90 P.A. Sabelli · B.A. Larkins
has been shown that light has a negative effect on endoreduplication in
Arabidopsishypocotyls (Gendreau et al. 1998; Gendreau et al. 1999). Endo-
reduplication in maize endosperm is inhibited by environmental stresses,
such as water deficit (Artlip et al. 1995; Setter and Flannigan 2001) and heat
stress (Engelen-Eigles et al. 2000). However, the reduction in endoreduplica-
tion is more a consequence of perturbation during early mitotic development,
rather than a direct result of the inhibition of the endoreduplication cell cycle
per se, reinforcing the idea that a tight relationship couples the mitotic and
endoreduplication phases of development mentioned above. A direct stimu-
latory effect of certain symbionts and parasites on endoreduplication in plant
cells has been well established in the case of nitrogen-fixing root nodules of
legumes infected with the bacteriumSinorhizobium meliloti, and in root-knot
galls induced by the endoparasitic root-knot nematodeMeloidogyne incog-
nita(Kondorosi and Kondorosi 2004) (see Sect. 2.3).
Various plant hormones impact endoreduplication differently. For ex-
ample, ethylene stimulates endoreduplication in bothArabidopsis(Joubes
and Chevalier 2000) and cucumber hypocotyls (Dan et al. 2003). It is well es-
tablished that the transition from a mitotic to an endoreduplication cell cycle
in maize endosperm involves a sharp decline in the cytokinin/auxin ratio
(Lur and Setter 1993; Sabelli et al. 2005b). Auxin is apparently involved in the
induction of endoreduplication, but does not seem limiting for its continua-
tion. Application of chemicals that inhibit auxin-triggered effects suggested
that auxin has a rather indirect effect in stimulating endoreduplication (Lur
and Setter 1993). In spite of a large body of circumstantial evidence for
endosperm cell cycle control by both environmental and hormonal factors,
information about the key elements linking them to the cell cycle machinery
is largely missing and this is a challenge for future research.
4.3
Parent-of-Origin Effects and Epigenetic Control
It is well known that endoreduplication in maize endosperm is under ma-
ternal control, in that the endoreduplication pattern in the offspring of
a cross between two inbred lines generally resembles that of the maternal
parent (Kowles et al. 1997; Dilkes et al. 2002). It is possible that differen-
tial gene dosage contributes to this “maternal effect”, as the genetic makeup
of the endosperm involves two maternal copies and one paternal copy of
the genome. An alteration to this 2 : 1 ratio is deleterious for the develop-
ment of the endosperm. A maternal excess causes a shortened mitotic phase
and a precocious endoreduplication period (but does not affect the extent of
endoreduplication). Conversely, excess of the paternal genome delays endo-
reduplication through an apparent extended mitotic hiatus prior to the switch
to endoreduplication, and it also inhibits DNA amplification in all endo-
reduplicating cells (Leblanc et al. 2002).