Plant Cytokinesis – Insights Gained from Electron Tomography Studies 275 Fig. 9 Immunogold labeling of callose in the cell plate lumen at different stages of
somatic-type cytokinesis.Atubulo-vesicular network cell plate. The few gold particles
(arrowheads) are located mostly over discrete, lighter cell plate (cp) regions.BTubu l a r
network. Immunolabeling is much more abundant over the callose deposits (cd) ho-
mogeneously distributed throughout the cell plate lumen.CTubular network. Newly
synthesized callose is deposited as a layer (cl) at the luminal side of the cell plate, sur-
rounding the previous cell plate material (darker contents).DMature cell wall. Massive
deposition of cellulose fibrils replaces callose as the main constituent of mature cell walls
(cw). Callose deposits (cd) are limited to the spaces around the plasmodesmata (pd).
gs:Golgistack;mt:microtubule;v:vesicle.Thescale barsrepresents 100 nm
of copious amounts of callose in the cell plate lumen (compare Fig. 9A and B),
the disappearance of the vesicular (spherical) cell plate domains (Fig. 4C),
and the formation of increasing numbers of wide tubules that develop into
fenestrated sheet-type cell plate domains (Fig. 4D).
The widening of the cell plate tubules into fenestrated sheets appears to
be driven by the deposition of newly synthesized callose, a linearb-1,3-linked
glucose polymer, into the cell plate lumen (Fig. 9B; Samuels et al. 1995). Cal-
lose synthases are large, membrane-spanning protein complexes that excrete
their product directly into the cell plate lumen (Verma and Hong 2001), and
as this happens, the callose is seen to spread over the membrane surface
in a∼ 15 nm thick layer that has been postulated to provide the force that
drives tubule widening and fenestrated sheet formation (Fig. 9C; Samuels
et al. 1995). At the end of this spreading phase, the cell plate resembles a pla-
nar sheet with numerous openings (fenestrae). Small, secondary CPAMs and
associated clusters of MTs assemble de novo over and around these openings,
guiding new vesicles to these regions to bring about closure of the gaps. How-
ever, where tubular strands of endoplasmic reticulum (ER) become trapped
within closing fenestrae, those fenestrae never completely close (Fig. 2F). In-
stead, they are transformed into the first symplastic connections between the
daughter cells, the primary plasmodesmata (Fengshan and Peterson 2001).
While callose synthesis is transforming the general architecture of the
cell plate, the concomitant removal of cell plate membrane by clathrin-
coated vesicles produces parallel changes in cell plate membrane composition
(Samuels et al. 1995; Rensing et al. 2002; Seguí-Simarro et al. 2004; Seguí-
Simarro and Staehelin 2006a). For example, upon completion of the rapid,
vesicle-mediated growth phase, the need for vesicle tethering and vesicle fu-
sion membrane components is greatly reduced, and thus these membrane
proteins have to be removed to allow for maturation of the cell plate mem-
brane to occur. Considering that the recycling of plasma membrane proteins
is typically carried out by clathrin-coated pits and vesicles (Alberts et al.
2002), and that the cell plate membrane is a precursor form of the plasma
membrane, the clathrin-coated vesicles seen budding from maturing cell
plates are assumed to be involved in membrane recycling. The parallel in-
crease of clathrin-coated vesicles and multivesicular bodies during cell plate