ABA and Its Derivatives: Chemistry and Physiological Functions - Abscisic Acid: Metabolism, Transport and Signaling - page 10

CO 2 H

OH

CO 2 H

HO Cl

25 27

N O

CO 2 H

C l O O

Cl

26 28

N

O O

S

S N

O O

H

O N

B r

29 30

Fig. 1.8 ABA mimics lacking an ABA-skeleton. Two molecular models extracted from the
PYL1-pyrabactin ( 29 ) and PYL2-quinabactin ( 30 ) crystal structures (PDB code: 3NEF and
4LA7, respectively)

1.2 Structural Requirements

1.2.1 For Exhibiting Biological Activity

The biological activities of many ABA analogues have been examined using vari-
ous plants and tissues under various conditions (Fig. 1.9 ). Although there are some
exceptions, the absence of an existing functional group, or the addition of a new
functional group, generally reduces the ABA activity. The most critical functional
group is the C-1 carboxy group, although ABA analogues that have a C-1 hydroxy
(ABAlc), aldehyde (ABAld), or methyl ester (Me ABA, 31 ) are relatively active
under some assay conditions. For example, the aba3 mutant, which is deficient in
molybdenum cofactor (MoCo) sulfurase necessary for ABAld oxidase (AAO3), is
insensitive to ABAlc and ABAld (Kepka et al. 2011 ). The relatively high activity