Plant Cytokinesis – Insights Gained from Electron Tomography Studies 255 Fig. 1 Different steps in the tomographic reconstruction and modeling process, com-
pared with conventional transmission electron micrographs.AElectron micrograph of
a 100 nm-thick section of an Arabidopsis meristem cell at the solid phragmoplast stage of
cytokinesis. Large organelles such as mitochondria (m), or structures with simple geome-
tries such as microtubules (mt) are clearly resolved, but irregular membranous structures
such as the cell plate (cp) often appear as dark, blurred spots. Due to the section thick-
ness, the cell plate assembly matrix (cpam) can only be distinguished at certain regions
around the cell plate. r: ribosome.BTwo-nm-thick virtual slice of a tomogram from
the same cell. The overall image is sharper, and the cell plate (cp) membranes, Golgi-
derived vesicles (v) and endoplasmic reticulum (er) membranes are better resolved. The
ribosome-excluding CPAM is now more clearly seen around the cell plate.CModeling of
the same tomographic slice. Each object within the tomographic volume is given a color,
and the contours are manually traced around them across the whole set of tomographic
slices. Each contour is then connected by a mesh to reconstruct the virtual surface of
each object in the model.DTomographic model. After object modeling and render-
ing, the whole set of objects can be visualized, giving a 3-D picture of their actual size,
distribution and associations between them. Thescale barsrepresents 500 nm
by then extending the reconstructions from one section to the next (Otegui
and Austin 2006). Most of the tomography-based models presented in this
review are based on serial section reconstructions.3
Structure and Function of the Phragmoplast Cytoskeleton
3.1
Phragmoplast Microtubules Undergo Changes in Organization
During Cell Plate FormationMicrotubules are the most visible component of the cytoskeletal elements in-
volved in cytokinesis, such as the premitotic pre-prophase band (Fig. 2A) and
the phragmoplast. They have been the subject of hundreds of studies of divid-
ing plant cells, both at the light and the electron microscope level of analysis.
Here we only review contributions made by electron microscopy to the study
of the phragmoplast and cell plate of cryofixed cells.
Figures 2 and 3 illustrate the organization of the MT arrays associated
with phragmoplast formation and maturation inArabidopsismeristem cells
(somatic-type cytokinesis). The two opposing sets of MTs in Fig. 3 are shown
in different colors so that their spatial organization can be more readily
discerned. The comparison between the late anaphase organization of the
mitotic spindle MTs and the organization of the MTs at the time of phrag-
moplast initial assembly during very late anaphase (Fig. 3A and B) highlights
how the phragmoplast initials arise from clusters of opposing sets of spindle
MTs. The distinction between the two types of arrays is based on the pres-
ence of multiple CPAMs in the equatorial plane of the phragmoplast initials