Plants produce a vast array of defense compounds to protect themselves from pathogen attack or herbivore predation. Saponins are a specific class of defense compounds comprising bioactive glycosides with a steroidal or triterpenoid aglycone backbone. The model legume Medicago truncatula synthesizes two types of saponins, hemolytic saponins and nonhemolytic soyasaponins, which accumulate as specific blends in different plant organs. Here, we report the identification of the seed-specific transcription factor TRITERPENE SAPONIN ACTIVATION REGULATOR3 (TSAR3), which controls hemolytic saponin biosynthesis in developing M. truncatula seeds. Analysis of genes that are coexpressed with TSAR3 in transcriptome data sets from developing M. truncatula seeds led to the identification of CYP88A13, a cytochrome P450 that catalyzes the C-16α hydroxylation of medicagenic acid toward zanhic acid, the final 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 genes encoding the identified biosynthetic enzymes are present in clusters of duplicated genes in the M. truncatula genome. This appears to be a common theme among saponin biosynthesis genes, especially glycosyltransferases, and may be the driving force of the metabolic evolution of saponins.

植物产生大量防御化合物，以保护自身免受病原体侵袭或食草动物的侵害。皂苷是一类特殊的防御化合物，其包含具有甾体或三萜类苷元骨架的生物活性糖苷。模型豆科植物run藜苜蓿合成两种类型的皂苷，溶血皂苷和非溶血性大豆皂苷，它们以特定的混合物形式积累在不同的植物器官中。在这里，我们报告鉴定的种子特异性转录因子TRITERPENE皂苷激活调节剂3（TSAR3），其控制发育中的M. truncatula种子的溶血皂苷生物合成。在发育中的M. truncatula转录组数据集中与TSAR3共表达的基因分析导致CYP88A13，细胞色素P450朝向zanhic酸催化medicagenic酸的C-16α羟基化的识别种子，在溶血皂苷生物合成分支的最终氧化步骤蒺藜苜蓿。此外，还鉴定了两个尿苷二磷酸糖基转移酶UGT73F18和UGT73F19，它们在C-3位糖基化溶血性皂苷元。编码已鉴定的生物合成酶的基因存在于截枝分枝杆菌基因组中重复的基因簇中。这似乎是皂苷生物合成基因特别是糖基转移酶中的一个共同主题，并且可能是皂苷代谢过程中的驱动力。