Iridoid Synthase Activity Is Common among the Plant Progesterone 5β-Reductase Family

Catharanthus roseus, the Madagascar periwinkle, synthesizes bioactive monoterpenoid indole alkaloids, among which the anti-cancer drugs vinblastine and vincristine. The monoterpenoid branch of the alkaloid pathway leads to the secoiridoid secologanin and involves the enzyme iridoid synthase (IS), a member of the progesterone 5β-reductase (P5βR) family. IS reduces 8-oxogeranial to iridodial. Through transcriptome mining, we show that IS belongs to a family of six C. roseus P5βR genes. Characterisation of recombinant CrP5βR proteins demonstrates that all but CrP5βR3 can reduce progesterone, and thus can be classified as P5βRs. Three of them, namely CrP5βR1, CrP5βR2 and CrP5βR4, could also reduce 8-oxogeranial, pointing to a possible redundancy with IS (corresponding to CrP5βR5) in secoiridoid synthesis. In depth functional analysis by subcellular protein localisation, gene expression analysis, in situ hybridisation and virus-induced gene silencing, indicates that besides IS, CrP5βR4 may also participate in secoiridoid biosynthesis. Finally, we cloned a set of P5βR genes from angiosperm plant species not known to produce iridoids and demonstrate that the corresponding recombinant proteins are also capable of using 8-oxogeranial as a substrate. This suggests that 'IS activity' is intrinsic to angiosperm P5βR proteins and has evolved early during evolution.

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.