292 A. Sanderfoot
fuses with the PM, either at the completion of the centrifugal expansion from
the center or closing like a curtain across the cytosol from one side to the
other (Cutler and Ehrhardt 2003).
Recently, the role of other organelles in the supply of membrane to the cell
plate has begun to be examined. For many years, EM-observations have in-
dicated that the cell plate is a major site of coated vesicle formation, likely
as part of an endocytic process (see Seguí-Simarro et al. 2004). This makes
sense considering that some aspect of the vesicle trafficking machinery must
be recycled during the cell plate forming processes, but also suggests the pos-
sibility that exocytosis from endosomes may also be an active participant in
supplying membrane to the cell plate. In fact, Dhonukshe et al. (2006) have
suggested that, rather than the Golgi, the endosomes are the major player in
cell plate initiation and expansion. Their results, based on dye tracers and
pharmacology, suggest that the major source of membrane and material for
the initiating cell plate is derived by flow through the endosomes from the
PM (Dhonukshe et al. 2006). Still, decades of results have supported a role
for delivery of Golgi-derived vesicles, and associated trafficking machinery
to the cell plate. Indeed, such alternate routes for cell plate membrane may
support the ability of cell plates (or at least fragments) to form in many
cytokinetic mutants, which might otherwise be expected to fail at the first
zygotic division. Such results are intriguing, and may allow a better funda-
mental understanding of the sources of vesicles that create the cell plate. Now
that the cell biological tools such as electron tomography, tracer dyes and
other well-known endocytic markers can be aimed at the problem, know-
ledge of the organellar players in cell plate formation can progress along with
the known molecular components of the vesicle trafficking machinery.
3
Cytokinetic Mutants Lead to the Vesicle Trafficking Machinery
Several years of forward and reverse genetic examination has provided
a glimpse of the molecular machinery involved in assembling the cell plate
compartment. Not surprisingly, the parallels between cell plate assembly
and regular secretion have become apparent; just as the parallels between
all aspects of vesicle trafficking among the endomembrane organelles have
indicated a deep conservation of the core machinery involved in moving
membrane from one part of the cell to another. Much of the mechanics
of operating the endomembrane system are conserved throughout eukary-
otic evolution (SNAREs, Rab-GTPases, SM-proteins, coatomers, etc.), though
many duplications and specializationshave occurred to coordinate different
aspects of trafficking (Sanderfoot and Raikhel 2003). Certainly, one expects
that some novel components will be necessary, and that future work will iden-
tify these factors. Yet, to this day, much of the known components of the