Phytoalexins salicylic acid

Three groups of phenolic compounds are involved in defense responses (1) the signal molecule salicylic acid [42], (2) phenylpropanoid compounds (phenolics and phytoalexins) [43], and (3) lignin and related polyphenolics [44,45],... 

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... 

Development of ethylene [64] starts about 40-60 min after mechanical perturbation, much earlier than generation of salicylic acid [53]. This perturbation is induced, for instance, just by wind [30], explaining the observation that the rice blast disease (induced by pathogens) is suppressed in seasons of strong wind (apparently the perturbation induces expression of ethylene, this stimulates generation of phytoalexins which prevent attack by fungi). 

Resistance inductors. R. i. are compounds which initiate the biosynthesis of phytoalexins or, respectively, of PR proteins. Besides certain synthetic compounds some low-molecular weight natural compounds such as salicylic acid , fatty acids methyl jasmonate as well as the peptide systemin act as resistance inductors in plants and initiate the plant s defensive mechanism against phytopathogens. lit. Mdtraux et al.. Induced Resistance in Cucumber in Response to 2,6-Dichloroisonicotinic Acid and Pathogens, in Hennecke Verma (eds.), Adv. Mol. Genet. Plant-Microb. Inter. vol. 1, p. 432-439, Dordrecht Kluwer 1991. J. Pesticide Sci. 15, 199-203 (1990). Plant Physiol. 106, 1269-1277 (1994). Science 250, lOOdff. (1990). Plant Physiol. 99, 799-803 (1992). Science 212, 67Jf. (1981). Trends Cell Biol. 2,236-241 (1992). Plant Physiol. 44,569-589 (1993). Science 255, 1570-1573 (1992). 

Pierce, M.L., Cover, E.C., Richardson, P.E., Scholes, VE. and Essenberg, M. (1996) Adequacy of cellular phytoalexin concentrations in hypersensi-tively responding cotton leaves. Physiol Mol. Plant Pathol. 48, 305-324 Pieterse, C.M.J., van Loon, L.C. (1999) Salicylic acid independent plant defense pathways. Trends Plant Sci., 4, 52-58... 

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