34 A.J. Wright · L.G. Smith
pears during G2 and disappears before metaphase. The PPB is a cortical band
of parallel microtubules (MTs) and microfilaments (MFs) that encircles the
cell at the future division plane. The PPB breaks down upon formation of the
mitotic spindle, which segregates chromosomes to daughter nuclei. Normally
the spindle forms so that its axis is perpendicular to the plane delineated
by the PPB. Upon completion of chromosome segregation, a plant-specific,
cytokinetic apparatus called a phragmoplast forms between the daughter
nuclei. A cytoskeleton-based structure containing both MTs and MFs, the
phragmoplast, acts as scaffolding for the building of a new cell wall (cell plate)
during cytokinesis. Following its initiation between the daughter nuclei, the
phragmoplast expands laterally, taking on the shape of a donut or torus as
MTs and MFs are disassembled from its interior, where the cell plate has al-
ready been deposited, and assembled at its exterior. Cytokinesis is completed
when the phragmoplast has expanded to the cell periphery where the cell
plate attaches to the mother cell wall at the former PPB site.
Understanding the spatial control of cytokinesis requires complete know-
ledge of how the PPB, spindle, and phragmoplast are formed and the forces
that influence their positioning. The last major reviews related to this subject
were published prior to 2002 and contain many valuable references (Mineyuki
1999; Kumagai and Hasezawa 2001; Brown and Lemmon 2001; Smith 2001).
In this review, we emphasize discussion of work published in the past five
years. During this period, imaging of GFP fusion proteins in living cells has
confirmed and extended previous discoveries and has permitted new and in-
formative observations regarding the spatial control of plant cell division.
Further characterizations of new and old mutants and the corresponding
gene products have also provided new insights into the mechanisms by which
plant cells orient their division planes.
2
Selection and Establishment of the Division Plane
The PPB was originally described as a cortical band of MTs that encircles
the mother cell perimeter at the future site of cell plate insertion (Pickett-
Heaps and Northcote 1966a,b; Fig. 1c). Formed during an arbitrarily defined
portion of the cell cycle called preprophase that corresponds to G2 or early
prophase, the PPB persists throughout prophase (Wick and Duniec 1984; Ven-
verloo and Libbenga 1987; Mineyuki et al. 1988). During preprophase and
prophase, cortical MTs are found only within the PPB (Mineyuki et al. 1991;
Granger and Cyr 2000). More recently, MFs have been identified as a com-
ponent of the PPB. Prior to the onset of mitosis, MFs become co-aligned with
PPB MTs while the density of MFs elsewhere in the cortex is reduced (Pale-
vitz 1987; Traas et al. 1987; McCurdy and Gunning 1990; Sano et al. 2005). In
large, vacuolated cells, formation of the PPB coincides with a global reorgani-