16-O-methyltransferase involved in the biosynthesis of vindoline. Highly specific for 16-hydroxytabersonine. No activity with tabersonine, 3-hydroxytyramine, 4-hydroxytyramine, 5-hydroxytryptamine (5HT), 2,3-dihydro-3-hydroxytabersonine, lochnericine, hoerhammericine, 16-hydroxy-2,3-dihydro-3-hydroxytabersonine, 16-hydroxylochnericine, 16-hydroxyhoerhammericine, quercetin, kaempferol and caffeic acid as substrates. 16-hydroxytabersonine + S-adenosyl-L-methionine = 16-methoxytabersonine + H(+) + S-adenosyl-L-homocysteine Optimum pH is 7.0-7.5. Alkaloid biosynthesis; vindoline biosynthesis. Homodimer. Expressed in leaves and flowers. Detected in stems and roots. In leaves, expressed in epidermal cells. Peak of expression in 8-day-old seedlings. Highest expression in the youngest leaf pair. Up-regulated by herbivory. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family. COMT subfamily.