Nicotianamine and histidine/proline are, respectively, the most important copper chelators in xylem sap of Brassica carinata under conditions of copper deficiency and excess.

The effect of two different copper conditions (deficiency and excess) on the amino acid composition in B. carinata xylem sap was analysed. When the Cu in the nutrient solution was increased from 0.12 to 2.5 or 5 microM, the concentrations of histidine, threonine, glutamine, proline, methionine, and glycine were much increased in the xylem sap. When Cu was made deficient in the nutrient solution by decreasing its concentration from 0.12 microM to 0 microM, nicotianamine, glutamine, and threonine were significantly increased in the xylem sap. Aqueous solutions containing different Cu-amino acid complexes (simulated saps) responded in a specific way to the changes in pH, providing a signature that was used to evaluate, by comparison with the real xylem sap, the importance of each amino acid in the xylem transport of Cu. For a single amino acid, the free solution Cu(2+) concentration versus pH titration curves for histidine and proline were the most similar to that for xylem under Cu excess. Under Cu deficiency, this Cu concentration versus pH titration curve appeared to be very similar to that for nicotianamine. It is concluded that increased Cu concentrations induced the selective synthesis of certain amino acids in the sap, of which histidine and proline are the most important. Under Cu deficiency, the concentration of nicotianamine was induced the most. The fact that nicotianamine is induced under Cu starvation and not under Cu excess, is in contrast to similar studies indicating species-specific reactions. However, the induction of nicotianamine under Cu starvation is in line with recent molecular data of the role of nicotianamine in intracellular Cu delivery.