Protein Kinases and Phosphatases Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 154

important, negative regulators in ABA signaling, including ABI1, ABI2, HAB1,
HAB2, AHG1, and AHG3/PP2CA.
Early forward genetic approaches allowed to isolate the abi1 - 1 and abi2 -
1 mutants that show dominant, strong ABA-insensitive phenotypes in the major
ABA responses including reduced seed dormancy, ABA-induced inhibition of
seed germination, postgermination growth arrest, promotion of stomatal closure,
and inhibition of stomatal opening (Koornneef et al. 1982 ; Leung et al. 1994 ,
1997 ; Meyer et al. 1994 ). The mutations were shown, by the map-based cloning
technique, to take place in the genes encoding two homologous PP2C proteins,
named ABI1 and ABI2 (Leung et al. 1994 , 1997 ; Meyer et al. 1994 ). Further stud-
ies revealed that ABI1 and ABI2 function redundantly as negative regulators of
ABA signaling essentially because loss of function of these genes results in ABA-
hypersensitive phenotypes and double mutation increases the ABA hypersensitiv-
ity (Sheen 1998 ; Gosti et al. 1999 ; Merlot et al. 2001 ). ABI1 and ABI2 contribute
nearly 50 % of the ABA-induced PP2C activity, suggesting that these two PP2C
members play essential roles in the PP2C-mediated ABA signaling, but other
PP2Cs are also involved in ABA signaling (Merlot et al. 2001 ), which is veri-
fied by pursuant studies. HAB1, HAB2 (Leonhardt et al. 2004 ; Saez et al. 2004 ,
2006 ; Robert et al. 2006 ), and AHG1 (Nishimura et al. 2007 ) and AHG3/PP2CA
(Cherel et al. 2002 ; Kuhn et al. 2006 ; Yoshida et al. 2006a ; Lee et al. 2009 ) were
also identified as negative ABA-signaling regulators, all of which belong to the
clade-A PP2Cs (Schweighofer et al. 2004 ). The hab1 pp2ca and abi1 pp2ca dou-
ble mutants show ABA-hypersensitive phenotypes in seed germination, postgermi-
nation growth, reduced water loss and enhanced resistance to drought stress, and
two triple mutants hab1 abi1 abi2 and hab1 abi1 pp2ca show an extreme ABA
hypersensitivity in the major ABA responses, impaired growth and partial consti-
tutive response to endogenous ABA (Rubio et al. 2009 ). These genetic data reveal
that these six members of the clade-A PP2Cs, functioning negatively and redun-
dantly, play central roles in ABA signaling.
The remaining three PP2Cs, HAI1, HAI2/AIP1, and HAI3 (with Arabidopsis
genome locus At5g59220, At1g07430, and At2g29380, respectively), were
recently reported to have both effect on ABA-independent low water poten-
tial phenotypes and effect on ABA-sensitivity phenotypes in seed germination
and seedling growth; however, double or triple mutants of the HAI genes have
moderate ABA hypersensitivity in seedling growth, while these mutants have
ABA-insensitive seed germination, which contrasts with the phenotypes of other
clade-A PP2C mutants (Antoni et al. 2012; Bhaskara et al. 2012 ). Only was HAI1
identified clearly as a negative regulator of ABA signaling (Antoni et al. 2012).
This suggests a level of functional differentiation among the clade-A PP2Cs in
ABA signaling (Bhaskara et al. 2012 ).
Recent advances, especially with discovery of the cytosolic PYR/PYL/RCAR
receptors for ABA (reviewed in Cutler et al. 2010 , and see Chap. 6 of this
topic), have uncovered how the clade-A PP2Cs work in ABA signaling. Early
studies found that the point mutations of both abi1 - 1 and abi2 - 1 in ABI1 and
ABI2 PP2Cs, respectively, involve substitution of the same amino acid residue