Abstract
The saponins of the model legume Medicago truncatula are glycosides of at least five different triterpene aglycones, soyasapogenol B/E, medicagenic acid, hederagenin and bayogenin. These aglycones are most likely derived from β-amyrin, a product of the cyclization of 2,3-oxidosqualene. Mining M. truncatula EST datasets led to the identification of sequences putatively encoding three early enzymes of triterpene aglycone formation; squalene synthase(SS), squalene epoxidase(SE), and β-amyrin synthase(β-AS). SS was functionally characterized by expression in E. coli, two forms of SE by complementation of the yeast erg1 mutant, and β-AS by expression yeast. Transcript encoding β-AS, SS and one form of SE were strongly and co-ordinately induced, associated with accumulation of triterpene, upon exposure of M. truncatula cell suspension cultures to methyl jasmonate. Molecular verification of induction of triterpene pathway in a cell culture system provides a new tool for saponin pathway gene discovery by DNA array-based approaches.
