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and soybean (Simaei et al.
2011
). Contradictory results for the same species
suggested that the biochemical effect of SA depends not only on the species, but on
several other factors, such as the mode of application, the concentration, the
environmental conditions, etc. Different SA concentrations had different effects on
the enzyme activity in pepper (Capsicum annuum L.). Concentrations of 0.7, 1.5
and 3 mM SA decreased catalase activity, but concentrations of 6 and 9 mM
increased it (Mahdavian et al.
2007
). These contradictory results can be explained
not so much by differences in the SA concentrations applied, but by differences in
the binding of SA to various catalase isoenzymes in different plant species. In
tobacco all the catalase isoenzymes are inhibited by SA (Durner and Klessig,
1996
), while in maize and rice differences were found between the catalase iso-
enzymes in their sensitivity to SA. A substantial level of non-competitive inhi-
bition was caused by 2 mM SA in the activity of the CAT1 isoenzyme of maize,
while in the case of CAT2 the inhibition was competitive and weak (Horváth et al.
2002
). In rice SA inhibited the activity of the CATb isoenzyme, but not that of
CATa (Chen et al.
1997
). The CAT1 isoenzyme of maize and the CATb isoen-
zyme of rice, both of which are sensitive to SA, exhibited considerable sequence
homology with tobacco catalase, which is also inhibited to a great extent by SA.
The tissue-specific expression of various catalase isoenzymes may lead to differ-
ences in the effect of SA on the given tissue if catalase does indeed play a role in
transmitting the effect of SA.
There is evidence that not only may SA cause a rise in the quantity of ROS in
the cell, but ROS may also lead to the accumulation of SA (León et al.
1995
,
Enyedi
1999
). This observation suggested the existence of a self-induced SA-H
2
O
2
cycle, resulting in the accumulation of ROS and the death of the cell (Van Camp
et al.
1998
). As H
2
O
2
treatment alone did not cause such a great extent of oxidative
damage, and as dimethyl-thiourea treatment reduced the damaging effect of SA
treatment (Rao et al.
1997
; Luo et al.
2001
) by reducing the H
2
O
2
level, it could be
assumed that the effect of SA is only mediated in part by H
2
O
2
. In fact, it is
thought that SA might act as an electron donor, diverting catalase to the slower
peroxidative pathway. At low levels of H
2
O
2
this is manifested as inhibition, while
at damaging levels of H
2
O
2
it protects the enzyme (Durner and Klessig
1996
).
During SA binding to catalase, SA itself was also converted into a free radical
leading to lipid peroxidation. Both the higher H
2
O
2
level caused by catalase
inhibition and the lipid peroxidation arising in the course of inhibition are thought
to be involved in the signal transduction process leading to SA-dependent resis-
tance (Anderson et al.
1998
). SA also influences the activity of other antioxidant
enzymes. In some cases SA stimulates the activity of the Cu- and Zn-SOD
enzymes, which again may contribute to a rise in the H
2
O
2
level (Krantev et al.
2008
; Sahu and Sabat
2011
).
Besides CAT, the H
2
O
2
level is also regulated by G-POD, which was found to
exhibit increased activity after SA application in various plant species (Ananieve
et al.
2004
; Tas gín et al.
2006
; Mahdavian et al.
2007
; Ahmad et al.
2012
).
Although the total activity did not increase substantially in maize treated with SA,
a new peroxidase isoform was detected (Janda et al.
1999
). It should also be
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