Isolation and Characterization of Genes for the Synthesis of Chlorogenic Acid in Artichoke

It is well known that artichoke possesses a number of beneficial properties, mainly attributable to polyphenols, a class of compounds with beneficial effects on human health. These are secondary metabolites including hydroxycinnamates, particularly caffeic acid, chlorogenic acid (CGA), di-caffeoylquinic acids, and flavonoids (e.g. luteolin). Chlorogenic acid, in particular, is a potent antioxidant very abundant in some vegetables, included artichoke. In artichoke the synthesis of CGA has not been completely elucidated yet. There are three proposed routes for the synthesis of CGA in plants, where the first common enzymatic step is the deamination of phenylalanine by means of the enzyme phenylalanine ammonia-liase (PAL), while the enzyme hydroxycynnamoyl-quinate transferase (HQT), belonging to the BAHD family is reported to be involved in the last step of the biosinthesys of CGA in some plant species. Recently, four members of the artichoke pal gene family have been isolated (De Paolis et al., 2008); in the present contribution, we report on the relative expression of three PAL isoforms in different plant parts, at different developmental stages and under stress simulation by mechanical wounding, and also on the isolation and characterization of two artichoke full-length hqt and hqt2 cDNA sequences. The HQT sequences showed a high level of similarity to homologous genes from other plant species, particularly tobacco and tomato. The transcripts of the two artichoke genes were heterologously expressed and biochemical assays were performed, in order to assess specificity of the enzymes for various substrates.