functions including transcription, replication, DNA repair, gene transposition, and
cell differentiation. Also, modifications in chromatin and generation of small RNAs
have been shown to be involved in transcriptional and posttranscriptional control of
gene expression, critical for stress responses. The expression of genes can also be
altered by transgenerational or heritable modifications in chromatin. DNA methylation is controlled by hormonal fluxes, which are in turn influenced by various
abiotic and biotic factors (Zhang et al. 2012) resulting in plant adaptation. Thus,
deciphering how the methylation and functions of epigenetic machinery will be
based. It will also provide a valuable platform for potential applications including
genetic manipulation of plants toward enhanced tolerance to environmental stresses. The mechanism of establishment of DNA methylation is the covalent transfer
of methyl group from S-adenosyl methionine to the 50 position of cytosine, thus
result in converting cytosine into 5-methylcytosine (5 mC). Plants have high levels
of 5 mC, ranging from 6 to 25 % of total cytosine, depending on the species. This
epigenetic memory is accumulated by plants during their vegetative phase under the
influence of the environment; it is passed on to the next generation by germline
cells, which are later established during development. DNA is methylated at promoter as well as gene body regions, thus allowing the gene to be in a repressed state
(Saze et al. 2003). Thus, a decline in the level of methylation is likely to lead to an
increase in gene expression. Further, enzymes participating in cytosine methylation
are grouped into three distinct categories: methyltransferase1 (MET1), chromomethylase3 (CMT3), and domains rearranged methylase (DRM). Transgenerational
epigenetic inheritance (TEI) is referred to the inheritance of expression states and
the traits not determined by the DNA sequence, the molecular mechanisms
involved in the process are only rarely verified. This especially applies to the
heritability of environmentally induced traits, which have gained interest over the
last years. The decision about which epigenetic marks are reset and which ones are
not crucial for the understanding of TEI. We will consider examples of epialleles
induced by genetic and/or environmental factors used in experimental setups. For
example, the progenies of stress-treated plants showed increased global hypermethylation even in the absence of stress, but these transgenerational effects did not
persist in successive generations during the absence of stress. In Arabidopsis, the
level of cytosine methylated DNA was measured in progenies of treated and
untreated plants for two generations (Boyko et al. 2010). Higher 5mC levels were
maintained in the progenies of treated plants in response to stressed as well as
unstressed conditions, relative to the progenies of untreated plants of the same
generation, suggesting that DNA methylation decreases during the absence of
stress. Viral infection in tobacco and exposure of UV-C and flagella in Arabidopsis
have shown to stimulate the transgenerational inheritance of stress tolerance even to
the untreated progeny via increased homologous recombination frequency and
global genome methylation (Boyko et al. 2010).
440
L. A. Cruz-Ramírez et al.
cell differentiation. Also, modifications in chromatin and generation of small RNAs
have been shown to be involved in transcriptional and posttranscriptional control of
gene expression, critical for stress responses. The expression of genes can also be
altered by transgenerational or heritable modifications in chromatin. DNA methylation is controlled by hormonal fluxes, which are in turn influenced by various
abiotic and biotic factors (Zhang et al. 2012) resulting in plant adaptation. Thus,
deciphering how the methylation and functions of epigenetic machinery will be
based. It will also provide a valuable platform for potential applications including
genetic manipulation of plants toward enhanced tolerance to environmental stresses. The mechanism of establishment of DNA methylation is the covalent transfer
of methyl group from S-adenosyl methionine to the 50 position of cytosine, thus
result in converting cytosine into 5-methylcytosine (5 mC). Plants have high levels
of 5 mC, ranging from 6 to 25 % of total cytosine, depending on the species. This
epigenetic memory is accumulated by plants during their vegetative phase under the
influence of the environment; it is passed on to the next generation by germline
cells, which are later established during development. DNA is methylated at promoter as well as gene body regions, thus allowing the gene to be in a repressed state
(Saze et al. 2003). Thus, a decline in the level of methylation is likely to lead to an
increase in gene expression. Further, enzymes participating in cytosine methylation
are grouped into three distinct categories: methyltransferase1 (MET1), chromomethylase3 (CMT3), and domains rearranged methylase (DRM). Transgenerational
epigenetic inheritance (TEI) is referred to the inheritance of expression states and
the traits not determined by the DNA sequence, the molecular mechanisms
involved in the process are only rarely verified. This especially applies to the
heritability of environmentally induced traits, which have gained interest over the
last years. The decision about which epigenetic marks are reset and which ones are
not crucial for the understanding of TEI. We will consider examples of epialleles
induced by genetic and/or environmental factors used in experimental setups. For
example, the progenies of stress-treated plants showed increased global hypermethylation even in the absence of stress, but these transgenerational effects did not
persist in successive generations during the absence of stress. In Arabidopsis, the
level of cytosine methylated DNA was measured in progenies of treated and
untreated plants for two generations (Boyko et al. 2010). Higher 5mC levels were
maintained in the progenies of treated plants in response to stressed as well as
unstressed conditions, relative to the progenies of untreated plants of the same
generation, suggesting that DNA methylation decreases during the absence of
stress. Viral infection in tobacco and exposure of UV-C and flagella in Arabidopsis
have shown to stimulate the transgenerational inheritance of stress tolerance even to
the untreated progeny via increased homologous recombination frequency and
global genome methylation (Boyko et al. 2010).
440
L. A. Cruz-Ramírez et al.