Plant Cytokinesis – Insights Gained from Electron Tomography Studies 279 Fig. 10Developmental stages of syncytial-type cytokinesis in the Arabidopsis endosperm.
AOrganization of the syncytial endosperm into nuclear cytoplasmic domains (ncd) de-
fined by radial systems of microtubules (mt) prior to cytokinesis.BMini-phragmoplast
assembly at the boundaries of adjacent nuclear cytoplasmic domains and vesicle fusion
at the division plane.CFormation of wide tubular networks (wtn) in multiple cell plate
assembly sites.DFormation of a coherent cell plate by fusion of multiple assembly sites
and maturation of the central cell plate domains into convoluted sheets (cs).EFusion of
the cell plate with the parental plasma membrane and conversion of convoluted sheets
into planar fenestrated sheets (pfs). cpam: cell plate assembly matrix; cw: cell wall;
db: dumbbell-shaped intermediate; ge: cell plate growing edge; gs: golgi stack; mvb: mul-
tivesicula r body; n: nucleus; ne: nuclear envelope; nu: nucleolus; pm: plasma membrane;
v: golgi-derived vesicle
Before syncytial-type cytokinesis occurs, microtubules assemble on MT-
organizing centers associated with the outer membrane of the nuclear enve-
lope (Canaday et al. 2000) and radiate outwards, intersecting the MTs extend-
ing from adjacent nuclei (Figs. 10A, 11A). The cytoplasmic region defined by
a radial array of microtubules is called nuclear-cytoplasmic domain (Brown
et al. 1994; Pickett-Heaps et al. 1999; Brown and Lemmon 2001). In male
meiocytes (microsporocytes), four nuclear cytoplasmic domains correspond-
ing to the four haploid nuclei are established, whereas up to several hundreds
cytoplasmic domains are established in nuclear endosperms. Syncytial-type
cell plates originate in CPAMs associated with mini-phragmoplasts (Figs. 10B,
11B; Otegui and Staehelin 2000b, 2004; Otegui et al. 2001). In structural
terms, mini-phragmoplasts resemble phragmoplast initials. However, unlike
the phragmoplast initials that arise from clusters of anaphase spindle MTs,
mini-phragmoplast, MT clusters form at the boundaries of adjacent nuclear
cytoplasmic domains (Brown and Lemmon, 1992; Pickett-Heaps et al. 1999).
Each mini-phragmoplast contains on average 2× 10 MTs, and the (+)-ends
of these MTs overlap in the region where the CPAM develops (Otegui and
Staehelin 2000b). In contrast, the number of MTs associated with the phrag-
moplast initials tends to be much higher and more variable, and MT overlap
is only seen during the earliest stages of the spindle to phragmoplast transi-
tion period (Fig. 3A).
Another difference pertains to the architecture of the cell plate assembly
intermediates and their mechanism of maturation. Thus, whereas somatic-
type cytokinesis first involves the assembly of a single, coherent, tubulo-
vesicular cell plate, which is rapidly followed by the centrifugal expansion
along the cell plate periphery (Figs. 2,4), syncytial type cell plate formation
occurs simultaneously in all of the many mini-phragmoplasts that are aligned
along the edges of each nuclear-cytoplasmic domain (Figs. 10B, 11B), and
only at a later stage do the multiple cell plate assemblies merge into a coherent
and continuous cell plate membrane system (Figs. 10D, 11D).
In the endosperm, just before cellularization begins (Fig. 10A), the nuclei
are positioned in a peripheral cytoplasmic layer lining the plasma membrane