Division Plane Orientation in Plant Cells 35
zation of the cytoplasm to form the phragmosome, a plate-like arrangement
of transvacuolar cytoplasmic strands connecting the cortex/plasma mem-
brane to the nucleus (Sinnott and Bloch 1940; Venverloo and Libbenga 1987).
A variety of other cellular components and activities are enriched in the
PPB/phragmoplast zone including Golgi (Nebenführ et al. 2000; Dixit and Cyr
2002a), endocytosis (Dhonukshe et al. 2005b), and endoplasmic reticulum in
gymnosperms only (Zachariadis et al. 2001, 2003). Thus, while the PPB is of-
ten thought of as a MT structure, it is really a complex assembly associated
with many local changes in cellular organization.
InthewidevarietyofcelltypeswherePPBshavebeenobserved,they
faithfully predict the future division plane (for review, Mineyuki 1999). More-
over, pharmacological (Hoshino et al. 2003; Vanstraelen et al. 2006) or ge-
netic (Traas et al. 1995) disruption of PPBs causes cells to divide in aber-
rant orientations, supporting the conclusion that the PPB plays a key role
in determining division planes. The PPB has long been thought to function
during prophase to establish a cortical “division site” that somehow guides
the expanding phragmoplast (Pickett-Heaps and Northcote 1966a,b; Gunning
1982), but it is still a mystery how its position is determined, how it forms,
and how it marks the cell cortex so that the expanding phragmoplast can be
guided to its former location during cytokinesis.
2.1
Selection of the Division Plane
While selection of the division plane is not well understood, the preprophase
nucleus, cell geometry, cell polarity, and extrinsic signals all appear to play
arole.
2.1.1
A Role for the Nucleus
In addition to PPB/phragmosome formation, another important, early event
in division plane establishment is migration of the nucleus into the division
plane, if it is not already located there. Nuclear migration in symmetrically
dividing cells is dependent on intact MTs, while most studies show little
or no effect of actin depolymerizing drugs (Venverloo and Libbenga 1987;
Mineyuki and Furuya 1986; Katsuta et al. 1990; but also see contrasting report
of Miyake et al. 1997). MTs grow out from the nuclear surface to the cortex
during preprophase/prophase, initially in all directions but becoming grad-
ually restricted to the future division plane as the nucleus is centered and
the PPB forms (Fig. 1c). This arrangement of cytoplasmic MTs is observed in
both large, vacuolated cells with phragmosomes, where the MTs are present
within phragmosomal strands, and also in small cells with no recognizable
phragmosome (Fig. 1d; Wick and Duniec 1983; Katsuta et al. 1990; Kutsuna