Agriculture Reference
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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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