Ascorbate efflux as a new strategy for iron reduction and transport in plants [Metabolism]

December 17th, 2013 by Grillet, L., Ouerdane, L., Flis, P., Hoang, M. T. T., Isaure, M.-P., Lobinski, R., Curie, C., Mari, S.

Iron (Fe) is essential for virtually all living organisms. The identification of the chemical forms of Fe (the speciation) circulating in and between cells is crucial information to further understand the mechanisms of Fe delivery to its final targets. Here we have analyzed how iron is transported to the seeds, by the chemical identification of Fe complexes that are delivered to embryos, followed by the biochemical characterization of the transport of these complexes by the embryo, using pea (Pisum sativum) as a model species. We have found that Fe circulates as ferric complexes with citrate and malate (Fe(III)3Cit2Mal2, Fe(III)3Cit3Mal1, Fe(III)Cit2). Since dicotyledonous plants only transport ferrous iron, we have checked if embryos were capable of reducing iron of these complexes. Indeed, embryos did express a constitutively high ferric reduction activity. Surprisingly, Fe(III) reduction is not catalyzed by the expected membrane-bound ferric reductase, instead, embryos efflux high amounts of ascorbate that chemically reduce Fe(III) from citrate-malate complexes. In vitro transport experiments on isolated embryos using radiolabeled 55Fe demonstrated that this ascorbate-mediated reduction is an obligatory step for the uptake of Fe(II). Moreover, the ascorbate efflux activity was also measured in Arabidopsis embryos suggesting that this new Fe transport system may be generic to dicotyledonous plants. Finally, in embryos of the ascorbate deficient mutants vtc2-4, vtc5-1 and vtc5-2 the reducing activity and the Fe concentration were significantly reduced. Taken together, our results have identified a new Fe transport mechanism in plants that could play a major role to control Fe loading in seeds.