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Abstract The inversion of C3 stereochemistry in monoterpenoid indole alkaloids (MIAs), derived from the central precursor strictosidine (3S), is essential for synthesizing numerous 3RMIAs and oxindoles, including the antihypertensive drug reserpine found inRauvolfia serpentina(Indian snakeroot) andRauvolfia tetraphylla(devil pepper) of the plant family Apocynaceae. MIA biosynthesis begins with the reduction of strictosidine aglycone by various reductases, preserving the initial 3Sstereochemistry. In this study, we identify and biochemically characterize a conserved oxidase-reductase pair from the Apocynaceae, Rubiaceae, and Gelsemiaceae families of the order Gentianales: the heteroyohimbine/yohimbine/corynanthe C3-oxidase (HYC3O) and C3-reductase (HYC3R). These enzymes collaboratively invert the 3Sstereochemistry to 3Racross a range of substrates, resolving the long-standing question about the origin of 3RMIAs and oxindole derivatives, and facilitation of reserpine biosynthesis. Notably,HYC3OandHYC3Rare located within gene clusters in both theR. tetraphyllaandCatharanthus roseus(Madagascar periwinkle) genomes, which are partially homologous to an elusive geissoschizine synthase (GS) gene cluster we also identified in these species. InR. tetraphylla, these clusters occur closely in tandem on a single chromosome, likely stemming from a single segmental duplication event, while inC. roseus, a closely related member of rauvolfioid Apocynaceae, they were later separated by a chromosomal translocation. The ancestral genomic context for both clusters can be traced all the way back to common ancestry with grapevine. Given the presence of syntenic GS homologs inMitragyna speciosa(Rubiaceae), the GS cluster, at least in part, probably evolved at the base of the Gentianales, which split from other core eudicots up to 135 million years ago. We also show that the strictosidine biosynthetic gene cluster, required to initiate the MIA pathway, plausibly evolved concurrently. The reserpine biosynthetic cluster likely arose much later in the rauvolfioid lineage of Apocynaceae. Collectively, our work uncovers the genomic and biochemical basis for key events in MIA evolution and diversification, providing insights beyond the well-characterized vinblastine and ajmaline biosynthetic pathways.