Microtubules in plant cell division

Immunofluorescence microscopy has proven to be a valuable accompaniment to electron microscopy for study of the cytoskeleton of plant cells. Whereas electron microscopy provides greater resolution and details of the spatial relationships of the cytoskeleton to other cellular components, fluorescence visualization makes it possible to see the three-dimensional organization of cytoskeletal elements without laborious reconstruction of views from serial sections. An area in which immunofluorescence microscopy has been useful is the investigation of how plant cells organize and position the various microtubule arrays that are utilized during mitosis, cytokinesis and cell expansion phases. One of the earliest indications of an impending division in a meristematic plant cell is the formation of a preprophase band of microtubules in the cell cortex, at the site where the new wall will be placed at the subsequent cytokinesis. At its later stages, the band is narrower than when first identifiable. In most cells, preprophase band microtubules have the same general orientation as the preceding interphase microtubules, and so preprophase band formation here could, in theory, be achieved by lateral bundling of microtubules.Cells in which the division site and the preprophase band that marks it are not oriented parallel to interphase microtubules are found in stomatal complexes of grass leaves . Fig. 1 illustrates the arrangement of two such cell types: the guard mother cell, which divides lengthwise to form two guard cells, side-by-side, and the subsidiary mother cell, which undergoes a very asymmetric division to produce one of the pair of lens-shaped subsidiary cells that flank the guard cells. Interphase and preprophase arrangements of microtubules for each cell type are diagrammed in Figs. 2-4. In order to examine how these cell types achieve the reorientation of microtubules that is necessary to progress from interphase to preprophase, sheets of epidermis containing actively dividing stomatal complex cells were examined with immunofluorescence microscopy using antibodies to tubulin. Thin epidermal slices of leaves were fixed and glued down to a slide, whereupon cell walls were enzymatically weakened so that unwanted cell layers could be removed . Because waves of division pass along grass leaves, cells of the same type in a given file tend to be at similar stages, which facilitates deduction of the developmental pattern.