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Finally, the transition from the gametes to the embryo and the endo-
sperm also implies abrupt chromatin changes (
Fig. 6.2
). Gametic H3 pro-
teins are rapidly eliminated following fertilization and replaced by newly
synthesized histones (
Ingouff et al., 2010
). DNA methylation and histone
PTMs also likely undergo changes in very early embryos (
Autran et al.,
2011
). Expression patterns of known DNMTs indicate that both the key
maintenance enzymes and
de novo
DNMTs are expressed in the embryo
(
Jullien et al., 2012
), which results in the rapid acquisition of DNA meth-
ylation patterns that are similar to those of the soma for both parental
genomes (
Calarco et al., 2012; Ibarra et al., 2012
). However, strong
methylome differences are observed in the endosperm between the mater-
nally and paternally inherited genomes, the maternal genome being signif-
icantly less methylated than the paternal genome in the CG context, with
“hot spots” of extensive demethylation. Most of the differences in CG
methylation between the two genomes derive from the activity of the
DNA glycosylase DME, in the central cell but not in the egg. In contrast
to CG, CHG methylation patterns are similar for both parental genomes,
except for targets of DME that has context-independent activity.
Thus, a key feature of plant gametophytic development is the establish-
ment in the mature gametophytes of epigenetically dimorphic cell types that
distinguishes germ cells from cells that have a transitory role in reproduction.
The resulting dimorphism emerges as a key innovation of plant reproductive
biology, as exemplified by its importance for the regulation of genomic
imprinting and genome reprogramming.
4.2. DNA methylation and small RNAs determine parental
imprinting in the seed
Imprinting refers to the differential expression of the maternal and paternal
alleles of a gene. In plants, imprinting mostly occurs in the endosperm
(which from that perspective is analogous to the extra-embryonic tissues
of mammals), affecting an estimated 50 loci (
Gehring, Missirian, &
Henikoff, 2011; Gehring et al., 2009; Hsieh et al., 2011
). A key actor in this
process is DME, the activity of which results in differentially methylated
regions (DMR) for most imprinted genes in the endosperm (
Calarco
et al., 2012; Choi et al., 2002; Gehring et al., 2009, 2006; Ibarra et al.,
2012
). Importantly, DME-dependent demethylation mostly targets small,
AT-rich repeats. In both rice and
Arabidopsis
, such sequence targets are
mostly found near genes (
Ibarra et al., 2012
). This suggests that imprinting
