The biochemical studies discussed above provided strong circumstantial evidence that ABA is synthesized via an epoxy-carotenoid precursor (Fig. 5).In recent years, this pathway and its regulation have become well established
through the identification and characterization of ABA-deficient mutants (Table1). Several phenotypes associated with an ABA deficiency are illustrated in Figure 7. These mutants have also been the basis for isolating most of the ABA biosynthetic genes