46 A.J. Wright · L.G. Smith
3
How do Cells Remember the Division Plane Following PPB Disassembly?
In a wide variety of plant cell types, the phragmoplast attaches to the cell plate
at the former location of the PPB despite the fact that the PPB was disassem-
bled upon entry into mitosis. As discussed earlier, it has been suggested that
the PPB leaves a mark on the plasma membrane during its brief existence that
is later recognized by the phragmoplast. The nature of this mark remains to
be fully elucidated, but several components have been identified.
3.1
Vesicles/Membrane Trafficking
In the vicinity of the PPB, vesicles and vesicle budding/fusion at the
plasma membrane were observed using electron microscopy suggesting some
form of vesicle transport might be required for establishment of the cor-
tical mark recognized during cytokinesis (Gunning et al. 1978b; Galatis
and Mitrakos 1979; Galatis et al. 1982; Eleftheriou 1996; Dhonukshe et al.
2005b). Testing a potential role for Golgi-dependent secretion, Dixit and Cyr
(2002a) examined BY-2 cells expressing fluorescently labeled tubulin and
N-acetylglucosaminyltransferase I, a resident Golgi enzyme. As previously
observed by Nebenführ et al. (2000), they saw a small increase in the number
of Golgi stacks in the PPB zone. However, treatment during PPB forma-
tion with brefeldin A, an inhibitor of Golgi-dependent secretion, followed by
washing out to permit cytokinesis to occur, did not disrupt cell plate orienta-
tion. This result suggests that Golgi activity is not necessary for the formation
of the PPB mark (Dixit and Cyr 2002a).
To probe a role for endocytosis, Dhonukshe et al. (2005b) applied FM-64,
a dye that marks endocytotic vesicles, to BY-2 cells and observed a belt of
labeled vesicles that co-localized with the PPB. One possible interpretation
of this result is that endocytosis may be involved in establishing the cortical
mark, perhaps by altering the characteristics of the plasma membrane in the
vicinity of the PPB.
3.2
Actin Depleted Zone (ADZ)
While the MF component of the PPB breaks down along with the MT com-
ponent, cortical actin is retained elsewhere, creating a zone of local F-actin
depletion corresponding to the former PPB site which persists and “neg-
atively” marks the division site throughout mitosis and cytokinesis. This
so-called actin-depleted zone (ADZ) was first observed in livingTrades-
cantiastamen hair cells injected with fluorescent phalloidin (Cleary et al.
1992) and in fixed root cells (Liu and Palevitz 1992). More recently, a similar