Activity, but not Expression, of Soluble and Cell Wall-Bound Acid Invertases Is Induced by Abscisic Acid in Developing Apple Fruit

The present experiment, involving both the in vivo injection of abscisic acid (ABA) into apple (Malus domestica Brohk.) fruits and the in vivo incubation of fruit tissues in ABA-containing medium, revealed that ABA activates both soluble and cell wall-bound acid invertases. Immunoblotting and enzyme-linked immunosorbent assays showed that this ABA-induced acid invertase activation is independent of the amount of enzyme present. The acid invertase activation induced by ABA is dependent on medium pH, time course, ABA dose, living tissue and developmental stage. Two isomers of cis-(+)-ABA, (-)-ABA and trans-ABA, had no effect on acid invertases, showing that ABA-induced acid invertase activation is specific to physiologically active cis-(+)ABA. Protein kinase inhibitors K252a and H7 as well as acid phosphatase increased the ABA-induced effects. These data indicate that ABA specifically activates both soluble and cell wall-bound acid invertases by a posttranslational mechanism probably involving reversible protein phosphorylation, and this may be one of the mechanisms by which ABA is involved in regulating fruit development.