Cell Division Control in Plants

Plant Cytokinesis – Insights Gained from Electron Tomography Studies 261

Fig. 4 Diagrams illustrating the structural changes in cell plate membrane and CPAM
organization during somatic-type cytokinesis based on tomographic models. The cell
plate membranes are depicted inyellow, the CPAM is outlined withred dots,andthecell
plate is shown in a face-on view. The microtubules have been omitted to facilitate view-
ing of the cell plate and CPAM.APhragmoplast initials stage. Different vesicle clouds
concentrate within individual CPAMs at the central region of the cell equator. Within
them, the first dumbbell-shaped cell plate intermediates are detected. Arrows indicate
the direction of expansion of the phragmoplast initials.BSolid phragmoplast stage. The
isolated early intermediates have now merged into a coherent tubulo-vesicular network
cell plate (cp) surrounded by a single, cocoon-like CPAM (cpam).CTransitional phrag-
moplast stage. The cell plate transforms into a CPAM-free tubular network at its central
domain, whereas the peripheral region actively expands outwards (arrows) surrounded
by a ring-shaped CPAM.DRing phragmoplast stage. The central cell plate region ma-
tures into a planar fenestrated sheet whereas the peripheral growth zone merges with
the parental plasma membrane within the ring-shaped CPAM. Small, secondary CPAMs
are created de novo around the larger perforations (fenestrae) of the planar sheet central
cell plate. Thegreen boxesoutline the region of the cell plate shown ininsetsin cross-
sectional views. cp: cell plate; cpam: cell plate assembly matrix; cw: cell wall; pm: plasma
membrane

initials that yields the solid phragmoplast is paralleled by an expansion and

merging of the individual CPAMs of the initials into a coherent CPAM across

the entire width of the solid phragmoplast MT array (Seguí-Simarro et al.

2004), encompassing the nascent tubulo-vesicular cell plate (Figs. 2C, 4B).

The breakdown and subsequent reassembly of the CPAM during the transi-

tional and the ring phragmoplast stages of cell plate formation (Figs. 2D,E,

4C,D) highlights further the close structural and functional relationship be-

tween the CPAM, the distribution of MTs, and the growing, but not the

maturing, cell plate domains.

Structurally,theCPAMcanbedefinedasafinefilamentousmatrixthat

excludes ribosomes to a distance of∼ 160 nm from the cell plate in somatic

cells, and∼ 100 nm in syncytial cells. The first insights into the composition

and the functional properties of the CPAM have come from a combination

of electron tomography and immunolocalization studies. A list of putative

components of the CPAM is presented in Table 1, and the functional impor-

tance of these molecules for cell plate formation is discussed in the following

sections.

4.2

The CPAM Stabilizes MT (+)-Ends

Phragmoplast MTs originate from MT organizing centers (MTOCs) located in

the vicinity of the nuclei (Vantard et al. 1990; Baskin 2000). This leads to the

(+)-ends of the MTs being oriented towards the cell plate (Euteneuer et al.

1982). Prior to the discovery of the CPAM, it was reported that the region

surrounding the cell plate, where the MT (+)-ends terminate, is critical for