Component of the seco-iridoid and derivatives monoterpenoid indole alkaloids (MIAs, e.g. catharanthine, tabersonine, vincadifformine, vindoline, vincristine, quinine and strychnine) biosynthesis pathway. Catalyzes iminium reduction on 4,21-dehydrogeissoschizine to produce 19E-geissoschizine, precursor of catharanthine and tabersonine derivatives (PubMed:29147812, PubMed:30256480). May also catalyze the production of reactive intermediate used by the HL1, HL2, HL3 and HL4 to form catharanthine, vincadifformine and tabersonine (PubMed:30256480). (19S)-geissoschizine + NADP(+) = 4,21-dehydrogeissoschizine + NADPH Binds 2 Zn(2+) ions per subunit. Alkaloid biosynthesis. Homodimer. Expressed in leaf epidermis. Induced by methyl jasmonate (MeJA, MJ) and silver nitrate (AgNO(3)). Accumulation of ajmalicine at the expense of catharanthine and vindoline production (PubMed:29147812, PubMed:30256480). Lower tabersonine-forming activity (PubMed:30256480). Belongs to the zinc-containing alcohol dehydrogenase family. Class-III subfamily.