Fungal Membrane Responses Induced by Plant Defensins and Thionins

Abstract

Treatment of hyphae of Neurospora crassa with antifungal 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.

Overexpression of an endogenous thionin enhances resistance of Arabidopsis against Fusarium oxysporum.

“…Transcripts for Pdf1.2 are highly induced by F. o. matthiolae as are Thi2.1 transcripts, and both genes are probably regulated by the same signal transduction pathway (Epple et al, 1997). The toxic mechanisms by which thionins and plant defensins exert their antimicrobial activity are different (Thevissen et al, 1996), and both THI2.1 and PDF1.2 might display a synergistic effect as has been found for hordothionin and other cysteine-rich proteins in vitro (Molina et al, 1993;Terras et al, 1993). PR proteins are induced by a variety of pathogens in a salicylate-dependent pathway and are thought to mediate a protective effect during SAR (Alexander et al, 1993).…”

Differential effects of five types of antipathogenic plant peptides on model membranes

“…Wheat a-thionin contains 45 amino acid residues, with a charge/mass ratio of +2X1CT 3 [4]. Plant defensins, which are phylogenetically separate but structurally similar to thionins, do not seem to cause substantial membrane permeabilization [6]. Defensin PTH1 from potato, to be used in this study, has 47 amino acid residues, and a charge/mass ratio of +1CT 3 [9].…”

“…Thionins, which were the first plant peptides for which an antipathogenic activity had been reported [3,4], have been shown to alter cell membrane permeability [5,6] and to interact with artificial liposomes containing phosphatidylserine [7,8]. Wheat a-thionin contains 45 amino acid residues, with a charge/mass ratio of +2X1CT 3 [4].…”

Recent advances in the development of transgenic papaya technology

“…To improve the resistance of papaya to Phytophthora, Zhu et al [26] introduced the defensin gene from Dahlia merckii by particle bombardment in embryogenic calli of papaya. The dahlia defensin has been shown to inhibit the in vitro growth of a broad range of fungi [88][89]. The defensin gene has also been introduced in different crops resulting in enhanced resistance to their respective fungal pathogens, e.g., radish defensin in tobacco vs. the leaf pathogen Alternaria longipes [90] and tomato vs. A. solani [91].…”