82 P.A. Sabelli · B.A. Larkins
generally involves incomplete replication of heterochromatin (Edgar and Orr-
Weaver 2001). That not withstanding, many studies have implicated CDKs in
preventing rereplication in baker’s yeast, fission yeast, fly, and frog. As already
mentioned, inSaccharomyces cerevisiaeCDK activity represses rereplication
by three overlapping mechanisms, and rereplication is stimulated by muta-
tions affecting CDC2/CycB kinase (Nguyen et al. 2001).
It has also been shown inArabidopsisthat CycA2;3 negatively regulates
endoreduplication and ploidy levels (Imai et al. 2006). The involvement of
CYCA2 in repressing endoreduplication was further confirmed by the obser-
vation that transcription of allCYCA2family members was repressed in the
increased level of polyploidy1-1Dmutant, in which endoreduplication is stim-
ulated (Yoshizumi et al. 2006). Although CYCA2 inhibits endoreduplication,
it does not seem to play a role in the mitosis/endoreduplication switch. How-
ever, the SIAMESE (SIM) protein, which interacts with and inhibits CycD and
CDKA;1, appears to control the balance (switch) between mitotic and endo-
reduplication cell cycles (Churchman et al. 2006).
CycD expression is associated with the mitotic cell cycle but not with the en-
doreduplication cell cycle. Overexpression ofCYCD3;1has an inhibitory effect
on endoreduplication and cell differentiation inArabidopsisleaves, suggesting
that aspects of G1/S-phase regulation are distinct in mitotic and endoredu-
plicating cells (Dewitte et al. 2003). Likewise, trichome endoreduplication
was inhibited by overexpression ofCYCB1;2,buttrichomesizewasmain-
tained through compensatory stimulation of cell proliferation (Schnittger et al.
2002b). A similar result was obtained upon downregulation of the mitotic cy-
clin inhibitor,CCS52A,inMedicago sativa(Cebolla et al. 1999). In contrast,
when the CDK inhibitorICK1/KRP1was overexpressed in trichomes, it caused
an inhibition of endoreduplication only (i.e., no effect on cell number) with
trichome downsizing and a simplified morphology (Schnittger et al. 2003).
Additional evidence involving the reduction of CDK activity by overexpress-
ing certain CKIs, such asNtKIS1a(Jasinski et al. 2002) andKRP2(De Veylder
et al. 2001), inArabidopsisconfirmed the important role played by CDKs in
endoreduplication. Consistent with these findings, downregulation of CDKA1-
associated kinase in developing maize endosperm resulted in an inhibition of
the endoreduplication cell cycle (Leiva-Neto et al. 2004).
3
Endoreduplication Model Systems
3.1
Maize Endosperm
Maize endosperm is a triploid tissue that originates from the fusion of one
sperm nucleus with two polar nuclei in the female gametophyte. Endosperm