ABA Regulation of Stomatal Movement - Abscisic Acid: Metabolism, Transport and Signaling - page 285

ABA hypersensitive phenotype in seed germination compared to abi1 , abi2, and
hab1 (Yoshida et al. 2006 ; Kuhn et al. 2006 ). The four PP2Cs involve in ABA
signaling transduction in guard cells by interacting with upstream ABA receptors
and downstream protein kinases as described above.
As the counterparts of PP2Cs, some protein kinases are the positive regulators
of ABA signaling network in guard cells. Currently, the known protein kinases
for ABA-induced stomatal movement are mainly from calcium-dependent pro-
tein kinase (CPK) family and SNF1-related kinases (SnRK) family. SnRK2.2, 2.3
and 2.6 are the three SnRK members functioning in guard cells in Arabidopsis .
The Arabidopsis OST1/SnRK2.6 is the ortholog of AAPK from Vicia faba , and
the mutant ost1 / snrk2.6 shows a strong ABA-insensitive phenotype in stoma-
tal closure (Mustilli et al. 2002 ; Yoshida et al. 2002 ). The double mutant of
snrk2.2/2.3 shows a weaker ABA-insensitive phenotype compared to ost1/snrk2.6
in stomatal closure (Fujii et al. 2007 ), and ABA insensitivity of the triple mutant
snrk2.2/2.3/2.6 is stronger than ost1 and snrk2.2/2.3 (Fujii et al. 2011 ), indicat-
ing the presence of functional redundancy within the three SnRKs for stoma-
tal closure regulation in Arabidopsis . In the absence of ABA, SnRKs can be
dephosphorylated/inactivated by PP2Cs and cannot phosphorylate downstream
S-type anion channel SLAC1 after being dephosphorylated (Belin et al. 2006 ;
Umezawa et al. 2009 ; Vlad et al. 2009 ; Weiner et al. 2010 ; Dupeux et al. 2011 ).
The increase of ABA level in leaves leads to the binding of ABA to ABA receptors
PYR/PYL/RCARA, which in turn inhibit the activity of PP2Cs by direct protein
interaction. SnRKs are then released from PP2C, activated by autophosphoryla-
tion, activate downstream S-type anion channel SLAC1 by phosphorylation, and
consequently, stomata are closed. The activation and inhibition of downstream
S-type anion channel SLAC1 by SnRKs and PP2Cs in an ABA-dependent manner
were verified by direct anion current recordings in Xenopus oocyte (Geiger et al.
2009 ) and structural analysis of protein complexes (Soon et al. 2012 ; Nishimura
et al. 2009 ). CPKs are a protein kinase family, which contain a Ca 2 + -binding
domain, can be activated by the binding of Ca 2 + , and further phosphorylate
downstream targets. The genome of model plant Arabidopsis encodes 34 CDPK
members, four of which were found functioning in stomatal closure regulation,
including CPK3, 4, 6, and 11, and the double mutants cpk3/cpk6 and cpk4/cpk11
show ABA insensitivity in stomatal closure (Mori et al. 2006 ; Zhu et al. 2007 ).
Further research confirmed the functions of CPK6 as an activator of anion chan-
nel SLAC1 by protein phosphorylation in Xenopus oocytes, and the phosphatases
ABI1, ABI2, and PP2CA can inhibit CPK6-mediated activation of SLAC1 (Brandt
et al. 2012 ), suggesting a different signaling branch from SnRKs. Interestingly,
SLAC1 can also be activated by CPK21 and CPK23 in Xenopus oocytes, but the
Arabidopsis mutant cpk21 and cpk23 showed a strong drought tolerance pheno-
type rather than drought hypersensitive phenotype compared to wild type (Franz
et al. 2011 ; Ma and Wu 2007 ), indicating that these two CPKs involve in sto-
matal movement regulation in a different manner compared to CPK3, 4, 6, and
11. Guard cell hydrogen peroxide-resistant 1 (GHR1) is a receptor-like kinase
localized in the plasma membrane of Arabidopsis guard cells (Hua et al. 2012 ).