Mitotic Spindle Assembly and Function 155
and GFP technology will likely reveal search-and-capture in plant mitotic
spindles.
4.4
The PPB as an Equatorial Organizer of the Prophase Spindle
Substantial evidence supports the notion that the PPB facilitates the bipolar
organization of the associated prophase spindle. In all reported cases where
the PPB is experimentally perturbed or absent, the accompanying prophase
spindle also exhibits abnormalities. Specifically, bipolarity of the prophase
spindle is absent or reduced in the following cases: (1) cells that naturally
lack PPBs, such as those of endosperm and meiotic tissues (Smirnova and
Bajer 1992); (2) cells of thetonneaumutant, which lack cortical MTs and
PPBs (Camilleri et al. 2002); (3)Arabidopsissuspension cells overexpressing
EB1::GFP, which frequently lack PPBs (Chan et al. 2005); (4) cells treated with
Cytochalasin D, which causes narrow PPBs to broaden (Mineyuki and Pale-
vitz 1990; Mineyuki et al. 1991; Eleftheriou and Palevitz 1992); (5) cells treated
with cycloheximide or kinase inhibitors, which also inhibit PPB narrowing
(Nogami et al. 1996; Nogami and Mineyuki 1999); (6) cells exhibiting double
PPBs (Yoneda et al. 2005); (7) cells treated with taxol, which interferes with
PPB narrowing and causes the MTs bridging the nucleus and PPB to become
more numerous and unevenly distributed (Panteris et al. 1995; Baluska et al.
1996); (8) caffeine-induced binucleate cells where one nucleus lacks a PPB
(Manandhar et al. 1996); (9) cells from theArabidopsis atk1-1mutant, which
exhibit abnormally broad PPBs (Marcus et al. 2003); (10) asymmetrically
dividing subsidiary cells ofZea mays, where the PPB does not encircle the nu-
cleus (Panteris et al. 2006); and (11) meristematic cells fromArabidopsisroot
tips overexpressing the MT reporter GFP::MAP4, which frequently exhibit an
absence of PPBs or lack of PPB narrowing (our unpublished observations).
Although there is little doubt that the PPB plays a role in establishing
prophase spindle bipolarity, studies are lacking that suggest a clear mech-
anism as to how this occurs. On the basis of the available data, we propose
that the cytoplasmic MTs bridging the PPB and spindle (hereafter referred to
as bridge MTs) are capable of transmitting forces that facilitate the sorting
of perinuclear MTs into two half-spindles, which are symmetrically mirrored
about the PPB plane. These bridge MTs may exert transient tensive forces be-
tween the PPB and perinuclear MTs, thereby providing a cue for both the
bipolar organization and proper orientation of the prophase spindle. Bridge
MTs connect directly to the PPB, where they may become bundled with PPB
MTs (Burgess 1970; Bakhuizen et al. 1985; Panteris et al. 2006). The ends of
the bridge MTs that reside inside the PPB, coaligned with cortical PPB MTs,
provide possible sites for anchorage or generation of force via MT-MT sliding
mechanisms. Indeed crossbridges between PPB MTs, which could represent
crosslinking factors or motors, have been observed (Hardham and Gunning