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

Abstract

The aim of this study was to examine more in detail the uptake of Fe²⁺ in non‐graminaceous (Strategy I plants) and graminaceous species (Strategy II plants). FeSO₄ (2 μM) labelled with ⁵⁹Fe was supplied in short‐term nutrient solution experiments (30 min, pH 5.0) to cucumber (Cucumis sativus L.), tomato (Lycopersicon esculentum Mill.), barley (Hordeum vulgare L.) and maize (Zea mays L.). A mutant of tomato (Tfer, defective in Fe deficiency inducible reductase) was also examined. Plant Fe nutritional status was modified by different Fe preculture (Fe adequate and Fe deficient, respectively). The Fe²⁺ uptake rates varied within a narrow range (0.25–0.40 μmol ⁵⁹Fe g⁻¹ root dry wt 30 min⁻¹) in both, Strategy I and Strategy II plants adequately supplied with Fe. Under Fe deficiency, the Fe²⁺ uptake rates were markedly increased (4–10‐fold) in the Strategy I plants (tomato and cucumber). 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 the graminaceous species this inducible system is absent or is less expressed. The kinetic studies on Fe²⁺ uptake in cucumber plants gave indications for two saturable uptake systems that were activated under Fe deficiency, a high‐affinity system with a K_m of 4.7±1.6 μM and V_max of 7.5±1.9 μmol ⁵⁹Fe g root dry⁻¹ wt 30 min⁻¹, and a low‐affinity system that operated at Fe²⁺ concentrations higher than 50 μM. Studies with cucumber plants on the specificity of the Fe deficiency activated high‐affinity Fe²⁺ transport system showed that a 10‐ and 100‐fold excess of Zn resulted in 42–83% inhibition of Fe²⁺ uptake, suggesting that Zn could also be transported by the Fe²⁺ transporter, although less efficiently. A lower inhibition of Fe²⁺ uptake was found in the control cucumber plants as well as in the control and 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 preference for Fe²⁺ (Fe²⁺ transporter) rather than by a general transport system for divalent cations.