J. Biol. Chem. doi:10.1074/jbc.M113.514828

Credit: STEPHANE MARI

Iron concentration must be carefully controlled in vivo to enable enzyme function but prevent oxidative stress. Plant roots import iron first by generation of Fe(III) via acidification, followed by reduction to Fe(II) by the enzyme ferric chelate reductase and uptake by a metal transporter. However, the mechanisms used in other parts of the plant for these processes are less clear. Grillet et al. now explore this question in seeds from the pea Pisum sativum. XANES spectroscopy of the embryo sac liquid, which directly feeds the seed, was consistent with a 4:1 Fe(III)/Fe(II) ratio, with citrate as a predominant ligand for the Fe(III) species. Mass spectrometry further identified the Fe(III) complexes as tri-iron compounds with varying proportions of citrate and malate ligands. In contrast, Fe(II) was complexed by nicotianamine, an aminopropyl polymer previously linked to iron transport in plants, demonstrating the use of different ligands for the two metal oxidation states. The authors confirmed the embryos were capable of reducing iron and, surprisingly, discovered this activity persisted when embryos were removed, suggesting the active ingredient was excreted from the cells. Indeed, mass spectrometry, influx assays with iron chelators and ascorbate oxidase, and knockout of metabolic genes confirmed that the small molecule ascorbic acid plays a central role in iron reduction and uptake. These results, which were also confirmed in Arabidopsis thaliana, provide new insights into iron speciation and transport.