Thaumatin-like pathogenesis-related proteins are antifungal

Tobacco pathogenesis-related (PR) proteins of group 5, namely PR-S and osmotin, were shown to be serologically related to zeamatin, an antifungal protein of maize seeds. These PR proteins, dubbed thaumatin-like because they show sequence homology with thaumatin, a sweet-tasting protein from fruits of Thaumatococcus daniellii, were demonstrated to have a direct antifungal activity, with specificity for different fungal species. Osmotin was particularly effective in inhibiting the growth of Candida albicans, Neurospora crassa and Trichoderma reesei. PR-S had no detectable activity against these fungi but was found the most potent antifungal protein against the plant pathogen Cercospora beticola. Osmotin caused rapid bursting of the hyphal tips of N. crassa, suggesting that tobacco thaumatin-like PR proteins are antifungal by a membrane permeabilization mechanism similar to that demonstrated previously for zeamatin. These results confirm and extend observations made by Woloshuk et al. [1] who showed that osmotin and a related protein from tomato had antifungal activity against the phytopathogen, Phytophthora infestans.