RNAi-mediated gene knockdown of progesterone 5β-reductases in Digitalis lanata reduces 5β-cardenolide content

Studying RNAi-mediated DlP5βR1 and DlP5βR2 knockdown shoot culture lines of Digitalis lanata, we here provide direct evidence for the participation of PRISEs (progesterone 5β-reductase/iridoid synthase-like enzymes) in 5β-cardenolide formation. Progesterone 5β-reductases (P5βR) are assumed to catalyze the reduction of progesterone to 5β-pregnane-3,20-dione, which is a crucial step in the biosynthesis of the 5β-cardenolides. P5βRs are encoded by VEP1-like genes occurring ubiquitously in embryophytes. P5βRs are substrate-promiscuous enone-1,4-reductases recently termed PRISEs (progesterone 5β-reductase/iridoid synthase-like enzymes). Two PRISE genes, termed DlP5βR1 (AY585867.1) and DlP5βR2 (HM210089.1) were isolated from Digitalis lanata. To give experimental evidence for the participation of PRISEs in 5β-cardenolide formation, we here established several RNAi-mediated DlP5βR1 and DlP5βR2 knockdown shoot culture lines of D. lanata. Cardenolide contents were lower in D. lanata P5βR-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 P5βR-RNAi lines than in the wild-type shoots. Cardenolide biosynthesis was restored by buthionine sulfoximine (BSO) treatment in Dl P5βR2-RNAi lines but not in Dl P5βR1-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 P5βR1 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).

PRISEs (progesterone 5β-reductase and/or iridoid synthase-like 1,4-enone reductases): Catalytic and substrate promiscuity allows for realization of multiple pathways in plant metabolism

PRISEs (progesterone 5β-reductase and/or iridoid synthase-like 1,4-enone reductases) are involved in cardenolide and iridoid biosynthesis. We here investigated a PRISE (rAtSt5βR) from Arabidopsis thaliana, a plant producing neither cardenolides nor iridoids. The structure of rAtSt5βR was elucidated with X-ray crystallography and compared to the known structures of PRISEs from Catharanthus roseus (rCrISY) and Digitalis lanata (rDlP5βR). The three enzymes show a high degree of sequence and structure conservation in the active site. Amino acids previously considered to allow discrimination between progesterone 5β-reductase and iridoid synthase were interchanged among rAtSt5βR, rCrISY and rDlP5βR applying site-directed mutagenesis. Structural homologous substitutions had different effects, and changes in progesterone 5β-reductase and iridoid synthase activity were not correlated in all cases. Our results help to explain fortuitous emergence of metabolic pathways and product accumulation. The fact that PRISEs are found ubiquitously in spermatophytes insinuates that PRISEs might have a more general function in plant metabolism such as, for example, the detoxification of reactive carbonyl species.

and their putative role in cardenolide biosynthesis

3β-Hydroxysteroid dehydrogenases (3βHSD) are supposed to be involved in cardenolide biosynthesis in plants. Erysimum crepidifolium Rchb., a member of the Brassicaceae accumulating cardenolides, is a close relative to Arabidopsis thaliana. Full length cDNAs encoding for three individual 3βHSDs (EcHSD1, EcHSD2, EcHSD3) were isolated from E. crepidifolium leaves. EcHSD1 and EcHSD2 encode proteins assembled from 257 amino acids whereas EcHSD3 encodes a protein assembled from 260 amino acids. All three proteins qualify as members of the short-chain dehydrogenases/reductases family of proteins (SDRs). EcHSD1 and EcHSD2 shared a high amino acid sequence identity of about 86% and 91% with putative 3βHSDs of A. thaliana (AT2G47140 and AT2G47130). EcHSD3 showed high homology to the A. thaliana SDRs AT2G47150 (74%) and AT2G47120 (81%). All three EcHSD genes were expressed in Escherichia coli and the recombinant enzymes were characterized biochemically. All three recombinant EcHSDs catalyzed the dehydrogenation of pregnenolone and the 3-reduction of 5α/β-pregnane-3,20-dione when NAD and NADH were used as cosubstrates, respectively. After exposure to different stress conditions, no increased transcription was seen for EcHSD1 whereas EcHSD2 was expressed four times higher under osmotic stress than under control conditions. EcHSD3 expression was 10 times and 6 times higher after osmotic stress and MeJA treatment, respectively, than in controls.