276 J.M. Seguí-Simarro et al.
maturation has been interpreted to suggest that these vesicles are recycled via
multivesicular bodies (Seguí-Simarro and Staehelin 2006a).
The final transformation of the planar fenestrated sheet-type cell plate into
a new primary cell wall (Fig. 9D) involves the progressive replacement of the
callose deposits by cellulose fibrils (Kakimoto and Shibaoka 1992; Samuels
et al. 1995; Otegui and Staehelin 2000b). Because the first cellulose molecules
can be detected via cellobiohydrolase I binding during the tubular network
phase of cell plate development (Samuels et al. 1995), and the large cellulose
synthase complexes would not be able to translocate through such a network
while spinning out cellulose microfibrils (Brown et al. 1996), it is possible
that the first cell plate cellulose molecules are produced by a different iso-
form of the cellulose synthase enzyme. Upon deposition in the cell plate
lumen/cell wall, the cellulose molecules and microfibrils are assumed to trig-
ger the assembly of the primary cell wall by the incorporation of the already
secreted hemicellulose molecules into a cellulose-hemicellulose network ad-
jacent to each daughter plasma membrane. In turn, the formation of this
cellulose-hemicellulose network could be the driving force behind the gen-
eral reorganization of the pectic polysaccharides and the formation of the
pectin-rich middle lamella (Carpita and McCann 2000).
5.5
Increasing Amounts of Endoplasmic Reticulum Progressively Associate
with the Cell Plate During the Maturation Stages of Cytokinesis
Cell plate-associated ER membranes were already described and discussed
in some of the earliest electron microscope studies of cytokinesis in plant
cells (Porter and Caulfield 1958; Hepler and Newcomb 1967; Bajer 1968).
However, the functional importance of these ER membranes in cytokinesis
remains an enigma. After the large lytic vacuole, the ER is one of the principal
stores of Ca2+in plant cells (Sanders et al. 2002). Calcium, in turn, is known
to regulate a number of cellular activities involved in cell plate formation,
including vesicle fusion (Hepler et al. 1990; Hepler 1994), the dynamics of cy-
toskeletal elements (Hepler et al. 1990), callose synthase activity (Kakimoto
and Shibaoka 1992; Verma and Hong 2001), and dynamin dynamics (Verma
2001), Thus, the cell plate-associated ER cisternae are generally assumed to
function in the local regulation of free Ca2+within the phragmoplast and to
thereby control cell plate assembly activities.
By defining in more precise terms the organization of the ER membranes
during cytokinesis, electron tomography studies of cryofixed cells have pro-
vided a means for narrowing the potential functions of the ER during differ-
ent stages of cell plate formation. For example, it was suggested that the ER
membranes regulate cell plate formation via control of the positioning and
fusion of the cell plate forming vesicles (Hepler 1982). A prediction of this hy-
pothesis is that ER plate-associated ER membranes should be very prominent