Alterations in the silymarin metabolism in transgenic Silybum marianum cultured cells by the heterologous expression of the Arabidopsis thaliana V-myb myeloblastosis viral oncogene homolog transcription factor MYB12 and Cicer arietinum chalcone synthase

Abstract

Silymarin (Sm), the main bioactive principle of Silybum marianum (L.) Gaernt. fruits, has strong antihepatotoxic properties and also exhibits antitumor and antiviral activity. This flavonolignan complex is synthesized from the flavonoid precursor taxifolin, which undergoes an oxidative coupling with the monolignol coniferyl alcohol. Attempts to produce Sm in plant cell suspensions have met with limited success, probably due to insufficient flux into the flavonoid branch pathway of Sm biosynthesis.In this study, it was attempted to increase Sm production in in vitro cultures of S. marianum by genetically engineering the flavonoid pathway. Using Agrobacterium tumefaciens strain C58C1, S. marianum cell suspension cultures were transformed either with the Arabidopsis thaliana transcription factor AtMYB12 (V-myb myeloblastosis viral oncogene homolog) (MYB) or with a chalcone synthase (CHS) gene from Cicer arietinum. A double transformant was also generated by introducing the CaCHS gene in AtMYB12-expressing cultures.The ectopic expression of AtMYB12 in cultures activated some of the genes involved in the flavonoid pathway, notably chalcone isomerase (CHI) and flavonoid 3′-hydroxylase (F3’H), whereas the secondary metabolites altered by the inserted gene were chlorogenic acid glycoside derivatives. Expression of CaCHS in the cultures led to the production of the flavanone naringenin (Ng), which accumulated as glycosides. The simultaneous expression of AtMYB12-CaCHS resulted in the production of lower amounts of chlorogenic derivatives but similar levels of Ng glycosides compared with the CaCHS lines. Both CaCHS lines and the double transformant could be maintained over prolonged subcultures without Ng production losses, confirming long-term and stable CaCHS transgene expression.In no case did the inserted genes have a discernable effect on Sm production compared with the untransformed cultures, neither under control culture conditions or in cultures elicited with MeJA.