38 A.J. Wright · L.G. Smith
Clearly the position of the PPB and premitotic nucleus are interrelated,
but what is the cause and effect relationship between them? When the pre-
mitotic nucleus ofAdiantumprotonema cells was displaced by centrifugation
from its normal, apical position, the majority of cells formed a PPB around
the displaced nucleus instead of at the apical location (Murata and Wada
1991). Similar results were previously reported for centrifuged wheat root
cells (Burgess and Northcote 1968). These experiments clearly indicate an im-
portant role for the premitotic nucleus in dictating the site of PPB formation
in some cells (Murata and Wada 1991). In contrast, when the premitotic nu-
cleus was displaced in asymmetrically dividing cells, the PPB still formed
in the usual location (Pickett-Heaps 1969; Galatis et al. 1984). Moreover, in
asymmetrically dividing cells, the premitotic nucleus can occasionally be ob-
served outside of the future division plane delineated by the PPB without
experimental manipulation (e.g. Panteris et al. 2006). Thus, whether the nu-
cleus leads or follows the PPB may depend on cell type, with the nucleus
leading in symmetrically dividing cells and following in asymmetrically di-
viding cells. In this regard, it is interesting that nuclear migration to the
division plane appears to depend primarily on MTs in symmetrically dividing
cells and on MFs in asymmetrically dividing cells.
While the centering of the nucleus by a MT-based mechanism appears to
be an important part of the division plane selection process in some cells,
nuclear position alone cannot be sufficient to determine the division plane.
For example, an elongated cell may divide symmetrically in either transverse
or longitudinal planes—in either case, the premitotic nucleus will be located
centrally. As discussed in the next sections, cell geometry and cell polarity
may also be important factors in division plane selection.
2.1.2
A Role for Cell Geometry
Most plant cell divisions appear to be constrained by cell geometry in two key
ways (reviewed by Lloyd 1991). First, cell plates do not attach to the mother
cell wall at the same point as a mature, neighboring cell wall, preventing the
formation of 4-way junctions (Fig. 2a; Sinnot and Bloch 1941). Second, the
plane of cell division is often aligned with the shortest axis of the cell, al-
though many exceptions to this rule exist (Hofmeister 1863). Experimental
treatments in which round cells were forced to adopt an elongated shape by
externally applied pressure have further reinforced the notion that cell geom-
etry can be an important determinant in division plane selection (Lynch and
Lintilhac 1997). Since it is known that the PPB and phragmosome predict the
future plane of division, it is the position of their formation, and not that of
the cell plate itself, that must be influenced by these rules.
How might cells “read” their geometries in order to follow these division
plane rules? Cytoplasmic strands, and presumably the MTs and MFs they con-