RESUMEN
KEY MESSAGE: Studying RNAi-mediated DlP5ßR1 and DlP5ßR2 knockdown shoot culture lines of Digitalis lanata, we here provide direct evidence for the participation of PRISEs (progesterone 5ß-reductase/iridoid synthase-like enzymes) in 5ß-cardenolide formation. Progesterone 5ß-reductases (P5ßR) are assumed to catalyze the reduction of progesterone to 5ß-pregnane-3,20-dione, which is a crucial step in the biosynthesis of the 5ß-cardenolides. P5ßRs are encoded by VEP1-like genes occurring ubiquitously in embryophytes. P5ßRs are substrate-promiscuous enone-1,4-reductases recently termed PRISEs (progesterone 5ß-reductase/iridoid synthase-like enzymes). Two PRISE genes, termed DlP5ßR1 (AY585867.1) and DlP5ßR2 (HM210089.1) were isolated from Digitalis lanata. To give experimental evidence for the participation of PRISEs in 5ß-cardenolide formation, we here established several RNAi-mediated DlP5ßR1 and DlP5ßR2 knockdown shoot culture lines of D. lanata. Cardenolide contents were lower in D. lanata P5ßR-RNAi lines than in wild-type shoots. We considered that the gene knockdowns may have had pleiotropic effects such as an increase in glutathione (GSH) which is known to inhibit cardenolide formation. GSH levels and expression of glutathione reductase (GR) were measured. Both were higher in the Dl P5ßR-RNAi lines than in the wild-type shoots. Cardenolide biosynthesis was restored by buthionine sulfoximine (BSO) treatment in Dl P5ßR2-RNAi lines but not in Dl P5ßR1-RNAi lines. Since progesterone is a precursor of cardenolides but can also act as a reactive electrophile species (RES), we here discriminated between these by comparing the effects of progesterone and methyl vinyl ketone, a small RES but not a precursor of cardenolides. To the best of our knowledge, we here demonstrated for the first time that P5ßR1 is involved in cardenolide formation. We also provide further evidence that PRISEs are also important for plants dealing with stress by detoxifying reactive electrophile species (RES).