270 J.M. Seguí-Simarro et al.
5
Membrane Events Associated with Cell Plate Formation
5.1
The Generation of a Cell Plate From Vesicles and its Transformation
into a Cell Wall Requires Many Coordinated Cellular Activities
Cell plates are the precursors of cell walls, and understanding how new cell
walls are produced from Golgi-derived vesicles is central to the understand-
ing of plant cell cytokinesis. Somatic-type cytokinesis starts with the appear-
ance of the cell plate initials during late anaphase (Figs. 2B, 4A). During the
subsequent growth and transformation of these initials, the cell plate accumu-
lates not only new membrane, which is destined to give rise to new plasma
membrane, but also large quantities of polysaccharides and proteoglycans,
structural and enzymatic proteins, and diverse types of lipidic compounds to
create a new cell wall (Whaley and Mollenhauer 1963; Verma 2001). Whereas
some of the polysaccharides, such as pectins and hemicelluloses, are pro-
duced by enzymes in the Golgi apparatus and delivered in vesicles to the cell
plate (Staehelin and Moore 1995; Sorensen et al. 2000), others like callose and
cellulose are produced by enzymes located in the cell plate membrane and
deposited directly into the cell plate lumen to give rise to the new cell wall
(Samuels et al. 1995; Verma 2001). Upon fusion of a cell plate with the mother
cell wall, mobile cellulose synthase rosette complexes translocate laterally
within the plasma membrane from the old to the new plasma membrane re-
gions while spinning out new cellulose microfibrils, which, together with the
matrix polysaccharides, firmly anchor the new cross wall into the cell wall
system as a whole (Brown et al. 1996).
During this carefully orchestrated cell plate assembly and maturation pro-
cess, many logistic, mechanistic and biochemical problems have to be solved,
including the timely delivery of the precursor molecules to the site of cell
plate formation (Ueda et al. 2003), the control of the vesicle fusion processes
to ensure that a planar cell plate and not a vacuole-like balloon is created
(Seguí-Simarro et al. 2004), and the regulation of the timing of the onset of
polysaccharide (e.g., callose) synthesis (Samuels et al. 1995) and of membrane
recycling (Seguí-Simarro and Staehelin 2006a). All of these changes coincide
with the appearance of characteristic cell plate intermediates, which are dis-
cussed in greater detail in the following sections.
5.2
Cell Plate Formation Requires Vesicular Building Blocks that do not Balloon
when Fused Together
The fusion of spherical cytoplasmic vesicles typically leads to the forma-
tion of larger spherical vesicles. However, if a dividing cell wants to form