SALICYLIC-ACID HAS A DUAL ROLE IN THE ACTIVATION OF DEFENSE-RELATED GENES IN PARSLEY

Systemic acquired resistance is an inducible plant defence state, the activation of which depends mostly on the accumulation of salicylic ac id (SA). During the past several years, it has been demonstrated that pretreatment of cultured parsley cells with SA potentiates the elicita tion of several defence responses that are local in whole plants, incl uding the accumulation of phenylpropanoid products. Here it is reporte d that while anionic peroxidase and mannitol dehydrogenase encoding ge nes are directly responsive to SA, pretreating parsley cells with SA n ot only enhances elicitation of the phenylpropanoid genes phenylalanin e ammonia-lyase and 4-coumarate:CoA ligase but also of genes for PR-10 and a hydroxyproline-rich glycoprotein. Enhanced induction of these g enes was seen at low levels of endogenous free SA. Enhancement of phen ylalanine 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 correl ated with their biological activity to promote plant disease resistanc e. In summary, these results emphasize that SA has at least a dual rol e in plant defence gene activation.