Agriculture Reference
In-Depth Information
Another source of ROS - peroxisomes are small, spherical organelles with an oxidative type
of metabolism. There are two sites of O
2
•−
generation in peroxisomes: in the organelle ma‐
trix, where xanthine oxidase (XOD) catalyzes the oxidation of xanthine and hypoxanthine to
uric acid and in the membrane, by components of peroxisomal ETC. The main metabolic
processes responsible for H
2
O
2
generation in peroxisomes are photorespiratory glycolate ox‐
idase reaction, fatty acid β-oxidation, enzymatic reaction of
fl
avin oxidases and dispropor‐
tionation of superoxide radicals [87].
ROS are also generated in the apoplast by NADPH oxidases residing in the plasma mem‐
brane and generating superoxide radicals. The extracellular O
2
•−
is quickly mutated into
H
2
O
2
or converted to
•
OH. The latter initiates a series of reactions that cause a plasma mem‐
brane damage, finally leading to cell death. Two
Arabidopsis
respiratory burst oxidase genes,
RBOHD
and
RBOHF
, that encode NADPH oxidases have been proven to be responsible for
ROS production during the HR. Enzymes such as cell wall peroxidases, germin-like oxalate
oxidases and amine oxidases have been proposed as a source of hydrogen peroxide in the
apoplast. The alkalization of apoplast upon elicitor recognition precedes the production of
H
2
O
2
by pH-dependent cell wall peroxidases [88].
The peroxidation of lipids is considered as one of the most damaging processes occurring in
the cell. The damage of membrane is often considered as a parameter determining the level
of cell destruction under various stresses. Upon ROS overproduction, polyunsaturated pre‐
cursors undergo lipid peroxidation, forming small hydrocarbon fragments such as ketones
or aldehydes. LPO in both cellular and organellar membranes affects proper cellular func‐
tions and aggravates oxidative stress by the production of lipid-derived radicals [89]. This
process often affects PUFA, since they contain multiple double bonds in between which lie
methylene (-CH2-) groups with reactive hydrogens. Hydroxyl or perhydroxyl radicals com‐
bining with a hydrogen atom produce water and a fatty acid radical. The fatty acid radicals
are unstable and react rapidly with molecular oxygen, creating a peroxyl-fatty acid radical
(ROO
•
). Once initiated, ROO
•
can further propagate the peroxidation chain reaction by ab‐
stracting a hydrogen atom from PUFA side chains. The resulting lipid hydroperoxide easily
decomposes into several reactive species including: lipid alkoxyl radicals, MDA, alkanes
and lipid epoxides. Thus, LPO generates multiple peroxide molecules and results in the
membrane
fl
uidity decrease, its leakiness to substances that do not normally cross it, the
damage of membrane proteins and ion channels. It has been found that such PUFAs as lino‐
leic and linolenic acids are particularly susceptible to ROS attack [90]. Increased level of LPO
has been demonstrated in many abiotic stress studies, for instance under salt stress in
Oryza
sativa
[91].
Apart from lipid peroxidation, the accumulation of ROS leads to protein oxidation. Only
few types of these covalent modifications are reversible, most of them are irreversible [92]. A
widely used marker for protein oxidation is their carbonylation level. The oxidation of ami‐
no acids such as arginine, histidine, lysine, proline, threonine and tryptophan causes the for‐
mation of free carbonyl groups, that may inhibit or alter the protein activity and increase the
susceptibility towards proteolytic attack [90]. Proteins with sulfur-containing amino acids
and thiol groups are often the target for ROS. Cysteine and methionine are especially reac‐
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