Biology Reference
In-Depth Information
de novo
methylation during both gamete formation and early stages of
embryogenesis. Current knowledge of reprogramming in plants remains
limited (Reviewed in
Bourc'his & Voinnet, 2010; Wollmann & Berger,
2012
).
As mentioned above, the repertoire of parental H3 proteins is largely
reset following fertilization. Yet, it is generally assumed that at least part
of the chromatin-based information present in the parental genomes can
be inherited in plants. This is the case, for example, for DNA methylation
states at transgenes and repeats (
Matzke, Kanno, Daxinger, Huettel, &
Matzke, 2009
). This has long been viewed as an argument against a global
resetting of epigenetic state through generations. However, this does not
imply that DNA methylation is not reprogrammed during reproduction,
and the stable inheritance of epigenetic states might also imply successive
steps of erasure and selective reestablishment of marks. Analyzing the
dynamics of chromatin changes during plant reproductive development is
technically challenging, and the current data lack sufficient temporal reso-
lution to discriminate between the two possibilities. However, it has been
shown that some reprogramming (a decrease followed by reestablishment)
of DNA methylation at non-CG sites occurs during the late stages of sperm
development and is dependent on RNAi (
Calarco et al., 2012; Ibarra et al.,
2012
). Similarly, genetically hypomethylated repeats can be gradually rem-
ethylated through generations by an RNAi-dependent process that likely
occurs during reproduction, possibly in the early embryo (
Teixeira &
Colot, 2010; Teixeira et al., 2009
). This indicates at least that DNA meth-
ylation states are not passively transmitted through the germ cells.
In addition, genome-wide DNA demethylation of companion cells as a
consequence of both active (DNA glycosylate dependent) and passive
(absence of maintenance methyltransferase) mechanisms may serve to stabi-
lize the germline epigenome. DNA demethylation of the vegetative cell in
the pollen grain and the central cell in the female gametophyte are proposed
to trigger massive production of TE-associated small RNAs. These small
RNA molecules possibly move into the germ cells and the embryo, with
a role not only in protecting the genome against transposable elements
but also in reconfiguring the embryo epigenome via RdDM activity
(
Calarco et al., 2012; Gehring et al., 2009; Hsieh et al., 2009; Ibarra
et al., 2012; Slotkin et al., 2009
). As most siRNAs present in the seed are
of maternal origin (
Mosher et al., 2009
), these are possibly involved in
restorating CHH methylation patterns in the embryo, where maternal
RdDM components regulate genome activity according to parental origin
