The origin of copper-induced medicarpin accumulation and its secretion from roots of young fenugreek seedlings are regulated by copper concentration

Isoflavonoid pterocarpans, like medicarpin (M), are produced by leguminous plants in response to biotic or abiotic elicitation from either their glycosidic conjugate pools or by de novo synthesis. In an attempt to clarify M origin in response to copper elicitation, intact Trigonella foenum-graecum (fenugreek) seedlings and cell suspension cultures were treated with CuCl2, and the accumulated isoflavonoid aglycones and their glycosides were determined by High Performance Liquid Chromatography (HPLC). Our results show that copper induces the de novo synthesis of M in a concentration dependent manner. In roots, where copper accumulated in high amounts, only part of M was formed de novo, while another part was formed at the expense of its malonyl glucoside (MGM). In contrast, when tissue copper concentration was low, like in shoots, or in roots treated with low CuCl2, M was formed only by de novo synthesis. The increase of phenylalanine ammonia lyase activity (PAL) as well as the accumulation of chalcone synthase (CHS) and vestitone reductase (VR) specific transcripts are consistent with the de novo synthesis of M induced by copper. The non-linear negative correlation of the studied copper concentrations to the amount of M excreted in the seedling growth medium suggests the existence of an M secretion process which is regulated by copper concentration. The possible involvement of an ATP-dependent transporter in the copper-induced M excretion is discussed.