G1/S Transition and the Rb-E2F Pathway 65
2005). In addition or in accordance with multiple members of the E2F fam-
ily, genome-wide analysis identified a high number of E2F target genes in-
volved in cell-cycle regulation, DNA replication and repair, chromatin dy-
namics, cell-wall biogenesis, and others (Ramirez-Parra et al. 2003; Van-
depoele et al. 2005), implying a broad role of E2F family proteins beyond
the cell-cycle control.
3.2
Rb
While only one Rb homolog exists inArabidopsis, at least three different ones
arepresentinmaize.ThepresenceofmultipleRBR(Rb-Related)proteins
in grasses was speculated to explain their recalcitrance to genetic trans-
formation (Sabelli et al. 2005). Plant RBR proteins exhibit a similar domain
organization as animal Rb, with highest conservation within the pocket do-
mains A and B that bind E2F. Physical interaction between RBR and E2F has
been shown in yeast two-hybrid assays (reviewed in Shen 2002) and a protein
complex containing RBR, E2Fb and DPa has been demonstrated in transgenic
Arabidopsis(Magyar et al. 2005). Loss-of-function mutations in the RBR gene
inArabidopsiswere gametophytic lethal (Ebel et al. 2004), precluding the as-
sessment of its precise role in cell-cycle progression. Downregulation of the
RBR expression by RNAi (RNA interference) in tobacco (Park et al. 2005)
andArabidopsis(Wildwater et al. 2005) as well as titration of RBR protein
by overexpression of the geminivirus RepA protein in tobacco and maize
(Gordon-Kamm et al. 2002; Sabelli et al. 2005) andArabidopsis(Desvoyes
et al. 2005) stimulated cell division. Consistent with its repressor role, the
reduction of RBR increased E2F activity and upregulated expression of E2F-
target genes (Desvoyes et al. 2005; Park et al. 2005; Sabelli et al. 2005).
In mammals, hyperphosphorylation of Rb, sequentially by cyclin D, cy-
clin E and cyclin A CDK kinases, ensures the G1/S transition. In plants, both
CYCA-CDK and CYCD-CDK kinases phosphorylate RBR (Nakagami et al.
1999, 2002; Roudier et al. 2000; Boniotti and Gutierrez 2001), which is thought
to release the activity of RBR-bound E2F in transcription. Overall, it appears
that CYCD-CDK and subsequently CYCA-CDK kinases phosphorylated RBR,
resulting in the activation of the plant Rb-E2F pathway at the G1/S transi-
tion. A canonical Rb-E2F pathway appears critical in stem-cell maintenance
inArabidopsisroots (Wildwater et al. 2005).
On the basis of findings in mammals and conservation of regulators in
plants, I previously proposed that chromatin remodelling could play a signifi-
cant role in the Rb-E2F pathway (Shen 2002). In support of this assumption,
Williams et al. (2003) found that the chromatin at E2F-target genes was de-
condensed before activation of these genes during dedifferentiation and entry
into the S-phase of tobacco leaf protoplasts. Rossi et al. (2003) showed that
the maize histone deacetylase ZmRpd3I binds RBR and that the two proteins