Classification, biosynthesis, and biological functions of triterpene esters in plants.

Triterpene esters comprise a class of secondary metabolites that are synthesized by decorating triterpene skeletons with a series of oxidation, glycosylation, and acylation modifications. Many triterpene esters with important bioactivities have been isolated and identified, including those with applications in the pesticide, pharmaceutical, and cosmetic industries. They also play essential roles in plant defense against pests, diseases, physical damage (as part of the cuticle), and regulation of root microorganisms. However, there has been no recent summary of the biosynthetic pathways and biological functions of plant triterpene esters. Here, we classify triterpene esters into five categories based on their skeletons and find that C-3 oxidation may have a significant effect on triterpenoid acylation. Fatty acid and aromatic moieties are common ligands present in triterpene esters. We further analyze triterpene ester synthesis-related acyltransferases (TEsACTs) in the triterpene biosynthetic pathway. Using an evolutionary classification of BAHD acyltransferases (BAHD-ATs) and serine carboxypeptidase-like acyltransferases (SCPL-ATs) in Arabidopsis thaliana and Oryza sativa, we classify 18 TEsACTs with identified functions from 11 species. All the triterpene-skeleton-related TEsACTs belong to BAHD-AT clades IIIa and I, and the only identified TEsACT from the SCPL-AT family belongs to the CP-I subfamily. This comprehensive review of the biosynthetic pathways and bioactivities of triterpene esters provides a foundation for further study of their bioactivities and applications in industry, agricultural production, and human health.

Deciphering the network of cholesterol biosynthesis in Paris polyphylla laid a base for efficient diosgenin production in plant chassis.

Cholesterol serves as a key precursor for many high-value chemicals such as plant-derived steroidal saponins and steroidal alkaloids, but a plant chassis for effective biosynthesis of high levels of cholesterol has not been established. Plant chassis have significant advantages over microbial chassis in terms of membrane protein expression, precursor supply, product tolerance, and regionalization synthesis. Here, using Agrobacterium tumefaciens-mediated transient expression technology, Nicotiana benthamiana, and a step-by-step screening approach, we identified nine enzymes (SSR1-3, SMO1-3, CPI-5, CYP51G, SMO2-2, C14-R-2, 8,7SI-4, C5-SD1, and 7-DR1-1) from the medicinal plant Paris polyphylla and established detailed biosynthetic routes from cycloartenol to cholesterol. Specfically, we optimized HMGR, a key gene of the mevalonate pathway, and co-expressed it with the PpOSC1 gene to achieve a high level of cycloartenol (28.79 mg/g dry weight, which is a sufficient amount of precursor for cholesterol biosynthesis) synthesis in the leaves of N. benthamiana. Subsequently, using a one-by-one elimination method we found that six of these enzymes (SSR1-3, SMO1-3, CPI-5, CYP51G, SMO2-2, and C5-SD1) were crucial for cholesterol production in N. benthamiana, and we establihed a high-efficiency cholesterol synthesis system with a yield of 5.63 mg/g dry weight. Using this strategy, we also discovered the biosynthetic metabolic network responsible for the synthesis of a common aglycon of steroidal saponin, diosgenin, using cholesterol as a substrate, obtaining a yield of 2.12 mg/g dry weight in N. benthamiana. Our study provides an effective strategy to characterize the metabolic pathways of medicinal plants that lack a system for in vivo functional verification, and also lays a foundation for the synthesis of active steroid saponins in plant chassis.