GAs promote seed germination, stem elongation, flowering, and cone production and retard leaf and fruit senescence. They also induce de novo synthesis of numerous enzymes, including -amylase, in the aleurone layer of barley. The enzymes catalyzing the synthesis of geranylgeranyl diphosphate and its conversion to ent kaurene, which is the first committed step in GA biosynthesis, are plastid-localized terpene cyclases. ABA, a C 15 compound, is associated with desiccation tolerance, suppression of vivipary, and the closure of stomata induced by water stress. Like GAs, ABA is a product of the terpenoid pathway.
ABA is metabolized to phaseic acid, dihy-drophaseic acid, and dihydrophaseic acid glucoside.
ABA-deficient mutants typically exhibit a wilty or viviparous phenotype. Although many such mutants are known, many of the genes encoding ABA biosynthetic enzymes have yet to be cloned. Cytokinins, in conjugation with auxin, promote cell division and determine cell differentiation. They also are associated with the senescence of plant organs, apical dominance, and stomata opening. IAA affects apical dominance, tropisms, shoot elongation, the induction of cambial cell division, and root initiation.