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

Authors: COHEN, CLARA - CORNELL UNIVERSITY; Norvell, Wendell; Kochian, Leon

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Iron (Fe) is an essential nutrient for both plants and humans, and Fe deficiency is a worldwide problem for both. Acquiring sufficient iron from many soils is difficult for plants; hence, not enough Fe is available in plant foods to satisfy an adult human s nutritional requirements when eaten nin normal amounts. Thus, we are working to expand our understanding of the mechanisms plants use to acquire Fe from the soil in order to ultimately improve the content of bioavailable Fe in plant-based foods. In this paper we studied a mechanism that is turned to assist plants to acquire Fe from the soil. This mechanism involves an enzyme in the outer cellular membrane of root cells that converts oxidized Fe in the soil to a reduced form that is available for uptake into root cells (an iron reductase enzyme). It has been generally accepted in the literature that this mechanism is only turned on by Fe deficiency, but we have recently shown that it is also turned on by copper deficiency in peas. In this study, we investigated whether deficiencies of other essential mineral nutrients in pea seedlings also turns on this Fe acquiring system. We looked at the effect of imposing deficiencies in potassium, calcium, magnesium, zinc, or manganese (as well as iron and copper) on the regulation of this system in pea roots. It was found that only Fe and copper deficiency turned on this Fe reductase. Detailed studies of this system indicated that the same Fe reducing enzyme is turned on by Fe and copper deficiencies. This research provides important insights into how Fe acquisition by plant roots is regulated by the mineral nutrient status of the plant.

Technical Abstract: Induction of ferric-reductase activity in dicots and non-grass monocots is a well-recognized response to Fe deficiency. Recent evidence has shown that Cu deficiency also induces plasma-membrane iron reduction. In this study, we investigated whether other nutrient deficiencies could also induce ferric-reductase activity in roots of pea seedlings (Pisum sativum L. cv Sparkle). 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 if the same reductase is induced by Fe and Cu deficiency, concentration- and pH-dependent kinetics of root ferric reduction were compared in plants grown under control, -Fe and -Cu conditions. Additionally, rhizosphere acidification, another process induced by Fe deficiency, was quantified in pea seedlings grown under the three regimes. Control, Fe-deficient, and Cu-deficient plants exhibited no major differences in pH optima or Km for the kinetics of ferric reduction. However, the Vmax for ferric reduction was dramatically influenced by plant nutrient status, increasing by 16 to 38 fold under Fe deficiency and 1.5 to 4 fold under Cu deficiency, 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 iron or copper status. Rhizosphere-acidification rates in the copper-deficient plants were similarly intermediate between those of the control and Fe- deficient plants. These results suggest that Cu deficiency induces the same suite of responses induced by Fe deficiency in peas.