Division Plane Orientation in Plant Cells 49
4
Phragmoplast Guidance to the Division Site
After spindle breakdown, the phragmoplast arises from the remnants of the
spindle and begins to expand, ultimately attaching the cell plate at the di-
vision site predicted by the former PPB. As discussed earlier, the spindle
is initially suspended within the division plane with its long axis perpen-
dicular to this plane and is usually maintained in this position throughout
mitosis, apparently via cytoplasmic actin cables (Traas et al. 1987; Lloyd and
Traas 1988; Sano et al. 2005). Thus, in most cells, the phragmoplast is already
aligned with the division plane upon inception and relatively little guidance
of its expansion may be needed for cell plate attachment at the former PPB
site. This may be especially true if the phragmoplast expands within a phrag-
mosome. However, if the spindle or early phragmoplast becomes misaligned
with the division plane, the expanding phragmoplast can be guided to the for-
mer PPB site (Galatis et al. 1984; Granger and Cyr 2001). For example, in some
cell types, spindles normally rotate to an oblique orientation during mitosis.
When this occurs, the phragmoplast is initially oriented obliquely, but rotates
as cytokinesis proceeds so that the cell plate attaches at the former PPB site
(Palevitz and Hepler 1974a; Palevitz 1986; Cleary and Smith 1998). Further-
more, phragmoplasts displaced from the division plane via centrifugation can
often migrate back to the former PPB site as they expand (Ota 1961; Gun-ˆ
ning and Wick 1985). However, under experimental conditions that result in
the spindle forming far away from the former PPB site, the subsequent phrag-
moplast fails to be guided to this site. Thus, it appears that phragmoplast
guidance can only operate within a relatively short distance of the cortical di-
vision site (Galatis et al. 1984; Granger and Cyr 2001). Very little is known
about how the expanding phragmoplast “finds” the division site, but most in-
vestigation has focused on defining roles for MFs and MTs. Before discussing
these studies, we first discuss two alternate modes of cytokinesis with differ-
ent implications for mechanisms of phragmoplast guidance.
4.1
Symmetric Versus Polarized Cytokinesis
According to the “textbook” view of cytokinesis just discussed, the phragmo-
plast and associated nuclei are initially positioned in the middle of the cell and
the phragmoplast expands extensively before making contact with the cell cor-
tex (Fig. 1i). In this situation, initial guidance of the expanding phragmoplast
would have to involve a long-range interaction with the cortex (e.g., as dis-
cussed later, perhaps mediated by MFs or MTs connecting the phragmoplast
to the cell cortex at this stage). Subsequently, when the expanded phragmoplast
makes contact with the cortex, phragmoplast position could be “fine tuned” via
direct interactions between the phragmoplast edges and the cortical mark.