title: Phytosiderophore efflux transporters are crucial for iron acquisition in graminaceous plants.
abstract: Eukaryotic organisms have developed diverse mechanisms for the acquisition of iron, which is required for their survival. Graminaceous plants use a chelation strategy. They secrete phytosiderophore compounds, which solubilize iron in the soil, and then take up the resulting iron-phytosiderophore complexes. Bacteria and mammals also secrete siderophores to acquire iron. Although phytosiderophore secretion is crucial for plant growth, its molecular mechanism remains unknown. Here, we show that the efflux of deoxymugineic acid, the primary phytosiderophore from rice and barley, involves the TOM1 and HvTOM1 genes, respectively. Xenopus laevis oocytes expressing TOM1 or HvTOM1 released (14)C-labeled deoxymugineic acid but not (14)C-labeled nicotianamine, a structural analog and biosynthetic precursor of deoxymugineic acid, indicating that the TOM1 and HvTOM1 proteins are the phytosiderophore efflux transporters. Under conditions of iron deficiency, rice and barley roots express high levels of TOM1 and HvTOM1, respectively, and the overexpression of these genes increased tolerance to iron deficiency. In rice roots, the efficiency of deoxymugineic acid secretion was enhanced by overexpression of TOM1 and decreased by its repression, providing further evidence that TOM1 encodes the efflux transporter of deoxymugineic acid. We have also identified two genes encoding efflux transporters of nicotianamine, ENA1 and ENA2. Our identification of phytosiderophore efflux transporters has revealed the final piece in the molecular machinery of iron acquisition in graminaceous plants.
authors.affiliation: Department of Global Agricultural Sciences, Graduate School of Agricultural and Life Sciences, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657 Japan.
authors.forename: Tomoko
authors.initials: T
authors.lastname: Nozoye
authors.forename: Seiji
authors.initials: S
authors.lastname: Nagasaka
authors.forename: Takanori
authors.initials: T
authors.lastname: Kobayashi
authors.forename: Michiko
authors.initials: M
authors.lastname: Takahashi
authors.forename: Yuki
authors.initials: Y
authors.lastname: Sato
authors.forename: Yoko
authors.initials: Y
authors.lastname: Sato
authors.forename: Nobuyuki
authors.initials: N
authors.lastname: Uozumi
authors.forename: Hiromi
authors.initials: H
authors.lastname: Nakanishi
authors.forename: Naoko K
authors.initials: NK
authors.lastname: Nishizawa
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001383:Azetidinecarboxylic Acid / Q000378:metabolism; D001692:Biological Transport / Q000502:physiology; D015870:Gene Expression; D001467:Hordeum / Q000235:genetics / Q000378:metabolism*; D007501:Iron / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009865:Oocytes; D012275:Oryza / Q000235:genetics / Q000378:metabolism*; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D017262:Siderophores / Q000235:genetics / Q000378:metabolism*; D014982:Xenopus laevis
chemical_list: D026901:Membrane Transport Proteins; D010940:Plant Proteins; D017262:Siderophores; D001383:Azetidinecarboxylic Acid; D007501:Iron