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mentioned that the treatment of Ficus carica leaves with SA by submerging the
leaves into SA solution did not cause any significant increase in the mRNA level
of peroxidase (Kim et al.
2003
a).
The enzymes involved in the ascorbate-glutathione cycle also play an important
role in scavenging stress-induced ROS. These antioxidant enzymes showed
increased activityin response to several abiotic and biotic stresses (Tuteja
2010
;
Bhattacharjee
2005
). Furthermore, the overexpression of these enzymes improved
tolerance to various stresses (Sharma et al.
2012
). APX, which is part of this cycle, is
thought to play the most important role in scavenging ROS, as it has higher affinity
for H
2
O
2
than catalase. Other components of the cycle are monodehydroascorbate
reductase (MDHAR) and GR. MDHAR plays a role in the transformation of de-
hydro-ascorbate (created when H
2
O
2
is reduced to H
2
O) to ascorbate. GR catalyses
the reduction of GSSG to GSH in a NADPH-dependent reaction, thus maintaining
the GSH pool. Increased APX (Agarwal et al.
2005a
,
b
; Krantev et al.
2008
) and GR
activity (Agarwal et al.
2005a
) was found after SA treatment in several plants.
Although the APX and GR activity declined in pepper after treatment with less than
3 mM SA, application a concentration of 6 or 9 mM led to an increase (Mahdavian
et al.
2007
). Tobacco plants growing in vitro in the presence of 0.01 or 0.1 mM SA
also showed increased glutathione reductase and dehydroascorbate reductase
activity in the shoots, although there was no significant effect on APX. SA at 0.1 mM
also increased the MDHAR activity (Dat et al.
2000
). In another study it was found
that the mRNA level of BcMdhar (encoding a polypeptide showing a high level of
identity to cytosolic MDHAR) increased in response to the oxidative stress invoked
by H
2
O
2
, SA, paraquat or ozone (Yoon et al.
2004
).
GPXs have broad substrate specificities and are able to use H
2
O
2
as a substrate.
The expression of GPX genes showed a strong response to abiotic stress and was
also affected by several plant hormones, including SA (Borsani et al.
2001
; Milla
et al.
2003
). Interestingly, chloroplastic GPX depletion appears to causes com-
pensatory changes in antioxidant levels in order to cope with the stress, as the
accumulation of ascorbate, glutathione and SA could be detected in Arabidopsis
(Chang et al.
2009
).
Glutathione S-transferases (GST) form a large family of non-photosynthetic
enzymes known to function in the detoxification of xenobiotics. Exposure to heavy
metal and salt stress caused a substantial increase in GST activity (Halušková et al.
2009
). The effect of SA is ambiguous in the case of the GST enzyme. The in vitro
activity of the enzyme is inhibited non-competitively by SA (Watahiki et al.
1995
),
which however, stimulates its expression. In rice seedling roots the expression of
the osgstu4 and osgstu3 genes, which are induced by various stresses (PEG, heavy
metal, salt, H
2
O
2
), can also be triggered by plant hormones, such as ABA, JA,
auxin and SA (Moons,
2003
). Similarly, SA caused an increase in the expression
of the Gnt35 gene coding for GST in tobacco cells. Furthermore, the SA-
responsive component, as-1 was identified in the promoter region of some GST
genes, which is activated not only by SA but also by auxin and methyl JA via ROS
(Garretón et al.
2002
). In Arabidopsis, it was found that GSTs exhibit class-
specific responses to SA treatment, suggesting that several mechanisms act to
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