Cooperative effect of root exudates on the toxicity of Cu(II) ions towards plants

Copper is a structural and catalytic component of many redox enzymes like plastocyanin, oxidases, laccases, and dismutases and is therefore considered an essential element for plants. However, accumulation of toxic levels of copper in soil has become an increasing problem since its absorption by plants in amounts higher than certain threshold can cause structural damages or inhibit enzymatic activities with consequent alterations of cellular metabolism. Much of the Cu(II) accumulates within roots where it is strongly bound to cell walls and can be displaced by similar cations e.g. Pb(II) or by different organic ligands.To verify the role of certain cell wall components and of selected organic aliphatic and/or aromatic acids commonly occurring in root exudates on Cu(II) availability for plants, we studied several systems made up by Cu(II), polygalacturonic, malic and caffeic acids at pH 5.8 in the presence of 1 mM CaCl2. Our results indicate a high capacity of polygalacturonic acid (PGA) to bind Cu(II) and a high redox activity of caffeic acid towards Cu(II). The spectrophotometric data indicate that Cu(II) ions interact with caffeic acid (CAF) to form a CAF‐Cu(II) complex where Cu(II) is reduced to Cu(I) through an inner‐sphere electron transfer. The redox activity is strongly influenced by the presence of malic acid which shows a higher affinity towards Cu(II) stabilizing it in its higher oxidation state. Contrarily to CAF, malic acid was able to mobilize Cu(II) from PGA. To verify the importance of such reactions on Cu(II) phytotoxicity we performed different plant growth experiments using triticale (x Triticosecale Wittmack). Preliminary results indicate that PGA and with a lesser extent CAF reduce Cu(II) phytotoxicity.