Inhibition of iron deficiency stress response in cucumber by rare earth elements

We investigated the influence of the trivalent scandium (Sc), chromium (Cr), gallium (Ga), yttrium (Y) and lanthanum (La) on both the function and activity of ferric chelate reductase (FCR) in cucumber (Cucumis sativus L.) roots. Cucumber seedlings were grown for 1 week in a nutrient solution without Fe or in some experiments with 10 μM FeEDTA. Intact root systems were assayed for FCR activity in a medium at pH 5.0 containing 100 μM FeEDTA with the ferrous chelating agent Ferrozine. Addition of 100 μM concentrations of the EDTA complexes of Sc, Cr, Ga, Y and La did not inhibit FCR in Fe-deficient roots. When Fe-deficient roots were grown with 10 μM LaCl3, ScCl3, or YCl3 for 3 days, FCR activity decreased to 23%, 15% and 1%, respectively, of the activity of Fe-deficient plants grown without trivalent metal addition. Additionally, these trivalent metals suppressed proton secretion. Growth of Fe-deficient plants with 80 μM Ga2(SO4)3 decreased FCR activity to 35% of the control activity while 80 μM CrEDTA did not affect FCR activity. With the addition of either FeEDTA or YCl3, FCR activity decreased to less than 5% of the activity of the Fe-deficient control roots in 3 days. Addition of FeEDTA, but not Y, resulted in recovery from Fe deficiency as indicated by increasing chlorophyll content of leaves.