Acetic acid acts as an elicitor exerting a chitosan-like effect on xanthone biosynthesis in Hypericum perforatum L. root cultures

Hypericum perforatum L. (Hypericaceae), popularly known as St. John’s wort, is a medicinal plant widely used in folk medicine [1]. It is known mainly for its antidepressant activity, and nowadays St. John’s wort preparations are among the most widely prescribed drugs for depression in many European countries [2]. Since the secondary metabolites responsible for the antidepressant activity (e.g. hyperforins and hypericins) are mainly accumulated in leaves and flowers, the chemical composition and the medicinal properties of aerial parts have been extensively investigated [3]. Differently the root has been poorly studied and only recently it has been recognized as an attractive source of bioactive secondary metabolites [4]. We have recently demonstrated that H. perforatum root cultures constitutively produce xanthones at higher levels than the root of the plant and that they respond to chitosan (CHIT) elicitation with a significant increase in xanthone production [5]. Xanthones are a wide and structurally diverse group of polyphenols produced by a restricted number of plants, fungi, lichens, and bacteria with multiple bioactivities [6]. In the present study, CHIT was administered to H. perforatum root cultures using three different elicitation protocols, and the increase in xanthone production was evaluated through HPLC. The best results (550 % xanthone increase) were obtained by subjecting the roots to a single elicitation with 200 mg l-1 CHIT and maintaining the elicitor in the culture medium for 1 week. To discriminate the effect of CHIT from that of the solvent, control experiments were performed by administering acetic acid alone at the same concentration used for CHIT solubilization. Unexpectedly, acetic acid caused an increase in xanthone production comparable to that observed in response to CHIT. Feeding experiments with 13C-labeled acetic acid demonstrated that this compound is not incorporated into the xanthone skeleton. Other short-chain monocarboxylic acids (i.e. propionic and butyric acid) had little or no effect on the production of xanthones. These results indicate that acetic acid acts as a specific signal molecule, able to significantly enhance xanthone biosynthesis in H. perforatum root cultures.