Salt Tolerance in Apple Seedlings is Affected by an Inhibitor of ABA 8′-Hydroxylase CYP707A

The effects of an ABA 8'-hydroxylase inhibitor, (+/-)abscinazole-E2B (Abz-E2B), were examined in "Fuji" apple seedlings exposed to sodium chloride (NaCl). Water potential, stomata aperture, and endogenous ABA, proline, and sodium (Na+) levels were analyzed, along with expression of the 9-cis-epoxycarotenoid dioxigenase (MdNCED) and ABA 8'-hydroxylase (MdCYP707A) genes. Water potential and stomatal aperture in the leaves treated with Abz-E2B and NaCl (Abz-E2B(+)NaCl(+) group) were lower than those in the untreated controls, but remained higher than those in the leaves treated with NaCl alone (Abz-E2B(-)NaCl(+) group). Endogenous ABA levels were higher in the Abz-E2B(+)NaCl(+) leaves at 4 days after treatment, whereas those in the Abz-E2B(-)NaCl(+) leaves increased with time and peaked at 12 days after treatment. Expressions of the MdNCED1 and MdNCED2 genes remained lower in Abz-E2B(+)NaCl(+) compared with Abz-E2B(-)NaCl(+) leaves, especially at 12 days after treatment. Proline levels increased sharply at 100-mM NaCl, and Na+ levels increased with time. The Na+ concentrations in the Abz-E2B(-)NaCl(+) leaves increased fourfold compared to those in the Abz-E2B(+)NaCl(+) leaves. The expressions of MdCYP707A1 and MdCYP707A2 were higher in Abz-E2B(+)NaCl(+) at 4 days, but declined at 12 days. The present data suggest that the increased endogenous ABA during the early stages of Fuji apple cultivation under salinity-stress conditions might be a good approach to prevent long-lasting damage and allow plants to recover.