Fungal membrane responses induced by plant defensins and thionins

Treatment of hyphae of Neurospora crassa with anti fungal plant defensins, i.e. Rs-AFP2 and Dm-AMP1 isolated from radish and dahlia seed, respectively, induced a rapid K+ efflux, Ca2+ uptake, and alkalinization of the incubation medium, The Rs-AFP2-induced alkalinization of the incubation medium could be inhibited with G-protein inhibitors. alpha-Hordothionin, an antifungal thionin from barley seed, caused a sustained increased Ca2+ uptake at subinhibitory concentrations but only a transient increased uptake at inhibitory concentrations, alpha-Hordothionin also caused increased K+ efflux and alkalinization of the medium, but these fluxes occurred more rapidly compared to those caused by plant defensins. Furthermore, alpha-hordothionin caused permeabilization of fungal hyphae to the non-metabolite alpha aminoisobutyric acid and, in addition, altered the electrical properties of artificial lipid bilayers, consistently leading to rupture of the lipid bilayers, The plant defensins did not form ion permeable pores in artificial membranes and did not exhibit substantial hyphal membrane permeabilization activity, Our results are consistent with the notion that thionins inhibit fungal growth as a result of direct protein-membrane interactions, whereas plant defensins might act via a different, possibly receptor-mediated, mechanism.