Phragmoplast Microtubules Research Articles

Pre- and postmitotic reorientation of microtubule arrays in youngSphagnum leaflets: Transitional stages and initiation sites

The microtubules (MTs) of developingSphagnum leaflets rearrange from the interphase array into the preprophase band without obvious participation of definite initiation sites. At late prophase, additional MTs appear along the nuclear envelope, with the same orientation as in the peripherally situated preprophase band. Spindle formation begins along the nuclear envelope; spindle MTs run perpendicular to preprophase band MTs and converge in several focus points with indistinct polar bodies. After cytokinesis, most spindle and phragmoplast MTs disappear. Interphase MTs reappear at first along the central part of the new cell wall, in a region which was occupied before by the initial phragmoplast; their orientation is perpendicular to the phragmoplast MTs. Also here, distinct MT organizing centers could not be observed. Then the MT spread out over the cell periphery. The observations suggest that diffuse MT organizing “zones” rather than definite MT organizing “centers” play a role in the rearrangement of the different MT arrays during the cell cycle.

Organization of microtubules in stabilized meristematic plant cells revealed by a rat monoclonal antibody reacting only with the tyrosinated form of α-tubulin

A rat monoclonal antibody against yeast tubulin (clone YL 1 2 ; Kilmartin et al., 1982) that reacts specifically with mammalian α-tubulin carrying a carboxyterminal tyrosine residue (Wehland et al., 1983) was used to localize microtubules in plant cells derived from onion root apices ( Allium cepa L.). YL 1 2 reacted with all types of microtubular arrays known to occur in higher plant meristematic cells such as interphase cortical microtubules, pre-prophase bands, the mitotic spindle and phragmoplast microtubules. The specific labeling of microtubules in isolated cells from onion root tips by YL 1 2 indicates that plant cells like animal cells contain tubulin tyrosine ligase, the enzyme which posttranslationally modifies α-tubulin. This enzyme could be involved in the dynamic regulation of microtubular arrays in all eukaryotic cells.

Immunofluorescence microscopy of organized microtubule arrays in structurally stabilized meristematic plant cells.

Cells were prepared for indirect immunofluorescence microscopy after paraformaldehyde fixation of multicellular root apices and brief incubation in cell wall-digesting enzymes. This allowed subsequent separation of the tissue into individual cells or short files of cells which were put onto coverslips coated with polylysine. Unlike spherical protoplasts made from living tissues, these preparations retain the same polyhedral shape as the cells from which they are derived. Cellular contents, including organized arrays of microtubules, are likewise structurally stabilized. Antibodies to porcine brain tubulin react with all types of microtubule array known to occur in plant meristematic cells, namely, interphase cortical microtubules, pre-prophase bands, the mitotic spindle, and phragmoplast microtubules. The retention of antigenicity in permeabilized, isolated, stabilized cells from typical, wall-enclosed plant cells has much potential for plant immunocytochemistry, and in particular should facilitate work on the role of microtubules in the morphogenesis of organized plant tissues.

Polarity of midbody and phragmoplast microtubules.

A newly discovered method (Heidemann and McIntosh, 1980, Nature [Lond.] 286:517) for displaying the molecular polarity of microtubules (MTs) has been slightly modified and applied to the midbodies of cultured mammalian cells and the phragmoplasts of Haemanthus endosperm. The method involves the decoration of preexisting MTs in lysed cells with curved ribbons of tubulin protofilaments; the direction of curvature of these C-shaped appendages as seen in cross section reflects the intrinsic polarity of the MTs. In travsverse sections of midbodies from HeLa and PtK cells, we find that essentially all the MTs in a given region of the structures have the same direction of hook curvature, and hence the same polarity. The midbody MTs that lie on one side of the spindle equator show the opposite polarity from those on the other side, indicating that the midbody is constructed from two families of antiparallel MTs. Midbody MTs are arranged with their fast-growing ends overlapping at the spindle equator, consistent with the hypothesis that the midbody is formed by the interdigitation of aster MTs. The polarities of the MTs from the phragmoplast of endosperm cells are the same as those found in the mammalian midbody. Our results eliminate one model for mitosis, but are consistent with others. The systematic and reproducible polarities observed favor the concept that MT polarity is an important factor in the formation and/or the function of these two mitotic structures.

MITOSIS AND CYTOKINESIS IN SESSILE SPORANGIUM OFTRENTEPOHLIA AUREA(CHLOROPHYCEAE)1

ABSTRACTAn ultrastructure study of mitosis and cytokinesis in the sessile sporangium ofTrentepohlia aurea(L.) Mart, was made to clarify the phylogenetic position of the alga. Mitosis was closed and centric at late anphase with cytokinesis involving the production of cleavage membranes by dictyasames between the numerous, well‐separated daughter nuclei. Neither phycoplast nor phragmoplast microtubules were observed during cytokinesis. The lack of phycoplast microtubules and the presence of multilayered structures in flagellated cells suggestTrentepohliais phylogenetically related to those green algae thought to have given rise to the land plants. The primitive type of mitosis and the lack of microbodies suggest that the ancestors ofTrentepohliamay have branched off from this line relatively early.

Evidence for initiation of microtubules in discrete regions of the cell cortex in Azolla root-tip cells, and an hypothesis on the development of cortical arrays of microtubules

Complexes of microtubules, vesicles, and (to varying degrees) dense matrix material around the microtubules were seen along the edges of cells in root apices of Azolla pinnata R.Br. (viewing the cells as polyhedra with faces, vertices and edges). They are best developed after cytokinesis has been completed, when the daughter cells are reinstating their interphase arrays of microtubules. They are not confined to edges made by the junction of new cell plates with parental walls, but occur also along older edges. Similar matrices and vesicles are seen amongst phragmoplast microtubules and where pre-prophase bands intersect the edges of cells. It is suggested that the complexes participate in the development of cortical arrays of microtubules. The observations are combined with others, made on pre-prophase bands and on the substructure of cortical arrays lying against the faces of cells, to develop an hypothesis on the development of cortical microtubules, summarised below: Microtubules are nucleated along the edges of cells, at first growing in unspecified orientations and then becoming bridged to the plasma membrane. Parallelism of microtubules in the arrays arises by inter-tubule cross-bridging. Lengths of microtubule are released from, or break off, the nucleating centres and are moved out onto the face of the cell by intertubule and tubule-membrane sliding, thus accounting for the presence there of short tubules with randomly placed terminations. The nucleating zones along cell edges might have vectorial properties, and thus be able to control the orientation of the microtubules on the different faces of the cell. Also, localised activation could generate localised arrays, especially pre-prophase bands in specified sites and planes. Two possible reasons for the spatial restriction of nucleation to cell edges are considered. One is that the geometry of an edge is itself important; the other is that along most cell edges there is a persistent specialised zone, inherited at cytokinesis by the daughter cells when the cell plate bisects the former pre-prophase-band zone.

Cytokinesis in the developing wheat grain; Division with and without a phragmoplast

Cell wall formation during the transition from free-nuclear to cellular endosperm of wheat (Triticum aestivum L. cv. Heron) was investigated using correlated light and electron microscopy. Partitioning of the multinucleate syncytium that lines the inner periphery of the embryo sac is initiated 1-2 days after anthesis. Wall ingrowths, at first recognizable in sections as minute wall pegs, furrow inward from the edge of the embryo sac through the vacuolate cytoplasm which, to the inside, is clearly delimited by the central vacuole. Growth of the walls at this stage is independent of a phragmoplast and in this respect is reminiscent of the cleavage processes of lower plant cells. Intense fluorescence of the walls after staining with aniline blue suggests that callose may be a principal component. The growing walls branch and eventually meet on the side nearest the central vacuole. Cellularization of the peripheral layer of endosperm cytoplasm is thus complete about 2 days after anthesis. Between 2 and 3 days after anthesis, the peripheral layer of cells commences to divide both radially and tangentially and by 4 days the entire embryo sac is cellular. Cytokinesis during this phase entails the formation of a cell plate between sister nuclei. At the periphery of a forming cell plate, "vesicles" appear scattered amongst an array of phragmoplast microtubules. This mechanism of wall growth differs markedly from the initial infurrowing of the first-formed walls. The overall timing and the manner of cell wall deposition vary in a number of important respects from the model recently proposed by Mares et al. whose work was based largely on light microscopy (D.J. Mares; K. Norstog; A.B. Stone: Aust. J. Bot. 23, 311-326, 1975).