154 J.C. Ambrose · R. Cyr
1996). A somewhat similar situation has been observed in plants, the differ-
ence being that the MTs are nucleated randomly about the prophase nuclear
surface, instead of around chromatin or DNA-coated beads. In cells lacking
PPBs or exhibiting aberrant PPB organization, the prophase spindle exhibits
reduced bipolarity—with randomly organized MTs accumulating about the
NE during prophase—creating apolar or multipolar spindles. These MTs later
become sorted (presumably via motor proteins) during prometaphase into
bipolar spindles, although establishment of a metaphase spindle takes more
time compared to cells with bipolar prophase spindles (Chan et al. 2005;
Yoneda et al. 2005). In theXenopusextract system, sorting into a bipolar
spindle is mediated by Eg5 (Kinesin-5), while Kinesin-14 and cytoplasmic
dynein participate in spindle stability and pole focusing (Walczak et al. 1998).
A similar situation is likely to exist in plants, given the existence of Kinesin-5
motors (Asada et al. 1997; Barroso et al. 2000), and given that spindles lack-
ing ATK1 or ATK5 exhibit splayed spindle poles (Chen et al. 2002; Ambrose
et al. 2005; Ambrose and Cyr 2007). Given the likely involvement of motors
in sorting of random perinuclear MTs, the self-organizational pathway can
be generalized to include both the chromatin-based MT nucleation as well as
nuclear-envelope-based MT nucleation. In both cases, motor-dependent sort-
ing of random MT arrays occurs, regardless of the nucleating structure (i.e.
chromatin or NE).
4.3
Evidence for Search-and-Capture Pathways in Plants
The term “search-and-capture” was initially coined as a theoretical explan-
ation for the functional significance of MT dynamic instability, the in-
trinsic property wherein individual MTs stochastically alternate between
phases of growth and shortening (Mitchison and Kirschner 1984; Kirschner
and Mitchison 1986). Search-and-capture of kinetochores was later demon-
strated both in vitro (Mitchison and Kirschner 1985) and in vivo in ani-
mal cells (Tulu et al. 2006). Although search-and-capture has not been di-
rectly demonstrated in plant cells, its presence can be inferred from sev-
eral lines of evidence. First, the bipolar organization of plant prophase
spindles poises MT plus ends such that they point predominately inward,
muchinthesamewaythatcentrosomesdo,allowingdynamicMTsto
probe the three-dimensional cellular space for kinetochores after NEB. Sec-
ond, GFP::EB1 moves from the poles toward the midzone during mitosis,
consistent with polar nucleation and growth toward the spindle midzone,
as described by the search-and-capture model (Dixit et al. 2006). Third,
when the bipolar organization of prophase spindles is disrupted, the sub-
sequent duration of metaphase spindle formation is prolonged, consistent
with a less efficient search-and-capture than from pre-established spindle
poles (Chan et al. 2005; Yoneda et al. 2005). Advances in live-cell imaging