Influence de l\'acide salicylique sur la synthèse de la phénylalanine ammonia-lyase, des polyphénoloxydases et l\'accumulation des composes phénoliques chez le manioc ( Manihot esculenta Crantz)

Pour etudier la reaction de defense du manioc contre les agents pathogenes, nous avons evalue l'activite de la phenylalanine ammonia-lyase (PAL) et des polyphenoloxydases (PPO), et dose les composes phenoliques totaux ethano-solubles accumules avant et apres l'inoculation de l'acide salicylique (AS) dans des plants âges de six semaines. La synthese des enzymes a ete precocement induite (1-4 h). L'activite de la PAL a ete 4 a 13 fois stimulee et celle des PPO, 3 a 11 fois plus, selon les cultivars. Les taux de composes phenoliques totaux ethano-solubles ont subi une augmentation significative des l'inoculation des plantes. Ces taux sont restes quasi constants malgre la baisse de l'activite de la PAL. La synthese de l'enzyme a ete egalement stimulee dans les feuilles immediatement situees audessus (fis) et en dessous (fii) de la feuille inoculee (fi). Les composes phenoliques ont subi aussi une augmentation dans ces feuilles. Concernant l'âge de la feuille inoculee (4 et 6 semaines), la synthese de la PAL et la quantite des composes phenoliques ont ete plus elevees dans les feuilles adultes. To study the reaction of defence of cassava against pathogens, we have evaluated the activity of phenylalanine ammonia-lyase (PAL), polyphenol oxidases (PPO) and dosed total ethano-soluble phenolic compounds accumulated before and after inoculation of salicylic acid in six weeks old seedlings. Enzymes synthesis was precociously induced (1–4 h). PAL activity was 4 to 13 times stimulated and those PPO, 3 to 11 fold higher, according to varieties. Dosed total phenolic compounds undergone a significant increase since the plants inoculation, and this rate has remained almost constant despite the decrease of PAL activity. The enzyme synthesis was equally stimulated in immediately situated leaves above (fis) and in under (fii) of the inoculated leaf (fi). Phenolic compounds have also undergone an increase in these leaves. Concerning the age of the inoculated leaf (4 and 6 weeks), the PAL synthesis and the phenolic compounds quantity have become more important in adult leaves. Keywords : cassava, phenolic compounds, PAL, PPO, salicylic acid, reaction of defence Sciences & Nature Vol. 5 (1) 2008: pp. 1-13