2. Biosynthesis and metabolism of SA

Salicylic acid biosynthesis can occur through two distinct pathways viz. isochorismate (IC) pathway and phenylalanine ammonia-lyase (PAL) pathway (Figure 1). Both IC and PAL pathways are started with chorismic acid. Chorismic acid is the end product derived from the shikimic acid pathway in plastid [22–24]. Chorismic acid is converted to IC by isochorismate synthase (ICS) as reported in several plant species [25–27]. Isochorismate pyruvate lyase (IPL) supposed to catalyze the conversion of IC to SA (but the mechanism is not clear) [28].

In PAL pathway, deamination of phenylalanine is accomplished by the activity of PAL which generates trans-cinnamic acid. Trans-cinnamic acid is converted to intermediate product orthocoumaric acid or producing the benzoic acid which later on produces SA [29–31].

After biosynthesis, SA can be modified into different other forms. Glucosylation of SA generates salicyloyl glucose ester (SGE) and salicylic acid 2-O-β-glucoside (SAG) where the activity of UDP-glucosyltrasferase is involved [32]. The SAG can be stored in the vacuole. Methylation of SA is occurred by SAM-dependent carboxyl methyltransferase to produce methyl salicylate (MeSA) [33]. After production, MeSA is transported to different parts of the plant. Through amino acid conjugation with SA, salicyloyl-L-aspartic acid (SA-Asp) generates (GH3-like phytohormone amino acid synthetase is proposed enzyme catalyze this reaction). SA-Asp can undergo through further catabolism [34]. The conversion of SA to SA-2-sulfonate is proposed to catalyze by sulfotransferase and this process is termed as sulfonation [35]. Hydroxylation of SA is responsible for the production of 2,5-dihydroxybenzoate (Gentisic acid) but the enzyme is unknown [36, 37].

Salicylic Acid: An All-Rounder in Regulating Abiotic Stress Responses in Plants http://dx.doi.org/10.5772/intechopen.68213 33

caused by abiotic stress is vital. This can be implemented through various approaches and one

Salicylic acid or orthohydroxy benzoic acid is ubiquitously distributed plant growth regulator [5]. Salicylic acid has positive effects on plant growth and developmental processes [5–7]. Research findings demonstrated its roles in seed germination, glycolysis, flowering, fruit yield [8], ion uptake and transport [9], photosynthetic rate, stomatal conductance (gs), and in transpiration [10]. Salicylic acid can modulate antioxidant defense system thereby decreasing oxidative stress [11]. Photosynthesis, nitrogen metabolism, proline (Pro) metabolism, production of glycinebetaine (GB), and plant-water relations in abiotic stress affected plants were regulated by SA [12–14]. Induction of defense-related genes and stress resistance in biotic stressed plants have also been reported [15]. Moreover, exogenously applied SA showed putative positive effects on stressed plants [16–20]. Salicylic acid induced genes encoding chaperone, heat shock proteins (HSPs), antioxidants, and secondary metabolites of different types. Moreover, SA was involved in mitogen-activated protein kinase (MAPK) regulation, and in the expression [21]. There is no doubt about the vital roles of SA under abiotic stress condition. So, we will review and cover the area regarding the biosynthesis, involvement, and

Salicylic acid biosynthesis can occur through two distinct pathways viz. isochorismate (IC) pathway and phenylalanine ammonia-lyase (PAL) pathway (Figure 1). Both IC and PAL pathways are started with chorismic acid. Chorismic acid is the end product derived from the shikimic acid pathway in plastid [22–24]. Chorismic acid is converted to IC by isochorismate synthase (ICS) as reported in several plant species [25–27]. Isochorismate pyruvate lyase (IPL) supposed to catalyze the conversion of IC to SA (but the mechanism is

In PAL pathway, deamination of phenylalanine is accomplished by the activity of PAL which generates trans-cinnamic acid. Trans-cinnamic acid is converted to intermediate product ortho-

After biosynthesis, SA can be modified into different other forms. Glucosylation of SA generates salicyloyl glucose ester (SGE) and salicylic acid 2-O-β-glucoside (SAG) where the activity of UDP-glucosyltrasferase is involved [32]. The SAG can be stored in the vacuole. Methylation of SA is occurred by SAM-dependent carboxyl methyltransferase to produce methyl salicylate (MeSA) [33]. After production, MeSA is transported to different parts of the plant. Through amino acid conjugation with SA, salicyloyl-L-aspartic acid (SA-Asp) generates (GH3-like phytohormone amino acid synthetase is proposed enzyme catalyze this reaction). SA-Asp can undergo through further catabolism [34]. The conversion of SA to SA-2-sulfonate is proposed to catalyze by sulfotransferase and this process is termed as sulfonation [35]. Hydroxylation of SA is responsible for the production of 2,5-dihydroxybenzoate (Gentisic acid) but the enzyme

coumaric acid or producing the benzoic acid which later on produces SA [29–31].

of those is the application of exogenous phytoprotectant molecule.

32 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

role of salicylic acid on abiotic stress affected plants.

2. Biosynthesis and metabolism of SA

not clear) [28].

is unknown [36, 37].

Figure 1. Proposed pathways for SA biosynthesis and metabolism. Biosynthesis of SA is occurred by isochorismate (IC) or phenylalanine ammonia-lyase (PAL) pathways. Salicylic acid is also metabolized into different forms. Isochorismate synthase (ICS), BA2H (benzoicacid-2-hydroxylase), IPL (isochorismate pyruvate-lyase); MeSA (methylsalicylate), SA-Asp (salicyloyl-L-aspartic acid), SAG (salicylic acid 2-O-β-glucoside), SGE (salicyloyl glucose ester) are involved in either or biosynthesis or metabolic pathway of SA. Here, 1 indicates UDP-glucosyltransferase, 2 indicates SAM-dependent carboxyl methyltransferase, 3 indicates GH3-like phytohormone amino acid synthetase, and 4 indicates sulfotransferase.

Modifications of SA often render it inactive but these modifications are also related to accumulation, function, and/or mobility. Glucosylation inactivates SA and allows vacuolar storage. Methylation inactivates SA and increases its membrane permeability, volatility which is vital for long-distance transport of this defense signal. Amino acid conjugation of SA is involved in SA catabolism [37].
