252 J.M. Seguí-Simarro et al.
kinesis and cytokinesis are both temporarily and structurally separate events
(Otegui and Staehelin 2000a).
Our current concept of the plant cytokinetic machinery includes three
distinct structures, each of which is an essential component of the cytoki-
netic apparatus: the phragmoplast, the cell plate assembly matrix (CPAM),
and the cell plate. The phragmoplast is a cytoskeletal array of anti-parallel
microtubules (MTs) and actin filaments. The principal function of the MTs
is to deliver both the Golgi-derived vesicles and the CPAM molecules to
the cell plate assembly region. A major function of the actin filaments is to
guide the cell plate to the proper site of fusion of the cell plate with the cell
wall (Gunning 1982; Whalen et al. 2001; Molchan et al. 2002). The CPAM is
a transient, membrane-less cytoplasmic domain that can be recognized in
electron micrographs of cryofixed and freeze-substituted cells as a ribosome-
excluding zone that encompasses the fusing vesicles of growing cell plates
(Samuels et al. 1995; Otegui et al. 2001; Otegui and Staehelin 2004; Seguí-
Simarro et al. 2004). It is brought to the cell plate assembly region together
with the Golgi-derived, cell plate-forming vesicles, and it appears to con-
sist of scaffolding proteins and enzymatic, structural and regulatory proteins
that are required for cell plate assembly. The cell plate itself is a transient
membranous organelle that arises within the CPAM from the fusion of Golgi-
derived vesicles during late anaphase and telophase, and progresses through
a defined sequence of maturational steps to give rise to the new cell wall
(Samuels et al. 1995; Otegui et al. 2001; Otegui and Staehelin 2004; Seguí-
Simarro et al. 2004).
The focus of this review is on the structural events associated with the as-
sembly of both somatic- and syncytial-type cell plates as seen in electron to-
mograms of cells preserved by high pressure freezing and freeze-substitution
(HPF-FS) techniques. In particular, we review the structure of the membrane
assembly intermediates associated with the different maturational stages of
cellplateassembly,aswellasdiscusstheconcomitantchangesintheorgani-
zation and function of the CPAM and how the CPAM influences MT (+)-end
dynamics.
2
Techniques: High Pressure Freezing and Electron Tomography
Have Ushered in a New Era in Plant Cytokinesis Research
The cytokinetic apparatus and growing cell plates are highly dynamic mo-
lecular assemblies that undergo changes in organization in the time range
of seconds and even fractions of seconds. Preserving the assembly inter-
mediates of the cell plate for structural analysis has been a central goal of
electron microscopists for half a century (reviewed in Seguí-Simarro and
Staehelin 2006b). Classical chemical fixatives, such as glutaraldehyde and os-