258 J.M. Seguí-Simarro et al.
methods, is that there is no systematic overlap between the MT (+)-ends in
somatic-type cytokinesis phragmoplast arrays, but there is overlap between
the MTs in the mini-phragmoplasts of endosperm cells (Otegui and Stae-
helin 2000b; Austin et al. 2005). The reason for this difference is unknown,
but it could be related to the fact that the mini-phragmoplasts formed dur-
ing syncytial-type cytokinesis contain only∼ 2 ×10 MTs versus∼ 2 ×400 to
500 in the solid phragmoplasts formed during somatic-type cytokinesis. In-
terdigitation of the MT ends might be required for the stabilization of the
mini-phragmoplast MT assemblies, which have to remain operative during
the simultaneous assembly of all of the syncytial cell walls, a process that
wouldbeexpectedtotakemoretimethantheassemblyofasinglecellplate
and cell wall during somatic-type cytokinesis (Otegui and Staehelin 2000b).
Upon completion of the tubulo-vesicular stage of cell plate formation
(see below), the solid phragmoplast MT array breaks down in conjunction
with the disassembly of the CPAM. This stage is called the transitional stage
(Figs. 2D, 3D), because it signals the transition from a solid phragmoplast to
a ring phragmoplast type of MT organization. Formation of the ring phrag-
moplast MT arrays follows the reformation of the CPAM around the edges
of the centrifugally expanding cell plate (Figs. 2E, 3E). The few MTs and sec-
ondary CPAMs that are seen to interact with the central cell plate region at
this stage of development are focused on the remaining pores in the fenes-
trated sheet-type cell plate region (Seguí-Simarro et al. 2004), which have to
be closed to produce a new cell wall (Fig. 2F).
3.2
Transport of Cell Plate-Forming Vesicles to the Future Site
of Cell Plate Assembly Starts during Metaphase
In addition to illustrating the organization of the MTs inArabidopsismeris-
tem cells during the anaphase stage of mitosis and at different stages of
somatic-type cytokinesis, Fig. 3 shows the positions of all of the Golgi-
derived, cell plate-forming vesicles in the same samples. One of the unex-
pected discoveries to result from the analysis of these tomograms was that
the transport of the vesicles towards the equatorial plane starts already during
mitosis, i.e., much earlier than the onset of cytokinesis. Indeed, Golgi stacks
accumulate at the equatorial plane in a belt-shaped manner as soon as in
prometaphase (Seguí-Simarro and Staehelin 2006a). Initiation of this vesicle
trafficking starts at metaphase and is largely completed by the end of the solid
phragmoplast stage of cell plate formation. This explains how it is possible for
a meristematic cell to produce a cell plate from∼20 000vesicles within less
than 5 minutes of the onset of phragmoplast assembly.
In this context, the following simple calculations provide interesting in-
sights into the rate of vesicle production and cisternal turnover by individual
Golgi stacks during cytokinesis. With a diameter of∼ 9 μm, a new cross