Pathogenesis related (PR) proteins, which are induced by various agents ranging from ethylene to pathogens, are structurally diverse and apparently ubiquitous in plants : they include thaumatin, osmotin, tobacco major and minor PR proteins, alpha-amylase/trypsin inhibitor, and P21 and PWIR2 soybean and wheat leaf proteins. The proteins are involved in systematically acquired resistance and stress response in plants, although their precise role is unknown . Thaumatin is an intensely sweet tasting protein (about 100,000 times sweeter than sucrose ) found in the West African shrub Thaumatococcus daniellii: it is induced by attack by viroids, which are single-stranded unencapsulated RNA molecules that do not code for protein.
Like other PR proteins, thaumatin is predicted to have a mainly beta structure, with a high content of beta-turns and little helix .
Tobacco cells exposed to gradually increased salt concentrations develop a greatly increased tolerance to salt, due to the expression of osmotin, a member of the PR protein family. Wheat plants attacked by barley powdery mildew express a PR protein (PWIR2), which results in resistance against that infection . The similarity between this and other PR proteins to the maize alpha-amylase/trypsin inhibitor has suggested that PR proteins may act as some form of inhibitor .