Regulation of L-Phenylalanine Ammonia-lyase in Germinating Lettuce Seeds: Effect of Substrate Analogues and Phenylpropanoid Compounds

The effect of various substrate analogues and some of the intermediates and endproducts of the phenylpropanoid pathway on the development of L-phenylalanine ammonia-lyase (PAL) in relation to the growth of the embryonic axes in germinating lettuce seeds was investigated. Externally supplied L-phenylalanine and its D-isomer did not affect extractable PAL activity at 50 and 100 μM concentrations, whereas at 500 μM L-phenylalanine, there was a slight but significant inhibition of PAL activity. In the presence of 500 μM D-phenylalanine a slight stimulation of PAL activity was observed. Two other substrate analogues, p-fluorophenylalanine and tryptophan, strongly inhibited both the growth of the radicle as well as the development of PAL activity, whereas tyrosine significantly stimulated both the radicle elongation and PAL activity. All the intermediates of the phenylpropanoid pathway used were inhibitory to growth and PAL activity, the order of effectiveness for inhibition being cinnamic acid > p-coumaric acid > ferulic acid > caffeic acid. Some of the end products of the phenylpropanoid pathway, viz., coumarin, quercetin and chlorogenic acid were also found to be very strong inhibitors both of radicle elongation and PAL activity. In all these treatments a strong correlation between PAL activity and radicle elongation was observed supporting the earlier observations that in germinating lettuce seeds any chemical or physical treatment that inhibits radicle elongation also inhibits PAL activity.