Agriculture Reference
In-Depth Information
reductaseregionoftheETCalsoproduces•O
2
-
from
oxygen. Fully reduced ubiquinone donates an electron
to cytochrome
c
1
resulting in the formation of an
ubisemiquinone radical, which is highly unstable and
favours electron leakage to O
2
and thus finally results in
theformationof•O
2
-
(Murphy, 2009). The ETC and ATP
synthases are strongly coupled in aerobic conditions but
various stress factors lead to inhibition and modification
of the various components, leading to over-reduction of
electron carriers and, hence, increased production of
ROS (Noctor
et al.,
2007; Blokhina & Fagerstedt, 2010).
Some of the enzymes like aconitase produce ROS
directly whereas some others like 1-galactono-γ-lactone
dehydrogenase (GAL) furnish electrons to ETC
(Andreyev
et al.,
2005; Rasmusson
et al.,
2008). Being
the primary ROS formed by monovalent reduction in
theETC,•O
2
-
gets quickly converted to H
2
O
2
either by
the MnSOD (mitochondrial form of SOD) or by APX.
H
2
O
2
is a relatively stable and membrane-permeable
ROS; it can be further converted to the extremely active
hydroxylradical(OH•)viatheFentonreaction.
normal metabolism. Biochemical and electron spin
resonance spectroscopy (ESR) methods have identified
twositesof•O
2
-
production in peroxisomes of pea
leaves and watermelon cotyledons: in the organelle
matrix, where xanthine oxidase (XOD) catalyses the
oxidation of either xanthine or hypoxanthine to uric
acid, and in the peroxisomal membranes, where and
ETC made up of a flavoprotein NADH and Cyt
b
is
involvedintheproductionof•O
2
-
. Integral membrane
polypeptides (PMPs) of peroxisomes having molecular
masses of 18, 29 and 32 kDa were found to be involved
in•O
2
-
generation. The 18 kDa and 32 kDa PMPs use
NADHaselectrondonorfor•O
2
-
generation while the
29 kDa PMP is capable of reducing cytochrome
c
with
thehelpofNADPHaselectrondonorfor•O
2
-
genera-
tion (López-Huertas
et al.,
1999). Of the three integral
polypeptide membranes of peroxisomes, the 18 kDa
PMPwasthemainproducerof•O
2
-
.The•O
2
-
generated
by all three PMPs of peroxisomes is quickly converted
into H
2
O
2
via SOD.
7.4.2.5 Plasma membranes
Plasma membranes loaded with electron-transporting
oxidoreductases also produce ROS. EPR spin-trapping
techniques and specific dyes illustrate the production of
ROS from isolated plasma membranes, especially ones
from the growing and non-growing zones of hypo-
cotyls, roots of etiolated soybean seedlings, and
coleoptiles and roots of etiolated maize seedlings (Heyno
et al.,
2011).Theproductionof•O
2
-
is mediated by
NAD(P)H via NADPH oxidase and quinone reductase in
soybean plasma membranes (Heyno
et al.,
2011).
NADPH oxidase is responsible for the transfer of elec-
trons from cytoplasmic NADPH to O
2
resulting in
•O
2
-
generation.The•O
2
-
formed is rapidly converted to
H
2
O
2
either spontaneously or enzymatically via SOD
activity. NADPH oxidase has been found to play an
important role in the production and accumulation of
ROS in plants thriving under stress conditions (Apel &
Hirt, 2004; Torres
et al.,
2002; Kwak
et al.,
2003).
7.4.2.3 Endoplasmic reticulum
Cytochrome P450 is involved in NAD(P)H-dependent
electrontransportandproduces•O
2
-
in endoplasmic
reticulum (Mittler, 2002). Organic substrate, RH, is first
reduced by a flavoproteins after reacting with Cyt P450
to form a radical intermediate (Cyt P450 R
-
). This radical
intermediate can readily react with triplet oxygen as
each has one unpaired electron. This oxygenated com-
plex (Cyt P450-ROO
-
) may either be reduced by
cytochrome
b
or occasionally may become dissociated
resultingintheproductionof•O
2
-
.
7.4.2.4 Peroxisomes
Peroxisomes, as a result of their essentially oxidative
type of metabolism, are probably the major sites of
intracellular H
2
O
2
production (Del Río
et al.,
2006). The
glycolate oxidase reaction, β-oxidation of fatty acids, the
enzymatic reactions of flavin oxidases, and the dispro-
portionationof•O
2
-
radicals are the main metabolic
processes occurring in different types of peroxisomes
and are responsible for the production of H
2
O
2
.
Photorespiration occurring in peroxisomes involves the
oxidation of glycolate by glycolate oxidase resulting in
the production of the majority of H
2
O
2
(Noctor
et al.,
2002). In peroxisomes, like mitochondria and chloro-
plasts,•O
2
-
is also produced as a consequence of their
7.4.2.6 Cell walls
Cell walls also play an active role in the production of
ROS. Peroxidases associated with the cell walls are
involved in the production of H
2
O
2
. Peroxidases associated
with isolated cell walls of horseradish catalyse the produc-
tion of H
2
O
2
in the presence of NADH, and this reaction
is enhanced by various monophenols, particularly
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