Agriculture Reference
In-Depth Information
nontelomeric functions such as transcriptional regulation, have been proposed for
hTERT whose ectopic expression causes remarkable alterations in gene activity and
the recent study by Park et al. (
2009
) has demonstrated that hTERT is a component
of the transcription activator complex of the Wnt/APC/β-catenin pathway.
As for plants, experimental evidence concerning the telomere-independent ac-
tivities of telomerase is missing. However, it has been reported that tobacco BY-2
cells, exposed to cadmium sulfate and showing chromosome fragmentation, are
able to overcome the heavy metal treatment and complete recovery was concomi-
tant with a 2.5-fold increase in telomerase activity, thus suggesting the involvement
of this enzyme in DNA repair (Fojtova et al.
2002
). In a different work, Riha et al.
(
2002
) investigated DSB repair in
Arabidopsis
plants defective in Ku70, a critical
component of the Non Homologous End Joining (NHEJ) pathway localized at the
telomere cap, are hypersensitive to gamma irradiation. Differently from animals, in
the
Arabidopsis
Ku70 mutant the control of telomere length is altered to increase
significantly by the second generation.
Restoration of telomeres at chromosome ends occurs early during seed imbibi-
tion, when DNA repair is required to ensure genome integrity and it has been sug-
gested that a level of telomerase might be essential for the control of seed viability
during long-term storage (Watson and Riha
2010
). Till date, only a few reports are
available dealing with the possible role of telomerase in seed aging and for this
reason additional studies are required.
9 Modulation of DNA Repair in Response to Genotoxic
Stress: Role of small Regulatory RNAs
Recent studies have highlighted the correlation between the ability to withstand en-
vironmental stresses and the complex mechanisms of RNA silencing and epigenetic
activities occurring in plants (Riuz-Ferrer and Voinnet
2009
). Gene expression can
be modulated by small regulatory RNAs, a broad class of molecules with regulatory
functions which include microRNAs (miRNAs), small interfering RNAs (siRNAs),
and transactivating siRNAs (ta-siRNAs; Mallory and Bouche
2008
).
The miRNAs represent a highly conserved 18-22 bp RNA family that bind to
mRNA, blocking transcription or promoting mRNA degradation, a unique mecha-
nism for the regulation of gene expression. As for the possible involvement of miR-
NAs in the response to genotoxic stress, most information is currently available
from animal systems. The study from Simone et al. (
2009
) carried out in human
fibroblasts suggests that miRNA expression is a common feature of the response
to genotoxic agents and the authors hypothesize that DNA damage might modulate
miRNA expression by either inducing the transcription of miRNA genes or directly
interacting with the processing and maturation machinery of miRNAs.
Yao et al. (
2010
) showed that
Arabidopsis
mutants impaired in siRNA biogen-
esis are more sensitive to genotoxic stress. siRNA biogenesis depends on various
enzymes, among which are Dicer-like proteins (DCL2, DCL3, and DCL4), ribo-
nucleases that process double-stranded RNA into small RNAs. DCL2 and DCL3
