Evolutionary role of a vacuolar metal transporter for hypertolerance/hyperaccumulation in Arabidopsis halleri.

VMT is a tonoplast metal transporter participating in vacuolar sequestration that is involved in metal hyperaccumulation/hypertolerance. The gene is constitutively expressed in metal hyperaccumulators, as Arabidopsis halleri, with higher transcript levels than in the corresponding non-accumulator species. In A. halleri, three different promoter sequences were identified for VMT, suggesting the presence of multiple gene copies, while a single copy is present in Arabidopsis thaliana. VMT promoter activity was compared in A. thaliana and A. halleri by GUS assay. All promoters are active in roots and guard cells, but A. halleri members drive GUS expression also in leaf mesophyll and trichomes. In silico analysis highlights, in the 5’UTR of the A. halleri promoters, a dimer of MYB-binding motifs, which is mutated in a single nucleotide in the A. thaliana sequence. Promoter mutation analysis indicates that this motif is likely involved in trichome-specific expression. The high VMT transcription levels observed in trichomes of A. halleri, counteracted by its absence in A. thaliana trichomes, suggest a putative evolutionary role of VMT in the hypertolerance/hyperaccumulation trait.