AtMHX is a vacuolar transporter encoded by a single gene in Arabidopsis. Electrophysiological analysis showed that it exchanges protons with Mg 2+, Zn2+, and Fe2+ ions. The physiological impact of AtMHX was examined so far only in tissue-culture grown seedlings of tobacco plants overexpressing this transporter. Here we investigated the impact of AtMHX on growth, response to different metals, and metal accumulation of mature tobacco plants, as well as Arabidopsis plants in which we overexpressed this transporter. The analyses were carried out in hydroponic growth-systems, in which the mineral composition could be effectively controlled, and the metal content of roots could be examined. Transformed tobacco plants showed necrotic lesions and apical burnings upon growth with increased levels of Mg 2+, Zn2+, and Cd2+ ions. This suggested that AtMHX can carry in planta not only Mg2+ and Zn2+ ions, as previously deduced based on observations in tissue-culture, but also Cd 2+ ions. Transformed plants of both tobacco and Arabidopsis showed a reduction in plant size. However, the overall response of Arabidopsis to AtMHX overexpression was minor. No change was detected in the mineral content of any organ of the transgenic tobacco or Arabidopsis plants. The necrotic lesions in tobacco resembled those seen in plants with perturbed proton balancing, raising the assumption that AtMHX can affect the proton homeostasis of cells. In agreement with this assumption, the transformed tobacco plants had increased expression and activity of the vacuolar H+-ATPase. The relative significance of AtMHX for metal and proton homeostasis still has to be elucidated.
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Acknowledgments The authors thank H. Sze for the V-ATPase antibodies, R. Serrano for the P-ATPase antibodies, M. Maeshima for the V-PPase antibodies, and R. Mittler for helpful discussions. This work was supported by the Israel Science Foundation (Grant no. 682/ 03).
- Metal and proton homeostasis
- Vacuolar transporter