Molecular Analysis of the Cell Plate Forming Machinery 309
plate formation is not blocked, suggesting the presence of a pool of vesicles
that participate in the initial cell plate formation. The disruption of micro-
filaments causes oblique cell plates, but does not prevent the formation of
the cell plateper se(Schopfer and Hepler 1991). Thus, a coordinated deliv-
ery of the Golgi vesicles is essential for the formation of the cell plate. Little
is known about the vesicle fusion machinery at the forming cell plate except
the identification of the phragmoplastin GTPase (Gu and Verma 1996, 1997;
Zhang et al. 2000) and a pair of syntaxin/Sec1-like proteins (KNOLLE/KEULE;
Lauber et al. 1997; Assaad et al. 2001). Other proteins must be required for the
various events in vesicle fusion.
3.2
Homotypic Membrane Fusion
Homotypic fusion refers to the merging of two membranes of the same type.
Cell plate formation involves two types of homotypic membrane fusion, i.e.,
creation of tubular structures from vesicle-vesicle fusion, and formation of
honeycomb-like network via end-to-end fusion of tubular structures. Phrag-
moplastin and its associated proteins are known to play pivotal roles in
creating the dumbbell-shaped tubular structures, whilst Knolle syntaxin is re-
quired for the formation of honeycomb-like tubular network (Lauber et al.
1997; Jurgens 2004). In cell plate formation, the end-to-end fusion of tubular
structures is unique in the aspect that the fusion involves the alignment of
several tubular structures. Under electron microscope, the junction of these
tubular structures look like a “star” (Hepler 1982). Mutant cells defective at
Knolle cannot form tubular networks at the cell plate (Lauber et al. 1997).
Thus, homotypic fusion of vesicles is an essential step in building the cell plate.
Proteins required for homotypic fusion has been studied in other sys-
tems such as the yeast mitotic vacuole fusion. After mitosis in yeast, small
vacuoles fuse with each other, giving rise to the formation of bigger, round
vacuoles (Wickner 2002; Jun et al. 2006). In contrast, homotypic fusion at the
cell plate results in the formation of the “dumbbell-shaped” structures due
to the presence of phragmoplastin-like DRP1 proteins. These structures are
then expended and connected with each other in a unique end-to-end fusion
manner. The termini of several dumbbell-shaped structures align together to
form “star”-like junctions, which were observed by electron microscopy three
decades ago (Hepler 1982). Interconnection of tubular structures gives rise to
a tubulo-vesicular network (TVN) (Staehelin and Hepler 1996). Merging of
these tubular termini is also thought to occur via homotypic membrane fu-
sion (Verma 2001; Verma and Hong 2005). Thus homotypic fusion of vesicles
is an essential step in building the cell plate. Moreover, the homotypic fusion
machinery must be highly efficient because the completion of the cell plate
takes about an hour. During this time, not only plate must be flattened but
callose must be formed to fill the growing cell plate.