Cadmium and iron transport by members of a plant metal transporter family in Arabidopsis with homology to Nramp genes

Metal cation homeostasis is essential for plant nutrition and resistance to toxic heavy metals. Many plant metal transporters remain to be identified at the molecular level. In the present study, we have isolated AtNramp cDNAs from Arabidopsis and show that these genes complement the phenotype of a metal uptake deficient yeast strain, smf1. AtNramps show homology to the Nramp gene family in bacteria, yeast, plants, and animals. Expression of AtNramp cDNAs increases Cd(2+) sensitivity and Cd(2+) accumulation in yeast. Furthermore, AtNramp3 and AtNramp4 complement an iron uptake mutant in yeast. This suggests possible roles in iron transport in plants and reveals heterogeneity in the functional properties of Nramp transporters. In Arabidopsis, AtNramps are expressed in both roots and aerial parts under metal replete conditions. Interestingly, AtNramp3 and AtNramp4 are induced by iron starvation. Disruption of the AtNramp3 gene leads to slightly enhanced cadmium resistance of root growth. Furthermore, overexpression of AtNramp3 results in cadmium hypersensitivity of Arabidopsis root growth and increased accumulation of Fe, on Cd(2+) treatment. Our results show that Nramp genes in plants encode metal transporters and that AtNramps transport both the metal nutrient Fe and the toxic metal cadmium.