Mitotic Spindle Assembly and Function 149
Mor1/Map200 (Dis1/XMAP215/TOG).TheArabidopsis MOR1gene was
identified in a screen for plants with aberrant MT organization and was found
to share homology to the Dis1/XMAP215/TOG family of MAPs (Whittington
et al. 2001), which are known to facilitate polymerization of MTs (Whitting-
ton et al. 2001; Hamada et al. 2004). Although initially characterized by its
disruption of cortical MT arrays at restrictive temperature, additionalmor1
defects during mitosis have been reported (Kawamura et al. 2006). At restric-
tive temperature, cells ofmor1mutants frequently fail to form PPBs, and in
these cells, the subsequent orientation of the spindle and phragmoplast is
aberrant (Eleftheriou et al. 2005; Kawamura et al. 2006). Spindles ofmor1mu-
tants are also significantly shorter than normal, providing additional support
for its role as an MT-polymerizing factor. This finding presents the first ge-
netic alteration of spindle length in plants, and emphasizes the universality of
MT dynamics as a key governor of spindle length; which is also becoming ap-
parent in animal systems (Goshima et al. 2005). The tobacco MOR1 homolog,
TMPB200, has been localized to all MT arrays, and purified TMBP200 has
the ability to crosslink MTs in vitro, suggesting a possible role in structural
organization of MT arrays, in addition to modulating dynamics (Yasuhara
et al. 2002; Hamada et al. 2004). Although structural and motor proteins fa-
cilitate the integrity and functioning of spindles, alterations of MT dynamic
properties generally result in the most dramatic changes in spindle length
(Goshima et al. 2005).
Interestingly, another member of this family, XMAP215, fromXenopus
acts antagonistically in mitotic spindles with the MT-destabilizing kinesin,
XKCM1, in regulating MT length (Tournebize et al. 2000). A similar scenario
exists in yeast, where the MT-destabilizing kinesin Kar3p counteracts the MT-
stabilizing effects of another kinesin, Kip1p (Huyett et al. 1998). Whether an
analogous situation exists in plants remains to be seen; although it is likely,
given that the counterbalancing of MT polymerizing and depolymerizing fac-
tors is emerging as a general mechanism in regulating MT structures and
fine-tuning of MT dynamics in vivo (Valiron et al. 2001).
EB1.It is the founding member of a conserved group of plus end tracking
proteins (+TIPs) involved in modulation of MT dynamics and attachment of
MT plus ends to cellular structures such as kinetochores and cortical division
sites (Tirnauer et al. 2002; Tirnauer et al. 2004). ThreeArabidopsisEB1 hom-
ologs have been cloned and shown to localize to mitotic spindles, although no
functional data have yet been reported (Chan et al. 2003; Mathur et al. 2003;
Van Damme et al. 2004a; Chan et al. 2005; Dixit et al. 2006). It will be interest-
ing to learn the roles of plant EB1, given the presence of the novel MT arrays
seen in plants.
Ta n g l e d .The maizeTa n g l e dgene encodes a MAP with distant homology to
the animal EB-1 binding partner APC (Smith et al. 2001). Mutants in thetan-
gledgene form normal mitotic arrays, but the trajectory of the phragmoplast
is aberrant, leading to oblique cell plates and disorganized cell files (Cleary