Untargeted investigation of Silybum spp.: exploring the specialized metabolome beyond silymarin

The genus Silybum (tribe Cardueae, family Asteraceae) currently includes two species, S. marianum (L.) Gaertn (milk thistle), well-known for its use in folk medicine, and the less common S. eburneum Coss. & Durieu. The achenes of both species accumulate high amounts of a mixture of flavonolignans derived from the oxidative coupling of taxifolin and coniferyl alcohol, collectively known as silymarin. The strong hepatoprotective properties of silymarin have focused phytochemical research in these taxa almost exclusively on flavonolignans, and on the mechanisms regulating their synthesis and accumulation. Notably, exhaustive characterizations for other components of the specialized metabolome of Silybum spp. are still scarce. In this study, we employed an untargeted metabolomics approach to characterize and compare the specialized metabolomes of two genotypes of S. marianum (leaves and achenes) and one genotype of S. eburneum (achenes). The UPLC-qTOF-MS analysis of the methanolic extracts revealed expected differences in silymarin profiles, and specific accumulation patterns for several metabolite classes. In the achenes, these included uncommon derivatives of hydroxycinnamoylquinic acids, lignanamides, and glycosides and dehydrated forms of flavonolignan isomers. Interestingly, two sulfated glycosides of syringic acid and two hydroxicinnamoylamides were exclusively found in S. eburneum. In the leaves of S. marianum, mainly characterized by luteolin and apigenin glucuronides, chlorogenic and dicaffeoylquinic acids, no silymarin flavonolignans were detected. Nonetheless, the presence of various metabolites possessing the guaiacyl (e.g., pinoresinol glycosides) and syringil moieties suggests a selective use of the monolignol precursors in S. marianum, primarily directed toward the synthesis of flavonolignan in the achenes and other lignans/neolignans in the leaves