INDUCTION OF THE ROOT CELL PLASMA-MEMBRANE FERRIC REDUCTASE - AN EXCLUSIVE ROLE FOR FE AND CU

Induction of ferric reductase activity in dicots and nongrass monocots is a well-recognized response to Fe deficiency. Recent evidence has s hown that Cu deficiency also induces plasma membrane Fe reduction. In this study we investigated whether other nutrient deficiencies could a lso induce ferric reductase activity in roots of pea (Pisum sativum L. cv Sparkle) seedlings. Of the nutrient deficiencies tested (K, Mg, Ca , Mn, Zn, Fe, and Cu), only Cu and Fe deficiencies elicited a response . Cu deficiency induced an activity intermediate between Fe-deficient and control plant activities. To ascertain whether the same reductase is induced by Fe and Cu deficiency, concentration- and pH-dependent ki netics of root ferric reduction were compared in plants grown under co ntrol, -Fe, and -Cu conditions. Additionally, rhizosphere acidificatio n, another process induced by Fe deficiency, was quantified in pea see dlings grown under the three regimes. Control, Fe-deficient, and Cu-de ficient plants exhibited no major differences in pH optima or K-m for the kinetics of ferric reduction. However, the V-max for ferric reduct ion was dramatically influenced by plant nutrient status, increasing 1 6- to 38-fold under Fe deficiency and 1.5- to 4-fold under Cu deficien cy, compared with that of control plants. These results are consistent with a model in which varying amounts of the same enzyme are deployed on the plasma membrane in response to plant Fe or Cu status. Rhizosph ere acidification rates in the Cu-deficient plants were similarly inte rmediate between those of the control and Fe-deficient plants. These r esults suggest that Cu deficiency induces the same responses induced b y Fe deficiency in peas.