Abstract
In the rice seedling elongation inhibition assay, 5'α,8'-cyclo-ABA (CycloABA) exhibited stronger and longer-lasting activity than ABA. This may be explained in view of (1) uptake, (2) affinity for the ABA receptor, and (3) catabolic inactivation. In the light of structure-activity relationships of ABA, we focused the metabolism of CycloABA. In rice seedlings, CycloABA was accumulated seven times as much as ABA. In radish seedlings, the major metabolite of ABA was phaseic acid that is produced by the 8'-hydroxylation, whereas we found no significant oxidative products of CycloABA. The major metabolite of CycloABA was the C-1 glucose conjugate, which was the minor metabolite of ABA. In this study, we uncovered the complete resistance of CycloABA to 8'-hydroxylation. CycloABA is expected to be a useful tool for studying plant physiology in which ABA metabolism involved.
