Division Plane Orientation in Plant Cells 39
Fig. 2Geometry and plant cell division.aA new cell wall (dashed line) avoids connecting
to the mother cell wall at a pre-existing cell wall junction and forming a four-way junction
(on the left). Instead the connection between the new cell wall and the mother cell will
often form a three-way junction (on the right).bThe avoidance of four-way junctions is
hypothesized to be due to the cytoskeleton elements extending from the nucleus to the
cortex seeking the shortest distance due to tension during PPB/phragmosome formation.
Previous junctions have an out-pocketing which places the cortex in that area farther way
from the nucleus
tain, are known to be under tension in preprophase (Hahne and Hoffman 1984;
Goodbody et al. 1991). Since elements under tension with mobile attachment
points will tend to adopt the shortest path across the cell, tension could provide
a simple explanation for alignment of the division plane with the cell’s short
axis. Tension could also explain avoidance of four-way junctions. As a new cell
wall ages and strengthens, it creates an inward-protruding vertex in its own cell
file and a corresponding outward-protruding vertex in the neighboring files
(Fig. 2b). Thus, cytoskeletal elements may simply avoid these vertexes because
attaching at a vertex would mean spanning a longer distance (Flanders et al.
1990; Lloyd 1991). Since PPB position is correlated with the MTs extending from
the nucleus to the cortex (often contained within phragmosomal strands), the
connecting MTs may interpret the geometry via tension and then influence the
position of the forming PPB (Flanders et al. 1990).
2.1.3
A Role for Cell Polarity
While it seems that geometrical rules canexplain division plane selection in
many cells, they may be descriptive rather than instructive. In Sect. 2.1.4, be-
low, we discuss asymmetric divisions where cell polarity clearly influences
division plane selection. However, it is interesting to consider the possibil-