Mitotic Spindle Assembly and Function 157
an MTOC essential to mitotic spindle formation, to one of an organelle that
facilitates proper spindle positioning and enhances the fidelity of cell divi-
sion, thus becoming important in cell survival and viability of the organism
(Wadsworth and Khodjakov 2004). In animal cells, which are malleable and
constantly change shape, centrosomes act to position and orient the spindle
via their associated astral MTs, which attach to the cortex and anchor the
spindle (Wittmann et al. 2001). In these regards, centrosomes bear several
functional similarities to the PPB.
The general effects of loss or disruption of PPBs include misoriented and
wobbly spindles (Chan et al. 2005), lack or reduction of prophase/prometa-
phase spindle bipolarity (Nogami et al. 1996; Nogami and Mineyuki 1999;
Chan et al. 2003; Marcus et al. 2003), prolonged duration of prometaphase/
metaphase, while MTs sort into bipolar spindles (Yoneda et al. 2005), and
misguided phragmoplast trajectory during cytokinesis (Camilleri et al.
2002; Chan et al. 2005). Similarly, in animal cells wherein the centrosomes
have been removed either genetically or mechanically, bipolar spindles still
form, although this occurs in a slower and less efficient manner (Khod-
jakov et al. 2000). Furthermore, these spindles frequently become mis-
oriented, which can lead to abortive cytokinesis (Khodjakov et al. 2000;
Khodjakov and Rieder 2001). In all cases, as with loss of centrosomes in
animal cells, plant cells lacking PPBs are capable of forming spindles, but
the efficiency of formation and subsequent orientation of the spindles are
compromised.
On the basis of these observations, we hypothesize that the PPB and
centrosomes represent derived structures that act complementarily to the an-
cestral self-organizational pathways to facilitate efficient spindle formation
and orientation/positioning, as well as to enhance the fidelity of cytokinesis.
Both the PPB and centrosome facilitate bipolar organization of the prophase
spindle, which poises dynamic MTs to undergo search-and-capture of kineto-
chores and interpolar MTs during prometaphase. In this regard, both the PPB
and centrosome function within the search-and-capture pathway of spin-
dle formation, although these two structures achieve spindle bipolarity by
very different mechanisms; the centrosomes acting at the poles, and the PPB
acting at the equator. In this regard, components that act in the search-and-
capture pathway may be grouped intopolar organizers(e.g. centrosomes,
spindle pole bodies, plastids) andequatorial organizers(i.e. the PPB). All of
these structures share the common function of facilitating prophase spindle
bipolarity, which aids search-and-capture during prometaphase/metaphase
spindle formation.
Another major function of both centrosomes and the PPB is cortical an-
chorage and orientation of the spindle. Centrosomes achieve this through-
out mitosis via interactions of their associated astral MTs with the cortex,
whereas in plants the function of astral MTs is supplanted by two cytoskeletal
components: F-actin and bridge MTs. In contrast to astral MTs, the pres-