In order to investigate the role of frankial siderophores in minimizing metal toxicity, Frankia strains were grown at different concentrations of Mg 2þ , Cu 2þ , and Zn 2þ . Growth was observed up to 500 mmol L À1 Mg 2þ , 10 mmol L À1 Cu 2þ , and 10 mmol L À1 Zn 2þ while the maxima were at 200 mmol L À1 Mg 2þ , 1 mmol L À1 Cu 2þ , and 1 mmol L À1 Zn 2þ . The siderophore production was increased up to 500 mmol L À1 Mg 2þ , 10 mmol L À1 Cu 2þ , 100 mmol L À1 (hydroxamate type), and 200 mmol L À1 (catecholate type) Zn 2þ while maximum production was found at Mg 2þ (200 mmol L À1 ), Cu 2þ (10 mmol L À1 ), and Zn 2þ (10 mmol L À1 ). The results suggested that the growth was protected at higher concentrations of Mg 2þ (up to 500 mmol L À1 ), Cu 2þ (10 mmol L À1 ), and Zn 2þ (10 mmol L À1 ), possibly due to enhanced siderophore production. Thus, siderophores minimize the metal-induced inhibition of growth in Frankia, likely due to regulation of nutritional imbalances and metabolic processes during adaptation towards metal stress and/or metal toxicity.