Biochemistry of Salicylic Acid and its Role in Disease Resistance

Salicylic acid (SA) is involved in the establishment of systemic acquired resistance (SAR) in many plant including tobacco. Considering the important role of SA in disease resistance, biosynthetic and metabolic pathways of SA in tobacco have been studied extensively: The initial step for biosynthetic pathway of SA is conversion of phenylalanine to trans-cinnamic acid, followed by decarboxylation of trans-cinnamic acid to benzoic acid and ie subsequent ring hydroxylation at the C-2 position to form SA. In TMV inoculated tobacco, most of the newly synthesized SA is glucosylated or methylated. Methyl salicylate has been identified as a biologically active, volatile signal. In contrast, the two glucosylated forms accumulate in the vicinity of lesions and consist of SA glucoside, a major metabolite, and SA glucose ester, a relatively minor from. Two enzymes involved in SA biosynthesis and metabolism have been purified and characterized : benzoic acid 2-hydroxylase which catalyzes conversion of benzoic acid to SA; UDP-Glucose: SA 1-O-D glucosyltransferase which converts SA to SA glucose ester. Further studies of the biosynthetic and metabolic pathways of SA will help to elucidate the SAR signal transduction pathway and provide potential tools for the manipulation of disease resistance.