Salicylic acid

There are two possible biosynthetic routes to SA, one plant-specific, and the other more similar to the one found in bacteria (Figure 3-5). The 'plant-specific' biosynthetic route to SA starts with phenylalanine (3.27), which, as part of the general phenylpropanoid pathway (see Section 7), is converted to cinnamic acid (3.29) by the enzyme phenylalanine ammonia lyase (PAL). Cinnamic acid is converted to benzoic acid (3.40), probably through a process similar to P-oxidation of fatty acids. The hydroxylation of C2, catalyzed by the enzyme benzoic acid 2-hydroxylase (BA2H), results in SA (3.41). Characterization of purified tobacco BA2H showed that this enzyme is a cytochrome P450-dependent oxygenase (Léon et al., 1995). Alternatively, BA2H may hydroxylate cinnamic acid (3.29) on C2 to produce coumaric acid (3.42), which, after oxidation of the propane side chain, results in SA (3.41).

Figure 3-5. Biosynthesis of salicylic acid. The enzymes involved in this pathway are: (a) chorismate mutase (E.C. 5.4.99.5), (b) prephenate aminotransferase (E.C. 2.6.1.78 and E.C. 2.6.1.79), (c) arogenate dehydratase (E.C. 4.2.1.91), (d) phenylalanine ammonia lyase (E.C. 4.3.1.5), (e) presumed P-oxidation by a yet to be identified enzyme, f) benzoic acid 2-hydroxylase, (g) isochorismate synthase (E. C. 5.4.4.2), and (h) a putative plant pyruvate lyase.

Figure 3-5. Biosynthesis of salicylic acid. The enzymes involved in this pathway are: (a) chorismate mutase (E.C. 5.4.99.5), (b) prephenate aminotransferase (E.C. 2.6.1.78 and E.C. 2.6.1.79), (c) arogenate dehydratase (E.C. 4.2.1.91), (d) phenylalanine ammonia lyase (E.C. 4.3.1.5), (e) presumed P-oxidation by a yet to be identified enzyme, f) benzoic acid 2-hydroxylase, (g) isochorismate synthase (E. C. 5.4.4.2), and (h) a putative plant pyruvate lyase.

An alternative biosynthetic pathway towards SA was hypothesized to exist after experiments with labeled benzaldehyde, benzyl alcohol, and phenylalanine resulted in lower incorporation of the label in SA than expected (Ribnicky et al., 1998). Sequence analysis of the Arabidopsis genome revealed two genes with homology to the bacterial gene encoding isochorismate synthase, an enzyme involved in SA production in bacteria such as Pseudomonas aeruginosa. One of these genes, ICS1 was shown to be up-regulated when Arabidopsis leaves were infected with bacterial pathogens (Wildermuth et al., 2001). In addition, two mutants that were lacking a functional copy of the ICS1 gene produced less SA, exhibited a reduced pathogenesis-related gene expression, and were more susceptible to pathogens. The ICS1 protein contains a putative plastid transit sequence and cleavage site that would be consistent with biosynthesis of SA from the plastid-synthesized pool of chorismate (3.24). In bacteria isochorismate (3.43) is converted to SA and pyruvate (3.10) by the enzyme pyruvate lyase. It is yet unclear whether plants would use a similar enzyme. Wildermuth et al. (2001) speculated that the SA synthesized by isochorismate synthase is crucial for SAR, whereas SA synthesized from phenylalanine may be important in mediating cell necrosis in response to certain pathogens or fungal elicitors.

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