40 A.J. Wright · L.G. Smith
ity that symmetric division planes are governed by polarizing cues as well.
Hofmeister’s rule states that new cell walls are typically formed perpendicu-
lar to the mother cell’s axis of elongation (Hofmeister 1863). The connection
between the plane of division and the growth polarity of a cell may be medi-
ated in part by the hormone auxin (Dhonukshe et al. 2005a). This connection
is suggested by the observation that mutations in genes encoding proteins
involved in polar auxin transport often have disrupted planes of division
(Mayer et al. 1993; Shevell et al. 1994; Willemsen et al. 2003). Additionally,
application of auxin efflux inhibitors causes abnormal division planes, which
are predicted by abnormally oriented PPBs, in elongated cells that would nor-
mally divide symmetrically and transversely (Petrásek et al. 2002; Dhonukshe
et al. 2005b). Polarized auxin flow may link the plane of cell division with the
polarity of cell growth, allowing for changes in growth and division patterns
in response to developmental and environmental cues. Moreover, differing
patterns of polarized auxin transport might explain why some symmetrically
dividing, elongated cells divide transversely while others divide longitudi-
nally.
2.1.4
A Role for Extracellular Signals
Some cells do not abide by the geometrical rules that predict the division
planes of most plant cells. Divisions that occur in response to wounding are
a clear example. Upon injury, cells close to the wound are induced to divide
and the plane of division parallels the wound even if that plane is oblique rela-
tive to the axis of the dividing cell or the growth axis of the plant (Sinnott and
Bloch 1941; Venverloo and Libbenga 1987; Goodbody and Lloyd 1990). It is
unknown what signals orient wound-induced division planes. An intriguing
recent study suggests that during wound-induced divisions inColeus,both
nuclear migration and PPB formation follow other, earlier determinants of
the division plane that respond more directly to wound-induced extracellular
cues. The cortical cytoplasmic ring, or CCR, is a ring of cytoplasm contain-
ing MFs and ER that encircles the future division plane following wounding
ofColeuspith tissue (Panteris et al. 2004). Nuclear migration into the di-
vision plane occurs after CCR formation coincident with a proliferation of
transvacuolar strands, some of which connect the nucleus to the CCR, form-
ing a phragmosome. The MT component of the PPB forms in alignment with
the CCR and phragmosome. It would be very interesting to know whether fac-
tors directing CCR formation have any role in more conventional modes of
plant cell division.
Asymmetric divisions are another clear example where dividing cells do
not follow standard geometrical rules in selecting their division planes (see
Bisgrove and Kropf, this volume). In some cases, these divisions appear to be
oriented in relation to extracellular cues. A good example is the asymmetric