Cowpea (Vigna unguiculata) is an important grain legume in sub-Saharan Africa, but its production is constrained by the parasitic weed Striga gesnerioides. S. gesnerioides germinate in response to orobanchol, an strigolactone released from cowpea roots. Our research on orobanchol biosynthesis in cowpea revealed that carlactonoic acid (CLA) is converted to orobanchol through 18-oxo-CLA. We identified the enzymes responsible for these conversions as VuCYP722C and VuSRF. In addition, we discovered a paralog (VuSRF-like) of VuSRF in the cowpea genome. VuSRF-like converts orobanchol to orobanchonoic acid, which was also found in cowpea root exudates.
Germination-inducing assays revealed that orobanchonoic acid is much less effective than orobanchol in stimulating the germination of S. gesnerioides seeds. Analysis of root exudates from azuki bean and mung bean, both closely related leguminous plants to cowpea, showed the presence of orobanchol but not orobanchonoic acid in these legumes. Azuki bean and mung bean, which are not parasitized by S. gesnerioides, have only one copy of SRF in the putative syntenic regions to the cowpea genome. Since the conversion of orobanchol to orobanchonoic acid reduces its ability to stimulate the germination of S. gesnerioides seeds, the neofunctionalization of VuSRF-like is advantageous for cowpea as it makes its roots less susceptible to attachment by the germinated parasite seeds. Further understanding of the other functions of orobanchol and orobanchonoic acid in the rhizosphere will provide more insight into the conversion.