Figure 1.

Potential pathways for the biosynthesis of salicylic acid in plants.

The isochorismate (IC) pathway (Route 1, green) is the primary route for SA production in Arabidopsis thaliana. The phenylalanine ammonia-lyase (PAL) pathway from trans-cinnamic acid (Route 2, tan) has been implicated in SA synthesis in a number of species and plays a minor role, either directly or indirectly, in SA production in Arabidopsis. Pathway products branching from precursors and intermediates in the proposed SA biosynthetic pathway are shown, with focus on Arabidopsis compounds. Open arrows indicate flux to these pathways, with larger arrows indicating greater flux. Results from studies in C. roseus using labeled glucose are consistent with the bulk of induced SA being synthesized via the IC pathway, with retention of the ¹³C label in SA (shown above). For the PAL pathway, there are a number of possible routes to SA (2a, 2b, 2c-1, 2c-2). Synthesis of SA from BA could also include glycosylated intermediates (not shown). Enzymes (red) are abbreviated as follows: aldehyde oxidase (AAO), 4-amino-4-deoxychorismate synthase (ADCS), anthranilate synthase (AS), benzoic acid 2-hydroxylase (BA2H), benzoyl-CoA ligase (BZL), cinnamate 4-hydroxylase (C4H), 4-coumarate: CoA ligase (4CL), chorismate mutase (CM), isochorismate synthase (ICS), isochorismate pyruvate lyase (IPL), and phenylalanine ammonia-lyase (PAL). Enzymes involved in modification of SA are not included (see Figure 2). For details refer to the text.