Agriculture Reference
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mitochondria (LaIoi et al.
2004
). Impairment of the components in this chain leads
to the accumulation of ROS in mitochondria. Given that the mitochondrial elec-
tron transport chain also contains an alternative oxidase that can be activated under
stress to remove toxic ROS (Maxwell et al.
1999
), ROS produced from mitochon-
dria are present at only low concentrations in green tissues (Apel and Hirt
2004
).
ABO6
encodes a DEXH box RNA helicase that regulates the splicing of several
genes that encode components of complex I in mitochondria. Compared with
wild-type (WT) plants, the
abo6
mutant accumulated more ROS in mitochondria,
but two dominant-negative mutations in the
ABA
-
insensitive1
(
abi1
-
1
) and
abi2
-
1
mutants greatly reduced ROS production in mitochondria. These findings provide
molecular evidence for the interplay between ABA and auxin through the produc-
tion of ROS from mitochondria (He et al.
2012
).
Besides chloroplasts and mitochondria, ROS are also generated extracellu-
larly in apoplasts in plants. pH-dependent cell wall peroxidases, germin-like oxa-
late oxidases and amine oxidases have been proposed as sources of H
2
O
2
in the
apoplasts of plant cells (Bolwell and Woftastek
1997
). Alkaline pH activates pH-
dependent cell wall peroxidases, which produce H
2
O
2
in the presence of reduct-
ant. Apoplast alkalisation upon elicitor recognition precedes the oxidative burst,
and H
2
O
2
production by a pH-dependent cell wall peroxidase has been proposed
as an alternative pathway of ROS production during biotic stress (Bolwell and
Woftastek
1997
).
ROS are also generated at the plasma membrane. ROS derived from NADPH-
dependent oxidases (NADPH oxidases) in the plasma membrane participate in
the oxidative burst associated with localised nonspecific pathogen responses,
incompatible plant-pathogen interaction and hormone signalling in plants. ROS
generated by NADPH oxidase RbohC, RbohD and RbohF function in plant
development, defence and ABA signalling (Foreman et al.
2003
; Monshausen
et al.
2007
). ABA induces the generation of H
2
O
2
in
Vicia
guard cells, and
the sources of H
2
O
2
are chloroplastic and plasma membrane NADPH oxidase
(Zhang et al.
2001a
,
b
). Another study suggested that the accumulation of ROS
occurred upon ABA-induced stomatal closure in
Arabidopsis
(Pei et al.
2000
).
Two NADPH oxidase subunit genes,
AtrbohD
and
AtrbohF
, function in ABA-
mediated stomatal closure, and the
atrbohD/F
double mutant shows reduced sto-
matal closure and ROS production compared with wild-type plants upon ABA
treatment (Kwak et al.
2003
). The ABA-activated SnRK2 protein kinase open
stomata 1 (OST1) acts upstream of ROS in guard cell ABA signalling. OST1
interacts with AtrbohF and can phosphorylate its Ser13 and Ser174 to activate
NADPH oxidase (Sirichandra et al.
2009
), which then causes the production of
H
2
O
2
(Santiago et al.
2009
; Kepka et al.
2011
). It was established that ABA-
insensitive mutants play different function in ROS production in ABA-regulated
stomatal movement. The observation that
abi2
, but not
abi1
, mutants can gener-
ate ROS, suggests that the
abi2
-
1
mutation impairs ABA signalling downstream
of ROS production (Murata et al.
2001
).
RbohD
is also required for ROS-
dependent signal propagation in response to abiotic stresses, such as HL, heat
and wounding (Miller et al.
2009
).
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