Protein Kinases and Phosphatases Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 145

2010 ; Brandt et al. 2012 ; Acharya et al. 2013 ). As described earlier, whereas
Ca 2 + -dependent activation of such ion channels is directly associated with the
CDPKs (Mori et al. 2006 ; Geiger et al. 2010 , 2011 ; Brandt et al. 2012 ; Scherzer
et al. 2012 ; Demir et al. 2013 ), the Ca 2 + -independent activation of the ion chan-
nels is mainly mediated by SnRK2.6 (Lee et al. 2009 , 2013 ; Geiger et al. 2010 ,
2011 ; Brandt et al. 2012 ; Acharya et al. 2013 ).
It has been shown that SnRK2.6/OST1 can interact with and phosphorylate an
inward K + channel KAT1, indicating that KAT1 is a target of SnRK2.6/OST1 (Sato
et al. 2009 ; Acharya et al. 2013 ). ABA inhibition of inward K + channels and light-
induced stomatal opening are reduced in ost1 mutants, while transgenic plants over-
expressing OST1 show ABA hypersensitivity in these responses, suggesting that
SnRK2.6/OST1 negatively regulates KAT1 to induce stomatal closure and inhibit
stomatal opening in response to ABA (Acharya et al. 2013 ). A recent report showed
that SnRK2.6/OST1 interacts with and phosphorylates a K + uptake transporter
KUP6, suggesting that this kinase may also regulate stomatal movement through
the KUP6 K + transporter (Osakabe et al. 2013 ). Importantly, like CPK3/6/21/23
(Mori et al. 2006 ; Geiger et al. 2010 , 2011 ; Brandt et al. 2012 ; Scherzer et al. 2012 ),
SnRK2.6/OST1 interacts with and phosphorylates guard cell anion channel SLAC1,
and OST1 coexpression activates SLAC1 anion channels in a heterologous system
( Xenopus oocytes ) (Geiger et al. 2009 , 2010 ; Lee et al. 2009 , 2013 ; Brandt et al.
2012 ; Acharya et al. 2013 ), while ost1 mutant shows strongly reduced slow anion
currents in Arabidopsis guard cells and is insensitive to ABA-induced stomatal clo-
sure and regulation of slow anion currents, but OST1 overexpressing lines are hyper-
sensitive for these responses, resulting in accelerated stomatal closure in response to
ABA (Geiger et al. 2009 ; Acharya et al. 2013 ). Thus, the CDPK (as described ear-
lier) and OST1 branch of ABA signal transduction in guard cells seem to converge
on the level of SLAC1. Additionally, the ost1 mutation reduces ABA activation of
rapidly activating (QUAC1) anion currents in guard cells (Imes et al. 2013 ), consist-
ent with the idea that SnRK2.6 plays a central role in Ca 2 + -independent ABA regu-
lation of stomatal behavior by targeting key ion channels in guard cells.
Recently, two independent groups, using phosphoproteomic approach and
working in Arabidopsis , identified a set of new potential substrates of SnRK2 pro-
tein kinase, including proteins involved in a variety of cell metabolic and signal-
ing processes such as flowering time regulation, RNA and DNA binding, miRNA
and epigenetic regulation, signal transduction, chloroplast function, and many other
cellular processes (Wang et al. 2013 ; Umezawa et al. 2013 ). Wang and coworkers
confirmed that, consistent with the SnRK2 phosphorylation of flowering time regula-
tors, the snrk2.2/2.3/2.6 triple mutant flowers significantly earlier than the wild-type
plants (Wang et al. 2013 ). Umezaw and coworkers observed that SnRK2 promotes
the ABA-induced activation of the mitogen-activated protein kinases AtMPK1
and AtMPK2 and also identified a previously unknown protein, SnRK2-substrate
1 (SNS1), which is phosphorylated by SnRK2s and negatively regulates ABA
responses in Arabidopsis (Umezawa et al. 2013 ). These studies suggest a highly com-
plicated SnRK2-mediated phosphorylation network and are helpful to elucidate the
functional mechanisms of the SnRK2 protein kinases in ABA-signaling pathways.