The induction of pathogenesis-related proteins by pathogens and specific chemicals

Abstract

Pathogenesis-related proteins (PRs) are induced in tobacco and other plant species by both biotic and abiotic agents, comprising necrotizing and non-necrotizing viruses, viroids, fungi, bacteria, specific physiological conditions and a variety of chemicals. Both ethephon and the natural precursor of ethylene, 1-aminocyclopropane-1-carboxylic acid, are good inducers and induction under all conditions investigated so far appears to be mediated by ethylene, except treatment with benzoic acid or its derivatives salicylic acid, aspirin, and 2,6-dihydroxybenzoic acid. Whereas the production of ethylene appears to result from a general reaction to stress, the mechanism by whicho-hydroxylated benzoic acids induced PRs is different. In ‘Samsun NN’ tobacco, tobacco mosaic virus (TMV) and ethephon induced PRs in both the treated and the untreated leaves at 20°C, but not at 32°C. However, salicylic acid induces PRs only in the treated leaves, but is as effective at 32°C as it is at 20°C. It has been proposed, therefore, that ethylene leads to the temperature-sensitive synthesis of a, presumably aromatic, compound that mimics the action of salicylic acid and functions as the natural inducer of PRs.