A Seed-Specific Regulator of Triterpene Saponin Biosynthesis in Medicago truncatula.

Plants produce a vast array of defense compounds to protect themselves from pathogen attack or herbivore predation. A specific class of defense compounds are the saponins, bioactive glycosides with a steroidal or triterpenoid aglycone backbone. The model legume Medicago truncatula synthesizes two types of saponins, hemolytic saponins and non-hemolytic soyasaponins, which accumulate as specific blends in the different organs of the plant. Here we report the identification of a seed-specific transcription factor, TRITERPENE SAPONIN ACTIVATION REGULATOR 3 (TSAR3), which controls the hemolytic saponin biosynthesis in developing M. truncatula seeds. Co-expression analyses with TSAR3 in transcriptome datasets from developing M. truncatula seeds led to the identification of CYP88A13, a cytochrome P450 that catalyzes the C-16alpha hydroxylation of medicagenic acid towards zanhic acid, the final, yet unknown, oxidation step of the hemolytic saponin biosynthesis branch in M. truncatula. In addition, two uridine diphosphate glycosyltransferases, UGT73F18 and UGT73F19, which glucosylate hemolytic sapogenins at the C-3 position, were identified. The identified biosynthesis enzymes are organized in clusters of duplicated genes on the M. truncatula genome. This appears to be a quite common theme among saponin biosynthesis genes, especially glycosyltransferases, and may be the driving force of saponin metabolic evolution.