Salicylic acid (SA) is one of plant hormone, whose function in biotic stress response has been well studied (Vlot et al. 2009). SA mediates plant defense against biotrophic pathogens, such as Pseudomonas syringae by induction of pathogenesis-related proteins. The name of SA is derived from the word Salix, the scientific name of the willow tree. In the fourth century bc, Hippocrates encouraged woman to chew willow leaves to relieve the pain of childbirth (Raskin 1992). The plants containing SA were used for therapeutic purpose throughout the ancient world. In 1828, salicin, the glucoside of salicylic alcohol, was purified from willow bark. Then, sali-cin is splitted into a sugar and an aromatic compound, salicylice. One of widely used drugs, aspirin is a synthetic derivative of SA.
SA is generally present in plants in quantities of few mg/g freshweight or less (Raskin et al. 1990), either in a free state or in the form of gly-cosylated, methylated, glucose-ester, or amino acid conjugates (Lee et al. 1995). SA is synthesized via two distinct pathways (Fig. 13.1). Chorismate can be converted to isochorismate by ICS (isochorismate synthase), and IPL (isocho-rismate pyruvate lyase) catalyzes the conversion of isochorismate to SA (Fig. 13.1; Wildermuth et al. 2001; Strawn et al. 2007). This is the major pathway for the synthesis of SA in Arabidopsis, Nicotiana benthamiana, and tomato (Wildermuth et al. 2001; Uppalapati et al. 2007; Catinot et al. 2008). Because an Arabidopsis ics1 ics2 double mutant contains residual SA (Garcion et al. 2008), the ICS pathway is not the only source of SA. Another pathway starts from phenylalanine, which is deaminated by PAL (phenylalanine ammonia lyase) to yield cinnamic acid. Cinnamic acid is converted to SA via O-coumaric acid or benzoic acid. BA2H (benzoic acid 2-hydroxylase) catalyzes the conversion from benzoic acid to SA (Fig. 13.1; Garcion and Metraux 2006).
SA is a well-known plant hormone for plant immunity and SAR (systemic acquired resistance) (Vernooij et al. 1994). After infection with biotrophic pathogens, plants increase accumulation of endogenous SA in the necrotic lesion and surrounding tissues (Enyedi et al. 1992).
Accumulation of SA induces HR (hypersensitive response)-like cell death and the expression of PR (pathogenesis-related) genes (Vlot et al. 2009). Development of SAR requires SA because transgenic tobacco plants expressing nahG, which encodes salicylate hydroxylase to catalyze the conversion of SA to catechol, were unable to develop SAR (Gaffney et al. 1993). But, the first plant physiological processes in which SA was shown to be involved are growth regulation (DeKock et al. 1974) and flowering induction (Cleland 1974; Cleland and Ajami 1974). Evidence suggesting that SA is involved in abiotic stresses has also been increasing.
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