Steroidal scaffold decorations in Solanum alkaloid biosynthesis.

Steroidal glycoalkaloids (SGAs) are specialized metabolites produced by hundreds of Solanum species, including important vegetable crops such as tomato, potato and eggplant. Though SGAs are better known for their role in defence in plants and 'anti-nutritional' effects (e.g., toxicity and bitterness) to humans, many of these molecules have documented anti-cancer, anti-microbial, anti-inflammatory, anti-viral and anti-pyretic activities. Among these, α-solasonine and α-solamargine isolated from black nightshade (Solanum nigrum), are reported to have potent anti-tumor, anti-proliferative and anti-inflammatory activities. Notably, α-solasonine and α-solamargine, along with the core steroidal aglycone solasodine are the most widespread SGAs produced among the Solanum plants. However, it is still unknown how plants synthesize these bioactive steroidal molecules. Through comparative metabolomic-transcriptome guided approach, biosynthetic logic, combinatorial expression in Nicotiana benthamiana and functional recombinant enzyme assays, here we report the discovery of 12 enzymes from S. nigrum that converts the staring cholesterol precursor to solasodine aglycone, and the downstream α-solasonine, α-solamargine and malonyl-solamargine SGA products. We further identified 6 enzymes from cultivated eggplant that catalyse the production of α-solasonine, α-solamargine and malonyl-solamargine SGAs from solasodine aglycone, via glycosylation and atypical malonylation decorations. Our work provides the gene tool box and platform for engineering the production of high value, steroidal bioactive molecules in heterologous hosts using synthetic biology.

A BAHD-type acyltransferase concludes the biosynthetic pathway of non-bitter glycoalkaloids in ripe tomato fruit.

Tomato is the highest value fruit and vegetable crop worldwide, yet produces α-tomatine, a renowned toxic and bitter-tasting anti-nutritional steroidal glycoalkaloid (SGA) involved in plant defense. A suite of modifications during tomato fruit maturation and ripening converts α-tomatine to the non-bitter and less toxic Esculeoside A. This important metabolic shift prevents bitterness and toxicity in ripe tomato fruit. While the enzymes catalyzing glycosylation and hydroxylation reactions in the Esculeoside A pathway have been resolved, the proposed acetylating step remains, to date, elusive. Here, we discovered that GAME36 (GLYCOALKALOID METABOLISM36), a BAHD-type acyltransferase catalyzes SGA-acetylation in cultivated and wild tomatoes. This finding completes the elucidation of the core Esculeoside A biosynthetic pathway in ripe tomato, allowing reconstitution of Esculeoside A production in heterologous microbial and plant hosts. The involvement of GAME36 in bitter SGA detoxification pathway points to a key role in the evolution of sweet-tasting tomato as well as in the domestication and breeding of modern cultivated tomato fruit.

Spatial localization of monoterpenoid indole alkaloids in Rauvolfia tetraphylla by high resolution mass spectrometry imaging.

Monoterpenoid indole alkaloids (MIAs) are a large group of biosynthetic compounds, which have pharmacological properties. One of these MIAs, reserpine, was discovered in the 1950s and has shown properties as an anti-hypertension and anti-microbial agent. Reserpine was found to be produced in various plant species within the genus of Rauvolfia. However, even though its presence is well known, it is still unknown in which tissues Rauvolfia produce reserpine and where the individual steps in the biosynthetic pathway take place. In this study, we explore how matrix assisted laser desorption ionization (MALDI) and desorption electrospray ionization (DESI) mass spectrometry imaging (MSI) can be used in the investigation of a proposed biosynthetic pathway by localizing reserpine and the theoretical intermediates of it. The results show that ions corresponding to intermediates of reserpine were localized in several of the major parts of Rauvolfia tetraphylla when analyzed by MALDI- and DESI-MSI. In stem tissue, reserpine and many of the intermediates were found compartmentalized in the xylem. For most samples, reserpine itself was mainly found in the outer layers of the sample, suggesting it may function as a defense compound. To further confirm the place of the different metabolites in the reserpine biosynthetic pathway, roots and leaves of R. tetraphylla were fed a stable-isotope labelled version of the precursor tryptamine. Subsequently, several of the proposed intermediates were detected in the normal version as well as in the isotope labelled versions, confirming that they were synthesized in planta from tryptamine. In this experiment, a potential novel dimeric MIA was discovered in leaf tissue of R. tetraphylla. The study constitutes to date the most comprehensive spatial mapping of metabolites in the R. tetraphylla plant. In addition, the article also contains new illustrations of the anatomy of R. tetraphylla.

Carbaborane-substituted 1,2,3-triphospholanes and 1-aza-2,5-diphospholane: new synthetic approaches.

New phosphorus-containing, five-membered P,P,P and P,N,P heterocycles were synthesized and fully characterized. The P,P,P heterocycles, 1,2,3-triphospholanes, can be synthesized by two different facile pathways, whereas the P,N,P compound, a 1-aza-2,5-diphospholane, can only be obtained with silylamine.