Gene discovery and virus-induced gene silencing reveal branched pathways to major classes of bioactive diterpenoids in Euphorbia peplus

Most macro-and polycyclic Euphorbiaceae diterpenoids derive from the common C20 precursor casbene. While the biosynthetic pathway from casbene to the lathyrane jolki-nol C is characterized, pathways to other more complex classes of bioactive diterpenoids remain to be elucidated. A metabolomics-guided transcriptomic approach and a geno-mics approach that led to the discovery of two casbene-derived diterpenoid gene clusters yielded a total of 68 candidate genes that were transiently expressed in Nicotiana ben-thamiana for activity toward jolkinol C and other lathyranes. We report two short -chain dehydrogenases/reductases (SDRs), identified by RNA sequencing to be highly expressed in Euphorbia peplus latex. One of these, EpSDR-5, is a C3-ketoreductase, converting jolkinol C to the lathyrane jolkinol E. Gene function of EpSDR-5 was fur-ther confirmed by heterologous expression in Saccharomyces cerevisiae. To investigate the in vivo role of EpSDR-5, we established virus-induced gene silencing (VIGS) in E. peplus, resulting in a significant reduction in jatrophanes and a corresponding increase in ingenanes. VIGS of Casbene Synthase results in a major reduction in both jatrophanes and ingenanes, the two most abundant classes of E. peplus diterpenoids. VIGS of CYP71D365 had a similar effect, consistent with the previously determined role of this gene in the pathway to jolkinol C. These results point to jolkinol C being a branch point intermediate in the pathways to ingenanes and jatrophanes with EpSDR-5 responsible for the first step from jolkinol C to jatrophane production.

Automated summary

The majority of macro-and polycyclic Euphorbiaceae diterpenoids are derived from the common C20 precursor casbene, but the biosynthetic pathways for some of these diterpenoids are not yet clear. Using a metabolomics-guided transcriptomic approach, two casbene-derived diterpenoid gene clusters were discovered, with one gene identified as playing a crucial role in converting casbene to lathyrane diterpenoids.