The Endoreduplication Cell Cycle: Regulation and Function 91
Genomic imprinting, which involves the differential expression of mater-
nally and paternally derived alleles, may be involved in the maternal effect
on endosperm development in general, and endoreduplication in particu-
lar (Guitton and Berger 2005). InArabidopsis, several Polycomb group genes
that are involved in chromatin remodeling and epigenetic regulation of gene
expression, includingFIS,FIE,andMEA, are imprinted during early seed
development (Sorensen et al. 2001). After fertilization, only the maternal alle-
les of these genes are expressed in the endosperm, while the paternal alleles
are silenced. MaizeFIEhomologs have been identified (Danilevskaya et al.
2003), but it is not known if and how they might be involved in endoredu-
plication. A potential pathway linking imprinting, Polycomb group genes,
and endoreduplication may involve histone modification around replication-
origin bound ORC (Machida et al. 2005), chromatin remodeling complexes,
and interaction between RBR and Polycomb subunits (Ach et al. 1997b;
Mosquna et al. 2004).
5
Functions of Endoreduplication
It has long been recognized that endoreduplication is often correlated with
increased nuclear size, cell size, cell differentiation, and metabolic activity.
Therefore it has been tempting to propose a general role for endoredupli-
cation in driving growth and development. Indeed, there is strong evidence
supporting this view (for an in-depth discussion on cell size control by
endoreduplication, see Sugimoto-Shirasu and Roberts 2003). For example,
a clear relationship between ploidy, nuclear size, and cell size was observed
in floral apices ofDatura stramonium(Satina and Blakeslee 1941),Arabidop-
sisepidermis (Melaragno et al. 1993), and in petal development in cabbage
(Kudo and Kimura 2002). Endoreduplication is the first visible cellular event
in trichome development prior to cell growth and branching (Hulskamp
et al. 1994) and decreased ploidy content in transgenic trichomes expressing
ICK1/KRP1results in reduced cell size (Schnittger et al. 2003). In leaf epi-
dermis of durum wheat it promotes cell expansion and precedes cell growth
(Cionini et al. 1983). Endoreduplication in the pericarp could contribute sub-
stantially to the growth of the tomato fruit (Cheniclet et al. 2005), and it
is correlated with nuclear and cell size as well as starch and storage pro-
tein accumulation during endosperm development in cereals such as maize
(Larkins et al. 2001) and sorghum (Kladnik et al. 2006). These (and other)
observations are consistent with the “karyoplasmic ratio” theory, according
to which nuclear size controls cell size through regulation of cytoplasm vol-
ume. For example, inArabidopsis gl3mutant trichomes both DNA content
and cell size are reduced, but the nuclear size to cell size ratio is similar
to wild type.