Independent investigations of two different physiological phenomena in two different laboratories
across the Atlantic Ocean led to the identification and discovery of abscisic acid (ABA) as the causal
agent. Wareing and his associates in Wales worked for two decades or more on the seasonal changes in
bud dormancy in woody plants, particularly of sycamore (Acer pseudoplatanus), and identified a com-
pound that was named dormin[121]. Concurrently, a team led by Carns and Addicott in California, work-
ing on natural control of abscission in cotton, identified two compounds, which they named abscisin I and
abscisin II [122,123]. However, by 1965 these two independent but diverse paths converged on the dis-
covery that ABA was the hormone involved in both phenomena [124]. Like other hormones, abscisic acid
is also ubiquitous among vascular plants and has been found to occur in some mosses, algae, and fungi.
A. Chemical Nature
The naturally occurring enantiomorph is (S)-ABA, which is a sesquiterpenoid (a 15-carbon compound)
and by its biogenesis is related to monoterpenes, diterpenes (gibberellins), carotenoids, and triterpenes.
Endogenous (S)-ABA is optically active, having one center of asymmetry at C-1 , while synthetic ABA
is racemic and composed of equal amounts of (S)- and (R)-enantiomers. The synthetic (R)-ABA accounts
for 50% of the racemic mixtures of ABA and has biological activity equal to that of the natural (S)-ABA
(Figure 5) in most cases, except in stomatal closure, where it is inactive. Because the catabolism of the
(S)- and (R)-enantiomers is different, it is necessary to identify which compound is being used (R, S, or
RS). In such a situation, care must be taken to use only natural (S)-ABA for metabolic studies.
B. Metabolism
The typical sesquiterpene nature of ABA indicates that its endogenous synthesis is through mevalonic acid
(MVA) as a precursor. Two pathways for its biosynthesis have been suggested. First is via farnesyl py-
rophosphate, from which GAs are also derived. Through this pathway, MVA is converted to mevalonate
5-phosphate→mevalonate-5-pyrophosphate→^3 -isopentenyl pyrophosphate (IPP). This compound is
converted either directly to geranyl pyrophosphate or through 3,3-dimethylallyl pyrophosphate (DMAPP)
to geranyl pyrophosphate →farnesyl pyrophosphate and finally, to ABA. However, use of radioactive
MVA has yielded low amounts of ABA in only a few systems [125]. The second pathway is known to oc-
cur through the degradation of certain (40-carbon) carotenoids. Although this pathway is indirect, it seems
to produce major amounts of ABA via ABA-aldehyde in perhaps all plants [126,127]. Zeevaart et al. [128],
using various tissues incubated in an atmosphere containing^18 O 2 , have demonstrated that xanthophylls
rather than farnesyl pyrophosphate are the precursors of ABA. In the xanthophyll cycle, 9 - cis-neoxanthin
is converted to xanthoxin →ABA-aldehyde, which is finally oxidized to ABA.
Abscisic acid is catabolized to more polar compounds by conjugation, oxidation, hydroxylation, or
isomerization. However, it seems that each plant species has its own system to regulate its free ABA level.
This regulation is further dependent on the kind of organ tested as well as the physiological state. This
regulation of ABA may operate through conjugation with sugar(s) to form glucoside or glycosyl ester or
an acylated form. It can also be inactivated by oxidation to more polar free acids such as phaseic and di-
hydrophaseic acids. Both of these metabolites possess low or no growth-regulating activities and are de-
rived via 6 -hydroxymethyl ABA.
C. Transport
Abscisic acid is known to be translocated through the xylem and phloem to the actively growing regions
(apical buds, root tips) and also in the parenchyma outside the vascular tissues. Thus, ABA is not translo-
cated polarly; instead, it moves bidirectionally short as well as long distances [129].
PLANT GROWTH HORMONES 519
Figure 5 Structure of natural abscisic acid.