Insights into Plant Epigenome Dynamics - Fruit Ripening: Physiology, Signalling and Genomics - page 301

pathways involving RNA cleavage or
translational suppression (Reinhart et al. ,
2002; Brodersen et al. , 2008), while siRNAs
are primarily associated with cytosine
methylation and gene repression, espe-
cially with regard to retroelements (Mosher
et al. , 2008).
siRNAs are capable of mediating
sequence-specifi c methylation in all
cytosine contexts (CG, CHG and CHH,
where H is A, C or T) by targeting via
DOMAINS REARRANGED METHYL-
TRANSFERASE2 protein (DRM2; Cao and
Jacobsen, 2002) to siRNA homologous
sites. As a result of base pairing, CG and
CHG are symmetrical, and cytosines in
these contexts have their methylation
maintained during DNA replication by the
cytosine methyltransferases MET1 and
CMT3 (Ronemus et al. , 1996; Lindroth et
al. , 2001), while CHH methylation is
probably maintained by siRNA-mediated
DMR2 activity at each replication event.
While mechanisms exist to maintain DNA
methylation through subsequent gener-
ations, these epigenome modifi cations are
not fi xed and can be altered by lack of
MET1, CMT3 and DRM2 maintenance
activities, or more directly through the
demethylation activities of the RE-
PRESSOR OF SILENCING1 (ROS1) and the
homologous DEMETER family genes (Zhu,
2009).
The complex machinery of plant
cytosine methylation, maintenance and
variation, capable of discretely targeting
specifi c DNA sequences at specifi c loci, is
an area of considerable interest and
increasing understanding. A substantial
proportion of RNA-mediated DNA methyl-
ation is conferred by the plant-specifi c
RNA polymerase IV (Pol IV) and Pol V,
which specifi cally produce non-coding
sRNA precursors. Pol IV-derived siRNAs
target cytosine methylation of Pol
V-transcribed regions through interaction
with ARGONAUTE (AGO) proteins to
develop a complex with Pol V transcripts
and Pol V itself (He et al. , 2009). This
complex recruits the DMR2 methyl-
transferase and possibly additional
chromatin remodelling proteins (Gao et al. ,

2010, and references therein). Mutations in
Arabidopsis Pol IV and V genes reveal that,
while phenotypic consequences ensue,
viable organisms can still be recovered
(Haag and Pikaard, 2011), indicating their
importance but also the likely redundancy
of equivalent activities. Interestingly, in the
cases of Pol IV, Pol V or Pol IV/V double
mutants, CHH cytosine methylation was
shown to occur but in different positions
and with increased activity in peri-
centromeric loci, suggesting that these
polymerases themselves also infl uence the
specifi ed targeting of cytosine methyl-
transferases (Wierzbicki et al. , 2012).
Additional components of targeted plant
cytosine methylation remain to be
identifi ed and will contribute along with
those already described to our under-
standing of the mechanisms and control of
epigenome architecture.

17.3 Epigenome Plasticity

While the epigenome harbours information
that can be transferred reliably from
generation to generation, the epigenome is
also dynamic and changeable, demon-
strating variation through aspects of
plant development, in response to stress,
and the potential for reprogramming
during reproduction. This dynamism
presents opportunities for long- and short-
term genetic adaptation, regulatory fi delity
during stress and developmental re-
sponses, and prospects for novel
approaches to genetic improvement via the
development of novel advanced crop
varieties.

17.3.1 Embryo development and heterosis:
insights into imprinting, memory and
epigenome heritability

DNA methylation status during plant
sexual reproduction can result in variation
from parent to offspring (Becker et al. ,
2011; Jullien et al. , 2012), furthering
additional interest into how DNA methyl-
ation status is managed through plant