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Metabolism and Enzymology

Interrelationships between trans-Plasma Membrane Electron/Proton Transfer Stoichiometry, Organic Acid Metabolism, and Nitrate Reduction in Dwarf Bean (Phaseolus vulgaris)

Department of Soil Science and Plant Nutrition, Wageningen Agricultural University, P.O. Box 8005, NL 6700 EC Wageningen, The Netherlands, Department of Plant Physiology, University of Amsterdam, Kruislaan 318, NL 1098 SM Amsterdam, The Netherlands

Iron deficiency in dwarf bean (Phaseolus vulgaris L.) induces an increased activity of a system in the rhizodermal cells, which reduces extracellular ferric salts, and an active proton efflux from the roots, which is coupled to accumulation of citrate and malate in the roots and subsequent export of these compounds in the xylem. During reduction of extracellular ferricyanide by Fe-deficient plants, the stoichiometry of electron transport to proton efflux is 2e^–/1H⁺, and citrate and malate levels in the roots are strongly decreased. Reduction of ferricyanide by Fe-sufficient plants has no influence on root and shoot levels of citrate and malate, but in such plants the process is characterized by a e^–/H⁺ efflux stoichiometry close to unity. Apparently, organic acid metabolism and transport are closely associated with the e^–/H⁺ efflux ratio. To assess the significance of organic acid metabolism as one of the direct intracellular components of the induced unbalanced e^–/H⁺ efflux by roots, we studied NO₃^– reduction in shoots and roots of Fe-deficient and Fe-sufficient plants. Nitrate reductase activity in the roots was positively correlated with the level of citrate and malate, whereas the enzyme activity in the leaves responded positively to the import of these organic acid anions.