Lupin pyranoisoflavones inhibiting hyphal development in arbuscular mycorrhizal fungi

Abstract

White lupin ( Lupinus albus L.), a non-host plant for arbuscular mycorrhizal (AM) fungi in the typically mycotrophic family Fabaceae, has been investigated for root metabolites that inhibit hyphal development in AM fungi. Four known pyranoisoflavones, licoisoflavone B ( 1), sophoraisoflavone A ( 2), alpinumisoflavone ( 3) and 3′-hydroxy-4′- O-methylalpinumisoflavone ( 4), together with three previously unknown pyranoisoflavones, lupindipyranoisoflavone A ( 5), 10′-hydroxylicoisoflavone B ( 6) and 10′-hydroxysophoraisoflavone A ( 7) were isolated from the root exudates of white lupin as an inhibitor of germ tube growth in the AM fungus Gigaspora margarita. Pyranoisoflavones 1, 2 and 3 strongly inhibited germ tube growth at 0.63, 1.25 and 0.63 μg/disc, respectively. The remaining compounds 4– 7 were either moderate or weak inhibitors that inhibited germ tube growth at concentrations higher than 10 μg/disc. Licoisoflavone B ( 1) and sophoraisoflavone A ( 2) completely inhibited hyphal branching induced by a lupin strigolactone, orobanchyl acetate, in G. margarita at 0.16 and 0.63 μg/disc, respectively.