ABA Signal Perception and ABA Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 102

ABA-dependent manner, although not the structurally related HAB1, thus func-
tions as a bona fide ABA receptor (Fuchs et al. 2014 ). Further study will be needed
to clarify these discrepancies.
The other 13 members of PYR/PYL/RCARs act undoubtedly as ABA recep-
tors, which function redundantly but with diverse functional specialization features
to regulate ABA signaling. First, PYR/PYL/RCAR receptors can be character-
ized on their oligo-merization states; some members of PYR/PYL receptors are
homo-dimeric (PYR1, PYL1, and PYL2), while others are monomeric (PYL4,
PYL5, PYL6, PYL8, PYL9, and PYL10) (Dupeux et al. 2011 ; Hao et al. 2011 ;
see also Chap. 7 ) . The homo-dimer assembly is required to regulate strictly the
ABA-dependent switching of the signal transduction by PYR/PYL/RCARs. The
intrinsic affinities of the receptors for ABA are closely related to their monomeric/
dimeric states (Dupeux et al. 2011 ). The Kd of the monomeric receptors is approx-
imately 1 ʼ M (Santiago et al. 2009a ; Szostkiewicz et al. 2010 ; Dupeux et al.
2011 ), whereas that of the dimeric receptors (Kd > 50 ʼ M) is almost two orders
of magnitude lower (Dupeux et al. 2011 ). This difference can be explained by the
unfavorable contribution of dimer dissociation to the thermodynamics of ABA
binding rather than by differences in the ABA-binding cavity (Dupeux et al. 2011 ).
However, both monomer and dimer bind ABA with high affinity when interacting
with PP2Cs (Kd ≈ 30-60 nM) (Ma et al. 2009 ; Santiago et al. 2009a , b ; see also
Chap. 7 ). It is noteworthy that the PYL3 receptor is a trans-dimer, which suffers a
cis- to trans-dimer transition upon ABA binding to facilitate the subsequent disso-
ciation to monomer (Zhang et al. 2012 ; see also Chap. 7 ).
Second, the 14 homologous PYR/PYL/RCARs may selectively interact with
six members of clade A PP2Cs that are implicated in ABA signaling (Fujii et al.
2009 ; Cutler et al. 2010 ; Raghavendra et al. 2010 ; see also Chap. 8 ), which may
form numerous combinations of the receptor-PP2C complexes. Importantly, some
of the closely related receptor complexes differ in their selectivity and sensitivity
to ABA or natural structural derivatives of ABA and are likely to target different
downstream components (Ma et al. 2009 ; Park et al. 2009 ; Santiago et al. 2009b ;
Hao et al. 2011 ; Kepka et al. 2011 ; Szostkiewicz et al. 2010 ). For example, in con-
trast to PYR1 and PYLs 1 to 4, PYL9 and PYL5 show constitutive binding interac-
tions with PP2Cs (Ma et al. 2009 ; Park et al. 2009 ; Santiago et al. 2009b ). PYR1,
PYL1, PYL2, and PYL3 inhibit PP2Cs, such as ABI1, HAB1, HAB2, and PP2CA,
in an ABA-dependent manner, while PYL5, PYL6, PYL8, PYL9, and PYL10
inhibit PP2Cs even in the absence of ABA; PYL4 inhibits HAB2 PP2C activity
only in the absence of ABA; ABA-independent inhibition of PP2C requires the
PYLs to exist in a monomeric state (Hao et al. 2011 ). HAI1 PP2C (encoded by
At5g59220 gene) is selectively inhibited by some PYR/PYL/RCAR receptors, and
its close relatives, such as PP2CA/AHG3 and AHG1, show a contrasting sensitiv-
ity to PYR/PYL inhibition (Antoni et al. 2012 ). Different oligo-merization states of
PYR/PYL/RCAR receptors and multiple receptor complexes could potentially fine-
tune a multitude of ABA responses in different tissues or under various conditions.
Additionally, different expression patterns of the genes encoding differ-
ent members of PYR/PYL/RCARs suggest substantial functional differences of