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Salicylic Acid biosynthesis and metabolism - PubMed

Salicylic Acid biosynthesis and metabolism

D'Maris Amick Dempsey et al. Arabidopsis Book. 2011.

Abstract

Salicylic acid (SA) has been shown to regulate various aspects of growth and development; it also serves as a critical signal for activating disease resistance in Arabidopsis thaliana and other plant species. This review surveys the mechanisms involved in the biosynthesis and metabolism of this critical plant hormone. While a complete biosynthetic route has yet to be established, stressed Arabidopsis appear to synthesize SA primarily via an isochorismate-utilizing pathway in the chloroplast. A distinct pathway utilizing phenylalanine as the substrate also may contribute to SA accumulation, although to a much lesser extent. Once synthesized, free SA levels can be regulated by a variety of chemical modifications. Many of these modifications inactivate SA; however, some confer novel properties that may aid in long distance SA transport or the activation of stress responses complementary to those induced by free SA. In addition, a number of factors that directly or indirectly regulate the expression of SA biosynthetic genes or that influence the rate of SA catabolism have been identified. An integrated model, encompassing current knowledge of SA metabolism in Arabidopsis, as well as the influence other plant hormones exert on SA metabolism, is presented.

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Figures

Figure 1.
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.

Figure 2.
Figure 2.

Modifications of SA in plants. Arabidopsis proteins capable of catalyzing the described reaction are shown. Conversion back to SA is shown where evidence exists. All modifications excepting SA-2-sulfonate have been detected in plants including Arabidopsis. Note that while AtSOT12 is active on SA in vitro, it is also active on other substrates with much higher specific activity. While 2,3-DHBA (not shown) and 2,5-DHBA are synthesized via isochorismate in Arabidopsis, it is unclear whether SA is an intermediate in their biosynthesis. Refer to text for details. Abbreviations are as follows: uridine diphosphate (UDP), UDP-glucosyltransferases 74F1 and 74F2 (UGT74F1/2), salicylic acid glucosyltransferase 1 (SGT1), S-adenosyl methionine (SAM), S-adenosyl homocysteine (SAH), benzoic acid/ salicylic acid carboxyl methyltransferase 1 (AtBSMT1), adenosine triphosphate (ATP), amino acid (AA), adenosine monophosphate (AMP), WESO 1 (WES1), GH3 acyl adenylase family member 3.5 (GH3.5), 3′-phosphoadenosine 5′-phosphosulphate (PAPS), 3′-phosphoadenosine 5′-phosphate (PAP).

Figure 3.
Figure 3.

Model integrating current knowledge of biochemical and transcriptional regulation of SA synthesis, SA modification, and activation of robust defense-associated responses via reduced, nuclear-localized NPR1. SA-dependent NPR1-independent processes are not indicated here. Although all processes are shown in one cell, it is likely that spatial and temporal separation of responses is critical to their function. Dashed lines indicate transport across membrane(s). Concentrations shown reflect the Km, of the enzymes for SA. Feedback inhibition of enzyme activity is shown in red. Hormone-induced gene expression is indicated by thick green arrows, based on data from AtGenExpress Hormone and Chemical data series, Exogenous SA treatment of Whole Leaves experiment (NCBI Gene Expression Omnibus Acc. No. GSE33402), and the literature (in text). Transcription factors regulating gene activity are not shown. Although several other AtMES enzymes are active on SA, only the two most active are shown for brevity. Abbreviations are as follows: abscisic acid (ABA), benzoic acid/ salicylic acid carboxyl methyltransferase 1 (BSMT1), enhanced disease susceptibility 5 (EDS5), ethylene (ET), known and/or unknown members of the GH3 acyl adenylase family (GH3.5/GH3.X), indole-3-acetic acid (IAA), isochorismate synthase 1 (ICS1), jasmonic acid (JA), methyl esterase 1 and 9 (MES1/9), methyl salicylate (MeSA), oxidized or reduced forms of Non-expressor of Pathogenesis-Related genes 1 (NPR1ox/NPR1red), avrP-phB susceptible 3 (PBS3), pathogenesis-related 1 gene (PR-1), salicylic acid (SA), salicyloyl-L-aspartate (SA-Asp), SA 2-O-β-D-glucoside (SAG), salicylate glucose ester (SGE), salicylic acid glucosyltransferase 1 (SGT1), UDP-glucosyltransferase 74F1 (UGT74F1). See text for details and supporting information. Adapted from Okrent et al. (2011).

Figure 4.
Figure 4.

Spatial and temporal variation in free SA accumulation in TMVinfected tobacco. Leaves of TMV-infected tobacco undergoing a hypersensitive response (A) photographed under room light or (B) imaged in the dark for SA-induced bioluminescence from the infiltrated Acinetobacter sp. ADP1 SA biosensor. A false color coded SA concentration map based on In vitro concentration ladders is shown in (B). Dark spots in (A) are HR lesions. From Huang et al. (2006) Plant Journal 46: 1073–1083 with permission from John Wiley & Sons.

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