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production and oscillations in [Ca
2
+
]
cyt
during ABA-induced stomatal closure
suggest that ABA-induced ROS accumulation—not the constitutive accumulation
of ROS—causes ABA-controlled stomatal closure (Jannat et al.
2011a
,
b
). This is
not to say that constitutively accumulated ROS have no function; recently, a study
on a mutant lacking
APX1
(
KO
-
APX1
) showed high sensitivity to wounding or
MeJA treatment (Maruta et al.
2012
). Previously, we discussed that AtGPX3, the
redox state of which is regulated by H
2
O
2
, interacts strongly with ABI2 to control
stomatal aperture in an ABA-dependent manner (Miao et al.
2006
). Exogenously
applied ABA and 12-oxo-phytodienoic acid (12-OPDA, a precursor of jasmonic
acid), individually or combined, which promote stomatal closure of ABA and
allene oxide synthase biosynthetic mutants, albeit most effectively when com-
bined. The potency of this combination in inducing stomatal closure was veri-
fied using tomato (
Solanum lycopersicum
),
Brassica napus
and
Arabidopsis
.
These data have identified drought as a stress signal that uncouples the conver-
sion of 12-OPDA to JA and have revealed 12-OPDA as a drought-responsive reg-
ulator of stomatal closure, which functions most effectively together with ABA
(Savchenko et al.
2014
).
Guard cell turgor and the size of the stomatal aperture are determined by the
concentration of ions. ROS regulate stomatal channels and transporters in ABA
signalling. In a study to analyse the influence of ROS on ion channels by using
Vicia
guard cells, Zhang et al. (
2001a
) reported that exogenous H
2
O
2
suppressed
inwards K
+
current (K
in
+
). Meanwhile, a study that involved
Vicia
guard cells
showed that K
+
channels responded differently to ABA and H
2
O
2
: whereas ABA
depressed the activity of K
in
+
channels in a reversible manner, H
2
O
2
irreversibly
depressed the activities of both K
in
+
+
channels for guard cells immersed in
1-50 ﾵM H
2
O
2
(Khler et al.
2003
). Another example of ROS-regulated channel
activity is the Ca
2
+
-permeable (I
Ca
) channels, which is activated by H
2
O
2
through
ABA signalling release Ca
2
+
into the cytoplasm and then triggers stomatal closure
(Pei et al.
2000
). A recent study indicated that PYR/PYL/RCAR ABA receptors
regulate K
+
and Cl
−
channels through ROS-mediated activation of Ca
2
+
chan-
nels at the plasma membrane of
Arabidopsis
guard cells. In a
pyr1/pyl1/pyl2/pyl4
quadruple mutant, the basal activity of Ca
2
+
channels was not affected, but ABA-
induced ROS-enhanced accumulation was impaired and ABA-evoked Ca
2
+
chan-
nel activity was lost (Wang et al.
2013b
). By investigating activation of the plasma
membrane, a significantly smaller proportion of
atrbohD/F
guard cells displayed
ABA-induced [Ca
2
+
]
cyt
increase than wild-type guard cells (Kwak et al.
2003
). It
is thus likely that H
2
O
2
signalling converges with the ABA pathway at the point of
Ca
2
+
activation.
Receptors-like kinase has also function in both ROS and ABA signalling
in the regulation of stomatal behaviour. In the ABA pathway, H
2
O
2
is known
to be located downstream of OST1 in guard cells because ABA-induced ROS
production does not occur in
ost1
mutants (Mustilli et al.
2002
). A plasma
membrane receptor-like kinase, guard cell H
2
O
2
-resistant1 (GHR1), activates
ABA- and H
2
O
2
-regulated S-type anion currents and stomatal closure (Hua
et al.
2012
). It was also established that GHR1 phosphorylated and activated
and K
out
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