Catharanthus roseus, the Madagascar periwinkle, synthesizes bioactive
monoterpenoid indole alkaloids, including 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. Characterization 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, can also reduce 8-oxogeranial, pointing to a possible redundancy with IS (corresponding to CrP5βR5) in secoiridoid synthesis. In-depth functional analysis by subcellular
protein localization, gene expression analysis, in situ hybridization, and virus-induced gene silencing indicate that besides IS, CrP5βR4 may also participate in secoiridoid biosynthesis. 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.