142 J.C. Ambrose · R. Cyr
Fig. 1 Cartoons illustrating simplistic and realistic models of the higher plant mitotic
spindle during metaphase and anaphase
sets of MTs oriented with their minus ends near the poles and their plus ends
in the midzone, where they terminate at chromosomal kinetochores, form-
ing kinetochore MTs (kMTs), or overlap in an antiparallel manner with MTs
from the opposite half-spindle, forming interpolar MTs. Multiple kMTs as-
sociate to form kinetochore MT fibers (k-fibers), which mediate congression
of chromosomes to a point equidistant from each pole during prometaphase,
maintain them at the metaphase plate, and later shorten during anaphase
A to pull sister chromatids to the poles (Fig. 1, left panel). Interpolar MT
bundles contribute to the structural integrity of the spindle, to spindle elon-
gation during anaphase B, and play a pivotal role in spindle formation. This
simplistic description is sufficient for initially orienting the reader, however,
in reality, individual k-fibers and interpolar MT bundles exhibit substan-
tial branching and lateral interactions with one another (Fig. 1, right panel).
As will be discussed, these interactions likely play a fundamental role in
ensuring the integrity of spindle structure and the fidelity of chromosome
segregation.
2.2
Premitotic Nuclear Migration and PPB Formation
A prominent indication of ensuing cell division in plant cells is the migration
of the nucleus during S/G2 phase to the future cell division site (Sinnott and
Bloch 1940). Nuclear migration is mediated via cytoplasmic strands that em-
anate out from the nucleus and anchor at the cortex. These strands contain
actin microfilaments (MFs) and MTs, both of which are involved in nuclear
migration, although the relative contributions of each vary between species