Division Plane Orientation in Plant Cells 47
F-actin distribution was observed in BY-2 cells expressing an actin-binding
domain from fimbrin fused to GFP (Sano et al. 2005). However, in these
cells local accumulations of MF fluorescence were observed at both edges
of the ADZ creating MF “twin peaks” (Sano et al. 2005). Although this fea-
ture was not pointed out, other published images of ADZs in living cells
also show local enrichments of F-actin at the edges of the ADZ (e.g. Val-
ster and Hepler 1997; Cleary 1995; Vanstraelen et al. 2006). Thus, MF twin
peaks may be a general property of ADZs in living cells, but difficult to
preserve by chemical fixation. In any case, a key question is what is the sig-
nificance of the ADZ? To address this question, semi-synchronized cultures
of BY-2 cells were treated with actin depolymerizing drugs then the drugs
were washed out at various times during cell division (Hoshino et al. 2003;
Sano et al. 2005). The drugs had a maximum disruption to cell plate orien-
tation when applied for a period spanning the peak of ADZ formation, while
drug treatment during cytokinesis had little effect. These experiments sug-
gest that the presence of the ADZ during cytokinesis is not essential for
phragmoplast guidance, but that either the ADZ or PPB F-actin plays an
important role in establishment or maintenance of the division site prior
to cytokinesis.
3.3
KCA1 Depleted Zone (KDZ)
A kinesin, KCA1, has recently been shown to serve as a second negative
marker of the division site (Vanstraelen et al. 2006). KCA1 shows a strong in-
crease in plasma membrane localization in dividing cells and also localizes
to the newly forming cell plate during cytokinesis (Vanstraelen et al. 2004,
2006). Interestingly, from the time the PPB forms until the completion of cy-
tokinesis, KCA1 localization is reduced at the future division site relative to
the rest of the plasma membrane. The KCA1 depleted zone (KDZ) forms ear-
lier than the ADZ but then co-localizes with it following PPB disassembly
(Vanstraelen et al. 2006). Drug studies showed that while actin and MTs are
not required for KDZ maintenance during mitosis and cytokinesis, MTs are
required for KDZ formation (Vanstraelen et al. 2006). In cells where PPBs
were destroyed by MT inhibitors and failed to reform upon drug wash out,
no KDZ was present and cell plates failed to insert at the position of the for-
mer PPB (Vanstraelen et al. 2006). This experiment is consistent with the
conclusion that KDZs play an important role in phragmoplast guidance but
does not prove this point since MT PPBs were also destroyed by drug treat-
ment. A definitive demonstration of the role of KCA1 in the spatial control of
cytokinesis awaits further studies.