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becomes evident at the
ʱ
-oriented, axial methyl group (C-8′). This means that the
enantiomeric recognition of ABA by ABA-binding proteins depends on their sen-
sitivity to the location of this methyl group (Fig.
1.5
). The crystal structure of the
PYL3-
R
-ABA complex shows that
R
-ABA is oriented in the pocket in a manner
similar to
S
-ABA (Zhang et al.
2013
), indicating that the ability to accommodate
C-8′ provides PYL with a relatively high affinity for
R
-ABA. However, although
some PYL proteins have measurable affinities for
R
-ABA, generally this affin-
ity is weaker than that for
S
-ABA (Park et al.
2009
; Zhang et al.
2013
). Of the
Arabidopsis PYL proteins, PYL5 shows the strongest binding affinity for
R
-ABA,
whereas PYL9 does not function in the presence of
R
-ABA (Zhang et al.
2013
).
On the other hand, recombinant ABA 8′-hydroxylases (CYP707A enzymes),
which oxidise the C-8′ of ABA, do not bind
R
-ABA (Kushiro et al.
2004
). This
is as expected, since CYP707A must recognise C-8′ in order to conduct oxida-
tion, so stereospecific ligand binding is critical for this enzyme. Interestingly, C-8′
recognition by CYP707A seems to depend on the ring enone of ABA (Ueno et al.
2007
). The difference in sensitivity to the orientation of C-8′ between CYP707A
and PYL proteins may explain the comparable biological activity of
R
-ABA and
S
-ABA in some bioassays.
R
-ABA is metabolized to (
−
)-7′-hydroxy-ABA (
21
)
(Cowan and Railton
1987
; Boyer and Zeevaart
1986
; Balsevich et al.
1994a
)
and (
+
)-PA (
23
) (Gillard and Walton
1976
; Balsevich et al.
1994b
; Dashek et al.
1979
; Okamoto and Nakazawa
1993
) (Fig.
1.6
). Since the recombinant CYP707A
enzymes do not bind
R
-ABA (Kushiro et al.
2004
), these metabolites may be pro-
duced by the nonspecific oxidation of
R
-ABA.
1.1.2 Biosynthetic Precursors
The biosynthetic precursors that have an ABA-skeleton are xanthoxin (
10
), absci-
sic alcohol (ABAlc,
16
), and abscisic aldehyde (ABAld,
11
) (Fig.
1.6
). The side
chain of xanthoxin cannot be axial owing to the 1′,2′-epoxide, so the conforma-
tion of xanthoxin must be quite different from that of ABAlc, ABAld, and ABA.
Crystal structures of the PYL-ABA complex suggest that these precursors, which
have no carboxylic acid moiety, generally have poor affinity to PYL proteins
because the salt bridge between the C-1 carboxylate of ABA and the
ʵ
-ammonium
group of Lys of PYL is critical for forming a stable PYL-ABA complex. In fact,
these precursors induce a slight reduction in PP2C activity in the presence of PYL
proteins (Kepka et al.
2011
).
1.1.3 Catabolites
Common catabolites of ABA in many plants include phaseic acid (PA,
13
),
dihydrophaseic acid (DPA,
14
),
epi
-dihydrophaseic acid (
epi
-DPA,
15
),
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