Plant Cytokinesis – Insights Gained from Electron Tomography Studies 277
during early stages of cell plate assembly when most vesicles are delivered to
the forming cell plate. This is not the case. Instead, during the early stages of
cytokinesis, ER membranes in the cell plate forming region are very sparse
and the thin tubules are oriented at nearly right angles to the plane of the
cell plate (Seguí-Simarro et al. 2004). Thus, it is unlikely that the ER is either
involved in the positioning or the control of vesicle fusion during the initial
stages of cell plate assembly.
Increasing numbers of ER cisternae begin to appear at about the same time
that the deposition of callose in the cell plate lumen is observed, suggesting
that ER cisternae could be involved in the control of the Ca2+-activated cal-
lose synthases in the cell plate membranes (Samuels et al. 1995; Verma and
Hong 2001). In addition, the later appearing ER cisternae most likely serve
as templates for the formation of primary plasmodesmata (Lopez-Saez et al.
1966; Robards and Lucas 1990; Fengshan and Peterson 2001). Finally, the ap-
pearance of ER cisternal domains that are attached through linkers to late
stage cell plate membranes is consistent with observations in other systems
where membrane lipids are recycled by means of a lipid hopping mechan-
ism (reviewed in Staehelin 1997; Seguí-Simarro et al. 2004). This hypothesis,
however, has yet to be tested.
A recent study of living tobacco NT-1 cells stably expressing ER targeted
GFP has provided experimental support for the tomography results showing
that the cell plate associated ER increases during cytokinesis (Gupton et al.
2006). In particular, during late anaphase, when the chromosomes approach
the spindle poles and the phragmoplast initials are being formed (Fig. 2B),
the amount of ER membranes in the equatorial plane is very low, and only
during mid telophase (transitional phragmoplast stage, Fig. 2D) is a dramatic
increase in the cell plate-associated ER signal seen.
5.6
Syncytial- and Somatic-Type Cell Plate Assembly
Involves Different Structural Intermediates but Similar Molecular Mechanisms
Cells such as nuclear endosperms (Fig. 10) and meiocytes (Fig. 11) that exhibit
syncytial-type cytokinesis, do not couple nuclear and cytoplasmic division
as somatic cells do. Prior to cytokinesis, these cells undergo several cycles of
nuclear division, repositioning of the nuclei, and then the simultaneous devel-
opment of multiple cell walls between sister and non-sister nuclei (Figs. 10A,
11A; Otegui and Staehelin 2000b; Brown and Lemmon 2001). In addition, no
preprophase bands are formed in syncytial cells. Thus, although the syncytial-
and somatic-type cytokinesis mechanisms utilize many of the same enzymatic
and regulatory molecules and the process of cell plate assembly exhibits many
commonalities (Jürgens 2005b), there are also important differences and adap-
tations of the cell plate assembly process that can only be understood in the
spatial and temporal context in which the cell plates are assembled.