ABA Regulation of Stomatal Movement - Abscisic Acid: Metabolism, Transport and Signaling - page 284

phosphorylation; the anion efflux further activates outward-rectifying plasma
membrane K + channel GORK; finally, the efflux of anions and cations leads to
the stomatal closure. In this signaling pathway, ABA receptors are the “starting
line” for ABA signaling transduction in guard cells.
15.5 Components of ABA Signaling Network
Between ABA Receptors and Downstream
Plasma Membrane Ion Channels
15.5.1 Components of ABA Signaling in Guard Cells
Protein phosphorylation and dephosphorylation were found being important for
ABA signaling a few decades ago, and several protein phosphatases from pro-
tein phosphatase 2C (PP2C) family, including ABI1, ABI2, HAB1, and PP2CA,
were then identified as essential regulators of ABA signaling in Arabidopsis
guard cells, in which ABI1 and ABI2 are the most well-studied phosphatases.
Arabidopsis mutants abi1 (ABA insensitive 1) to a bi5 (ABA insensitive 5) were
isolated over 30 years ago by screening seed germination and seedling growth
in the presence of 10 ʼ M ABA (Koornneef et al. 1984 ). ABI3 to ABI5 encode
transcriptional factors, and mainly function in other cell types, not guard cells
(Finkelstein and Lynch 2000 ; Giraudat et al. 1992 ; Shkolnik-Inbar et al. 2013 ).
ABI1 and ABI2 are known as two essential negative regulators of ABA signaling
transduction in guard cells. The abi1 and abi2 mutants show excessive water loss
phenotype (Finkelstein and Somerville 1990 ), indicating their functions in ABA-
induced stomatal closure. Interestingly, it turned out that abi1 and abi2 mutants
are resulted from the same point mutation of a substitution of a Gly by an Asp in
the conserved catalytic domain in two different PP2C genes (Leung et al. 1994 ,
1997 ). This point mutation in the two homologs reduces the phosphatase activity
(Leung et al. 1997 ). ABI1 and ABI2 have a calcium-binding domain (EF hand)
and were suggested to be a Ca 2 + -regulated component of ABA signaling in guard
cells (Leung et al. 1994 ). However, ABA-induced cytosolic Ca 2 + increases were
impaired in abi1 and abi2 guard cells, but Ca 2 + activation of anion channel cur-
rents and external Ca 2 + -induced stomatal closure were not affected in the two
mutants (Allen et al. 1999 ), suggesting that ABI1 and ABI2 function upstream of
Ca 2 + signal in ABA signaling pathway. HAB1 and PP2CA are another two phos-
phatases from PP2C family. HAB1 was initially named as AtP2C-HA, the disrup-
tion of AtP2C-HA leads to an ABA hypersensitive phenotype in seed germination,
but stomatal closure is not obviously affected (Leonhardt et al. 2004 ; Saez et al.
2004 ). However, the abi1hab1 double mutants showed a reduced transpirational
water loss compared to abi1 and hab1 single mutants under a drought condition,
suggesting a cooperative negative regulation of the two phosphatases for ABA
signaling in guard cells (Saez et al. 2006 ). PP2CA is more active than other three
phosphatases in seed germination, and the mutation of PP2CA leads to a strongest