Agriculture Reference
In-Depth Information
dif
cult to determine until recently (Paszkowski and Grossniklaus
2011). The total evidence available still strongly raises the likely possi-
bility that a major global clearing and resetting of the methylation pattern
and perhaps other epimarks occurs during both gametogenesis and at the
early stage embryo (Feng et al. 2010b; Law and Jacobsen 2010). There
then likely exists in plants a similar situation for allowing mistakes as in
animals (Saze et al. 2003; Feng et al. 2010b; Jullien and Berger 2010; Law
and Jacobsen 2010; Paszkowski and Grossniklaus 2011).
Several aspects of the entire control of setting, maintaining, and
resetting epimarks remain unclear, in both plants and animals. Most
importantly, the control of the speci
city of epimark changes and
possible direct effects of the environment on this speci
city, which
are both central to the entire concept of epigenetics and evolution, are
still not well understood. Epigenetic error rates such as mismethylation
of DNA in humans during mitosis and transgenerational errors can occur
1,000 times more frequently than DNA sequence mutations (Richards
2006). However, some mitotic epigenetic errors are as infrequent as those
of sequence mutations suggesting that errors (Richards 2006) are not
stochastic processes, but are controlled.
Besides the recognition of hemimethylated and CpG island sites (Cao
and Jacobsen 2002), current models for controlling maintenance (copy-
ing) and
de novo
DNA methylation patterns in
Arabidopsis
point to at
least a partial dependence on Small interfering RNAs (siRNAs) to direct
methylation during DNA replication and providing template nucleotide
pairing-based speci
city (Cao and Jacobsen 2002; Sugiyama et al. 2005;
Slotkin et al. 2009; Autran et al. 2011). The discovery of ROS3, a small
RNA binding protein that is involved in DNA demethylation pathways,
strongly supports the possibility that small RNAs are involved in con-
trolling epimark speci
city (Zheng et al. 2008).
siRNAs regulate
de novo
RdDM, which in turn is associated with
imprinting at speci
c endosperm loci. siRNAs and DNA methylation
could therefore be common mechanisms in plants and mammals for the
selection of imprinted loci through evolution (Vu et al. 2013). RdDM has
been shown to be involved in wheat grain development (Sun et al. 2013).
The change from immunity to susceptibility to viral infection in
transgenic tomato is associated with the disappearance of transgene-
speci
c transcripts and siRNAs, and with hypermethylation of the
transgene (Catoni et al. 2013). Disappearance of transgene-speci
c tran-
scripts and siRNAs and hypermethylation of the transgene proceed
gradually through the generations. This process mediates the transition
from posttranscriptional gene silencing to transcriptional silencing of
the transgene, which eventually will abolish the transgenically induced
