Ancient gene clusters govern the initiation of monoterpenoid indole alkaloid biosynthesis and C3 stereochemistry inversion

The inversion of C3 stereochemistry in monoterpenoid indole alkaloids (MIAs), derived from the central precursor strictosidine (3 S ), is a critical step for the biosynthesis of numerous 3 R MIAs and spirooxindoles, including the antihypertensive drug reserpine. While early MIA biosynthesis preserves the 3 S configuration, the mechanism underlying C3 inversion has remained unresolved. Here, we identify and biochemically characterize a conserved oxidase-reductase pair in the Gentianales order: the heteroyohimbine/yohimbine/corynanthe C3-oxidase (HYC3O) and C3-reductase (HYC3R), which together invert the 3 S stereochemistry to 3 R across diverse substrates. Notably, HYC3O and HYC3R reside in gene clusters in Rauvolfia tetraphylla and Catharanthus roseus , homologous to an elusive geissoschizine synthase (GS) cluster we also uncovered. In R. tetraphylla , these clusters are in tandem on a single chromosome, likely derived from segmental duplication, whereas in C. roseus they reside on separate chromosomes due to translocation. Comparative genomics indicate the GS cluster originated at the base of Gentianales (∼135 Mya), coinciding with the evolution of the strictosidine synthase cluster, while the reserpine cluster arose later in rauvolfioid Apocynaceae. Together, these findings uncover the genomic and biochemical basis for key events in MIA evolution and diversification, providing insights beyond the canonical vinblastine and ajmaline biosynthetic pathways.