Agriculture Reference
In-Depth Information
tive with
1
O
2
and
•
OH. Activated oxygen radical can abstract the hydrogen atom from cys‐
teine residue to form a thiyl radical that cross-links to a second thiyl radical and leads to the
formation of disulphide bridges. Oxygen can also be added onto the methionine residue to
form a methionine sulphoxide. The best characterized response to the oxidation of peptide
residues is the induction of proteases that break down the oxidized proteins [93].
DNA damage, triggered by ROS is particularly dangerous for the cell since it causes replica‐
tion errors and genomic instability. From all ROS,
•
OH has the most damaging effect to
DNA as it can modify all components of the nucleic acid molecule: purines, pyrimidines and
the deoxyribose backbone [94]. The major types of DNA damage resulted from oxidative
stress are the formation of dimers between adjacent pyrimidines, cross-links, base deletion,
strand breaks and base modifications such as alkylation and oxidation. To counteract the
DNA damage, plant cells evolved mechanisms for the DNA repair in both nucleus and mi‐
tochondria. These include the direct inversion of modifications or the replacement of the
whole nucleotide [95].
To protect themselves against toxic oxygen intermediates, plant cells possess a vast antioxi‐
dant system. Stress-induced ROS accumulation is counteracted by both enzymatic and non-
enzymatic antioxidants. Enzymatic ROS scavengers include superoxide dismutases (SOD),
catalases (CAT), ascorbate peroxidases (APX), glutathione reductases (GR), monodehy‐
droascorbate reductases (MDHAR), dehydroascorbate reductases (DHAR), glutathione per‐
oxidases (GPX) and glutathione-S- transferases (GST). Low-molecular, non-enzymatic
antioxidants include ascorbic acid (AsA), glutathione (GSH), proline, α-tocopherol, carote‐
noids and
fl
avonoids [73].
Metalloenzyme SOD is the most effective enzymatic antioxidant which is ubiquitous in all
subcellular compartments. SODs remove O
2
•−
by catalyzing its dismutation (Figure 4A):
O
2
•−
+ O
2
•−
+ 2H
+
→ 2H
2
O
2
+ O
2
This reaction eliminates O
2
•−
and hence decreases the risk of
•
OH formation. SODs are classi‐
fied into three types, depending on their metal cofactor: copper/zinc (Cu/Zn-SOD), manga‐
nese (Mn-SOD) and iron (Fe-SOD). Different types of SODs are located in different cellular
compartments [96].
Arabidopsis thaliana
genome encodes three Fe-SOD (FSD1, FSD2 and
FSD3), three Cu/Zn-SOD (CSD1, CSD2 and CSD3) and one Mn-SOD (MSD1) [97]. Mn-SOD
has been found in mitochondria and peroxisomes of plant cells [98]. Cu/Zn-SOD isoenzymes
have been found in the cytosol and in chloroplasts of higher plants. Fe-SODs are usually as‐
sociated with chloroplasts [99]. The upregulation of SODs during biotic or abiotic stress-trig‐
gered oxidative stress has a critical role in the overcoming of adverse conditions and in the
plant survival. Many reports indicate that the overexpression of different SODs leads to the
generation of abiotic stress-tolerant plants, e.g. Mn-SOD overexpressing
Arabidopsis
has
shown increased salt tolerance [100] and Cu/Zn-SOD overexpressing transgenic tobacco has
demonstrated multiple stress tolerance [101]. Interestingly, FSD2 and FSD3 play also an es‐
sential role in the chloroplast development, protecting chloroplast nucleoids from ROS
[102].
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