50 A.J. Wright · L.G. Smith
In contrast, a novel form of cytokinesis, termed asymmetric or polarized
cytokinesis, is common in large, vacuolated cells as well as smaller, meris-
temic cells (Venverloo and Libbenga 1987; Cutler and Ehrhardt 2002; Panteris
et al. 2004; Chan et al. 2005). In polarized cytokinesis, the prophase nucleus
is found at the edge of the cell resulting in a lateral location of the subse-
quent spindle and phragmoplast (Cutler and Ehrhardt 2002). After the initial
anchoring of the cell plate to the mother cell cortex, the phragmoplast con-
tinues to expand across the mother cell creating a plane that bisects it (Fig. 1j;
Cutler and Ehrhardt 2002). In this mode of cytokinesis, the edges of the ex-
panding phragmoplast may follow a track suggested by the cortical mark at
the former location of the PPB without any need for long-range interactions
between the phragmoplast and cortex. What causes the initial asymmetric
positioning of the prophase nucleus that initiates polarized cytokinesis is of
great interest (Cutler and Ehrhardt 2002). While the studies to be discussed in
Sects. 4.2 and 4.3 have mostly not considered the question of whether the cells
being examined were undergoing symmetric or polarized cytokinesis, future
studies should consider this issue carefully because of possible mechanistic
differences in phragmoplast guidance in these two types of divisions.
4.2
Role of Actin in Phragmoplast Guidance
MFs are a major component of the phragmoplast and they co-align with MTs.
Nevertheless, phragmoplast MFs appear not to play as critical a role in cell
plate formation as MTs because actin depolymerizing drugs cause distortion
or misalignment of cell plates rather than disrupting cell plate formation per
se (Palevitz and Helper 1974b; Gallagher and Smith 1999; Granger and Cyr
2001; Baluska et al. 2001). Actin bundles that could potentially be involved
in phragmoplast guidance have been observed to connect the edge of the
phragmoplast to the cortex in fixed cells, cells injected with florescent phal-
loidin, and cells expressing fluorescent actin binding proteins (Traas et al.
1987; Lloyd and Traas 1988; Valster and Hepler 1997; Sano et al. 2005). How-
ever, there are other MF populations that may be important for division plane
control (e.g. the MF PPB or ADZ), and experiments in which drugs are ap-
plied continuously do not reveal which MF population(s) are critical for cell
plate orientation.
Experiments employing timed treatments with actin depolymerizing
drugs have been more informative as to the potential role of MFs in di-
vision plane determination. Palevitz and Hepler (1974b) applied cytocha-
lasin to dividing cells after prophase was complete and found that the nu-
clear/phragmoplast complex was positioned correctly in a majority ofAllium
guard mother cells examined. Actin inhibitor experiments previously dis-
cussed in Sect. 3.2 also suggest that the important role played by MFs in
division plane control is early in the cell cycle and that MFs do not play a large