Source of electrons for extracellular Fe(III) reduction in iron‐deficient bean roots

Iron‐deficient Phaseolus vulgaris L. cv. Prelude developed a high reducing capacity for extracellular Fe(III) at the root surface. This reduction was competitively inhibited by Nitro‐Blue Tetrazolium salt (Nitro‐BT) which was deposited as a blue precipitate within the epidermis cells of the youngest root parts. Root respiration was not influenced by Nitro‐BT. The intracellular reduction of Nitro‐BT could largely be prevented by excess extracellular Fe(III)EDTA. Iron‐sufficient control plants reduced both extracellular Fe(III)EDTA and intracellular Nitro‐BT at a much slower rate. A role of cytosolic NADH or NADPH as direct electron donors for the enhanced Fe(III) reduction at the plasmalemma is indicated. NAD+‐3‐phosphate dehydrogenase activity was higher in preparations from iron‐deficient root parts than in preparations from control root parts. Ferricyanide, dichlorophenolindophenol and phenazine methosulfate were also reduced at an increased rate by iron‐deficient roots. We conclude that a trans‐plasma membrane electron transfer, mediated by a membrane‐bound reductase, is responsible for the reduction of extracellular Fe(III).