The Endoreduplication Cell Cycle: Regulation and Function 89
cyclins and CDKA;1. Plants overexpressingSIMhave enlarged epidermal cells
with an increased ploidy resulting from enhanced endoreduplication. It was
proposed that they possess a CDK inhibitory activity with a key function in the
mitosis-to-endoreplication transition (Churchman et al. 2006).
Mutations at theGLABRA3(GL3) locus also reduce endoreduplication,
trichome branching, and trichome cell size, whereas mutations at theTRYP-
TICON(TRY)andKAKTUS(KAK) loci increase endoreduplication and tri-
chome cell size (Hulskamp et al. 1994).KAKencodes a protein similar to
the HECT domain proteins involved in ubiquitin-mediated protein degrada-
tion (El Refy et al. 2004). In addition, a CUE protein variant that controls
the endocycle leading to hypocotyl elongation was recently identified genet-
ically (Tsumoto et al. 2006). In theincreased level of polyploidy1-1D(ilp1-1D)
mutant, hypocotyls showed increased polyploidy under both light and dark
conditions.ILP1encodes a protein similar to the mammalian GC binding
factor, which functions as a transcriptional repressor for all members of
theCYCA2family. Thus ILP1 regulates endoreduplication through control
ofCYCA2(Yoshizumi et al. 2006). When theArabidopsis STEROL METHYL-
TRANSFERASE 2gene function is mutated, such as in thefrill1mutant,
sterol composition is altered and this results in ectopic endoreduplication in
petal cells (which normally do not undergo endoreduplication) and in rosette
leaves (Hase et al. 2005).
Mutations in theArabidopsissubunits of chromatin assembly factor CAF-
1 revealed a role for this complex in limiting DNA endoreduplication (Exner
et al. 2006). Additional mutations affecting endoreduplication inArabidopsis
have been reviewed elsewhere (Sugimoto-Shirasu and Roberts 2003).
In maize, analysis of defective kernel (dek) mutants, in which both the mi-
totic and endoreduplication cell cycle are inhibited, highlighted an elevated
degree of coordination between the mitotic and endoreduplication phases of
endosperm development (Kowles et al. 1992). However, mitosis and endo-
reduplication are also genetically uncoupled to some extent in this system,
as shown by the phenotype of theminiature1(mn1) mutant (Vilhar et al.
2002), in which the amount of energy available for endosperm development
is substantially decreased. In this mutant the activity of the cell wall INCW2
invertase, which is involved in providing hexose sugars to the developing
endosperm, is compromised, resulting in reduced mitotic activity and cell ex-
pansion, but leaving endoreduplication largely unaffected. This suggests that
endoreduplication is somewhat more resistant to certain perturbing factors
than the mitotic cell cycle.
4.2
Environment and Phytohormones
Endoreduplication is affected by environmental factors and phytohormones
and this subject has been recently reviewed (Barow 2006). For example, it