Specific Fe2+ uptake system in strategy I plants inducible under Fe deficiency

The aim of this study was to examine more in detail the uptake of Fe2+ in n on-graminaceous (Strategy I plants) and graminaceous species (Strategy II p lants). FeSO4 (2 muM) labelled with Fe-59 was supplied in short-term nutrie nt solution experiments (30 min, pH 5.0) to cucumber (Cucumis sativus L.), tomato (Lycopersicon esculentum Mill.), barley (Hordeum vulgare L.) and mai ze (Zea mays L.). A mutant of tomato (Tfer, defective in Fe deficiency indu cible reductase) was also examined. Plant Fe nutritional status was modifie d by different Fe preculture (Fe adequate and Fe deficient, respectively). The Fe2+ uptake rates varied within a narrow range (0.25-0.40 mu mol Fe-59 g(-1) root dry wt 30 min(-1)) in both, Strategy I and Strategy II plants ad equately supplied with Fe. Under Fe deficiency, the Fe2+ uptake rates were markedly increased (4-10-fold) in the Strategy I plants (tomato and cucumbe r). In contrast, in the graminaceous species (barley and maize) as well as in the Fe inefficient tomato mutant (Tfer), this increase was significantly less pronounced (1.3-1.6-fold). Our results indicated that a Fe deficiency induced transport system for Fe2+ is operating in Strategy I plants. In th e graminaceous species this inducible system is absent or is less expressed . The kinetic studies on Fe2+ uptake in cucumber plants gave indications fo r two saturable uptake systems that were activated under Fe deficiency, a h igh-affinity system with a K-m of 4.7+/-1.6 muM and V-max of 7.5+/-1.9 mu m ol Fe-59 g root dry(-1) wt 30 min(-1), and a low-affinity system that opera ted at Fe2+ concentrations higher than 50 CIM Studies with cucumber plants on the specificity of the Fe deficiency activated high-affinity Fe2+ transp ort system showed that a 10- and 100-fold excess of Zn resulted in 42-83% i nhibition of Fe2+ uptake, suggesting that Zn could also be transported by t he Fe2+ transporter, although less efficiently. A lower inhibition of Fe2uptake was found in the control cucumber plants as well as in the control a nd Fe deficient barley plants at both Zn concentrations. Cadmium inhibited by 90%, the Fe2+ uptake in Fe deficient cucumber plants that may be partly related to its toxicity. Fe2+ transport was markedly less affected (by 12%) in Fe deficient barley plants even at 100-fold excess of Cd. in conclusion , it appears that Fe2+ is transported by an uptake system with distinct pre ference for Fe2+ (Fe2+ transporter) rather than by a general transport syst em for divalent cations.