266 J.M. Seguí-Simarro et al.
Fig. 6 Diagrammatic model of the putative mechanisms of CPAM-mediated vesicle teth-
ering and squeezing during cell plate growth. Based on Seguí-Simarro et al. (2004).
Tethering of the Golgi-derived vesicles to each other within the CPAM is mediated by
the assembly of Y-shaped, putative exocyst complexes. Once tethered and fused, dynamin
(DRP) spirals assemble around the waist of the unstable hourglass intermediates, thereby
stabilizing the hourglass configuration. Hydrolysis of the GTP bound to the DRP subunits
causes the spirals to expand and squeeze the constricted domain of the hourglass-shaped
vesicles, thereby converting them into dumbbell-shaped intermediates. The concomitant
loss of vesicle volume is most likely achieved through water expulsion from the dumbbell
lumen. After several rounds of assembly-expansion disassembly of the DRP springs, the
volume of the dumbbells is reduced by up to 50 %, and the concomitant loss of water ap-
pears to cause the contents of the vesicles (pectic polysaccharides and xyloglucans) to gel.
This gelling, in turn, appears to mechanically stabilize the elongated shape of the dumb-
bells. When new vesicles fuse with the ends ofthe stabilized dumbbells, the latter do not
swell, i.e., they retain their shape, thereby giving rise to increasingly complex vesicular-
tubular-type cell plate intermediates. In contrast, when the same types of vesicles fuse
outside of the CPAM, no DRP springs assemble around the necks of the hourglass inter-
mediates and the hourglass structures are rapidly transformed into larger, round vesicles.
These larger vesicles may also enter the CPAM at a later stage and fuse with the growing
cell plate
Very recently, two studies have demonstrated not only the existence of an
exocyst-like complex in plants, but also a functional role in polar exocytosis
in two distantly related plant species,Arabidopsis(Cole et al. 2005) and maize
(Wen et al. 2005). The maize rth1/sec3 mutant exhibits defects in root hair
elongation as well as delayed plant growth and flowering (Wen et al. 2005). In