Salicylic acid: An acquired resistance signal in plants

Salicylic acid (SA) was hypothesized to be an endogenous regulator of systemic acquired resistance (SAR) and an inducer of pathogenesis-related (PR) proteins in plants. Exogenously applied SA induces resistance and endogenous SA levels increase locally and systemically following TMV-inoculation of tobacco. Transgenic tobacco plants carrying a bacterial salicylate hydroxylase gene do not accumulate SA and therefore can not develop SAR, whereas Arabidopsis thaliana plants carrying the same gene show much higher susceptibility to pathogens. The results presented in this dissertation affirm the regulatory role of SA and MeSA in disease resistance in tobacco.; In the lower leaves of uninfected flowering Xanthi-nc tobacco which are resistant to TMV, levels of SA are positively correlated with developmental increases in resistance.; To determine the extent to which systemic increases in SA result from SA export from the inoculated leaf, SA produced in TMV-inoculated or healthy leaves was labeled with {dollar}sp{lcub}18{rcub}{lcub}rm O{rcub}sb2.{dollar} The spatial and temporal distribution of {dollar}sp{lcub}18{rcub}{dollar}O-SA indicates that most of the SA detected in healthy tissues is synthesized in the inoculated leaf. No significant increase in the activity of benzoic acid 2-hydroxylase is detected in upper uninoculated leaves. No increase in SA level in the upper uninoculated leaf is observed if the TMV-inoculated leaf is detached up to 60 h after inoculation. Apart from the inoculated tissues, the highest increase in SA is observed in the leaf located directly above the inoculated leaf. The systemic SA increase observed during SAR may be explained by phloem transport of SA from the inoculation sites.; Methylsalicylate (MeSA), a metabolite of SA, is volatilized under physiological conditions, and is a candidate airborne signal molecule. We have investigated the effects of applied MeSA as well as measuring endogenous production in order to evaluate its importance as a signal. Incubation of Xanthi-nc tobacco for 6 d in the presence of MeSA results in higher SA levels and resistance to TMV-infection. Production of endogenous MeSA increased from undetectable levels to 22 ng/hr/plant following TMV inoculation. The biosynthesis and metabolism of MeSA in tobacco plants were studied using {dollar}sp{lcub}14{rcub}{dollar}C-labeled SA and MeSA.