Arabidopsis INOSITOL TRANSPORTER4 mediates high-affinity H+ symport of myoinositol across the plasma membrane

Four genes of the Arabidopsis ( Arabidopsis thaliana) monosaccharide transporter-like superfamily share significant homology with transporter genes previously identified in the common ice plant ( Mesembryanthemum crystallinum), a model system for studies on salt tolerance of higher plants. These ice plant transporters had been discussed as tonoplast proteins catalyzing the inositol-dependent efflux of Na+ ions from vacuoles. The subcellular localization and the physiological role of the homologous proteins in the glycophyte Arabidopsis were unclear. Here we describe Arabidopsis INOSITOL TRANSPORTER4 ( AtINT4), the first member of this subgroup of Arabidopsis monosaccharide transporter-like transporters. Functional analyses of the protein in yeast ( Saccharomyces cerevisiae) and Xenopus laevis oocytes characterize this protein as a highly specific H 1 symporter for myoinositol. These activities and analyses of the subcellular localization of an AtINT4 fusion protein in Arabidopsis and tobacco ( Nicotiana tabacum) reveal that AtINT4 is located in the plasma membrane. AtINT4 promoter-reporter gene plants demonstrate that AtINT4 is strongly expressed in Arabidopsis pollen and phloem companion cells. The potential physiological role of AtINT4 is discussed.