Minisymposium 22: Phytoremediation

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Abs # M2202: MHX transporters show increased expression in leaves of the zinc hyperaccumulator Arabidopsis halleri than in leaves of Arabidopsis thaliana

Presenter:	Shaul, Orit       Contact Presenter
Authors	Shoshani-Knaani, Noa  (A)   Elbaz, Benayahu  (A)   David-Assael, Ora  (A)   Mizrachy-Dagri, Talya  (A)   Mizrahi, Keren  (A)   Saul, Helen  (A)   Gaash, Rachel  (A)   Brook, Emil  (A)   Berezin, Irina  (A)   Shaul, Orit  (A)
Affiliations:	(A): Faculty of Life Sciences, Bar-Ilan University

The molecular basis of Zn hyperaccumulation and tolerance in a few plant species, such as Arabidopsis halleri, is intensively sought out. Zn hyperaccumulator plants sequester this metal into their shoot vacuoles, while accumulation in root vacuoles is low. 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 A. halleri of a homolog of AtMHX, an Arabidopsis thaliana tonoplast transporter that exchanges protons with Mg, Zn, and Fe ions. AtMHX carries metals at the concentration range found in the cytosol of non-accumulating plants, i.e., the milli-molar range for Mg, and the micro-molar range for Zn and Fe. Accordingly, its affinity for Zn is higher than its affinity for Mg. A. halleri has a single copy of a homologous gene, nominated AhMHX. The encoded protein shares 98% identity with AtMHX, and localizes to the tonoplast. The levels of MHX proteins are much higher in leaves of A. halleri than in leaves of A. thaliana. Similar to other metal homeostasis genes, the high expression of AhMHX in A. halleri leaves is constitutive, and not significantly affected by the metal status of the plants. Interestingly, the levels of MHX transcript are similar in leaves of the two species. This suggests that the difference in MHX levels is regulated at the post-transcriptional level. Expression of AtMHX in A. thaliana is repressed at the translational level by the 5' untranslated region (5'UTR) of its gene. However, in vitro translation studies indicated that the difference in MHX expression in leaves of the two species is not likely to result from variations in the sequence of their 5'UTRs. In both species, MHX transcript levels are higher in leaves than in roots, but the difference is higher in A. halleri. In hyperaccumulator species, metal accumulation in root vacuoles is usually lower than in roots of related non-accumulator species. It was suggested that sequestration into root vacuoles inhibits metal translocation to the shoot. While Mg is the major free divalent metal cation in non-accumulating plants, A. halleri plants grown on Zn-rich soil include roughly equimolar levels of Zn and Mg. Although the affinity of AtMHX and AhMHX for Zn may differ, the high similarity between the two proteins, as well as the high levels of Zn in A. halleri, implicate AhMHX as a candidate gene in Zn accumulation or tolerance in A. halleri.