Ferric Reducing Activity in Roots of Fe-deficient Phaseolus vulgaris Source of Reducing Equivalents

Abstract

Summary Fe deficiency activates a system in the roots of dicotyledons which can reduce extracellular ferric chelates (ferric reductase); this system is located in the plasma membranes of the epidermal cells. In order to identify the source of reducing equivalents for the reduction of ferric chelates by roots of bean ( Phaseolus vulgaris L. var. Prelude), properties of NADP + reducing enzymes were studied and incubations were performed with glucose labeled with 14 C at positions Cl and C6. NADP-dependent isocitrate dehydrogenase was the most active of the enzymes tested in root protein extracts and it could reduce NADP+ even at very high NADPH/NADP + ratios. It is concluded that citrate, which accumulates in roots upon Fe deficiency, via transformation into isocitrate by mitochondrial aconitase, drives cytosolic NADP in bean roots to a strongly reduced state through isocitrate dehydrogenase. Upon addition of reducible ferric salts, the NADP couple becomes more oxidized making it possible for the oxidative pentose phosphate pathway to contribute to NADP + reduction.