Medicaol, a strigolactone identified as a putative didehydro-orobanchol isomer, from Medicago truncatula

A major strigolactone produced by the model legume Medicago truncatula (barrel medic) has been tentatively identified as a didehydro-orobanchol isomer. In this study, a putative didehydro-orobanchol isomer was isolated from root exudates collected from barrel medic grown hydroponically under phosphate-starved conditions. The structure and absolute configurations of this strigolactone, named medicaol, were determined by a combination of spectroscopic analysis and spectral comparison with 4-deoxymedicaol which was synthesized using solvolysis and rearrangement of hydroxymethylhexahydroindenone to tetrahydroazulenone as a key step. Medicaol has a seven-membered cycloheptadiene in the A ring instead of a typical six-membered cyclohexene. Medicaol and 4-deoxymedicaol showed activity comparable to their corresponding six-membered A ring relatives, orobanchol and 4-deoxyorobanchol (ent-2'-epi-5-deoxystrigol), in inducing hyphal branching of germinating spores of an arbuscular mycorrhizal fungus Gigaspora margarita. Plausible biosynthetic pathways from 4-deoxyorobanchol to medicaol are also proposed.