Plant Cytokinesis – Insights Gained from Electron Tomography Studies 271
a new cell wall, the cell plate-forming vesicles have to be coaxed into form-
ing a planar membrane structure. The mechanism by which this is achieved
involves exploiting the mechanical force of GTP-driven dynamin springs to
deform and dehydrate freshly -fused, hourglass-shaped vesicles as well as the
tubulo-vesicular membranes of growing cell plates (Figs. 6, 7; Seguí-Simarro
et al. 2004). As mentioned above, the dynamin-like proteins appear to be
confined to the CPAM, where all cell plate growth is seen to occur. The ev-
idence supporting this hypothesis is as follows. Dumbbell-shaped vesicles
with dynamin springs around their necks typically have a surface area that is
equivalent to the surface area of two small cell plate-forming vesicles. Thus,
thechangeinvesicleshapedoesnotinvolveachangeinmembranesurface
area. However, as the neck region of a dumbbell-shaped vesicle is extended,
the diameter and the surface area of the bulbous ends is reduced. The simul-
taneous reduction in volume can amount to 50 %, and this change in volume
is most likely achieved by the “squeezing out” of water from the bulbous
ends as the membrane is transiently stretched (Fig. 6). The concomitant de-
hydration of the vesicle contents cell wall forming pectic polysaccharides,
hemicelluloses and arabinogalactan proteins (Staehelin and Moore 1995) –
most likely causes gelling of the polysaccharides, especially of the esterified
pectins (Zhang and Staehelin 1992; Thakur et al. 1997). Such a gelling reac-
tion would be expected to mechanically stabilize the elongated vesicles, the
new cell plate building blocks. Supporting the idea of mechanically stabi-
lized building blocks is the observation that even as new vesicles fuse with
the ends of the dumbbell vesicles, the fused structures do not balloon. The
continued formation of dynamin springs around the tubular regions of the
tubulo-vesicular cell plate membrane networks, and the resulting contin-
ual compression of the cell plate contents, appears to ensure that the cell
plate retains a planar configuration during the rapid vesicle fusion stage
of cell plate assembly.
5.3
Assembly of the Tubulo-Vesicular Cell Plate Membrane Network
Occurs Exclusively Within the CPAM
Expansion and merging of the CPAM islands of the phragmoplast initials
leads to the formation of a large and continuous CPAM that spans the width
of the solid phragmoplast (Figs. 2C, 4B). This large CPAM defines where the
phragmoplast MT ends are stabilized, where vesicle delivery occurs, where
dumbbell-shaped vesicles are produced, and where cell plate growth oc-
curs. The initial cell plate has a tubulo-vesicular-type membrane architecture
(Figs. 2C, 4B, 7B). It is generated by the joining of the expanding tubulo-
vesicular membrane structures that arise from the fusion of spherical vesicles
with the dumbbell-shaped vesicles of the phragmoplast initials. Growth of
the tubulo-vesicular network involves both fusion of individual vesicles with