15.6.1 Genetically Induced Epialleles
The molecular mechanisms and factors involved in the creation, maintenance, and
stability of epialleles, in general, responsible for the continuity in the propagation
of epialleles during these various developmental transitions are poorly understood,
with the exception of cytosine methylation (Jullien et al. 2008). Therefore, the
formation and transmission of heritable epialleles is expected to correlate with
the different levels and/or distribution of CG methylation and such marks should
be resistant to transgenerational resetting. Indeed, MET1 seems to be essential for
the precision of TEI not only during vegetative growth but also during postmeiotic development of the gametophytes (Saze et al. 2003). The first classical
example of an epiallele is the gene variant causing the radial symmetry in peloric
phenotype of Linaria flowers, which are usually bilaterally symmetric, has been
attributed to hypermethylation of the CYCLOIDEA gene (Cubas et al. 1999). The
above-mentioned examples of epialleles, controlled through a genetic element in
cis or trans, provide the most convincing examples of TEI. However, these specific
examples do not address the molecular mechanisms and factors involved in the
creation, maintenance, and stability of epialleles in general. It is also not clear
whether there is a broader spectrum of loci that are subjected to similar mechanisms of epiallelic regulation within the genome (Paszkowski and Grossniklaus
2011). Experiments addressing these issues exploit mutations in genes encoding
various epigenetic regulators. Such studies show that the chief components
involved in the formation of epialleles, which can be propagated over many
generations in the absence of the mutation initially triggering the epigenetic
changes, are linked to the maintenance of CG methylation (Johannes et al. 2009).
The two best studied factors required for the maintenance of CG methylation are
DDM1 and MET1, such that ddm1 and met1 mutants were used to establish plant
strains with genome-wide alterations of epigenetic marks (Kakutani et al. 1999).
Recently, the role of DNA demethylation in Arabidopsis antibacterial defense has
been reported (Yu et al. 2013). Pseudomonas syringae pv. tomato DC3000
infection in Arabidopsis revealed that mutants met1 and ddc were unable to
develop the typical infection symptoms and showed the resistant phenotype
because of their loss cytosine methylation capability. Moreover, several pathogenresponsive genes were differentially regulated in these mutants, signifying the role
of DNA methylation in establishing plant defense against bacterial pathogen. A
gene named elongator complex subunit2 (ELP2) regulated the genomic DNA
methylation pattern in Arabidopsis, and subsequently stimulated the pathogeninduced DNA methylation alterations (Saze et al. 2003). Hypomethylation of DNA
during pathogen infection has been shown to influence the defense-related gene
expression. The rice R gene Xa21G, which was demethylated chemically exhibited
inherited resistance to Xanthomonas oryzae pv. Oryzae. Thus, epigenetic processes
may be a defense mechanism that allows plants to safeguard their offspring against
repetitive biotic stresses without stable inherent trait fixation. An enhanced
15 Plants as Bioreactors for Human Health Nutrients
441
The molecular mechanisms and factors involved in the creation, maintenance, and
stability of epialleles, in general, responsible for the continuity in the propagation
of epialleles during these various developmental transitions are poorly understood,
with the exception of cytosine methylation (Jullien et al. 2008). Therefore, the
formation and transmission of heritable epialleles is expected to correlate with
the different levels and/or distribution of CG methylation and such marks should
be resistant to transgenerational resetting. Indeed, MET1 seems to be essential for
the precision of TEI not only during vegetative growth but also during postmeiotic development of the gametophytes (Saze et al. 2003). The first classical
example of an epiallele is the gene variant causing the radial symmetry in peloric
phenotype of Linaria flowers, which are usually bilaterally symmetric, has been
attributed to hypermethylation of the CYCLOIDEA gene (Cubas et al. 1999). The
above-mentioned examples of epialleles, controlled through a genetic element in
cis or trans, provide the most convincing examples of TEI. However, these specific
examples do not address the molecular mechanisms and factors involved in the
creation, maintenance, and stability of epialleles in general. It is also not clear
whether there is a broader spectrum of loci that are subjected to similar mechanisms of epiallelic regulation within the genome (Paszkowski and Grossniklaus
2011). Experiments addressing these issues exploit mutations in genes encoding
various epigenetic regulators. Such studies show that the chief components
involved in the formation of epialleles, which can be propagated over many
generations in the absence of the mutation initially triggering the epigenetic
changes, are linked to the maintenance of CG methylation (Johannes et al. 2009).
The two best studied factors required for the maintenance of CG methylation are
DDM1 and MET1, such that ddm1 and met1 mutants were used to establish plant
strains with genome-wide alterations of epigenetic marks (Kakutani et al. 1999).
Recently, the role of DNA demethylation in Arabidopsis antibacterial defense has
been reported (Yu et al. 2013). Pseudomonas syringae pv. tomato DC3000
infection in Arabidopsis revealed that mutants met1 and ddc were unable to
develop the typical infection symptoms and showed the resistant phenotype
because of their loss cytosine methylation capability. Moreover, several pathogenresponsive genes were differentially regulated in these mutants, signifying the role
of DNA methylation in establishing plant defense against bacterial pathogen. A
gene named elongator complex subunit2 (ELP2) regulated the genomic DNA
methylation pattern in Arabidopsis, and subsequently stimulated the pathogeninduced DNA methylation alterations (Saze et al. 2003). Hypomethylation of DNA
during pathogen infection has been shown to influence the defense-related gene
expression. The rice R gene Xa21G, which was demethylated chemically exhibited
inherited resistance to Xanthomonas oryzae pv. Oryzae. Thus, epigenetic processes
may be a defense mechanism that allows plants to safeguard their offspring against
repetitive biotic stresses without stable inherent trait fixation. An enhanced
15 Plants as Bioreactors for Human Health Nutrients
441