Involved in monoterpene indole alkaloids (MIAs) biosynthesis (PubMed:29942076). Converts by aromatization the tetrahydro-beta-carboline alkaloids tetrahydroalstonine and ajmalicine to the corresponding beta-carboline alkaloids alstonine and serpentine, respectively (PubMed:29942076). A + O2 + reduced [NADPH--hemoprotein reductase] + tetrahydroalstonine = AH2 + alstonine + H(+) + 2 H2O + oxidized [NADPH--hemoprotein reductase] A + ajmalicine + O2 + reduced [NADPH--hemoprotein reductase] = AH2 + H(+) + 2 H2O + oxidized [NADPH--hemoprotein reductase] + serpentine Alkaloid biosynthesis. Highly expressed in stems (PubMed:29942076). Expressed at low levels in roots (PubMed:29942076). Belongs to the cytochrome P450 family.