A Single Oxidosqualene Cyclase Produces the -Triterpenoid α-Onocerin.

8,14--Triterpenoids are characterized by their unusual open C-ring. Their distribution in nature is rare and scattered in taxonomically unrelated plants. The 8,14--triterpenoid α-onocerin is only known from the evolutionarily distant clubmoss genus and the leguminous genus , which makes the biosynthesis of this -triterpenoid intriguing from an evolutionary standpoint. In our experiments with α-onocerin was detected only in the roots. Through transcriptome analysis of the roots, an oxidosqualene cyclase, OsONS1, was identified that produces α-onocerin from squalene-2,3;22,23-dioxide when transiently expressed in In contrast, in , two sequential cyclases, LcLCC and LcLCD, are required to produce α-onocerin in the transient expression system. Expression of in the lanosterol synthase knockout yeast strain GIL77, which accumulates squalene-2,3;22,23-dioxide, verified the α-onocerin production. A phylogenetic analysis predicts that OsONS1 branches off from specific lupeol synthases and does not group with the known α-onocerin cyclases. Both the biochemical and phylogenetic analyses of OsONS1 suggest convergent evolution of the α-onocerin pathways. When was coexpressed in leaves with either of the two squalene epoxidases, or , α-onocerin production was boosted, most likely because the epoxidases produce higher amounts of squalene-2,3;22,23-dioxide. Fluorescence lifetime imaging microscopy analysis demonstrated specific protein-protein interactions between OsONS1 and both squalene epoxidases. Coexpression of with the two s suggests that OsSQE2 is the preferred partner of OsONS1 in planta. Our results provide an example of the convergent evolution of plant specialized metabolism.