ABA and Its Derivatives: Chemistry and Physiological Functions - Abscisic Acid: Metabolism, Transport and Signaling - page 9

PA is a more rigid molecule than ABA. The conformation of the six-mem-
bered ring of PA is a rigid chair with an axial side chain. This conformation is
similar to that of ABA bound to PYL proteins; however, PA has much less affin-
ity with PYL compared to ABA (Kepka et al. 2011 ). PA has an ʱ -axial substitu-
ent at C-2′, like the substituent at C-6′ of R -ABA. However, the C-2′ substituent
of PA is not a methyl group, but an ether oxygen. This is a significant disadvan-
tage for interacting with PYL because the ether oxygen reduces the affinity of
PA with PYL due to steric hindrance and hydrophilicity. In fact, PA is inactive in
most bioassays. Although rarely mentioned, solutions of PA inevitably contain a
trace amount of 8′-hydroxy-ABA; the equilibrium ratio of 8′-hydroxy-ABA/PA is
2:98 at pH 3-10 (Todoroki and Hirai 2000a , b ). Consequently, a peak and signals
due to 8′-hydroxy-ABA are evident in the HPLC and 1 H NMR analyses of PA,
respectively.
1.1.4 ABA Mimics Lacking an ABA-skeleton
Although ABA is produced by fermentation using ABA-producing fungi at rela-
tively low cost, ABA mimics with a simpler structure than ABA would be prac-
tical and advantageous if their production costs were lower than that of ABA
(Fig. 1.8 ). In early studies, lunularic acid ( 25 ) was considered to play an ABA-
like physiological role in liverworts (Pryce 1972 ), although currently this hypoth-
esis is in question because ABA has been detected in two species of liverworts
(Nakayama et al. 1993 ). Some synthetic ABA mimics containing an aromatic ring
instead of the cyclohexenone ring of ABA ( 26 - 28 ) have been reported, although
their activities are lower than that of ABA (Bittner et al. 1977 ; Ladyman et al.
1988 ; Yoshikawa et al. 1992 ). The Lewis formulae of these compounds somewhat
resemble ABA. However, these molecules are relatively flat, so they bear little
resemblance to ABA sterically.
Pyrabactin ( 29 ), a key compound for identifying ABA receptors amongst PYL
proteins, exhibits weak ABA-like activity (Park et al. 2009 ). Although this com-
pound does not superficially resemble ABA, the bent conformation functions as
an effective gate-closing promoter in PYL1 (Melcher et al. 2010 ) (Fig. 1.8 ).
Interestingly, pyrabactin binds to PYL2 in a manner different from its binding to
PYL1 and does not induce closure of the gate. The ABA agonistic effect of pyra-
bactin is observed in in vitro experiments only for PYR1 and PYL1 (Okamoto
et al. 2013 ). Quinabactin ( 30 ), which contains aromatic rings and a sulfonamide
similar to pyrabactin, is a stronger ABA mimic than is pyrabactin (Okamoto
et al. 2013 ). In vitro analysis revealed that quinabactin activates mainly dimeric
receptors to elicit stomatal closure, ABA-regulated gene expression, and drought
tolerance. Quinabactin also adopts a bent conformation in PYL2 (Fig. 1.8 ). The
dimeric receptor-selective binding of quinabactin may depend on its slightly larger
molecular size compared to ABA.