Rhizosphere acidification as a response to iron deficiency in bean plants

Iron deficiency in higher plants causes accumulation of salts of organic acids in the roots, the most characteristic being citrate. We show that citrate and malate accumulate in beans (Phaseolus vulgaris L. var Prélude), not because of a lack of the iron-containing enzyme aconitase (EC 4.2.1.3), but in close coupling to the extrusion of protons during rhizosphere acidification, one of the `Fe-efficiency' reactions of dicotyledonous plants. When proton excretion is induced in roots of control bean plants by addition of fusicoccin, only malate, not citrate, is accumulated. We propose that iron deficiency induces production of organic acids in the roots, which in beans leads to both proton excretion and an increased capacity to reduce ferric chelates via the induced electron transfer system in the root epidermis cells.