Molecular Mechanisms Regulating the Expression of Plant Magnesium/Proton Exchangers

Relatively little is known about magnesium transporters in plants. The Arabidopsis thaliana MHX transporter (AtMHX) is the first exchanger of protons with magnesium ions that had been cloned from any organism. Phylogenetics analysis showed that the MHX family is limited to plants, and constitutes a sixth family within the Ca²⁺/cation (CaCA) exchanger superfamily. AtMHX is localized in the membrane of the vacuole, a key organelle in the maintenance of metal and proton homeostasis in plants. AtMHX exchanges vacuolar protons (H⁺) with Mg²⁺, Zn²⁺ and Cd²⁺ ions. Arabidopsis halleri, one of the few species that can grow on Zn and Cd rich soil, is a close relative of A. thaliana. A. halleri has a much higher expression of its MHX homolog compared to A. thaliana. This higher expression, which is apparently one of the mechanisms for A. halleri adaptation to heavy metal rich soil, is regulated at the post-transcriptional level. AtMHX expression is controlled by several molecular mechanisms, including intron-mediated enhancement (IME) and nonsense-mediated mRNA decay (NMD). NMD is a post-transcriptional mechanism that controls mRNA stability. The 5` untranslated regions (5` UTR) of AtMHX has an upstream open-reading frame (uORF) that subjects AtMHX transcript to degradation by NMD. This uORF also represses AtMHX translation, in a mechanism that depends on the secondary structure of the 5` UTR. The uORF-mediated restriction of AtMHX expression at the translational level and by NMD is essential for normal cell expansion. The prevalence of uORFs in MHX genes is much higher than in most plant genes. This suggests that uORFs are evolutionally conserved elements that restrict the expression of MHX transporters, which can be harmful upon excessive expression.