Ethylene-dependent activity of phenylalanine ammonia-lyase and lignin formation in cucumber roots exposed to phenolic allelochemicals

Abstract

Seedling roots of cucumber were subjected to allelochemical stress by treating them with 0.1 mM solutions of ferulic, p-coumaric, p-hydroxybenzoic and vanillic acids. In the experiment the influence of derivatives of cinnamic acid (ferulic and p-coumaric acids) on the activity of phenylalanine ammonia-lyase (E.C. 4.3.1.5.) (PAL), lignin content, and cucumber root growth was compared to that of benzoic acid (p-hydroxybenzoic and vanillic acids. Roots treated with ferulic and p-coumaric acids were found to have an increased activity of PAL, a higher lignin content, and lower growth indices. No similar effect was recorded in the case of p-hydroxybenzoic and vanillic acids. The application of ethylene synthesis inhibitor, (aminooxy) acetic acid (AOA), cancelled out the effect of ferulic and p-coumaric acids on the activity of PAL. Lignin content in roots treated with ferulic acid remained on the control level when using the inhibitor, while that in roots treated with p-coumaric acid increased. The obtained results suggest that an increase in the activity level of PAL induced by the action of ferulic and p-coumaric acids, depended on ethylene synthesis. A consequence of changes in the studied enzyme activity was an increased lignin content, and in the case of p-coumaric acid also a direct participation of that compound in their synthesis. A consequence of the above changes was inhibition of the growth of cucumber roots treated with ferulic and p-coumaric acids.