Agriculture Reference
In-Depth Information
gans, meristems and some fruits. Mitochondria play a central role in the metabolism of AsA
in plants. Ascorbic acid reacts with oxidants such as O 2 •− and OH, transfering a single elec‐
tron and forming its own radical ion in the following reaction:
RO + C 6 H 7 O 6 → ROH + C 6 H 6 O 6 •−
The oxidized form of ascorbate - dehydroascorbate (DHA) is relatively unreactive and do
not cause cellular damage. However, it has a short lifetime and needs to be regenerated back
into AsA. Ascorbic acid antioxidative capacity provides a protection to membranes by direct
scavenging ROS and by regenerating α-tocopherol from tocopheroxyl radical. In chloro‐
plasts, AsA acts also as a violaxantin de-epoxidase cofactor, sustaining dissipation of excess
excitation energy [116].
Another powerful antioxidant - glutathione is a tripeptide (γ-glu-cys-gly). In plant tissues it
occurs in a reduced form (GSH) and plays a central role in several physiological processes,
including detoxi fi cation of xenobiotics, signal transduction, conjugation of different metabo‐
lites, differentiation, senescence and cell death regulation [117]. By serving as an electron do‐
nor, GSH is converted into oxidized form - two glutathione molecules linked by a disul fi de
bond (glutathione disulfide, GSSG). Once oxidized, glutathione can be reduced by gluta‐
thione reductases that use NADPH as an electron donor (Figure 4C). The GSH/GSSG ratio is
often used as a measure of cellular redox state. GSH is necessary to maintain reduced state
of cell, counteracting inhibitory effects of ROS. It plays a key role in the antioxidative de‐
fense system by regenerating other antioxidants like AsA via the glutathione-ascorbate cycle
(Figure 4B). GSH is particularly important in chloroplasts since it helps to protect the photo‐
synthetic apparatus from oxidative damage [118].
Proline is considered as another important antioxidant and potential inhibitor of PCD. It has
been well established that it acts as an osmoprotectant and protein-stabilizing agent. How‐
ever, it has been also proven to be the O 2 •− and OH scavenger and inhibitor of LPO [119].
Increased concentration of proline has been correlated with enhanced tolerance to various
abiotic stresses, e.g. transgenic tobacco cells with silenced proline dehydrogenase, accumu‐
lating more proline than wild-type cells, have shown improved osmotolerance [120]. Over-
expression of proline biosynthetic pathway genes has been also found to increase the
drought stress tolerance in transgenic soybean [121].
Out of four tocopherol isomers (α, β, γ, δ) found in plants, α-tocopherol (vitamin E) has the
highest antioxidative activity because of the presence of three methyl groups [122]. α-toco‐
pherol, a lipid soluble antioxidant molecule is considered as a potential scavenger of ROS
and lipid radicals in membranes. It has been shown to prevent the chain propagation step in
the lipid autooxidation reaction [123]. It has been demonstrated that oxidative stress acti‐
vates the expression of tocopherols synthesis pathway genes. Higher tocopherol level has al‐
so been reported during water stress [124].
Another group of plant compounds with antioxidant abilities are lipid soluble carotenoids.
They play various functions in the plant metabolism such as absorption of light at wave‐
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