Cytokinesis in Higher Plants

structure, the phragmoplast proper (Figure 1B). This *Department of Molecular, Cellular, structure subsequently shortens in length and expands and Developmental Biology in girth until it finally reaches the side wall (Figure 1C). University of Colorado During phragmoplast expansion there appears to be a Boulder, Colorado 80309–0347 shift in the organization of the MTs; whereas the initial † Department of Biology and Program of Molecular phragmoplast cylinder arises from preexisting MTs, the and Cellular Biology later stages of centrifugal growth appear to be driven University of Massachusetts in part by the formation of new MTs. This conclusion Amherst, Massachusetts 01003 gains support from the observations showing that fluorescently-labeled tubulin readily incorporates into phragmoplasts of live (Zhang et al., 1993) and permeabil-ized (Vantard et al., 1990) cells, and into phragmoplasts Cytokinesis is the process that leads to the production that have been isolated (Asada et al., 1991). Moreover, of two daughter cells from one parent. In animal cells this ␥ tubulin, thought to be associated with regions of MT is achieved by constriction of the plasma membrane, organization, is present in the phragmoplast in a pattern whereas in higher plants it is accomplished through a that is biased towards the minus ends of the MTs (Liu centrifugal process which involves an expanding cell et al., 1993). plate that must find and fuse with a predetermined zone of the plasma membrane to produce a new cell wall. The MTs of the phragmoplast are also dynamic, exhib-Cell walls control many aspects of plant life including iting rapid turnover (t 1/2 ϭ 60 sec) based on analysis plant morphogenesis, which depends on the precise using fluorescence redistribution after photobleaching placing of each new wall and careful regulation of cell (Hush et al., 1994). However, attempts to uncover a wall expansion. Every new wall arises in the middle of directional flux, consistent with the minus end transloca-the cell from the fusion of Golgi-derived vesicles into a tion of fluorescent tubulin in isolated phragmoplasts thin membranous plate, the cell plate, which undergoes (Asada et al., 1991), have been negative. In living cells an elaborate series of changes that transforms it into a the fluorescence redistributes uniformly and quickly fully functional and integrated new wall. The structure that gives rise to the cell plate is known as the phrag-moplast, a complex array of microtubules (MTs), micro-filaments (MFs), Golgi-derived vesicles, and endoplas-mic reticulum that assembles during late anaphase and …