Release of phenols from Lupinus albus L. roots exposed to Cu and their possible role in Cu detoxification

The mechanisms enabling plants to tolerate high concentrations of available Cu in their rhizosphere are still poorly understood. To better understand the mechanisms involved, Lupinus albus L. (white lupin) was grown over 40 days in a hydroponic system compelling roots to develop under sterile conditions in the presence of a nutrient solution containing 0.5, 20 or 62 M Cu. The following parameters were investigated in detail: low molecular weight phenols in nutrient solution (colorimetric assay), high molecular weight phenols in roots and in solution (HPLC-MS, HPLC-UV), pH, redox potential in solution (electrochemistry) and Cu distribution in the plant (AAS) as well as in apical root sections (EDX microanalysis). Finally, in vitro adsorption studies using voltammetry were conducted to evaluate the Cu adsorption behaviour of different phenolic compounds. When exposed to 62 M Cu, biomass production of white lupin was strongly reduced. Plants grown in the presence of 20 M Cu had a similar dry matter production compared to the control plants grown in a 0.5 M Cu solution. However, an increased release of soluble and high molecular weight phenols into the solution was observed. The concentration of polyphenolic compounds in the roots (particularly isoflavonoids like genistein and genistein-(malonyl)-glucoside) was significantly higher for lupins grown in a 20 M Cu solution compared to the control plants. As shown by an in vitro adsorption study, these phenolic compounds can bind Cu ions. In addition, plants exposed to 20 and 62 M Cu cumulated high Cu amounts in root cell walls whereas only low amounts reached the symplasm. Therefore, it is proposed that the complexation of Cu²⁺ ions in the rhizosphere and in the roots apoplasm by phenolic compounds could alleviate Cu-mediated toxicity.