Division Plane Orientation in Plant Cells 51
role in phragmoplast guidance during cytokinesis (Hoshino et al. 2003; Sano
et al. 2005).
In addition to studies focusing on actin itself, other investigations have
examined the potential role of myosin, an actin-based motor protein. Two
different myosin inhibitors delayed or inhibited the lateral expansion of the
phragmoplast beyond the width of the nuclei inTradescant iastamen hair
cells (Molchan et al. 2002). Application of BDM, an inhibitor suspected to
affect a greater array of myosins, also resulted in tilting of the nuclear/
phragmoplast complex (Molchan et al. 2002). These results suggest a role
for myosins in lateral expansion of the phragmoplast and perhaps also in
phragmoplast guidance, although drug application throughout the cell cycle
limits the interpretation of these results since it is possible that myosins are
needed for division plane establishment and/or maintenance. It was also ob-
served that application of one of the inhibitors disrupted the structure of the
phragmoplast, suggesting that the effects of myosin inhibitors on phragmo-
plast expansion/guidance may be indirect (Molchan et al. 2002). Thus, further
work will be needed to clarify when and how myosins may be involved in cy-
tokinesis or its spatial regulation. Genetic studies investigating the roles of
myosins would be very helpful in this regard but are complicated by the large
number of myosin genes, whose products may act redundantly.
4.3
Role of MTs in Phragmoplast Guidance
Phragmoplast MTs play an essential role in cell plate formation. Vesicles travel
along phragmoplast MTs, fuse with the developing cell plate, and provide
membrane and cell wall materials needed for its continuous expansion. Since
MTs are essential for cell plate formation, destruction of phragmoplast MTs
with MT depolymerizing drugs does not offer any clues about the roles of
MTs in phragmoplast guidance. However, recent observations of MTs radiat-
ing out from the phragmoplast/nuclear complex suggest that MTs may indeed
play a role. Chan et al. (2005) reported that inArabidopsiscultured cells,
EB1:GFP-labeled MTs link phragmoplast-associated nuclei to the cell poles,
while Dhonukshe et al. (2005b) reported that in tobacco BY-2 cells, MTs la-
beled with either EB1:GFP or MAP4:GFPconnect phragmoplast-associated
nuclei to the former PPB site and other areas of the cortex (Fig. 1g,h). We have
also observed MTs connecting the phragmoplast/nuclear complex to the cor-
tex in fixedArabidopsisroot tips cells labeled with an anti-tubulin antibody
(L.G. Smith, unpublished), alleviating concern that the GFP fusion proteins
used in live cell studies might be stabilizing MTs to create a MT popula-
tion that does not normally exist. Most likely, MT-based connections between
the phragmoplast and the cell cortex have not been previously reported be-
cause they are easily overlooked in the presence of the dense, brightly labeled
phragmoplast MT array.