Tandem gene duplication drives the molecular evolution of orobanchol into a novel noncanonical strigolactone with weak germination-inducing activity toward root parasitic weeds in cowpea

Cowpea (Vigna unguiculata), a vital grain legume in West Africa, faces substantial yield losses due to the root parasitic weed Striga gesnerioides. The germination of Strigarelies on cowpea-produced strigolactones (SLs), including orobanchol and orobanchyl acetate. In cowpea, SL biosynthesis involves a stereoselective BC-ring-forming factor (SRF), a dirigent domain-containing protein that catalyzes the conversion of 18-oxo-carlactonoic acid (18-oxo-CLA) to orobanchol. VuSRF-like, an SRF homolog, has been identified as a tandemly duplicated gene of VuSRFin the cowpea genome, although its function remains unclear. Herein, we identified VuSRF-like as an additional biosynthetic enzyme that catalyzes the two-step conversion of 18-oxo-CLA to a novel SL via orobanchol. Structural analysis revealed that this novel SL, designated as orobanchonoic acid, features a cleaved C-ring and an oxidized hydroxy group at C-4. Orobanchonoic acid was detected in substantial amounts in cowpea root exudates but exhibited lower germination-inducing activity toward S. gesnerioidesand other parasitic weeds compared to orobanchol. Genomic comparisons among related legume species suggested that the tandem duplication of SRFin cowpea drove the neofunctionalization of VuSRF-like, facilitating orobanchonoic acid biosynthesis. This study provides new insights into SL biosynthesis, providing new opportunities to investigate SL evolution and rhizosphere interactions.