Cytoskeletal Motor Proteins in Plant Cell Division 177
kinesins including Kinesin-7, -10, -13 all play a non-redundant role in this
process (Zhu et al. 2005). In plant cells, it is not known whether members
of the Kinesin-7 subfamily are associated with kinetochore or chromosome
arms. Because motor activity is required for chromosome congression, it is
only a matter of time before kinesins in charge of this event are revealed
experimentally in plants.
2.4
Kinesins in Kinetochore Fiber Disassembly
and Sister Chromatid Movement During Anaphase
The disassembly of kinetochore fiber leads to the segregation of genetic mate-
rial during anaphase among eukaryotes. A molecular model depicting motor-
driven kinetochore fiber disassembly and sister chromatid segregation has
been developed in animal cells. While the cytoplasmic dynein is the primary
motor for the pole-ward movement, MCAK/Kinesin-13 plays an assistant role
for microtubule disassembly at the kinetochore (Heald 2001). Direct evidence
has been obtained in cells of the flyDrosophila melanogaster(Sharp et al.
2000b). Thus, ideally in plant cells a minus end-directed kinesin is needed at
the kinetochore to fulfill the function of dynein in animal cells. Among 21
predicted minus end-directed kinesins encoded by theA. thalianagenome,
to date none of the studied ones has been detected at the kinetochore. Al-
ternatively, microtubules of the kinetochore fiber could be depolymerized
solely at the spindle pole during anaphase in plant cells. An example has been
shown in Drosophila cells in which a microtubule depolymerizing Kinesin-13
acting at the centrosome, and another Kinesin-13 at the kinetochore coop-
eratively drive sister chromatid segregation during anaphase (Rogers et al.
2004). Therefore, identification of kinesins at the kinetochore and the spindle
pole would be critical for us to determine the mechanism of sister chromatid
movement in plant cells.
2.5
Kinesins Acting on Midzone Microtubules for Spindle Elongation
Midzone microtubules are responsible for spindle elongation, which is the
hallmark of anaphase B in animal and fungal cells. In plant meristematic cells,
midzone microtubules are rather prominent although anaphase B often is not
obvious (Fig. 4A) (Palevitz 1993). However, rapid polymerization and reorga-
nization of the interpolar microtubules, usually occurring within 10 minutes,
are rather intriguing and allow us to speculate that kinesins likely play a role
during this process (Granger and Cyr 2000; Kumagai et al. 2001; Zhang et al.
1990). The ultimate consequence of midzone microtubule organization is the
birth of the phragmoplast microtubule array. Thus, microtubule sliding activ-
ity would be responsible for the formation of this anti-parallel array.