Reconstitution of monoterpene indole alkaloid biosynthesis in Nicotiana benthamiana

Monoterpene indole alkaloids (MIAs) are a diverse and important class of plant natural products that include a number of medicinally significant compounds, often present at low concentrations within their native plant species. The complex biosynthesis of MIAs requires the assembly of tryptamine with a secoiridoid to produce the central intermediate, strictosidine, from which all known MIAs derive. Structural complexity makes chemical synthesis challenging, but recent efforts to identify the biosynthetic enzymes provide options for pathway reconstruction in a heterologous host. Previous attempts have had limited success, with yield in microorganisms limited by the poor expression of some enzymes. Here, we reconstitute the pathway for strictosidine biosynthesis from central metabolism without the need for supplementation of any metabolite precursors or intermediates in Nicotiana benthamiana. The best yields were obtained by the co-expression of 14 enzymes, of which a major latex protein-like enzyme (MLPL) from Nepeta (catmint) was critical for improving flux through the secoiridoid pathway. The production of strictosidine in planta expands the range of MIA products amenable to biological synthesis.

Jasmonate promotes artemisinin biosynthesis by activating the TCP14-ORA complex inArtemisia annua

Artemisia annuaproduces the valuable medicinal component, artemisinin, which is a sesquiterpene lactone widely used in malaria treatment. AaORA, a homolog of CrORCA3, which is involved in activating terpenoid indole alkaloid biosynthesis inCatharanthus roseus, is a jasmonate (JA)–responsive and trichome-specific APETALA2/ETHYLENE-RESPONSE FACTOR that plays a pivotal role in artemisinin biosynthesis. However, the JA signaling mechanism underlying AaORA-mediated artemisinin biosynthesis remains enigmatic. Here, we report that AaORA forms a transcriptional activator complex with AaTCP14 (TEOSINTE BRANCHED 1/CYCLOIDEA/PROLIFERATING CELL FACTOR 14), which is also predominantly expressed in trichomes. AaORA and AaTCP14 synergistically bind to and activate the promoters of two genes,double bond reductase 2(DBR2) andaldehyde dehydrogenase 1(ALDH1), both of which encode enzymes vital for artemisinin biosynthesis. AaJAZ8, a repressor of the JA signaling pathway, interacts with both AaTCP14 and AaORA and represses the ability of the AaTCP14-AaORA complex to activate theDBR2promoter. JA treatment induces AaJAZ8 degradation, allowing the AaTCP14-AaORA complex to subsequently activate the expression ofDBR2, which is essential for artemisinin biosynthesis. These data suggest that JA activation of the AaTCP14-AaORA complex regulates artemisinin biosynthesis. Together, our findings reveal a novel artemisinin biosynthetic pathway regulatory network and provide new insight into how specialized metabolism is modulated by the JA signaling pathway in plants.

Uncoupled activation and cyclisation in catmint reductive terpenoid biosynthesis

Terpene synthases typically form complex molecular scaffolds by concerted activation and cyclization of linear starting materials in a single enzyme active site. Here we show that iridoid synthase, an atypical reductive terpene synthase, catalyses the activation of its substrate 8-oxogeranial into a reactive enol intermediate but does not catalyse the subsequent cyclisation into nepetalactol. This discovery led us to identify a class of nepetalactol-related short-chain dehydrogenase enzymes (NEPS) from catmint (Nepeta mussinii) which catalyse the stereoselective cyclisation of the enol intermediate into nepetalactol isomers. Subsequent oxidation of nepetalactols by NEPS1 provides nepetalactones, metabolites that are well known for both insect-repellent activity and euphoric effect in cats. Structural characterisation of the NEPS3 cyclase reveals it binds to NAD+ yet does not utilise it chemically for a non-oxidoreductive formal [4+2] cyclisation. These discoveries will complement metabolic reconstructions of iridoid and monoterpene indole alkaloid biosynthesis.