title: Ascorbate efflux as a new strategy for iron reduction and transport in plants.
abstract: Iron (Fe) is essential for virtually all living organisms. The identification of the chemical forms of iron (the speciation) circulating in and between cells is crucial to further understand the mechanisms of iron delivery to its final targets. Here we analyzed how iron is transported to the seeds by the chemical identification of iron complexes that are delivered to embryos, followed by the biochemical characterization of the transport of these complexes by the embryo, using the pea (Pisum sativum) as a model species. We have found that iron circulates as ferric complexes with citrate and malate (Fe(III)3Cit2Mal2, Fe(III)3Cit3Mal1, Fe(III)Cit2). Because dicotyledonous plants only transport ferrous iron, we checked whether embryos were capable of reducing iron of these complexes. Indeed, embryos did express a constitutively high ferric reduction activity. Surprisingly, iron(III) reduction is not catalyzed by the expected membrane-bound ferric reductase. Instead, embryos efflux high amounts of ascorbate that chemically reduce iron(III) from citrate-malate complexes. In vitro transport experiments on isolated embryos using radiolabeled (55)Fe demonstrated that this ascorbate-mediated reduction is an obligatory step for the uptake of iron(II). Moreover, the ascorbate efflux activity was also measured in Arabidopsis embryos, suggesting that this new iron 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 iron concentration were reduced significantly. Taken together, our results identified a new iron transport mechanism in plants that could play a major role to control iron loading in seeds.
authors.affiliation: From the Laboratoire de Biochimie et Physiologie Moléculaire des Plantes, Institut de Biologie Intégrative des Plantes, Centre National de la Recherche Scientifique (UMR5004), Institut National de la Recherche Agronomique, Université Montpellier II, Ecole Nationale Supérieure d'Agronomie, 34060 Montpellier Cedex 2, France and.
authors.forename: Louis
authors.initials: L
authors.lastname: Grillet
authors.forename: Laurent
authors.initials: L
authors.lastname: Ouerdane
authors.forename: Paulina
authors.initials: P
authors.lastname: Flis
authors.forename: Minh Thi Thanh
authors.initials: MT
authors.lastname: Hoang
authors.forename: Marie-Pierre
authors.initials: MP
authors.lastname: Isaure
authors.forename: Ryszard
authors.initials: R
authors.lastname: Lobinski
authors.forename: Catherine
authors.initials: C
authors.lastname: Curie
authors.forename: Stéphane
authors.initials: S
authors.lastname: Mari
mesh_terms: D017360:Arabidopsis / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D001205:Ascorbic Acid / Q000378:metabolism*; D001692:Biological Transport / Q000502:physiology; D038181:FMN Reductase / Q000378:metabolism; D005290:Ferric Compounds / Q000378:metabolism; D007501:Iron / Q000378:metabolism*; D007504:Iron Radioisotopes; D008293:Malates / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism; D009713:Nucleotidyltransferases / Q000235:genetics / Q000378:metabolism; D018532:Pisum sativum / Q000378:metabolism*; D010744:Phosphoric Monoester Hydrolases / Q000235:genetics / Q000378:metabolism; D010940:Plant Proteins / Q000378:metabolism; D012639:Seeds / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D005290:Ferric Compounds; D007504:Iron Radioisotopes; D008293:Malates; D008565:Membrane Proteins; D010940:Plant Proteins; C025314:ferric citrate; D007501:Iron; D038181:FMN Reductase; C023903:ferric citrate iron reductase; D009713:Nucleotidyltransferases; C530691:VTC5 protein, Arabidopsis; C522629:GDP-L-galactose phosphorylase, Arabidopsis; C512578:VTC4 protein, Arabidopsis; D010744:Phosphoric Monoester Hydrolases; D001205:Ascorbic Acid
keywords: Arabidopsis; Ascorbic Acid; Citrate; Iron; Malate; Membrane Transport; Reductase; vtc2–4