The Endoreduplication Cell Cycle: Regulation and Function 93
endoreduplicating cells chromatin is permanently decondensed, which could
favor increased gene expression by sustaining RNA synthesis. However, this
hypothesis is mainly supported by correlative studies rather than by direct ev-
idence, and recent experiments have shown that a decrease in ploidy in maize
endosperm nuclei does not seem to impact transcriptional activity (Leiva-Neto
et al. 2004). However, only a small set of genes was analyzed in detail, and
it is not known the effect of reduced endoreduplication on a genome-wide
basis. Studies on polyploid isogenic yeast strains have indicated that expres-
sion of certain genes is induced or repressed in a ploidy-dependent fashion,
possibly by ploidy-related mechanisms such as transient homologous pairing
(Galitski et al. 1999). Thus, endoreduplication may represent a mechanism for
modulating expression of a selected pool of genes rather than indiscriminately
increasing gene expression across the entire genome.
Other potential functions of endoreduplication involve a role in protecting
the genome from recessive mutations, adapting to salt stress (Ceccarelli et al.
2006), or marking cells for programmed cell death (Bennett 1973; Kowles and
Phillips 1988). The latter may relate to the ability of polyploid endosperm
cells to provide a large pool of nucleosides and phosphorus for the develop-
ment and germination of the embryo.
Is endoreduplication tied to cell differentiation? A large body of evi-
dence suggests that endoreduplication is associated with differentiation in
many cases (Edgar and Orr-Weaver 2001). One of the best examples comes
fromMedicago truncatula, in which the differentiation of nodules contain-
ing nitrogen-fixing cells in symbiotic bacteroids requires the APC/C activator
CCS52A.DownregulationofCCS52Ain transgenic plants resulted in lower
ploidy, reduced cell size, and affected the maturation of symbiotic cells, indi-
cating that large, endoreduplicated cells are an essential component of nodule
development (Vinardell et al. 2003). Likewise, terminally differentiated cells
in the maize starchy endosperm are highly endoreduplicated (Larkins et al.
2001). Although there is often a clear correlation between endoreduplica-
tion and cell differentiation in endoreduplicating tissues and organs, cells can
undergo terminal differentiation without endoreduplicating, which indicates
that endoreduplication and cell differentiation are not necessarily associated
(Evans and Van’t Hof 1975). Thus, the requirement for endoreduplication dur-
ing the differentiation of certain cell types depends largely on the biological
and physiological contexts.
6
Perspectives
Investigators have been aware of the occurrence of endopolyploid cells for
a long time, yet until recently most studies on endoreduplication have been
primarily correlative. However, the ongoing characterization of many plant