Catalyzes the oxidation of cytokinins, a family of N(6)-substituted adenine derivatives that are plant hormones, where the substituent is an isopentenyl group. A + H2O + N(6)-dimethylallyladenine = 3-methyl-2-butenal + adenine + AH2 Belongs to the oxygen-dependent FAD-linked oxidoreductase family.