DEHYDRATION TOLERANCE IN APPLE SEEDLINGS IS ADVANCED BY RETARDING ABA 8'-HYDROXYLASE CYP707A

The effects of an abscisic acid (ABA) 8’-hydroxylase inhibitor (ABAI) on ABA catabolism, stomatal aperture, and water potential were investigated in apple seed-lings under dehydration conditions. In addition, 9-cis-epoxycarotenoid dioxygenase (MdNCED) and ABA 8’-hydroxylase (MdCYP707A) genes were isolated and their expressions were analyzed under dehydration conditions. The stomatal aperture decreased for 4 h after spraying with ABAI and the stomatal aperture in the ABAI- treated leaves was lower than that in the untreated control- leaves during the dehydra¬tion conditions. The water potential was maintained at a higher level in the ABAI- treated leaves during dehydration, however that in untreated control-leaves decreased with the progress of dehydration. In both the ABAI-treated and untreated control- leaves, endogenous ABA concentrations increased with dehydration, but the ABA levels in the ABAI-treated leaves were higher than those in the untreated control- leaves throughout dehydration. In contrast, the phaseic acid (PA) concentrations in the ABAI-treated leaves were lower than those in the untreated control-leaves during dehydration. Regardless of the higher endogenous ABA concentrations, the expres¬sions of MdNCEDs in the ABAI-treated leaves were lower than those in the untreated control-leaves. In addition, the expressions of MdCYP707As in the ABAI-treated leaves were also lower than those in the untreated control-leaves. Higher 50% effective concentrations (EC50) were observed in the ABAI-treated leaves, which suggests that the oxidative damage under dehydration may be reduced by ABAI application. The results suggest that prompt stomata closure is required for survival under dehydra¬tion, and ABAI application may therefore be of practical use. The increase of endogenous ABA, which induced prompt stomata closure in ABAI-treated leaves, may depend on the retardation of the expression of MdCYP707As. Moreover, the results demonstrate a close relationship between MdNCEDs and MdCYP707As on ABA catabolism.