296 A. Sanderfoot
AAS, unpublished data). Interestingly, in cells of the expanding root, SYP71
is found polarly localized to the acropetal PM (i.e. towards the root meris-
tem; L. Conner and AAS, unpublished data), suggesting a role for this protein
in non-dividing cells, outside of cell plate formation. Though SYP71 is one
of three SYP7-like Qc-SNAREs in Arabidopsis, there appears to be little re-
dundancy in this gene family, sincesyp71mutants are embryo lethal (L. Con-
ner and AAS, unpublished data). Both the NPSN1- and the SYP7-group of
SNAREs are not found in animals or fungi (Sanderfoot et al. 2000), suggest-
ing that such a group may be uniquely placed for a novel function such as
cell plate formation. However, it should be noted that NPSN1- and SYP7-like
SNAREs are found in non-animal/fungal, i.e. ophistokont eukaryotes that do
not form cell plates during cytokinesis (Chlamydomonas,Dictyostelium,etc;
Sanderfoot, 2007), so this is not necessarily a direct relationship. Nonethe-
less, it is likely that KNOLLE+NPSN11+SYP71 represents a second Qa + Qb
- Qc-SNARE complex in dividing cells (Fig. 2B).
Still missing from both of these complexes is an R-SNARE. It is widely
believed, though not yet shown experimentally, that the R-SNARE for PM-
complexes is a member of the VAMP72 group. The VAMP72 group of SNAREs
is represented by seven genes in Arabidopsis, and appears to be unique to
green plants (Sanderfoot et al. 2000). Uemura et al. (2004) showed that fluor-
escent protein fusions to VAMP72-family members were generally found on
the PM when transiently expressed in leaf protoplasts, supporting the pos-
sibility that these may represent the R-SNARE(s) for a cell-plate complex.
Since protoplasts do not divide, their methods cannot help us to understand
whether (and which of ) the seven members may be involved. Further work
in plants is necessary to clearly indicate the proper R-SNAREs for the two
Qa+Qb+Qc-SNARE complexes on the cell plate.
3.3
KEULE and the First Signs of Regulation
Since the identification of KNOLLE as a SNARE involved in cytokinesis,
other SNARE and SNARE-interacting proteins have also been found to have
a role in cell plate formation. KEULE, a member of the SM-family of SNARE-
regulators, was found to interact with KNOLLE on the cell plate (Fig. 2B;
Assaad et al. 2001). Likeknollemutants,keulemutants are also seedling lethal
(Assaad et al. 1996). Indeed, it is somewhat surprising, considering what
should be an essential role, that embryos lacking eitherknolleorkeuleare
able to survive many rounds of division prior to their loss of viability at
the young seedling stage (Assaad et al. 1996; Lukowitz et al. 1996). Clearly,
there must be some level of redundancy to allow the embryos to survive long
enough to reach seedling stages. In the case of KEULE, two other PM-type
SM-proteins are found in the Arabidopsis genome (Sanderfoot et al. 2000),
whichalsomightservetoregulateKNOLLEatsomelevelsufficienttopro-