Strigolactones (SLs), first identified as germination stimulants for root parasitic weeds, act as endogenous phytohormones regulating shoot branching and as root-derived signal molecules mediating symbiotic communication in the rhizosphere. SLs are classified into two types, canonical and non-canonical. The core structure of canonical SLs consists of a tricyclic lactone ring (ABC ring) system linked to a methyl butenolide (D-ring), while non-canonical SLs possess incomplete ABC-ring systems.
Cowpea (Vigna unguiculata) is a high producer of orobanchol, the most widely distributed canonical SL, and the primary host of root parasitic weed Striga gesnerioides, which requires orobanchol for seed germination. The biosynthesis of orobanchol in cowpea involves the conversion of carlactonoic acid (CLA), a common precursor of SL biosynthesis, to 18-oxocarlactonoic acid (18-oxo-CLA) catalyzed by VuCYP722C. Subsequently, 18-oxo-CLA is stereospecifically cyclized by VuSRF to form orobanchol. We found a VuSRF-like gene (VuSRFL) in the cowpea genome. We discovered that VuSRFL is responsible for converting 18-oxo-CLA to a novel non-canonical SL designated as orobanchonoic acid via orobanchol. The novel non-canonical SL was also identified in cowpea root exudates. This discovery of a non-canonical SL formation from a canonical SL by cleaving the C-ring enhances our understanding of the SL biosynthesis process. The structure of the novel SL and its ability to induce germination towards root parasitic weeds will be presented.