Magainin 2 effects on the ultrastructure of five plant pathogens

Magainin 2 from the African clawed frog is a member of a host defense peptide family with broad-spectrum antimicrobial activity related to its effects on membrane integrity. This bioactive peptide was examined for its effects on the ultrastructure of the plant pathogens Thielaviopsis basicola, Rhizoctonia solani, Fusarium oxysporum, Verticillium dahliae, and Pythium ultimum. Magainin 2 at 0.05 μg/μL in potato dextrose broth in microtiter plate wells completely inhibited all hyphal growth observable with a dissecting scope at 40×, except for P. ultimum. These organisms were subsequently grown on potato dextrose agar and the leading edge of each colony treated with 1 μg of magainin 2 in 10μL. Hyphal plugs from the treated area were taken at 12 and 24 h after treatment for transmission electron microscopic examination. Observations on the ultrastructure of magainin 2-treated hyphae revealed changes in the size and shape of the mitochondria, degradation of the mitochondrial and cytoplasmic matrices, reduction in the number of ribosomes and vacuolization of cytoplasm after 12 h of treatment. The structural degradation of the cells was more severe after 24 h treatment. Magainin 2 at the concentration tested had no significant effect on the growth of P. ultimum, perhaps due to the disparate metabolic chemistry of the Oomycetes compared to the other organisms. The low molecular weight and bioactivity of magainin 2 makes it a candidate for additional investigation into its use in bioengineered host resistance.