Abstract
Triterpenoids, with increasing evidence of their health beneficial properties, is a diverse group of secondary metabolites. Legumes biosynthesize various triterpenoids, including soyasaponins and glycyrrhizin. Glycyrrhizin is a triterpenoid glycoside (saponin) derived from licorice that is used as a natural sweetener and as a crude drug. Most legume triterpenoid saponins are derived from the common triterpene skeleton, β-amyrin. Cytochrome P450 monooxygenases (P450s) play critical roles in site-specific oxidations of triterpene skeleton; thereby create structural variation of triterpenoid aglycones. P450s exist as large gene families in plants, making it difficult to predict potential involvement of specific P450s in triterpenoid biosynthesis. In this study, the analysis of licorice ESTs and Medicago truncatula gene co-expression data led to the identification of several β-amyrin-oxidizing enzymes, including CYP88D6 (β-amyrin 11-oxidase) as the initial P450 in glycyrrhizin biosynthesis. Our observations indicate that in legumes, the P450 enzymes involved in triterpenoid biosynthesis are recruited in at least four very distant CYP families.
