Mitotic Spindle Assembly and Function 153
4
Pathways of Mitotic Spindle Assembly and Function in Higher Plants
During cell division, two general mechanistic pathways coexist to facilitate
efficient and accurate segregation of chromosomes into daughter cells. The
self-organizational pathway involves nucleation of MTs around condensed
chromatin and subsequent sorting of these MTs into a bipolar spindle via the
action of motor proteins. The search-and-capture pathway predominates in
the presence of centrosomes, which provide dominant centers for nucleation
of MTs that dynamically probe the cytoplasm until they encounter a kineto-
chore, forming stable attachments. In this section, we discuss the prevalence
of these pathways in higher plants and introduce modifications to general-
ize the mechanistic processes, thereby including the phenomena observed in
higher plants.
4.1
Evidence for the Self-Organizational Pathway in Plants
Chromatin-mediated MT nucleation involves the small GTPase Ran, which
in its GTP-bound form facilitates the activation of various spindle-promoting
components such as XCTK2, NuMA, TPX2, and the gamma tubulin ring com-
plex (γTuRC), which initiates nucleation of MTs around chromatin (Wilde
and Zheng 1999; Ems-McClung et al. 2004). The concentration of Ran-GTP is
kept high in the vicinity of chromatin via the activity of the chromatin-bound
GTP-exchange factor RCC1, and kept low in the cytoplasm via the Ran GT-
Pase activating protein RanGAP. Thus, MT nucleation and organization are
maintained in the vicinity of the chromatin.
The best evidence for chromatin-mediated MT nucleation in plants comes
from drug studies where MTs are depolymerized, and then fixed and im-
munostained for MTs at various time points during recovery. Several studies
show MTs appearing during recovery, first as kinetochore-associated dots,
which then grow into small tufts and later into fir-tree structures (Cleary
and Hardham 1988; Falconer et al. 1988). Similarly, small tufts of MTs form
asynchronously between different granules of individual sister kinetochores
during early prophase inHaemanthusendosperm cells (Bajer 1987). The
presence of poleward flux in plant spindles also indicates probable MT plus
end assembly at or near kinetochores.
4.2
Self-Organization from Random Arrays via Motor-Driven MT Sorting
When DNA-coated beads are added toXenopusegg extracts, MTs are nu-
cleated randomly around the beads via chromatin-mediated nucleation and
sorted into a bipolar spindle by the action of motor proteins (Heald et al.