290 A. Sanderfoot
cient to cover the half of the volume given to each daughter cell. The apparent
“stretching” that accompanies cells which use a cleavage furrow may appear
to temporarily compensate, yet a “stretched” membrane is a leaky membrane.
Since a cell cannot be allowed to leak, cells must also add new membrane at
the point of cytokinesis (Xu et al. 2002). Recent investigations into the vesi-
cle trafficking machinery has shed light upon the mechanisms of cytokinesis,
and this review will attempt to incorporate this research into a better under-
standing of the components and functions of the secretory organelles and their
associated proteins in the assembly of the cell plate in land plants. First, I will
look into the source of the vesicles that make up the cell plate. Second I will
try to identify what is known about the molecular machinery that turns these
vesicles into the cell plate membrane. Finally, we will discuss what is known
about the contents of the vesicles, and how the vesicle contents work with the
new membrane to produce the organelle that separates the daughter cells.
2
The Secretory Organelles and the Source of the Cell Plate Membrane
The cell plate is essentially a new secretory organelle that is formed dur-
ing telophase from membrane derived from other organelles of the secretory
pathway. The major source of this membrane has always been thought to be
vesicles that bud from the Golgi, though certainly other organelles contribute
membrane as well (see below). It is generally thought that the cell plate was
invented in the algal ancestor to the land plants, and organelles with simi-
lar morphologies are observed to be involved in the cytokinetic process of
several green algae, including those considered to be the sisters to the land
plant lineage (see López-Bautista et al. 2003). On the other hand, many other
green algae have a cytokinetic process superficially similar to the cleavage
furrow-mediated mechanisms of animal cells (e.g.Chlamydomonasand re-
lated chlorophytes). In turn, a structure similar to the cell plate is found in
some brown algae from eukaryotic lineages that are unrelated to the green
plant lineage (e.g. Bisgrove and Kropf 2004), suggesting that there may be
other morphological constraints that encourage cytokinetic mechanisms out-
side of an actin-myosin cleavage furrow. Finally, some have even suggested
that some aspect of a cell plate-like structure is involved in completion of the
midbody cleavage in animal cells (Otegui et al. 2005), suggesting that all these
differences may simply be variations on a singular theme. Overall, that theme
seems to involve addition of membrane, specifically in the form of vesicles
derived from the organelles of the secretory system.
Traditionally, it has been believed that vesicles derived from the Golgi
are the source of the membrane that forms the cell plate. Many cell bio-
logical studies have shown that the Golgi apparati form specialized arrays
surrounding the nascent cell plate in a cell cycle-specific manner (Nebenführ