Abscisic acid (ABA) (Fig. 1) was discovered independently by several groups
in the early 1960s. Originally believed to be involved in the abscission of fruit
and dormancy of woody plants, the role of ABA
in these processes is still not clear. In later work
it became evident, however, that ABA is
necessary for seed development, adaptation to
several abiotic stresses, and sugar sensing. The
regulation of these processes is in large part
mediated by changes in de novo synthesis of
ABA. When leaves of mesophytic plants are
water-stressed, ABA levels increase strikingly
(Fig. 2). The elevated ABA levels lead to
stomatal closure, changes in gene expression,
and other adaptations that
increase the plant’s stress
tolerance (Chapters D6 & E4).
In seeds, elevated ABA levels
during mid-embryogenesis are
necessary for the expression of
genes needed for the
accumulation of storage
reserves and tolerance to
desiccation (Chapter E4).
To understand how the
cellular levels of ABA are
regulated, it is necessary to
elucidate the pathways of
A B A s y n t h e s i s a n d
degradation. Progress in
working out the biosynthetic
pathway of ABA has been
relatively slow, because