Cytoskeletal Motor Proteins in Plant Cell Division 173
Fig. 1 Organization of spindle microtubules in plant cells.AFluorescent images show-
ing microtubules and chromosomes in an onion cell at metaphase.Asterisksmark the
incompletely focused spindle poles.Arrowspoint at kinetochore microtubule fibers, and
arrowheads at interzonal microtubules which do not end at kinetochores.BProposed role
of minus end-directed Kinesin-14 in converging microtubule minus ends.Arrowsindicate
the direction of kinesin motility
development of the preprophase band (PPB) (Wick and Duniec 1983). These
nuclear envelope-associated microtubules are later organized into what is re-
ferred to as the “prophase spindle” with two clear poles. The formation of
these spindle poles, which are the new MTOCs, involves the reorganization of
microtubules with their minus ends focused at the polar region as indicated
by the concentration of minus end-specific proteinγ-tubulin at the poles (Liu
et al. 1993). Prior to the breakdown of the nuclear envelope, microtubules
form polar caps which mark the future spindle poles (Lloyd and Chan 2006).
The formation of polar caps would require that microtubules get translocated
so that their minus ends become converged. Such a microtubule converging
phenomenon has been well documented in the endosperm cells undergoing
mitosis in the African blood lily Haemanthus (Smirnova and Bajer 1998).
A microtubule gliding activity alonga parallel microtubule would account
for the convergence of these microtubules. Minus end-directed kinesins in the
Kinesin-14 subfamily would be ideal candidates for such an activity (Fig. 1B).
Members of the Kinesin-14 subfamily have their motor domain located at