High expression in leaves of the zinc hyperaccumulator Arabidopsis halleri of AhMHX, a homolog of an Arabidopsis thaliana vacuolar metal/proton exchanger

Zn hyperaccumulator plants sequester Zn into their shoot vacuoles. To date, the only transporters implicated in Zn sequestration into the vacuoles of hyperaccumulator plants are cation diffusion facilitators (CDFs). We investigated the expression in Arabidopsis halleri of a homolog of AtMHX, an A. thaliana tonoplast transporter that exchanges protons with Mg, Zn and Fe ions. A. halleri has a single copy of a homologous gene, encoding a protein that shares 98% sequence identity with AtMHX. Western blot analysis with vacuolar-enriched membrane fractions suggests localization of AhMHX in the tonoplast. The levels of MHX proteins are much higher in leaves of A. halleri than in leaves of the non-accumulator plant A. thaliana. At the same time, the levels of MHX transcripts are similar in leaves of the two species. This suggests that the difference in MHX levels is regulated at the post-transcriptional level. In vitro translation studies indicated that the difference between AhMHX and AtMHX expression is not likely to result from the variations in the sequence of their 5' untranslated regions (5'UTRs). The high expression of AhMHX in A. halleri leaves is constitutive and not significantly affected by the metal status of the plants. In both species, MHX transcript levels are higher in leaves than in roots, but the difference is higher in A. halleri. Metal sequestration into root vacuoles was suggested to inhibit hyperaccumulation in the shoot. Our data implicate AhMHX as a candidate gene in metal accumulation or tolerance in A. halleri.