ABA Regulation of Stomatal Movement - Abscisic Acid: Metabolism, Transport and Signaling - page 288

further confirmed (Brandt et al. 2012 ). On the contrary, Ca 2 + increases and the
activation of hyperpolarization-activated Ca 2 + channels were also impaired or
abnormal in the guard cells of gca2 , ost1 , abi1, and abi2 mutants (Allen et al.
1999 ; Mustilli et al. 2002 ), suggesting that these components may function
upstream of Ca 2 + . Recent research found that ABA induces stomatal closure in
Ca 2 + -dependent and Ca 2 + -independent manners (Siegel et al. 2009 ). ABA can
only induce 30 % stomatal closure response in the absent of cytosolic Ca 2 + ele-
vation, the exposure of guard cells to ABA can enhance the ability of Ca 2 + to
activate S-type anion channel currents and down-regulate inward K + channels,
and the ABA-insensitive mutant ost1 , abi2, and gca2 showed only partial ABA
response correlated to cytosolic Ca 2 + (Siegel et al. 2009 ). These data suggest an
ABA-priming hypothesis for ABA-induced stomata closure response.
ROS were thought to be toxic for plant cells. But research in the last two dec-
ades found that ROS can function in guard cells as important messengers medi-
ating ABA signaling, and hydrogen peroxide (H 2 O 2 ) is the most effective one
because of its long life time and stability. ABA-induced ROS production was
first observed in the guard cells of Vicia faba (Song et al. 2014 ) and Arabidopsis
(Pei et al. 2000 ). Further research identified two NADPH oxidases AtrbohD and
AtrbohF as the key enzymes for ROS production in Arabidopsis guard cells, and
the disruption of these two genes impairs ABA-induced ROS production, cyto-
solic Ca 2 + increases, activation of hyperpolarization-activated inward Ca 2 + chan-
nels, and stomatal closure (Kwak et al. 2003 ). AtrbohD and AtrbohF localize in
the plasma membrane of guard cells, but ROS production in chloroplast of guard
cells was observed (Zhang et al. 2001 ), suggesting that ROS can be produced in
multiple area of guard cells. In ABA signaling pathway, ROS production locates
downstream of ABI1 and OST1, but upstream of ABI2 and GCA2 (Murata et al.
2001 ; Mustilli et al. 2002 ; Pei et al. 2000 ). Because ROS is such small molecules
with simple structures, how essential core proteins of ABA signaling network in
guard cells, such as PP2Cs, GCA2 and kinases (SnRKs, CDPKs, and GHR1), per-
ceive the small molecule without specific structural character is still unknown, and
further work will be needed to answer the question (Song et al. 2014 ).
Nitric oxide (NO) plays roles in ABA-induced stomatal closure as a second
messenger. NO can trigger the increase of cytosolic Ca 2 + , the efflux of both anion
and cation, and stomatal closure (Gayatri et al. 2013 ). The application of either
ABA or ROS can induce the production of NO, and the disruption of AtrBOHD/F,
the enzyme catalyzing the production of ROS, impairs the production of both ROS
and NO (Gayatri et al. 2013 ). NO scavenger can impair ABA- and ROS-induced
stomatal closure, and NO donor can mimick ABA- and ROS-induced stomatal clo-
sure responses. Therefore, NO functions downstream of ROS in guard cells. Nitric
oxide synthase (NOS)-like enzyme is supposed to be the essential protein for NO
production, but the existence of NOS in plants is still under debate. Together, the
current results show that NO is an important second messenger-mediating ABA
signaling in guard cells and localizes downstream of ROS but upstream of cyto-
solic Ca 2 + in ABA signaling cascade. Further work will be needed to unravel
every detail of the mechanisms of NO in ABA-induced stomatal closure.