CHARACTERIZATION OF TOBACCO PLANTS EXPRESSING A BACTERIAL SALICYLATE HYDROXYLASE GENE

Transgenic tobacco plants that express the bacterial nahG gene encodin g salicylate hydroxylase have been shown to accumulate very little sal icylic acid and to be defective in their ability to induce systemic ac quired resistance (SAR). In recent experiments using transgenic NahG t obacco and Arabidopsis plants, we have also demonstrated that salicyli c acid plays a central role in both disease susceptibility and genetic resistance. In this paper, we further characterize tobacco plants tha t express the salicylate hydroxylase enzyme. We show that tobacco mosa ic virus (TMV) inoculation of NahG tobacco leaves induces the accumula tion of the nahG mRNA in the pathogen infected leaves, presumably due to enhanced stabilization of the bacterial mRNA. SAR-associated genes are expressed in the TMV-infected leaves, but this is localized to the area surrounding necrotic lesions. Localized acquired resistance (LAR ) is not induced in the TMV-inoculated NahG plants suggesting that LAR , like SAR, is dependent on SA accumulation. When SA is applied to nah G-expressing leave's SAR gene expression does not result. We have conf irmed earlier reports that the salicylate hydroxylase enzyme has a nar row substrate specificity and we find that catechol, the breakdown pro duct of salicylic acid, neither induces acquired resistance nor preven ts the SA-dependent induction of the SAR genes.