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

Triterpene esters comprise a class of secondary metabolites that are synthesised by decorating triterpene skeletons with a series of oxidation, glycosylation and acylation modifications. Many triterpene esters have been isolated and identified with important bioactivities, including 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, a summary of the biosynthetic pathways and biological functions of plant triterpene esters is currently missing. Here, we classified triterpene esters into five categories in accordance with their skeletons and found that C-3 oxidation might impose a significant effect on triterpenoid acylation. Fatty acid and aromatic moieties were identified as common ligands present in triterpene esters. We analysed triterpene esters synthesis-related acyltransferases (TEsACTs) in the triterpene biosynthetic pathway. By employing the evolutionary classification of BAHD acyltransferases (BAHD-ATs) and serine carboxypeptidase-like acyltransferases (SCPL-ATs) in Arabidopsis thaliana and Oryza sativa as criteria, we classified 18 TEsACTs with identified functions in 11 species. Remarkably, all the triterpene skeleton-related TEsACTs belong to clades IIIa and I of BAHD-ATs and the only one identified TEsACT from the SCPL-AT family belongs to CP-I. In general, we provide a comprehensive review of triterpene ester biosynthetic pathways and bioactivities, providing a foundation for subsequent studies on the bioactivities of triterpene esters and their commercial applications, agricultural production and biological disaster prevention.