Division Plane Orientation in Plant Cells 41
division of the subsidiary mother cells (SMCs) during stomatal development
in grasses. Prior to dividing, SMCs become polarized towards the adjacent
guard mother cell (GMC) as evidenced by the formation of a localized F-actin
patch at a site on the plasma membrane adjacent to the GMC, migration of the
SMC nucleus to this actin patch, and formation of an asymmetric PPB pre-
dicting the asymmetric division plane that will later form a small subsidiary
cell adjacent to the GMC and a much larger sister cell (for review, Smith 2001).
The orientation of the SMC division clearly violates the geometry rules be-
cause the division wall is curved, does not form perpendicular junctions with
the mother cell, and is not orientated perpendicular to the axis of cell growth.
However, in no case has a cell division-orienting signal been clearly identi-
fied,norarethereexampleswheretheresponsetosuchasignalisunderstood
as to how it positions the nucleus, PPB, or phragmosome.
2.2
PPB Formation
Studies of PPB formation have focused primarily on understanding the as-
sembly of the MT component of this array. Prior to the decision to undergo
mitosis and the formation of the PPB, interphase cells contain a cortical MT
array (Fig. 1a,b). In elongated cells, this array consists of ordered, parallel
MTs that are mostly orientated perpendicular to the axis of elongation. In iso-
diametric cells, the cortical MT array can be ordered or random. One of the
mysteries of PPB formation is how PPB MTs are confined to a band at the cor-
tex when previously the entire cortex was capable of supporting cortical MT
growth. It is known that the MT PPB is created from newly polymerized MTs,
not a rearrangement of pre-existing MTs (Cleary et al. 1992; Panteris et al.
1995). The lack of lateral MT movement also suggests that MT rearrangement
via the action of motor proteins is not required for MT PPB formation (Shaw
et al. 2003; Vos et al. 2004).
In highly vacuolated cells the PPB forms at the same time as the phrag-
mosome (Venverloo and Libbenga 1987). Additionally, incomplete or com-
plete PPBs were associated with incomplete or complete phragmosomes,
respectively, suggesting interdependence in their formation (Venverloo and
Libbenga 1987; Flanders et al. 1990). Unlike the PPB, the phragmosome is
maintained throughout mitosis, suggesting that it may aid in phragmoplast
guidance later during cytokinesis.
2.2.1
Insights from Studies of MT Dynamics in Living Cells
One theory to account for the spatially restricted appearance of the PPB
is that its formation results from local changes in MT dynamics. To quan-
tify differences in MT dynamics between interphase and preprophase cells,