Salicylate: a phytochemist's headache

Acetylsalicylate is one of the most successful drugs ever made, with still novel indications being discovered. It was developed on the basis of the use of Salix bark, which contains salicin which one may consider as a pro-drug for salicylate (SA). Interestingly it was found that SA acts as signal compound in plants, particularly in systemic acquired resistance (SAR) observed after infection with for example a virus. Despite extensive studies in the past 20 years the biosynthesis still poses many questions [1]. Most work has been on the phenylalanine pathway leading to SA. Several enzymes have been proposed to be involved, but the step(s) between the putative intermediate benzoic acid and phenylalanine remain uncertain. Microorganisms produce SA in two steps via the isochorismate pathway. Verberne et al. (2000) proposed that this pathway might also function in plants, and showed that by introduction of microbial genes this pathway can be introduced in tobacco, making the plant more resistant against viral and fungal infections. The effect of the constitutive expression of salicylate and TMV infection in tobacco was studied by means of NMR-metabolomics. This metabolomics approach showed clear differences for the production of phenylpropanoids. In case of TMV infection clear differences between infected leaves and SAR leaves could be detected.

In Arabidopsis it was shown that a gene encoding isochorismate synthase is correlated with the formation of SA and SAR. But still the direct chemical evidence is missing that SA is derived from isochorismate and not from phenylalanine. Catharanthus roseus (L.) G. Don. cell cultures produce both SA and the closely related 2,3-dihydroxybenzoic acid (DHBA) upon elicitation. Feeding the cultures with 1-¹³C-glucose we found by means of ¹³C-NMR-spectrometry that DHBA has a labeling pattern as expected for the isochorismate pathway [3]. However, in case of SA the labeling in the aromatic ring was such that it might be a mixture of both pathways.

References: 1. Verberne, M. Verpoorte, R. et al. (2000), Nature Biotechnology 18: 779–783. 2. Verberne, M.C. et al. (1999), Salicylic acid biosynthesis. In: Biochemistry and Molecular Biology of Plant Hormones. New Comprehensive Biochemistry. Vol. 33. P.J.J. Hooikaas, M.A. Hall, and K.R. Libbenga, (Eds.) Elsevier, Amsterdam, 1999, pp. 295–312. 3. Budi Muljono, R.A. et al. (2002), Plant Physiol. Biochem. 40: 231–234.