76 P.A. Sabelli · B.A. Larkins
2002), stem and leaf epidermis (Melaragno et al. 1993; Kudo and Kimura
2002), mesocarp of fruits (Cheniclet et al. 2005), and seedling hypocotyl
(Gendreau et al. 1997).
The extent to which endoreduplication occurs varies widely among plants.
For example, the average DNA content inArabidopsistrichomes is 32C (where
C represents the amount of unreplicated DNA in haploid cells of a given
species), whereas it can reach in excess of 384C in some maize endosperms,
to about 8000 CinthesuspensorofPhaseolus coccineousand24 576CinArum
maculatum(equivalent to 13 endoreduplication cycles). Endoreduplication
also occurs in organisms other than plants, from yeast to arthropods and,
relatively infrequently, in mammals, either as part of their normal develop-
ment or in response to genetic or physiological perturbations (Edgar and
Orr-Weaver 2001). Endoreduplication is generally an irreversible process; that
is, endoreduplicated cells cannot divide. However, in the epidermis of the
Manduca sextacaterpillar, endoreduplicated 32C cells reenter mitosis, with
a consequent reduction in their ploidy to 2C (Edgar and Orr-Weaver 2001). In
addition, a recent report indicates that certainArabidopsiscells, which were
induced to undergo endoreduplication by the misexpression ofICK1/KRP1,
can reenter mitosis (Weinl et al. 2005).
It has long been observed that the occurrence of endoreduplication is cor-
related with enlargement of differentiated cells, which are often highly active
in gene expression and metabolic output. Yet exceptions to this rule have
also been documented, which makes it difficult to establish a general and ne-
cessary relationship between endoreduplication, cell growth, differentiation,
and the organism’s development. In recent years, studies of endoreduplica-
tion in plants have made substantial progress. Thus, in this chapter we focus
on current knowledge of the genetic and molecular mechanisms controlling
endoreduplication in well-studied plant model systems, and discuss different
theories concerning the potential functions of endoreduplication. Studies in
nonplant organisms will be referred to primarily in order to provide a histor-
ical perspective or to illustrate specific issues where knowledge from plants is
currently insufficient or lacking.
2
Molecular Regulation of Endoreduplication
In the mitotic cell cycle DNA synthesis and chromosome segregation are
tightly coupled. However, during endoreduplication one or more rounds of
DNA synthesis occur in the absence of mitosis. This observation suggests,
therefore, that the factors responsible for coupling DNA synthesis to mito-
sis are deregulated during the endoreduplication cell cycle. Here we con-
sider how endoreduplication may be regulated at the level of replication
origin activity, the retinoblastoma/E2F pathway, the proteolysis of G1/S-