Cytokinesis in Higher Plants

Completion of the fusion tube–generated membrane network signals the end of the first stage of cell plate formation. What follows is a series of steps that both mechanically stabilizes the initial delicate, interwoven membrane network and then transforms it into two new plasma membranes and a new cell wall. Mechanical stabilization of the fusion tube–generated membrane network appears to involve two processes: assembly of a very dense fibrous coat onto the membranes, and consolidation of the membranes into a tubulo-vesicular network (Figure 2bFigure 2b). In the presence of caffeine, both of these processes are inhibited, and the delicate network quickly breaks up into vesicles that are eventually resorbed by the cells, indicating a possible role for Ca2+ in these assembly processes (Samuels and Staehelin 1996xSamuels, A.L. and Staehelin, L.A. Protoplasma, in press. 1996; See all ReferencesSamuels and Staehelin 1996).Transformation of the tubulo-vesicular network first into a tubular network and then into a fenestrated membrane sheet (Figure 2b-dFigure 2b-d) involves the formation of clathrin coated buds, the deposition of callose (a β-1,3 glucose polymer produced by a Ca2+-activated synthase; Kakimoto and Shibaoka 1992xKakimoto, T. and Shibaoka, H. Plant Cell Physiol. 1992; 33: 353–361See all ReferencesKakimoto and Shibaoka 1992) in the network lumen, the loss of the dense membrane coat, and the disassembly of associated MTs. Because the appearance of clathrin-coated buds coincides with the appearance of nearby multivesicular bodies, their main function appears to be the removal of excess membrane and of selected membrane proteins targeted for destruction. A possible function for callose is suggested by its deposition in the form of a dense coat over the lumenal surface of the cell plate–forming membranes. Based on the visco-elastic properties of callose, the membrane-associated callose layer could provide the spreading force that converts the tubulo-vesicular network into a fenestrated sheet and ultimately into a cell wall (Samuels et al. 1995xSamuels, A.L., Giddings Jr, T.H., and Staehelin, L.A. J. Cell Biol. 1995; 130: 1345–1357Crossref | PubMed | Scopus (328)See all ReferencesSamuels et al. 1995).During the centrifugal expansion of the cell plate, which occurs at a rate of up to 1 μm per min, new vesicles continuously arrive at and fuse with the plate periphery, while the older and more centrally located cell plate domains simultaneously mature as discussed above (Figure 2dFigure 2d). When the cell plate reaches the side wall, fusion is brought about by hundreds of fusion tubes that arise from the cell plate margin and fuse with the actin-depleted plasma membrane domain originally demarcated by the preprophase band of MTs (Samuels et al. 1995xSamuels, A.L., Giddings Jr, T.H., and Staehelin, L.A. J. Cell Biol. 1995; 130: 1345–1357Crossref | PubMed | Scopus (328)See all ReferencesSamuels et al. 1995).Although virtually all of the Golgi-derived vesicles deliver cell wall matrix polysaccharides (hemicelluloses and esterified pectic polysaccharides) to the forming cell plate, callose remains the dominant polysaccharide until it is enzymatically removed following completion of the new cell wall. Significant amounts of cellulose fibrils, the tensile elements of plant cell walls, can only be detected beginning with the fenestrated sheet phase of cell plate formation (Samuels et al. 1995xSamuels, A.L., Giddings Jr, T.H., and Staehelin, L.A. J. Cell Biol. 1995; 130: 1345–1357Crossref | PubMed | Scopus (328)See all ReferencesSamuels et al. 1995). Plasmodesmata, the intercellular communication channels of plants, are also formed during this last stage of cell plate formation.The significant progress made in understanding the process of cell plate formation in descriptive terms has now set the stage for the characterization of these dynamic events at the molecular level.