Biosynthetic origin of medicarpin in elicited fenugreek (Trigonella foenum-graecum L.) seedlings

Medicarpin (M), an isoflavonoid phytoalexin with antioxidant [1] and antifungal properties [2], is produced by leguminous plants mainly in response to biotic or abiotic elicitation [3]. The difference in the biosynthetic origin of copper-induced accumulation between tissues of the same or related species prompted the systematic investigation on the accumulation of M and its 3-O-glucoside-6'-O-malonate (MGM) in intact seedlings, as well as their exudation in the culture media. Methanolic extracts of roots and shoots from 6-days old seedlings, treated hydroponically with CuCl₂ (0.01, 0.05, 0.1, 0.5 and 1mM) for 24h, were analysed by RP-HPLC. At low [Cu] concentrations, roots produced high amounts of M, most of which exuded in the culture medium, while there was no significant decrease of MGM. Increase of [Cu] caused a progressive reduction of MGM to form free M, the excretion of which was significantly reduced. In contrast, M accumulation increased in shoots with increasing [Cu] concentration, while there were no significant changes in MGM. Atomic absorption measurements showed that [Cu] accumulated more in roots, than in shoots, which may explain the difference in the response of the two tissues, suggesting that mild [Cu] stress induces the biosynthesis of M, while severe stress conditions trigger the hydrolysis of MGM in order to form free M. The de novo synthesis of M in roots and shoots induced by copper elicitation was implied by the copper-induced increase of phenylalanine ammonia lyase enzyme activity, as well as the up-regulation of the M pathway genes, chalcone synthase and vestitone reductase, as monitored using RT-PCR.

Acknowledgements: A grant to D.T. in the the framework of „HERAKLEITOS“, which has been co-funded by 75% from E.E. and 25% from the Greek Ministry of Education and Religious Affairs is gratefully acknowledged.

References: 1. Wang, W. et al (2000) Food Chem. 71: 45–49. 2. Blount, J.W. et al (1992) Phys. Mol. Plant Pathol. 41: 333–349. 3. Dixon, R. (1999) Isoflavonoids: biochemistry, molecular biology and biological function Comprehensive Natural Products Chemistry, Vol. 1. Oxford, Elsevier, pp 773–823