Agriculture Reference
In-Depth Information
various metabolic pathways that are localised in different cellular compartments,
such as chloroplasts, mitochondria and peroxisomes (Del Río et al.
2006
; Navrot
et al.
2007
).
10.2.1 ROS Generation by the Photosynthetic Apparatus
Under steady-state conditions, oxygen generated in the chloroplasts during pho-
tosynthesis can accept electrons passing through the photosystems, thus forming
O
2
−
. The accumulation of ROS is a common response to environmental stresses.
The chloroplasts and peroxisomes are the major sites of ROS production upon
exposure to high light (HL) (Foyer and Noctor
2009
). In chloroplasts, O
2
−
and
−
1
O
2
are mainly produced by photosystems I and II (PSI and PSII). O
2
arises as
a result of single electron transfer to molecular oxygen in electron transfer chains,
principally during the Mehler reactions in PSI (Asada
2006
). O
2
−
can be dismu-
−
and H
2
O
2
, OH
is much reactive,
tated to form H
2
O
2
. Compared with both O
2
−
which can be formed from O
2
and H
2
O
2
in the presence of Fe, through catalysis
by the Haber-Weiss reaction (Bhattacharjee
2010
).
1
O
2
, an electronically excited
species of O
2
that is also very toxic, is continuously produced during photosyn-
thesis mainly in PSII. The enhanced generation of
1
O
2
is caused by photoin-
hibition of PSII under excess photochemical stress or light energy (Hideg et al.
2002
). Therefore, excess photochemical energy inevitably increases ROS produc-
tion. The chloroplast may be function as a sensor of environmental information;
its redox signalling allows the plant to acclimatize to the environmental stresses.
(Pfannschmidt
2003
).
Most (81 %) of 28 HL-responsive genes analysed in one study, including the
ASCORBATE PEROXIDASE 2
(
APX2
), require photosynthetic electron transport
for their expression and are responsive to ABA (68 %) (Bechtold et al.
2008
). This
suggests that both ABA and ROS are crucial in the expression of HL-responsive
genes. Furthermore, the analysis of mutants with altered ROS metabolism indi-
cated that the expression of 61 % of these genes, including
APX2
, might be
responsive to chloroplast-derived ROS (Bechtold et al.
2008
).
10.2.2 ROS Generated from Non-photosynthetic Systems
Besides chloroplasts, mitochondria are the other important organelles that are
sites for ROS production; they probably constitute the main source of ROS in
plants under dark conditions. The mitochondrial electron transport system can
produce O
2
, H
2
O
2
and OH
. Experiments on isolated mitochondria showed that
about 1-5 % of the O
2
used in oxidative respiration leads to H
2
O
2
production
(Møller
2001
). In the mitochondrial electron transport chain, complexes I and III
are major sites for ROS production under dark conditions and in tissues that lack
−
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