Plant Cytokinesis – Insights Gained from Electron Tomography Studies 259
wall in anArabidopsismeristem cell requires the deposition of∼ 130 μm^2 of
plasma membrane. Taking into account the dimensions of the Golgi-derived,
cell plate-forming vesicles (∼ 50 nm; Seguí-Simarro et al. 2004) and the mean
number of Golgi stacks per cell (∼35 during interphase and∼68 during G2
and mitosis; Seguí-Simarro and Staehelin 2006a), the assembly of a new cross
wall would require that each (mitotic stage) Golgi stack produce∼240 vesicles
for the new cell plate. However, because∼ 30 % of the cell plate membrane is
recycled via clathrin-coated vesicles during maturation (Seguí-Simarro et al.
2004), the actual number of Golgi vesicles required is∼310. Since each TGN
cisterna fragments into∼35 secretory-type (cell plate forming) vesicles (un-
published results), the 310 vesicles correspond to∼8.8 cisterna equivalents.
According to the live cell measurements of Ueda et al. (2003), it takesAra-
bidopsiscells 6 to 8 minutes to progress from metaphase through the solid
phragmoplast stage of cytokinesis. This means that each Golgi has to deliver
∼300 vesicles in<8 m inu tes to the cell plate forming region, which amounts
to∼37 vesicles or 1 TGN cisterna equivalent per minute. In conclusion, this
rough calculation suggests that during cytokinesis, each Golgi stack produces
approximately one new cisterna per minute.
4
Structure and Function of the Cell Plate Assembly Matrix (CPAM)
4.1
The CPAM Defines the Site of Cell Plate Assembly
The discovery of the CPAM is an example of a cellular structure whose
discovery can be traced to the use of HPF-FS techniques to preserve cells
undergoing cytokinesis for electron microscope analysis. Although early elec-
tron microscope studies of chemically fixed dividing cells provided hints of
a cell plate-associated matrix (reviewed in Gunning 1982), the first defini-
tive evidence for the presence of a ribosome-excluding zone around growing
regions of cell plates and associated MT ends was provided in an investi-
gation of tobacco BY-2 cells preserved by HPF-FS (Samuels et al. 1995). In
that study, a prominent CPAM was seen to encompass the tubulo-vesicular
network-type cell plates, and this ribosome-excluding material was reported
to abruptly disappear when the tubulo-vesicular cell plate matured into
a tubular network-type of cell plate. A similar ribosome-excluding matrix
was subsequently recognized in endosperm cells ofArabidopsisundergoing
syncytial-type cell plate formation (Otegui and Staehelin 2000b). In particu-
lar, it was shown that the formation of the first membrane tubules from cell
plate forming vesicles always occurred within a ribosome-free matrix, and
that this matrix persisted until the wide tubular stage cell plate network was
converted into a fenestrated sheet-type of cell plate.