Agriculture Reference
In-Depth Information
might be induced to prevent seed germination under stressful environments (Tilki
and Dirik
2007
).
The expression profiles of several DNA repair genes, namely
MtTdp1a
,
MtTd-
p1b
,
MtTFIIS-like
,
MtTFIIS
,
MtOGG1
and
MtFPG
, were evaluated in barrel medic
seeds imbibed with the osmotic agent polyethylene glycol (Balestrazzi et al.
2011c
).
This high molecular weight compound cannot cross the cell wall, avoiding cell
plasmolysis. When cells are exposed to salt stress, rapid accumulation of ions in the
intracellular environment takes place and membrane permeability is compromised.
Osmotic stress induced by PEG can be reduced as a consequence of osmoprotectant
accumulation (Zhao et al.
2010
).
Germination was significantly reduced by PEG which also induced consistent
7,8-dihydro-8-oxoguanine (8-oxo-dG) accumulation. All the genes were responsive
to osmotic stress during seed imbibition with the osmotic agent and most of them
revealed a delayed and significant up-regulation at 12-24 hours, when the highest
levels of DNA oxidative damage were also observed. As expected, there was a tem-
poral shift in water up-take and occurrence of DNA damage in PEG-treated seeds.
Seed germination can be severely affected by heavy metals. It has been reported
that cadmium uptake by germinating rice seeds resulted in osmotic effects asso-
ciated with oxidative damage. A proteomic approach revealed cadmium-induced
protein profiles with up-regulated protein profiles related to signal transduction
and antioxidant defense (Ahsan et al.
2007
). However, no reports dealing with the
role played by DNA repair genes in seeds exposed to heavy metals are currently
available.
8 Role of Telomerase in DNA Repair and Genotoxic
Protection
Telomerase, the enzyme required for telomere synthesis and maintenance, is a criti-
cal element for the prevention of aging in animal cell. Telomerase is a ribonucleo-
protein composed of a catalytic subunit owing reverse transcription activity (TERT)
associated with an RNA subunit (telomerase RNA component, TR). The latter acts
as a template for the addition of telomere repeat sequences carried out by TERT
(Tomas-Loba et al.
2008
).
There is recent evidence to show that telomerase also owns some telomere-in-
dependent activities, among which are DNA repair and protection of mitochondrial
DNA (mtDNA) against oxidative stress (Majerska et al.
2011
). In human fibroblasts,
increased oxidative stress accelerates telomere shortening and this is caused by the
reversible, stress-induced translocation of telomerase from the nucleus to mitochon-
dria (Haendeler et al.
2009
). It has been demonstrated that the human telomerase
(hTERT) is localized within the mitochondrial matrix where it binds and protects
mtDNA. It seems that both hTERT and hTR participate in the modulation of DNA
repair and, consequently, in the cell response to genotoxic stress by stimulating
the ATM and Rad3 (ATR)-related sensor kinase (Kedde et al.
2006
). Additional
