Agriculture Reference
In-Depth Information
to avoid oxidative injury. An intriguing aspect of ROS scavenging in plants chal-
lenged with heavy metals is related to the small metal-binding cysteine-rich proteins
metallothioneins (MTs; Cobbet and Goldsbrough
2002
). Besides their role in heavy
metal detoxification, MTs have recently been found to act as ROS scavengers and
signal molecules outside and inside the nucleus, highlighting their possible interac-
tion with the DNA repair machinery (Wang et al.
2010
; Balestrazzi et al.
2011b
).
Till date, there is only indirect evidence of the putative protective role played by
MTs in the nucleus. Balestrazzi et al. (
2009
) demonstrated that expression of the
PsMT
A1
gene, encoding a metallothionein-like protein from
P. sativum
, confers
protection against oxidative stress in the nucleus, reducing the level of oxidative
DNA damage. On the other hand, studies on animal cells have reported that MTs
are transported into the nucleus where they make zinc available to transcription
factors (Cherian and Apostolova
2000
). MTs can help overcome defective DNA
repair functions as suggested by Yeong et al. (
2004
) who demonstrated that the
overexpression of the
MTIII
gene, encoding a type 3 metallothionein, prevents ac-
cumulation of the oxidized base 8-oxoguanine in normal and OGG1(8-oxoguanine
DNA glycosylase/lyase)-depleted cells exposed to g-rays.
As for the direct involvement of DNA repair genes in the plant response to
heavy metals, evidence has been recently reported in
M. truncatula
by Macovei
et al. (
2010
,
2011
). The
NER
and
BER
genes up-regulated in aerial parts and roots
of barrel medic plantlets exposed to toxic copper doses, will be described in detail
in the following paragraph.
5 Nucleotide Excision Repair Genes Responsive
to Abiotic Stresses
Nucleotide excision repair is a versatile DNA repair system involved in the remov-
al of different helix-distorting DNA lesions (Liu et al.
2010
) which includes two
distinct subpathways, the global genome (GG)-NER which targets lesions at the
genome level, comprising the majority of NER activities, and the transcription-cou-
pled (TC)-NER which acts more rapidly and removes lesions from the transcribed
strand of active genes (Hanawalt
2002
).
The NER pathway is regulated at the transcriptional level by different mecha-
nisms, including circadian regulation (Kang et al.
2009
). At the post-translational
level, mono- or poly-ubiquitination or SUMOylation of NER components play a
key role in the cellular choice between DNA repair and error bypass (Ulrich
2009
).
Some genes belonging to TC-NER have recently demonstrated to be signifi-
cantly up-regulated in response to osmotic and heavy metal stresses (Macovei et al.
2010
,
2011
; Balestrazzi et al.
2011c
). The application of DNA repair enzymes to
sites of stalled transcription complexes and the related mechanisms of damage re-
moval is a relevant part of the cellular responses to genotoxic stress.
Transcription Elongation Factor SII (TFIIS), which interacts with RNA poly-
merase II (RNAPII) allowing the efficient synthesis of long transcripts, participates
