellular microspore rather than elongation and (4) inhibition of pollen tube germination in the mature pollen grain. This suggests that MFs play an important role in anchoring the nucleus in the unicellular microspore as well as the spindle apparatus during microspore mitosis, in microspore shape determination and in pollen tube germination.Abbreviations MF microfilament - MFB microfilament bundle - rhph rhodamine phalloidin Dedicated to the memory of Professor John G. Torrey     

Structural rearrangements of tubulin and actin during the cell cycle of the yeast Saccharomyces

The distribution of actin and tubulin during the cell cycle of the budding yeast Saccharomyces was mapped by immunofluorescence using fixed cells from which the walls had been removed by digestion. The intranuclear mitotic spindle was shown clearly by staining with a monoclonal antitubulin; the presence of extensive bundles of cytoplasmic microtubules is reported. In cells containing short spindles still entirely within the mother cells, one of the bundles of cytoplasmic microtubules nearly always extended to (or into) the bud. Two independent reagents (anti-yeast actin and fluorescent phalloidin) revealed an unusual distribution of actin: it was present as a set of cortical dots or patches and also as distinct fibers that were presumably bundles of actin filaments. Double labeling showed that at no stage in the cell cycle do the distributions of actin and tubulin coincide for any significant length, and, in particular, that the mitotic spindle did not stain detectably for actin. However, both microtubule and actin staining patterns change in a characteristic way during the cell cycle. In particular, the actin dots clustered in rings about the bases of very small buds and at the sites on unbudded cells at which bud emergence was apparently imminent. Later in the budding cycle, the actin dots were present largely in the buds and, in many strains, primarily at the tips of these buds. At about the time of cytokinesis the actin dots clustered in the neck region between the separating cells. These aspects of actin distribution suggest that it may have a role in the localized deposition of new cell wall material.     

Microtubule organization in germinated pollen of the conifer Picea abies (Norway spruce,Pinaceae)

The organization of microtubules in germinated pollen of the conifer Picea abies (Norway spruce, Pinaceae) was examined using primarily confocal microscopy. Pollination in conifers differs from angiosperms in the number of mitotic divisions between the microspore and the sperm and in the growth rate of the pollen tube. These differences may be orchestrated by the cytoskeleton, and this study finds that there are important functional differences in microtubule organization within conifer pollen compared to the angiosperm model systems. Pollen from P. abies contains two degenerated prothallial cells, a body cell, a stalk cell, and a vegetative cell. The body cell produces the sperm. In the vegetative cell, microtubules form a continuous network from within the pollen grain, out through the aperture, and down the length of the tube to the elongating tip. Within the grain, this network extends from the pollen grain wall to the body and stalk cell complex. Microtubules within the body and stalk cells form a densely packed array that enmeshes amyloplasts and the nucleus. Microtubule bundles can be traced between the body and stalk cells from the cytoplasm of the body cell to the adjoining cell wall and into the cytoplasm of the stalk cell. Body and stalk cells are connected by plasmodesmata. The organization of microtubules and the presence of plasmodesmata suggest that microtubules form a path for intercellular communication by projecting from the cytoplasm to interconnecting plasmodesmata. Microtubules in the elongating tube form a net axial array that ensheathes the vegetative nucleus. Microtubules are enriched at the elongating tip, where they form an array beneath the plasma membrane that is perpendicular to the direction of tube growth. This enriched region extends back 20 μm from the tip. There is an abrupt transition from a net perpendicular to a net axial organization at the edge of the enriched region. In medial sections, microtubules are present in the core of the elongating tip. The organization of microtubules in the tip differs from that seen in angiosperm pollen tubes.     

Side branch formation and orientation in the caulonema of the moss,Funaria hygrometrica: Normal development and fine structure

Summary The regular branching of theFunaria caulonema filaments is partly related to rhythms in nuclear and cell division. The formation and development of the branches were studied by light and electron microscopy with particular attention directed to the distribution of microtubules and the polar organization of the cytoplasm. The new side branch breaks through the wall of the mother cell. The site of branch development is determined by the position of the nucleus of the mother cell. In protonemata which grow in vertically placed Petri dishes gravity influences the position of nuclei and side branches, and also the direction of oblique cross walls in the caulonema filaments to a certain extent.     

The role of microtubules during the cytokinetic events of cell wall ontogenesis and papilla development inCarteria crucifera

Summary An ultrastructural study of cytokinesis, cell wall ontogenesis, and papilla development/form inCarteria crucifera Korsh. andChloromonas rosae Ettl was undertaken. After typical phycoplast-mediated cytokinesis, wall ontogenesis begins at the level of Golgi apparatus activation and secretion to the outside of the daughter cells of fibrillar wall precursors which self assemble into the typical chlamydomonad wall (sensuRoberts 1974). As wall ontogenesis approaches the flagellar region of the cell, several precisely timed events occur: flagellar apparatus formation, flagellar emergence, protoplasmic extension in the future papilla area underlined by series of parallel aligned microtubules, wall formation (at least the W₂–W₆ layers), retraction of the protoplasmic extension and loss of underlying microtubules, and final wall modification (gap filling by W₁ material) to yield the characteristic wall papilla. The transient cytoplasmic extensions mimic the shape of the future wall papilla and are maintained, at least inCarteria, by underlying microtubules. Structural and developmental properties of the papilla are characterized and phylogenetic implications are discussed.This research was supported by National Science Foundation Grant DEB 78-0554.