Salicylic acid has a dual role in the activation of defence-related genes in parsley.

Systemic acquired resistance is an inducible plant defence state, the activation of which depends mostly on the accumulation of salicylic acid (SA). During the past several years, it has been demonstrated that pretreatment of cultured parsley cells with SA potentiates the elicitation of several defence responses that are local in whole plants, including the accumulation of phenylpropanoid products. Here it is reported that while anionic peroxidase and mannitol dehydrogenase encoding genes are directly responsive to SA, pretreating parsley cells with SA not only enhances elicitation of the phenylpropanoid genes phenylalanine ammonia-lyase and 4-coumarate:CoA ligase but also of genes for PR-10 and a hydroxyproline-rich glycoprotein. Enhanced induction of these genes was seen at low levels of endogenous free SA. Enhancement of phenylalanine ammonia-lyase gene activation was proportional to the length of SA pretreatment. Furthermore, the ability of SA analogues to both potentiate elicited and directly induce defence gene activation correlated with their biological activity to promote plant disease resistance. In summary, these results emphasize that SA has at least a dual role in plant defence gene activation.