86 P.A. Sabelli · B.A. Larkins
stitutively expressed throughout endosperm development, whereas KRP;2
expression was downregulated after the onset of endoreduplication, suggest-
ing different roles for these two CKIs in the mitotic and endoreduplication
cycles (Coelho et al. 2005). It is possible that KRP;1 specifically plays an im-
portant role in providing the oscillation in CDK activity required to sustain
endoreduplication in maize endosperm by inhibiting S-phase CDK activity
after replication initiation, similarly to what has been demonstrated for the
Drosophilaendoreduplication cycle (Weiss et al. 1998).
In our laboratory, intense research is focused on the role that the RBR/E2F
pathway plays in cell cycle regulation during endosperm development. Maize
possesses at least two different RBR types of genes,RBR1(Grafi et al. 1996;
Xie et al. 1996) andRBR3(Sabelli et al. 2005a; Sabelli and Larkins 2006).
Athirdgene,RBR2(Ach et al. 1997a), belongs to the RBR1 type of sequences
and encodes a protein over 90 % identical to RBR1, and it is not clear whether
it has a distinct function from that of RBR1. RBR3, however, is clearly dis-
tinct from RBR1 and RBR2 both in structural and functional terms. In fact,
it shares only 50 % sequence identity with RBR1 and is repressed by RBR1,
suggesting that total pocket protein activity is determined by an intrinsic reg-
ulatory loop within theRBRgene family. Evidence for such a configuration of
the RBR protein family has been reported in plants for only the grass family
within monocots, and the significance of this finding is being actively inves-
tigated (Sabelli et al. 2005a; Sabelli and Larkins 2006). Although both RBR1
and RBR3 are expressed in developing endosperm, RBR1 expression is con-
stitutive and actually increases during late endoreduplication stages, whereas
RBR3 expression is dramatically downregulated after the onset of endoredu-
plication, indicating that it may be involved specifically in regulating the
mitotic cell cycle but is largely dispensable (or even counterproductive) in en-
doreduplicating cells. The upregulation of RBR1 expression during endoredu-
plication might indicate a role in regulating the alternation of G- and S-phases
during endoreduplication, or an involvement in inducing programmed cell
death. Early results suggested that RBR1 was hyperphosphorylated (and pre-
sumably inactivated) during endoreduplication (Grafi et al. 1996), and thus
it is not clear whether the total activity of this protein increases, decreases,
or remains constant during endosperm endoreduplication. It is expected that
forward genetic experiments aimed at down- or upregulatingRBR1andRBR3
expression in transgenic endosperms will provide an answer to this question.
3.2
ArabidopsisLeaf Trichome
Trichomes are unicellular, epidermal structures and endoreduplication is
associated with their differentiation (Hulskamp 2004). Fully developedAra-
bidopsistrichomes average a 32C ploidy level (indicative of four rounds of
endoreduplication), and typically have three or four branches. Trichome cell