Coordination of Cell Division and Differentiation 385
DPa-mediated hyperplasia is inhibited by co-expressing a dominant-negative
mutant ofCDKB1but not the endoreplication phenotype, which is actually
enhanced (Boudolf et al. 2004a).E2Faoverexpression up regulates a num-
ber of cell cycle genes, such asRBR,KRP3andKRP5in rosette leaves, and
this may contribute to the leaf phenotype observed including reduction in
cell number and increase in cell size (He et al. 2004). Genome-wide analy-
sis of plants expressing a dominant-negative version of DP (Ramirez-Parra
et al. 2003) or E2Fa- DPa (Vandepoele et al. 2005) have identified a few hun-
dred genes that are most likely direct E2F targets. These are, among others,
PCNA,RNR,CDC6,MCM3, CDC45, DNA polymerase and DNA primase genes
(DNA replication genes), andE2Fb,E2Fc,RBR1,E2Ff/DEL3,andCYCA3;2
(cell cycle genes), but also others whose relationship with cell proliferation, if
any, still needs to be established.
E2Fb, which prefers DPa for heterodimerization (Kosugi and Ohashi 2002;
Magyar et al. 2005), plays a role in controlling cell proliferation dependent
on auxin signaling. Coexpression ofE2Fb,butnotE2Fa,withDPastimulates
cell proliferation in the absence of auxin (Magyar et al. 2005). High levels of
E2Fb leads to phenotypes in roots, leaves and cotyledons consistent with hy-
perplasia, that correlates with up-regulation of a variety of E2F target genes
required for G1/S and G2/M (Sozzani et al. 2006). Furthermore, E2Fb itself
seems to be an E2F target as shown in chromatin immunoprecipitation ex-
periments (Sozzani et al. 2006). E2Fc and DPb interact in vitro (Kosugi and
Ohashi 2002) and in vivo (del Pozo et al. 2006). Overexpression ofE2Fcis
highly detrimental for development of leaf primordia (del Pozo et al. 2002),
while a strong reduction ofE2FcmRNA levels produces leaves with a reduced
ploidy level, suggesting that E2Fc-DPb regulate the switch from proliferation
to the endocycle program (del Pozo et al. submitted).E2Fe/DEL1is expressed
in proliferating cells and it has been implicated in restricting endocycle pro-
gression (Vlieghe et al. 2005). Interestingly, the endoreplication phenotype of
the E2Fa-DPa overexpressing plants, but not the hyperplasia, is reduced by
E2Fe/DEL1overexpression (Vlieghe et al. 2005). E2Ff/DEL3 is required for cell
expansion, a process clearly observed in hypocotyl cells, without apparently
affecting the endocycle program (Ramirez-Parra et al. 2004). Its role in dif-
ferentiated cells is mediated by negatively regulating the expression of genes
suchEXP3,EXP7,EXP9,andUGT,involvedincellwallbiosynthesis.
The pre-replication complex (pre-RC), required for initiation of chromo-
somal DNA replication, is constituted by the association of CDC6 and CDT1
with ORC, the six subunit origin recognition complex, and the MCM com-
plex. Ectopic expression of CDC6 (Castellano et al. 2001) or CDT1 (Castellano
et al. 2004) is sufficient to induce extra endocycles or cell division in a cell
type-specific manner (Castellano et al. 2004). It is worth mentioning that in-
creasing Cdt1 activity in animals, similar consequences are obtained (Del
Bene et al. 2004). Gametophytic development (see McCormick 2004; Yadegari
and Drews 2004) and the early embryonic stages seem to be highly sensitive