Agriculture Reference
In-Depth Information
ven to enhance the plant resistance, e.g. tobacco plants with higher chloroplast APX expres‐
sion are more tolerant to the salt stress and water de
fi
cit [101].
Glutathione reductases are oxidoreductases participating in the glutathione-ascorbate cycle
(Figure 4B). They play an essential role in the defense against ROS by sustaining reduced sta‐
tus of glutathione (GSH), a tripeptide molecule involved in many regulatory and antioxida‐
tive processes in plants. They are localized predominantly in chloroplasts, but small amounts
have been also found in mitochondria and cytosol [105]. GRs catalyze the NADPH-dependent
reduction of the oxidized form of glutathione (GSSG) (Figure 4B and C) thus are important for
maintaining the GSH pool. Increased GR activity has been demonstrated in various abiotic
stress studies, e.g. in drought stressed rice seedlings [106]. Transgenic plants with lower GR ac‐
tivity have shown enhanced sensitivity to oxidative stress while these with higher GR have
been proved to be abiotic stress tolerant. Elevated chloroplastic GR activity has been demon‐
strated to decrease chilling-induced photoinhibition in transgenic cotton [107].
Monodehydroascorbate reductase is an enzymatic component of the glutathione-ascorbate cy‐
cle (Figure 4B). MDHARs are present in chloroplasts, mitochondria, peroxisomes and cytosol,
where they participate in H
2
O
2
scavanging [108]. They exhibit high specificity for monodehy‐
droasorbate as the electron acceptor and use NADH as the electron donor (Figure 4B):
NADH + C
6
H
7
O
6
→ NAD
+
+ C
6
H
8
O
6
Overexpression of MDHAR in the transgenic tobacco has been demonstrated to increase the
tolerance against ozone, salt and osmotic stress [109].
Dehydroascorbate reductases function in the regeneration of ascorbic acid from the oxidized
form (Figure 4B) and therefore regulate cellular AsA redox state. DHAR overexpression has
been demonstrated to enhance salt tolerance in
Arabidopsis
[110] as well as drought and
ozone stress resistance in tobacco [111].
Glutathione S-transferases are a large and diverse group of enzymes with 54 members re‐
ported in
Arabidopsis
[112]. They catalyze the conjugation of electrophilic substrates with
glutathione. Plant GSTs are known to participate in herbicides detoxi
fi
cation, hormone ho‐
meostasis maintenance, sequestration of anthocyanin and regulation of PCD in response to
biotic and abiotic stimuli. They are mostly located in the cytoplasm but chloroplastic, nucle‐
ar and apoplastic isoforms have also been reported [113].
Glutathione peroxidases are another group of isoenzymes that use GSH to reduce H
2
O
2
, or‐
ganic and lipid hydroperoxides (Figure 4C). A family of seven related proteins (AtGPX1-
AtGPX7) residing in cytosol, chloroplasts, mitochondria and endoplasmic reticulum has
been identified in
Arabidopsis
[114]. Overexpression of GPX in transgenic tobacco has been
demonstrated to confer tolerance towards chilling and salt stress [115].
Apart from enzymes participating in redox homeostasis maintenance, plants possess a vast
number of non-enzymatic compounds acting as antioxidants. Ascorbic acid (vitamin C) is
present in all plant tissues but especially high levels occur in photosynthetically active or‐
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