Overexpression of AtMHX in tobacco causes increased sensitivity to Mg2+, Zn2+, and Cd2+ ions, induction of V-ATPase expression, and a reduction in plant size

Abstract

AtMHX is a vacuolar transporter encoded by a single gene in Arabidopsis. Electrophysiological analysis showed that it exchanges protons with Mg(2+), Zn(2+), and Fe(2+) 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+), Zn(2+), and Cd(2+) ions. This suggested that AtMHX can carry in planta not only Mg(2+) and Zn(2+) 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.

Magnesium stress signaling in plant: Just a beginning

“…uORF significantly inhibits AtMHX expression by degradation of AtMHX mRNA, 11,23 and these processes may be regulated by ABA and auxins. 11 The novel literatures presented that overexpression of AtMHX increased expression and activity of the vacuolar H C -ATPase, moreover, transgenic plants were sensitive to high Mg 2C level, and got severe necrotic lesions, 24,25 consistent with our results that the expression of AtMHX was inhibited by MgT. 1 These possibly indicate that AtMHX mainly maintains metal and proton homeostasis in cells, the homeostasis is essential for photosynthesis and numerous enzymatic reactions.…”

Gene limiting cadmium accumulation in rice

“…In contrast, probably because of the mutation of amino acid at the position of 80 in OsHMA3a from high Cd-accumulating cultivar (Anjana Dhan), this transporter has lost its function to sequester Cd into the vacuoles, resulting in high translocation of Cd from the roots to the shoots. Cd is also supposed to be transported into the vacuoles by Cd 2+ /H + antiporters, such as CAX2, CAX4, and MHX (16,17), but the importance of these transporter activities are not clear, although overexpression of these genes resulted in enhanced accumulation of Cd in tobacco roots. Cd may be formed as a complex with phytochelatins or glutathione and then transported to the vacuoles through an unidentified ABC transporter (18).…”

Ca2+/Cation Antiporters (CaCA): Identification, Characterization and Expression Profiling in Bread Wheat (Triticum aestivum L.)

“…Since the AA residues in α-repeat regions play important role in ion selectivity (Shigekawa et al, 2002; Ottolia et al, 2005), sequence variation in these might be responsible for diversification in ion selectivity of various CaCA proteins. AtMHX is earlier reported to be involved in vacuolar transportation of Mg 2+ , Zn 2+ , Cd 2+ , and probably Fe 2+ against protons (Shaul et al, 1999; Berezin et al, 2008). Since the plant MHXs share comparable structural organization with significant conservation, they may also be involved in analogous functions, but that needs to be established in individual proteins.…”

“…Furthermore, we could not observe significant expression of TaMHX in grain and root developmental stages. The over expression of AtMHX caused reduced growth in transgenic tobacco and Arabidopsis plants, which indicated their role in development (Berezin et al, 2008). An interesting expression pattern of TaNCL genes was observed.…”