Transgenic tobacco plants that express the bacterial nahG gene encoding salicylate hydroxylase have been shown to accumulate very little salicylic acid and to be defective in their ability to induce systemic acquired resistance (SAR). In recent experiments using transgenic NahG tobacco and Arabidopsis plants, we have also demonstrated that salicylic acid plays a central role in both disease susceptibility and genetic resistance. In this paper, we further characterize tobacco plants that express the salicylate hydroxylase enzyme. We show that tobacco mosaic virus (TMV) inoculation of NahG tobacco leaves induces the accumulation 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 nahG-expressing leave's SAR gene expression does not result. We have confirmed earlier reports that the salicylate hydroxylase enzyme has a narrow substrate specificity and we find that catechol, the breakdown product of salicylic acid, neither induces acquired resistance nor prevents the SA-dependent induction of the SAR genes.
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