Norharmane (Norharman), a β-carboline alkaloid, is a potent and reversible monoamine oxidase inhibitor, with IC 50 values of 6.5 and 4.7 μM for MAO-A and MAO-B , respectively. Norharmane causes antidepressant responses. Norharmane is also a prospective anti-cancer photosensitizer. Norharmane alters polar auxin transport (PAT) by inhibiting PIN2, PIN3 and PIN7 transport proteins, thus causing a significant inhibitory effect on the growth of Arabidopsis thaliana seedlings .