182 Y.-R.J.Lee·B.Liu
Once microtubules are arranged in two mirrored sets in the phragmo-
plast, they serve as tracks along which vesicles are delivered by kinesins. Early
studies on microtubule dynamics in the phragmoplast indicate that tubu-
lins are continuously added to the plus ends of phragmoplast microtubules
in the middle (Asada et al. 1991; Vantard et al. 1990). The new microtubule
segments would have to be pushed away. Otherwise, microtubule plus ends
would be located at sites other than the future cell plate position. Since
the plus end is located in the middle, a plus end-directed kinesin would be
needed to continuously translocate newly added microtubule segments away
from the middle of the phragmoplast. The PAKRP1 kinesin from Arabidop-
sis and rice exclusively decorates the plus end of phragmoplast microtubules
(Lee and Liu 2000). PAKRP1 belongs to the Kinesin-12 subfamily with pre-
dicted plus end-directed motility, and uncharacterized non-motor domains
(Lee and Liu 2004). In theA. thalianagenome, at least two genes encode
Kinesin-12 with an identical spatial and temporal localization pattern in
the phragmoplast (Pan et al. 2004). Inactivation of either Kinesin-12 by T-
DNA insertional mutations neither alter the localization of its homolog, nor
affect cytokinesis. Thus, these kinesins function redundantly during cytoki-
nesis. It would be intriguing how microtubule organization would be affected
when both kinesins are inactivated. We also ought to determine whether
Kinesin-12s act as a matrix component connecting anti-parallel microtubules
in the phragmoplast.
3.2
Kinesins and Microtubule Turnover in the Phragmoplast
During cell plate assembly, rapid turnover of phragmoplast microtubules in-
cludes depolymerization in the central region where cell plate materials have
been delivered, and concomitant polymerization at a peripheral area where
vesicle delivery is about to begin (Zhang et al. 1993). The molecular mech-
anism underlying this rapid turnover had been unclear until a MAP kinase
cascade was connected to microtubule dynamics through serendipitous in-
vestigations (Takahashi et al. 2004). Early studies of plant MAP kinases have
revealed a particular set of kinases which become localized to the division
site during cytokinesis (Bogre et al. 1999; Nishihama and Machida 2001).
Inactivation of this MAP kinase cascade by mutations in genes encoding in-
dividual kinases leads to incomplete cytokinesis in Arabidopsis and tobacco
(Krysan et al. 2002; Nishihama et al. 2001; Soyano et al. 2003). The MAPKKK
of this cascade interacts with two plant specific kinesins NACK1/HIN and
STD/TES/NACK2 (Nishihama et al. 2002). These two homologous kinesins
have a typical plus end-directed motor structure with the motor domain at
the N-terminus (Nishihama et al. 2002; Strompen et al. 2002; Yang et al. 2003).
Although they sometimes are placed inside the Kinesin-7/CENP-E subfam-
ily, their motor domains are divergent enough from animal members of this