Lupin pyranoisoflavones inhibiting hyphal development in arbuscular mycorrhizal fungi

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.