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

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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 Km of 4.7±1.6 μM and Vmax 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.