Agriculture Reference
In-Depth Information
development after fertilization since siRNA genome surveillance may
also occur in plants (Slotkin et al. 2009; Feng et al. 2010a).
Methylation pattern changes induced by mutation of methyltransfer-
ases and other epimark machinery are highly transgenerational
(Johannes et al. 2009). It has also been suggested that phenotypes
inheritance in plants is more likely to involve epigenetics with respect
to mammals (Bond and Baulcombe 2013). Speci
city is also indicated by
variable rates of methylation in methyltransferase and other mutant
backgrounds, which also depend on the methylation target locus (Lister
et al. 2008). The FWA locus requires several generations to reset and the
SDC locus can reset in one or two generations (Teixeira and Colot 2009,
2010). The B
ยด
locus involved in maize paramutation has an extremely
low reversion rate (less than one in several hundred thousands) com-
pared with its B1 allele (Richards 2006). Epigenetic aberrations (epi-
mutations) at speci
c loci have also been more carefully examined in
plants owning to the robust genetic model system
Arabidopsis
. The
passage of altered epimarks through gametogenesis into the next-gener-
ation embryo is well established in plants and several epimutations have
been identi
ed. There has been considerable interest in the possibility
that TFs may serve as a speci
city framework of methylation since they
have been shown to bind methyltransferases (Gibney and Nolan 2010).
Their intricate speci
city for DNA sequence site recognition makes
them appealing candidates to control epimark target speci
city. How-
ever, the evidence pointing to the need for transcript formation in DNA
surveillance (Yu et al. 2005) indicates that RNA binding proteins are
involved in speci
city. The interaction of methylated H3K9 histone
with methylated DNA and its recruitment of methyltransferases to the
DNA through HP1 class proteins provide an interconnection of DNA
and histone epimark speci
city. Such links between different layers
(histone and DNA methylation, etc.) are crucial to the functional com-
plexity of genome organization (Sharpiro and Von Sternberg 2005)
observed in complex ontogeny.
The other important question is the degree to which the error rate and
speci
city of epimutations is biased by the environment. This is essen-
tially the basis of Weisman
s Paradox for direct involvement of the
environment in evolution. Any controlled, nonstochastic aspect of epi-
marks leaves open the opportunity for the environment to participate
directly in affecting epigenetic phenotype changes (Richards 2006).
Examples of mechanisms of environment-directed changes in trans-
generational phenotypes have been increasingly reported (Hilbricht
et al. 2008) and we will describe some of them in detail later. Jablonka
and Raz (2009) have pointed out that involvement of the gene silencing
'
