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controlling stomatal closure during drought stress, substantial work has focused
on ABA signalling in guard cells (Cutler et al. 2010 ; Kim et al. 2010 ). It has also
been reported that H 2 O 2 acts as a key regulator that mediates stomatal movement
(Bright et al. 2006 ). Considering that ROS signalling has been shown to be inter-
twined with other hormonal signals, it is likely that ROS are the vital hub that con-
nects the external growth conditions with ABA signals. The relationships between
ROS and ABA signals mainly involve regulation of guard cell movement. Many
important advances have been made in unravelling the roles of ROS and their rela-
tionships with ABA signalling in guard cells (Fig. 10.1 ), reviewed detially by Song
et al. ( 2014 ).
H 2 O 2 can be induced by ABA and accumulates in guard cells (Zhang et al.
2001a , b ; Pei et al. 2000 ). The function of constitutive ROS accumulation in ABA
signalling has been studied by using CAT1, CAT2 and CAT3 gene mutants or the
application of 3-amino-1,2,4-triazole (3-AT, a CAT inhibitor). These mutants and
this treatment cause a constitutively higher ROS level; however, the accumu-
lated ROS do not affect the stomatal aperture in the absence of ABA or methyl
jasmonate (MeJA) (Jannat et al. 2011a , b , 2012 ). However, the enhanced ROS
Fig. 10.1 ROS signal in guard cells. ROS sensors and targets not only monitor intra- and extracel-
lular ROS levels, but also respond to ROS signals. Black and red lines indicate activation and sup-
pression, respectively. Dashed line indicates the exact details of the signal pathway are still unknown
(this figure has been adapted from a recent review with little modification: Song et al. 2014 )
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