The hypersensitive response of plants to infection by pathogens is ass
ociated with the induction of the expression of genes encoding pathoge
nesis-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 i
nto 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-act
ing regulatory elements involved in the co-ordinate induction of PR ge
nes by TMV and SA have been mapped in most detail in the PR-1a promote
r. This promoter contains a number of elements that bind a GT-1 like t
ranscription factor with different affinities. To study the role of et
hylene in the induction of PR gene expression, plants have been transf
ormed with sense and antisense constructs of tobacco cDNAs encoding AC
C-synthase and/or ACC-oxidase.