298 A. Sanderfoot
ments at early stages suggested a role for a member of the Dynamin family
in shaping and preparing the small fragments for subsequent fusion into
larger membranes (Otegui et al. 2001; Seguí-Simarro et al. 2004). Green plants
have a unique gene family of dynamin-like proteins, the DRP1 family (for-
merly called ADL1 or Phragmoplastin; Hong et al. 2003), and their essential
role in cell plate development has been confirmed by both cell biological
and genetic analysis (Gu and Verma 1997; Kang et al. 2001; Kang et al. 2003;
Seguí-Simarro et al. 2004). The analogous roles played by dynamins in the
cytokinetic processes of animal and plant cells have recently been reviewed
(Konopka et al. 2006). That a known membrane deforming/remodeling fac-
tor is involved in deforming and remodeling the cell plate membranes should
not be a surprise. That green plants have dedicated a specialized family of
dynamin-related proteins to this task underlines the importance of this pro-
cess in cell plate formation. But, who regulates the dynamins? What other
factors could be involved in the lipid-aspects of the cell plate? Are specialized
kinds of lipases, flippases, scramblases, etc. required for the various mem-
brane gymnastics that occur during the assembly and development of the cell
plate? Such things have not been thoroughly investigated, and much is left to
be discovered.
4
Just What Do the Vesicles Bring to the Cell Plate?
While it has been clear for a long time that vesicles supply membrane for the
cell plate, what those vesicles carry as cargo (besides their constitutive lipids)
is still an open question. While many proteins have been shown to associate
with the cell plate (see Jürgens 2005), most are soluble/cytosolic proteins, and
could presumably just associate directly with the cell plate membrane from
the cytosol. Of most relevance would be membrane proteins that would be
transported from other endomembrane organelles to the cell plate, though
very few of these types of proteins are known: the cell plate SNARE proteins
and CalS1.
Components of the vesicle trafficking machinery are found on the cell
plate, probably concomitant with cell plate formation (e.g. KNOLLE, SNAP33,
NPSN11, SYP71, etc.; see above), there must be some signal that allows them
to be redirected away from other endomembrane destinations toward the cell
plate vesicles. As one might expect, this signal is protein specific, and appears
to be dependent on the cell cycle. For example, expressingKNOLLEconsti-
tutively does not rescue its cytokinetic defect, and leads to localization of
the PM (Völker et al. 2001). This result suggests that the machinery that rec-
ognizes the signals in KNOLLE is only expressed (or functional) during the
later phases of the cell cycle. However, placing the late endosomal Qa-SNARE
SYP21 (i.e. AtPEP12) under the control of theKNOLLEpromoter does not