Vesicle Traffic at Cytokinesis 291
et al. 2000; and see Nebenführ, this volume), or form morphologically dis-
tinct stacks that seem to specialize in cell plate formation in some cell types
(Otegui et al. 2001; and see Segui-Simarro et al., this volume). Genetic and
molecular biological studies have implicated specialized components of the
traditional secretory pathway (see Sect. 3 below, Segui-Simarro et al., this vol-
ume, and Hong and Verma, this volume), leading to the suggestion that cell
plate formation is simply a modified form of secretion (Bednarek and Falbel
2002). Considering the essential role of the Golgi in the secretory pathway,
this also implicates the Golgi as the source of the cell plate membrane. More-
over, the Golgi would also be the natural source of the new cell wall material,
and newly synthesized proteins that are delivered to the cell plate. Thus, the
models tend to indicate a specialized form of vesicle carrying cargo bear-
ing “cell plate” signals is formed from the trans-Golgi network (TGN). These
vesiclesarethenbornetothenascentcellplateoncytoskeletalelements
using one of the many motor proteins implicated in cell plate formation (see
Hasezawa, this volume, and Lee and Liu, this volume). Upon arrival at the cell
plate, these vesicles fuse with each other (homotypic fusion) or with already-
assembled fragments of the nascent cell plate (Fig. 1). Eventually, the cell plate
Fig. 1 Likely sources of membrane for the cell plate. During both interphase and mito-
sis/meiosis, regular anterograde traffic delivers membrane and cargo from the Golgi to
the PM ( 1 ) or from the Golgi to the late endosome ( 2 ). Similarly, retrograde traffic carries
material from the PM to early and late endosomes ( 3 ). During the late stages of mito-
sis, a new kind of Golgi vesicle is observed that presumably carried membrane and cargo
for the nascent cell plate ( 4 ). These cell-plate targeted vesicles are thought to undergo
homotypic fusion to assemble larger membrane enclosed organelles which are subse-
quently reformed into sheets through the action of dynamins until they form a sheet like
membrane that expands across the point of division ( 5 ). Endocytosis and clathrin coated
vesicles have long been observed budding from the cell plate, and recent evidence has
suggested that exocytosis of material back from the endosomes may be a major source
of membrane for the cell plate ( 6 ). See text for more information and references