catabolite results from the 9'-hydroxylation of ABA to give 9'-hydroxy ABA.This unstable compound rearranges to form an isomer of PA, called neoPA(Fig. 12) (49). It is not known at present whether the same or different enzymes catalyze hydroxylation at the 8' and 9' positions.Abscisic acid may also be inactivated by the formation of its glucose ester
(ABA-GE), which is stored in the vacuole. The level of ABA-GE continued to increase in plants that were subjected to a series of stress and rewatering cycles.An ABA glucosyltransferase gene from adzuki bean has recently been cloned
(46). Interestingly, this gene is also up-regulated by ABA. The physiological significance of ABA-GE formation as well as the potential for engineering ABA levels by decreased catabolism may now be investigated.