Plant Cytokinesis – Insights Gained from Electron Tomography Studies 281Fig. 11 Developmental stages of syncytial-type cytokinesis in Arabidopsis microsporo-
cytes.AOrganization of four nuclear cytoplasmic domains (ncd) after meiosis. Six
division planes between the four nuclei are established.BFormation of multiple mini-
phragmoplasts and associated cell plate assembly sites across the entire division plane.
CAssembly of multiple wide tubular netwoks (wtn).DEarly fusion of the most peripheral
cell plate assembly sites with the parental plasma membrane. These peripheral domains
mature into convoluted sheets (cs) and stub-like projections by the accumulation of cal-
lose.EUpon completion of cytokinesis, six callose-rich cell walls with no plasmodesmata
are formed. cpam: cell plate assembly matrix; cw: cell wall; db: dumbbell-shaped interme-
diate;gs:golgistack;mt:microtubule;mvb:multivesicularbody;n:nucleus;ne:nuclear
envelope; nu: nucleolus; pm: plasma membrane
persists until all of the assemblies have merged into a coherent network sys-
tem (Fig. 10D). The maintenance of the wide tubular network tubules in the
endosperm ofArabidopsis, is presumably achieved by the transient associa-
tion of dynamin-like rings and spirals with the tubules (Otegui et al. 2001).
Onlyafterallofthewidetubularnetworkshavemergedarethetubules
converted into sheet-like cell plates and cell walls (Fig. 10E). The advantage
for syncytial cells of having cell plate assembly intermediates with a wide
tubular membrane network geometry instead of a tubulo-vesicular net-
work/fenestrated sheet geometry as in meristematic cells, might be that the
tubular network configuration provides more mechanical flexibility. Tubular
membrane networks can readily stretch, twist and be compressed without
undergoing major structural changes. As in the somatic-type cytokinesis sys-
tem, the transformation of the syncytial-type wide tubular networks into
convoluted and fenestrated sheets also seems to involve exploitation of the
spreading forces associated with callose deposition along the cell plate mem-
branes (Otegui and Staehelin 2000b). Cell plates forming in the thin layer of
cytoplasm located between the plasma membrane and the tonoplast of the
central vacuole fuse first with the parental plasma membrane and with the ad-
jacent cell plates, whereas the cell plate edges facing the central vacuole keep
on growing. This particular pattern of cellularization leads to the formation
of open alveoli containing one nucleus each. Once this first set of anticlinal
cell walls has formed, any additional cycles of nuclear division are coupled to
cytokinesis and thus follow the somatic-type cytokinesis paradigm (Sorensen
et al. 2002; Olsen 2004).
In cellularizing microsporocytes (Fig. 11A), the cell plate assembly sites
form across the entire division plane (Fig.11B).However,thosewidetubular
networks located in the cell periphery begin to fuse with the parental plasma
membranebeforethemorecentralcellplateassemblysitesbecomeorganized
into a coherent cell plate (Fig. 11C). Fusion of the peripheral cell plate do-
mains with the parental plasma membrane triggers a rapid accumulation of
callose at these fusion sites, and the concomitant transformation of the wide
tubular networks into stub-like projections that grow towards the center of
the syncytium (Fig. 11D). As this occurs, the stub-like projections expand