Salicylic acid biosynthesis along the phenylpropanoid pathway

Salicylic acid biosynthesis along the phenylpropanoid pathway (R.A. Budi Muljono, M.C. Verberne, and R. Verpoorte) 

In plants at the present time, SA is thought to be derived from phenylalanine, via trans-cinnamic acid (CA) and benzoic acid (BA) as shown in Fig. 2 [17-19]. In tobacco plants (Nicotiana tabaccum) the accumulation of SA has been studied in some more detail and various stress factors, such as UV-irradiation, ozone [20], hydrogen peroxide [21] and elicitation with a Phytopthora megasperma preparation [22], have been shown to induce SA accumulation. 

The first study on the biosynthesis of SA, involving feeding experiments with radiolabelled precursors, in Helianthus annuus, Solanum tuberosum, and Pisum sativum, indicated that SA and p-o-glucosylsalicylic acid arise from [ C]-BA [23]. Later, it was shown that SA in plants might derive from phenylalanine and CA via either BA or o-coumaric acid as an intermediate [4,17]. 

Chadha and Brown [18] reported that those two alternative pathways could be present in the same plant species (see Fig. 2) as the hydroxylation of the aromatic ring can occur before and after the chain-shortening reaction. Evidence for the existence of two pathways came from tomato plants infected with Agrobacterium tumefaciens. SA is synthezised from o-coumaric acid in infected plants, whereas, SA derives from BA in the healthy ones. 

The mechanism of decarboxylation of CA to the corresponding benzoic acids is not well understood. It has been proposed that the formation of BA from the corresponding CA is similar to the (3-oxidation of fatty acids [24]. This hypothesis is in accordance with 

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