Characterization of tobacco plants expressing a bacterial salicylate hydroxylase gene

Abstract

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.

Pipecolic acid enhances resistance to bacterial infection and primes salicylic acid and nicotine accumulation in tobacco

“…SA is a plant defense hormone that induces immunity to plant pathogens with biotrophic and hemibiotrophic infection modes in Arabidopsis, tobacco, and other plants. 2,33,34 It is therefore likely that the enhanced production of SA early after pathogen contact contributes to the increased Pstb-resistance of Pip-treated tobacco plants (Figs. 5A, 6A).…”

The Effects of Salicylic Acid and Tobacco Mosaic Virus Infection on the Alternative Oxidase of Tobacco

“…Alternative oxidase protein was measured in two transgenic lines (SH-L and NahG) that have very low levels of SA (Bi et al, 1995;Friedrich et al, 1995) due to expression of a bacterial salicylate hydroxylase. Both transgenic lines exhibited comparable amounts of alternative oxidase protein and the same alternative pathway activity as wild-type plants.…”

“…nc (referred to as cv Xanthi) or cv Samsun, which are both local lesion hosts for TMV. Salicylate hydroxylaseexpressing lines were either NahG (also called NahG-10; Friedrich et al, 1995), a transgenic cv Xanthi line, or SH-L, a transgenic cv Samsun line. Both lines catabolize SA to catechol due to the activity of salicylate hydroxylase from Pseudomonas putida (Bi et al, 1995;Friedrich et al, 1995).…”

Systemic Acquired Resistance.

“…These plants are not only unable to accumulate free SA, but they are incapable of mounting a SAR response to viral, fungal, or bacterial pathogens (Gaffney et al, 1993;Bi et al, 1995;Friedrich et al, 1995;Lawton et al, 1995), indicating that SA accumulation is required for SAR induction. Interestingly, in Arabidopsis, depletion of SA causes a breakdown of both SAR and gene-for-gene resistance.…”