The NRAMP6 metal transporter contributes to cadmium toxicity

Natural Resistance Associated Macrophage Protein (NRAMP) homologues are evolutionarily conserved divalent metal transporters. In Arabidopsis, AtNRAMP3 and AtNRAMP4 play a key role in iron nutrition of the germinating plantlet by remobilizing vacuolar Fe stores. Here we describe the molecular and physiological characterization of AtNRAMP6. AtNRAMP6 is predominantly expressed in the dry seed embryo and to a lesser extent in aerial parts. Its promoter activity is found diffusely distributed in cotyledons and hypocotyls as well as in the vascular tissue region of leaf and flower. We show that AtNRAMP6 transcript coexists with a partially spliced isoform in all tested shoot cell types. When expressed in yeast, AtNRAMP6, but not its misspliced derivative, increased sensitivity to cadmium without affecting Cd content in the cell. Likewise, Arabidopsis transgenic plants overexpressing AtNRAMP6 were hypersensitive to Cd although plant Cd content remained unchanged. Consistently, a null allele of AtNRAMP6, named nramp6-1, was more tolerant to Cd toxicity, a phenotype that was reverted by expressing AtNRAMP6 in the mutant background. We used an AtNRAMP6::HA fusion, shown to be functional in yeast, to demonstrate through immunoblot analysis of membrane fractions and immunofluorescence localization that, in yeast cells, AtNRAMP6 is targeted to a vesicular-shaped endomembrane compartment distinct from the vacuole or mitochondria. We therefore propose that AtNRAMP6 functions as an intracellular metal transporter, whose presence, when modified, is likely to affect distribution/availability of Cd within the cell.