Salicylic acid pathway

PI P Salicylic acid pathway Benzothiadiazole BTH Acibenzolar-S- methyl Indirect action on fungi,... 

Staurosporine, a kinase inhibitor, also suppressed PAL transcription and enzymatic activation, which indicates that the phosphorylation of still unknown (transcription) factors happened in response to stimulation by pectic fragments. However, another staurosporine-insensitive but salicylic acid responding pathway led to the transcription of pathogenesis related (PR) genes [26]. 

Once aspirin is hydrolyzed to salicylic acid, it follows the metabolic pathway of the latter. 

Phytohormones such as ethylene, salicylic acid (SA), JA, and abscisic acid (ABA) regulate responses of plants to stresses via action referred as signaling crosstalk. Moreover, reactive oxygen species (ROS), the toxic byproducts of aerobic metabolism, play the important role of signaling molecules. Usually, the defensive responses of plants depend on the interaction (positive or negative) between phytohormone signaling pathways rather than on the independent contribution of each of them. " ... 

Ament K, Van Schie CC, Bouwmeester HJ, Haring MA, Schumink RC (2006) Induction of a leaf specific geranylgeranyl pyrophosphate synthase and emission of (it -4,8,12-trimethyltrideca-l,3,7,ll-tetraene in tomato are dependent onboth jasmonic acid and salicylic acid signahng pathways. Planta 224 1197-1208... 

Fig. (3). Hypothetical representation of host defense activation by extracellularly produced H2O2 in GO-transgenic plant. Salicylic acid biosynthesis is elevated through stimulation of benzoic acid 2-hydroxylase activity. The expression of defense-related proteins is induced by increased salicylic acid and perhaps also by H2O2 itself via a separate pathway.

Studies have shown that phenylpropanoid metabolism can be stimulated by ozone. The activity of PAL increased in soybean [91], Scots pine (Pinus sylvestris L.) [92], and parsley (Petroselinum crispum L.) [93] soon after treatment with 150-200 nmol O3 mol 1. Rapid increases in transcript levels for PAL in response to ozone have been observed in parsley [93], Arabidopsis thaliana L. Heynhold [94] and tobacco (Nicoticma tabacum L.) [95]. Transcript levels for 4-coumarate CoA ligase (4CL), the last enzyme in the general phenylpropanoid pathway, increased commensurately with PAL transcripts in ozone-treated parsley seedlings [93]. Phenolic compunds reported to accumulate in leaf tissue in response to ozone include hydroxycinnamic acids, salicylic acid, stilbenes, flavonoids, furanocoumarins, acetophenones, and proanthocyanidins [85, 92, 93, 96, 97]. 

As indicated above, although less data is available that for herbicides, fungicides also have been implicated in the appearance of variations in phenolic metabolism. Molina et al. [116] have demostrated that the systemic acquired resistance signal transduction pathway, a salicylic acid-dependent plant-defence mechanism, mediates fungicide action in the plant. 

Salicylic acid - [GASTROINTESTINAL AGENTS] pol 12) -biotransformation pathway [PHARMACODYNAMICS] pol 18)... 

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.

Salicylic acid clearance is higher in users of oral contraceptives, owing to increases in the glycine and glucuronic acid conjugation pathways. In eight men, eight women,... 

The biosynthetic pathway for salicylic acid is not clear. At present, at least two pathways have been proposed. Each branches from phenyl-propanoid biosynthesis after phenylalanine has been converted to trans-cinnamic acid by phenylalanine ammonium lyase (PAL). In one scheme (Pathway 1 Fig. 4), tram-cinnamic acid would be converted to 2-hydroxy cinnamic acid (or 2-coumaric acid) by a cinnamate 2-hydroxylase. This compound could then be converted to salicylic acid via -oxidation possibly through an acetyl coenzyme A (CoA) intermediate. Alternatively, tram-cinnamic acid could be oxidized to benzoic acid and then hydrox-ylated via a postulated o-hydroxylase activity. The details of this pathway, particularly in tobacco and cucumber, deserve further study. 

In either of the proposed pathways, salicylic acid is synthesised from tram-cinnamic acid. This is an intriguing observation and may provide a clue as to how and why the induction of SAR is tightly linked to the formation of a necrotic lesion. When plants react hypersensitively to pathogen attack, many biochemical changes occur, including the induction of phenylpropanoid biosynthesis. In bean, as well as other plants, this induction seems to be at least partly caused by an increase in the synthesis of phenylalanine ammonium lyase and other enzymes involved in the biosynthesis of isoflavonoid phytoalexins, flavonoid pigments and... 

Fig. 4. Possible biosynthetic pathway for salicylic acid in plants. Structural names are given in bold and enzymes in small letters.

We thank Drs Kay Lawton, Scott Uknes, Tom Gaffney and Mary-Dell Chilton for critical review of the manuscript, and Dr Ray Hammer-schmidt for useful discussions concerning the biosynthetic pathway for the salicylic acid. We would especially like to thank the members of our laboratories who have supplied the data and to our colleagues who have helped with various phases of this work over the years. 

---