The hypersensitive response of plants to infection by pathogens is associated with the induction of the expression of genes encoding pathogenesis-related (PR) proteins and the development of a systemic acquired resistance of the plant to viruses, fungi and bacteria. The PR genes induced in Samsun NN tobacco mosaic virus (TMV) have been classified into five groups, each encoding acidic extracellular and basic vacuolar proteins. In addition to induction by TMV, many PR genes are induced by treating the plant with salicylic acid (SA). Genes encoding acidic PRs are systemically induced upon TMV infection whereas genes encoding basic PRs are not. In contrast to the genes encoding acidic PRs, the genes encoding basic PRs are induced by ethylene and wounding. Cis-acting regulatory elements involved in the co-ordinate induction of PR genes by TMV and SA have been mapped in most detail in the PR-1 a promoter. This promoter contains a number of elements that bind a GT-1 like transcription factor with different affinities. To study the role of ethylene in the induction of PR gene expression, plants have been transformed with sense and antisense constructs of tobacco cDNAs encoding ACC-synthase and/or ACC-oxidase.