Evidence for a Specific Uptake System for Iron Phytosiderophores in Roots of Grasses

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Abstract
Roots of grasses in response to iron deficiency markedly increase the release of chelating substances (`phytosiderophores') which are highly effective in solubilization of sparingly soluble inorganic FeIII compounds by formation of FeIIIphytosiderophores. In barley (Hordeum vulgare L.), the rate of iron uptake from FeIIIphytosiderophores is 100 to 1000 times faster than the rate from synthetic Fe chelates (e.g. Fe ethylenediaminetetraacetate) or microbial Fe siderophores (e.g. ferrichrome). Reduction of FeIII is not involved in the preferential iron uptake from FeIIIphytosiderophores by barley. This is indicated by experiments with varied pH, addition of bicarbonate or of a strong chelator for FeII (e.g. batho-phenanthrolinedisulfonate). The results indicate the existence of a specific uptake system for FeIIIphytosiderophores in roots of barley and all other graminaceous species. In contrast to grasses, cucumber plants (Cucumis sativus L.) take up iron from FeIIIphytosiderophores at rates similar to those from synthetic Fe chelates. Furthermore, under Fe deficiency in cucumber, increased rates of uptake of FeIIIphytosiderophores are based on the same mechanism as for synthetic Fe chelates, namely enhanced FeIII reduction and chelate splitting. Two strategies are evident from the experiments for the acquisition of iron by plants under iron deficiency. Strategy I (in most nongraminaceous species) is characterized by an inducible plasma membrane-bound reductase and enhancement of H+ release. Strategy II (in grasses) is characterized by enhanced release of phytosiderophores and by a highly specific uptake system for FeIIIphytosiderophores. Strategy II seems to have several ecological advantages over Strategy I such as solubilization of sparingly soluble inorganic FeIII compounds in the rhizosphere, and less inhibition by high pH. The principal differences in the two strategies have to be taken into account in screening methods for resistance to `lime chlorosis'.
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