ABA Signal Perception and ABA Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 99

binding, inhibit activities of the group-A PP2Cs, such as ABI1 and ABI2 and
thus antagonize the PP2Cs to positively regulate ABA signaling (Ma et al. 2009 ;
Park et al. 2009 ). The ABA receptor identity of the PYR/PYL/RCAR proteins and
PYR/PYL/RCAR-PP2C coupled ABA signaling was further confirmed by another
two groups who identified PYL5 by the yeast two-hybrid assay as an interaction
partner of HAB1 (Santiago et al. 2009b ), and several PYR/PYL proteins by co-
purification technique as interaction partners of ABI1 (Nishimura et al. 2010 ).
Structural studies provided compelling evidence to support that the
PYR/PYL/RCAR proteins interact with ABA and function as ABA receptors
(Melcher et al. 2009 ; Miyazono et al. 2009 ; Nishimura et al. 2009 ; Santiago
et al. 2009a ; Yin et al. 2009 ; Shibata et al. 2010 ; see also Chap. 7 ) . Consistent
with the biochemical observations that the interaction of PYL9 with ABI2 and
PYL5 with HAB1 stimulates ABA binding to PYL9 and PYL5, respectively
(Ma et al. 2009 ; Santiago et al. 2009b ), the structural studies provided submo-
lecular evidence that RCAR/PYR1/PYL proteins can form tight complexes with
PP2Cs and cooperate with PP2Cs in the ABA binding to PYR/PYL/RCAR,
which induces the enzymatic inactivation of the PP2Cs (Melcher et al. 2009 ;
Miyazono et al. 2009 ; Nishimura et al. 2009 ; Santiago et al. 2009a ; Yin et al.
2009 ; Shibata et al. 2010 ; see also Chap. 7 ). However, the cooperation of PP2Cs
with RCAR/PYR1/PYL proteins in the RCAR/PYR1/PYL ABA binding led to a
question whether the RCAR/PYR1/PYL-PP2C complex is a co-receptor for ABA.
In regard to this question, the structural studies support a model that ABA first
binds RCAR/PYR1/PYL proteins, which recruits PP2Cs to bind and stabilize the
ABA-RCAR/PYR1/PYL complexes (Melcher et al. 2009 ; Miyazono et al. 2009 ;
Nishimura et al. 2009 ; Santiago et al. 2009a ; Yin et al. 2009 ; Shibata et al. 2010 ;
see also Chap. 7 ) , which is consistent with the idea that the PYR/PYL/RCAR pro-
teins, rather than the RCAR/PYR1/PYL-PP2C complexes, are ABA receptors.
6.5.2 A PYR/PYL/RCAR-PP2C-SnRK2-Linked Linear
Signaling Pathway
Park and colleagues observed that the ABA-induced activation of SNF1-related
protein kinases (SnRK2s) is reduced in the pyr1 pyl1 pyl2 pyl4 quadruple mutant
(Park et al. 2009 ), which is confirmed in a further study showing that the pyr1 pyl1
pyl2 pyl4 pyl5 pyl8 sextuple mutant lacks ABA-mediated activation of SnRK2s
(Gonzalez-Guzman et al. 2012 ), revealing that, in addition to PP2Cs, SnRK2s
function is dependent on, and downstream of, PYR/PYL/RCAR receptors in
ABA signaling pathway. The SnRK2s, mainly including SnRK2.2, SnRK2.3, and
SnRK2.6/OST1, are key players in ABA signaling (see Chap. 8 ) and were shown
to be substrates of the clade A PP2Cs in many reports. Several groups found
that SnRK2.6/OST1 interacts with ABI1, and the SnRK2 activities induced by
ABA are impaired in abi1 - 1 mutants (Mustilli et al. 2002 ; Yoshida et al. 2006 ;
Umezawa et al. 2009 ). Consistent with this, Umezawa et al. ( 2009 ) provided clear