Agriculture Reference
In-Depth Information
virus resistance. DNA methylation is involved in the regulation of the
A.
thaliana
immune system as well. Stress-induced differentially methy-
lated regions, also associated with differential gene expression, were
identi
ed upon exposure to bacterial pathogen, avirulent bacteria, or
salicylic acid (SA). In response to these effectors, transposon-associated
regions with different level of methylations were coupled by upregula-
tion of 21-nt siRNAs, inducing in most cases transcriptional changes of
the associated transposon and/or gene in proximity (Dowen et al. 2012).
siRNAs may have critical functions in plants under stress as well. An
interesting example of siRNAs/methylation gene regulation has been
reported for the phenylpropanoid pathway (Lin et al. 2013). Wounding
can induce sRNA8105, which will bind to the
first intron of IbMYB1
RNA to methylate IbMYB1, cleave IbMYB1 RNA, and trigger production
of secondary siRNAs, which will further repress the IbMYB1 family
genes expression responsible for the regulation of the phenylpropanoid
pathway.
As mentioned earlier, growth vigor in plant hybrids has also been
associated with epigenetic variation and differential gene expression.
siRNAs of transposable elements origin were strongly associated with
differences between hybrids and parent lines, implicating an important
role for genome-wide epigenetic regulation of gene activity in maize
hybrids (He et al. 2013).
Involvement of gene silencing and its templating basis for speci
city
would be intriguingly similar to the RNA/RNA surveillance templating
that occurs in Tetrahymena development (Henderson et al. 2006;
Teixeira et al. 2009). The movement of siRNAs from the male gameto-
phyte vegetative cell nucleus (VN) to the sperm nucleus in plants
(Slotkin et al. 2009; Olmedo-Mon
l et al. 2010) is also astonishingly
reminiscent of the movement of transcripts from the ciliate germ line
micronucleus to the macronucleus where they are involved in surveil-
ling for new genome invasions. If the function of mobile plant siRNAs is
similar, as proposed by Slotkin et al. (2009), it could explain the
difference in siRNA movement between ciliates and plants. Neither
the VN nor the micronucleus directs somatic development. This is
accomplished by the ciliate macronucleus and the plant gamete nuclei
(after fusion). Release of epigenetic silencing of TFs in VN and subse-
quent silencing in the plant male gamete nucleus by the siRNAs
(Olmedo-Mon
l et al. 2010) would be equivalent to elimination of
TEs in the macronucleus by micronucleus siRNAs. In both instances
siRNAmovement is into the nucleus that controls somatic development.
It is tempting to speculate that, as in protists, an siRNA system
of surveilling and templating the epigenome occurs to help mediate