44 A.J. Wright · L.G. Smith
2005). Like PPB formation, PPB narrowing requires de novo tubulin poly-
merization (Panteris et al. 1995). The MF component of the PPB does not
undergo conspicuous narrowing, remaining much wider than the final MT
PPB throughout prophase (McCurdy and Gunning 1990; Mineyuki and Pale-
vitz 1990). While MTs are required for formation of the MF PPB, MFs are not
required for the formation of the MT PPB (Palevitz 1987; McCurdy and Gun-
ning 1990). However, application of actin depolymerizing drugs does inhibit
MT PPB narrowing (in fact the MT PPB widens in this situation) suggesting
MFs are needed both to initiate and to maintain narrow MT PPBs (Mineyuki
and Palevitz 1990). Actin depolymerizing drugs also result in misaligned di-
vision planes, suggesting a possible role for PPB narrowing in division plane
alignment (Venverloo and Libbenga 1987; Mineyuki and Palevitz 1990). To
test this hypothesis, a MT depolymerizing drug was applied to BY-2 cells
expressing a fluorescent MT marker after the initial formation of the PPB
but before narrowing (Marcus et al. 2005). The drug was then washed out
to allow for spindle re-formation, mitosis, and cytokinesis. While the re-
forming spindle was not always perpendicular to the plane of the former
PPB, the phragmoplast was able to successfully track back to the former
PPB site, resulting in normally oriented cell division (Marcus et al. 2005).
These results suggest that the key function(s) of the MT PPB in “marking”
the division site for subsequent phragmoplast guidance are accomplished
early, prior to PPB narrowing, but that the presence and/or narrowing of
the PPB later in prophase is important for consistent spindle orientation
(Marcus et al. 2005).
Other recent studies further support the notion that the PPB influences
spindle assembly and orientation. The spindle normally forms so that its axis
is perpendicular to the plane of division predicted by the PPB (Gunning et al.
1978a). Since spindle poles begin to form on the surface of the prophase nu-
cleus while the PPB is still present and MTs connect the spindle poles to the
PPB, it has long been thought that the PPB may influence the initial pos-
ition of the spindle poles (Fig. 1e; Wick and Duniec 1984; Mineyuki et al.
1991; Nogami et al. 1996). To investigate this possibility, spindle behavior was
examined in a unique population ofArabidopsissuspension cells expressing
a MT marker, EB1:GFP. In this population, a significant percentage of cells fail
to produce a PPB (Chan et al. 2005). In cells with a PPB, spindle poles marked
by an accumulation of EB1-GFP formed prior to PPB and nuclear envelope
disassembly, and the spindles that subsequently formed were always perpen-
dicular to the plane of the PPB. In contrast, in cells that did not form a PPB,
spindle formation was delayed until after nuclear envelope breakdown and
spindle orientations were unpredictable (Chan et al. 2005).
Investigating the same issue by a different approach, Yoneda et al. (2005)
examined spindle formation in BY-2 cells expressing a fluorescent tubulin
marker that sometimes formed two PPBs after synchronization with aphidi-
colin. While cells with a single PPB formed the expected bipolar spindle, cells