Cephalotaxinone enzymes reveal a whole plant model for homoharringtonine biosynthesis.

Homoharringtonine (HHT) is a plant-derived protein translation inhibitor used widely in ribosomal profiling and as a treatment for chronic myeloid leukemia approved by the US Food and Drug Administration. HHT is commercially semisynthesized from the alkaloid core cephalotaxine (CET) extracted from endangered Cephalotaxus species. Despite its importance, the CET/HHT biosynthetic pathway remains unresolved. Here, we use paired untargeted metabolomics (stable-isotope precursor feeding) and transcriptomics to elucidate a near-complete CET biosynthesis. We show that, while CET is biosynthesized only in growing root tips, CET and HHT accumulate throughout the plant. We identify seven pathway intermediates and six enzymes-including cytochrome P450s, an atypical short-chain dehydrogenase and a 2-oxoglutarate-dependent dioxygenase-that enable carbon excision and CET/HHT pentacyclic backbone formation to produce cephalotaxinone, the likely direct precursor of CET. This study establishes a metabolic route to the HHT core and suggests a whole-plant coordination model, in which cephalotaxinone is produced in root tips and distributed for elaboration into HHT.