Mitotic Spindle Assembly and Function 147
a constant length. Poleward MT flux refers to the situation wherein tubu-
lin subunits are incorporated at MT plus ends in the midzone, and then
transported to the poles via the combined activities of motor activity and
minus end depolymerization; the net result being that the spindle length
remains unchanged (Mitchison 2005). Although plant spindle MTs exhibit
increased dynamics compared to interphase MTs, initial experiments employ-
ing photobleached fluorescent tubulin subunits were unable to demonstrate
flux (Hush et al. 1994). Using newer technology, however, our lab has de-
tected poleward flux of bleached GFP::Tubulin subunits within BY-2 spindles
(our unpublished observation). More recently, a paper by Dhonukshe et. al
demonstrates this as well (Dhonukshe et al. 2006). This suggests that pole-
ward flux may be a conserved feature of spindle functioning in eukaryotic
cells, although it has not been detected to date in fungi; possibly due to
technological limitations and small spindle sizes (Mallavarapu et al. 1999;
Maddox et al. 2000). It is interesting to note that as early as the 1950s, one
of the pioneers of live-cell microscopy, Andrew Bajer, described poleward
movement of small particles, granules, and nucleoli, which he collectively
termed “acentrics”—effectively demonstrating poleward flux in plant spin-
dles before it had even been hypothesized (Bajer 1967; Allen et al. 1969; Bajer
and Molè-Bajer 1972).
2.5
Anaphase
Anaphase chromosome movements in plants are derived from two differ-
ent processes: (1) kinetochore-to-pole movement, which results from the
shortening of k-fibers; and (2) spindle elongation, which is derived from
the sliding apart of the overlapping parts of antiparallel interpolar MT bun-
dles. In animal cells, these processes have been described as anaphase A and
B, respectively, due to their sequential nature. In plants, however, the two
processes appear to be largely concurrent, with varying degrees of over-
lap in different cell types (Ota 1961; Harris and Bajer 1965; Fuseler 1975;
Ryan 1983). As k-fibers shorten during anaphase A, they become disorga-
nized, losing their fir-tree appearance and becoming splayed out from the
kinetochore into the pole region (Bajer 1968; Hard and Allen 1977). At the
same time, interpolar MT bundles in the midzone become more ordered
and densely packed as they elongate and slide apart, thus decreasing the
degree of overlap between half-spindles (Jensen and Bajer 1973; Euteneuer
and McIntosh 1980; Euteneuer et al. 1982). Formation of the cell plate often
begins in mid to late anaphase, as indicated by the appearance of vesicles
and cell-plate precursors near interdigitating regions of interpolar MTs (Ota
1961; Hepler and Jackson 1968; Fuseler 1975). MTs of the anaphase spin-
dle midzone persist into telophase, giving rise to the phragmoplast during
cytokinesis.