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Julia Quintana, María Bernal, Marleen Scholle, Heike Holländer-Czytko, Nga T. Nguyen, Markus Piotrowski, David G. Mendoza-Cózatl, Michael J. Haydon, Ute Krämer

• IRON-REGULATED TRANSPORTER1 (IRT1) is the root high-affinity ferrous iron (Fe) uptake system and indispensable for the completion of the life cycle of \(\textit {Arabidopsis thaliana}\) without vigorous Fe supplementation. Here we provide evidence supporting a second role of IRT1 in root-to-shoot partitioning of Fe. We show that \(\it irt1\) mutants overaccumulate Fe in roots, most prominently in the cortex of the differentiation zone in \(\it irt1-2\), compared to the wild type. Shoots of \(\it irt1-2\) are severely Fe-deficient according to Fe content and marker transcripts, as expected. We generated \(\it irt1-2\) lines producing IRT1 mutant variants carrying single amino-acid substitutions of key residues in transmembrane helices IV and V, Ser206 and His232, which are required for transport activity in yeast. Root short-term \(^{55}\)Fe uptake rates were uninformative concerning IRT1-mediated transport. Overall \(\it irt1\)-like concentrations of the secondary substrate Mn suggested that the transgenic Arabidopsis lines also remain incapable of IRT1-mediated root Fe uptake. Yet, IRT1\(_{S206A}\) partially complements rosette dwarfing and leaf chlorosis of \(\it irt1-2\), as well as root-to-shoot Fe partitioning and gene expression defects of \(\it irt1-2\), all of which are fully complemented by wild-type IRT1. Taken together, these results suggest a regulatory function for IRT1 in root-to-shoot Fe partitioning that does not require Fe transport activity of IRT1. Among the genes of which transcript levels are partially dependent on IRT1, we identify MYB DOMAIN PROTEIN10, MYB DOMAIN PROTEIN72 and NICOTIANAMINE SYNTHASE4 as candidates for effecting IRT1-dependent Fe mobilization in roots. Understanding the biological functions of IRT1 will help to improve Fe nutrition and the nutritional quality of agricultural crops.