36 A.J. Wright · L.G. Smith
Fig. 1 A transverse division in a “typical” plant cell is diagrammed.a–c,e–h,and
krepresent two-dimensional projections of the outer half of the cell, while d, i,
andjrepresent midplane views. Genes important for the transition to each stage are
indicated under thearrows.aCells in interphase/G1 have peripheral nuclei, ordered cor-
tical MT and MF arrays, cortical MFs arranged in a meshwork, and MFs that tether the
actin-coated nucleus (as indicated by thegray circlearound the nucleus) to the cortex.
bG2 occurs after nuclear migration to the center of the cell in preparation for mito-
sis. MTs are nucleated from the nuclear surface (as indicated by theblack circlearound
the nucleus) and connect the nucleus to the cortex in alignment with MFs.cIn pre-
prophase, breakdown of the ordered cortical MT and MF arrays occurs in concert with
PPB formation. The PPB consists of a cortical band of MTs and MFs. MTs and MFs ex-
tend from the nucleus to the PPB and to the poles of the cell.dThis top down view
highlights the cortical nature of the PPB and the MTs and MFs that connect the nu-
cleus to the cortex. In vacuolated cells, these cytoskeleton components are contained
within the transvacuolar strands that comprise the phragmosome.eDuring prophase,
the MT PPB narrows while the MF PPB remains the same width. By late prophase, MTs
nucleated at the nuclear surface have begun to organize themselves into a bipolar spin-
dle. MTs extend from the new spindle poles to the PPB.fBefore metaphase, the PPB
disappears leaving behind an actin-depleted zone at the cortex. The MT spindle is sur-
rounded by MFs that extend to the cortex at the former PPB site and to the poles to
stabilize spindle position.gDuring telophase, the phragmoplast forms from the rem-
nants of the spindle and is composed of two anti-parallel arrays of MTs and MFs. The
new cell plate is deposited where the two arrays meet. MTs nucleated at the former
spindle poles extend to the cortex, including the former site of the PPB. MFs connect
the phragmoplast to the cortex, including the former site of the PPB.hDuring late cy-
tokinesis, the phragmoplast makes adjustments so that it aligns with the former PPB
site. MTs nucleated from the nucleus contribute to the expansion of the phragmoplast
and continue to probe the cortex, but begin to focus on the former PPB site. MFs con-
tinue to connect the phragmoplast to PPB site.i–jTop down view of cytokinesis. Only
one nucleus is visible since the other one is hidden beneath the expanding cell plate.
iDuring symmetric cytokinesis, the phragmoplast is initiated in the center of the cell and
expands to create a donut shape while depositing new cell plate.jPolarized cytokinesis
begins off center so the new cell wall first becomes anchored to one side of the mother
cell. The phragmoplast continues to expand along the other sides.kAfter formation of
the new cell plate, the cytoskeletal arrangements in the daughter cells resemble that seen in
G1 cells. This figure is based predominantly on results reported in Wick and Duniec 1984;
Lloyd and Traas 1988; Katsuta et al. 1990; Mineyuki et al. 1991; Cleary 1995; Nogami et al.
1996; Cutler and Ehrhardt 2002; Dhonukshe et al. 2005b; Chan et al. 2005; Sano et al. 2005
and Hasezawa 2002; Dhonukshe et al. 2005b). As MTs are relatively stiff poly-
mers, they may center the nucleus simply by pushing it away from the cell
periphery as they extend from the nuclear surface and “hit” the plasma mem-
brane. Maintaining the new nuclear position initially requires both MTs and
MFs, but following the breakdown of the connecting cytoplasmic MTs dur-
ing mitosis, MFs alone are sufficient to maintain the position of the spindle
(Venverloo and Libbenga 1987; Lloyd and Traas 1988; Katsuta et al. 1990). In-
terestingly, in contrast to symmetrically dividing cells, nuclear migration to
the division plane in asymmetrically dividing cells requires actin, but not MTs
(Mineyuki and Palevitz 1990; Kennard and Cleary 1997).