Division Plane Orientation in Plant Cells 45
with two PPBs formed multipolar spindles. Extra spindle poles were most
likely to be formed in the region between the double PPBs instead of directly
underneath them. On the basis of this and other observations, it has been
suggested that the bipolar nature of MT accumulations on the surface of the
nuclear envelope in cells with a PPB could be due to inhibition by the PPB
of MT polymerization or bundling in adjacent areas of the nuclear surface
(Granger and Cyr 2001; Yoneda et al. 2005; Chan et al. 2005). Bipolar spindle
formation also requires the kinesin ATK1 sinceArabidopsiscells lacking this
protein have multi-polar spindles (Marcus et al. 2003). Thus, through its in-
fluence on the initiation of spindle poles, the PPB appears to determine the
initial orientation of the spindle.
Although the spindle does not determine the division plane in plant cells
as it does in animal cells, its position can impact the final position of the cell
plate. Since the phragmoplast arises from the remnants of the spindle, initi-
ation and maintenance of the spindle in an orientation perpendicular to the
former PPB ensures that the newly initiated phragmoplast is already aligned
with the division plane, thereby facilitating attachment of the cell plate at the
former PPB site (see further discussion, Sect. 4).
2.4
PPB Breakdown
Although it faithfully predicts the future division plane, the PPB disappears
during mitosis well before the initiation of cytokinesis. The signal that initiates
the breakdown of the PPB is unknown, but one hypothesis is that some fac-
tor released from the nucleus upon nuclear envelope breakdown is responsible
(Mineyuki et al. 1991; Murata and Wada 1991). This hypothesis was suggested
by experiments with centrifugedAdiantumprotonemata. After PPB forma-
tion, centrifugation of these elongated cells to displace the nucleus from the
site of the PPB caused PPB breakdown to be delayed until metaphase or after
mitosis was complete (Murata and Wada 1991). Similar results were seen in
centrifuged onion cells (Mineyuki et al. 1991). Following up on these results,
Dixit and Cyr (2002b) determined precisely the temporal relationship between
nuclear envelope breakdown and PPB disappearance in BY-2 cells. By ob-
serving cells expressing fluorescently tagged MTs and nuclear envelopes, they
found that nuclear envelope breakdown precedes PPB breakdown by approxi-
mately 2.3 minutes. Interestingly, if nuclei were asymmetrically positioned
within the plane of the PPB, the region of the PPB closest to the nucleus broke
down first suggesting a signaling connection between the nucleus and the
PPB that influences MT stability (Dixit and Cyr 2002b). Given the evidence
discussed earlier in Sect. 2.2.2 that phosphorylation destabilizes PPB MTs, an
interesting possibility to explain these findings is that release of a kinase from
the nucleus upon nuclear envelope breakdown plays an important role in PPB
disassembly.