Strychnine is a natural product that, through isolation, structural elucidation and synthetic efforts, shaped the field of organic chemistry. Currently, strychnine is used as a pesticide to control rodents¹because of its potent neurotoxicity^2,3. The polycyclic architecture of strychnine has inspired chemists to develop new synthetic transformations and strategies to access this molecular scaffold⁴, yet it is still unknown how plants create this complex structure. Here we report the biosynthetic pathway of strychnine, along with the related molecules brucine and diaboline. Moreover, we successfully recapitulate strychnine, brucine and diaboline biosynthesis inNicotiana benthamianafrom an upstream intermediate, thus demonstrating that this complex, pharmacologically active class of compounds can now be harnessed through metabolic engineering approaches.

The evolution of chemical complexity has been a major driver of plant diversification, with novel compounds serving as key innovations. The species-rich mint family (Lamiaceae) produces an enormous variety of compounds that act as attractants and defense molecules in nature and are used widely by humans as flavor additives, fragrances, and anti-herbivory agents. To elucidate the mechanisms by which such diversity evolved, we combined leaf transcriptome data from 48 Lamiaceae species and four outgroups with a robust phylogeny and chemical analyses of three terpenoid classes (monoterpenes, sesquiterpenes, and iridoids) that share and compete for precursors. Our integrated chemical-genomic-phylogenetic approach revealed that: (1) gene family expansion rather than increased enzyme promiscuity of terpene synthases is correlated with mono- and sesquiterpene diversity; (2) differential expression of core genes within the iridoid biosynthetic pathway is associated with iridoid presence/absence; (3) generally, production of iridoids and canonical monoterpenes appears to be inversely correlated; and (4) iridoid biosynthesis is significantly associated with expression of geraniol synthase, which diverts metabolic flux away from canonical monoterpenes, suggesting that competition for common precursors can be a central control point in specialized metabolism. These results suggest that multiple mechanisms contributed to the evolution of chemodiversity in this economically important family.

Solanaceae plants produce two major classes of valuable sterol-derived natural products—steroidal glycoalkaloids and steroidal saponins—from a common cholesterol precursor. Attempts to heterologously produce these molecules have consistently failed, although the genes responsible for each biosynthetic step have been identified. Here we identify a cellulose synthase–like protein, an unexpected biosynthetic component that interacts with the early pathway enzymes, enabling steroidal scaffolds production in plants. Moreover, knockout of this gene in black nightshade, Solanum nigrum , resulted in plants lacking both steroidal alkaloids and saponins. Unexpectedly, these knockout plants also revealed that steroidal saponins deter serious agricultural insect pests. This discovery provides the missing link to engineer these high-value steroidal molecules and also pinpoints the ecological role for steroidal saponins.