AtMHX is an Arabidopsis vacuolar transporter that exchanges protons with Mg²⁺, Zn²⁺ and Fe²⁺ ions. Tobacco (Nicotiana tabacum (L.)) plants that overexpressed AtMHX showed necrotic lesions, similar to those shown by plants having increased proton influx from the apoplast into the cytosol. This raised the assumption that AtMHX affects the proton homeostasis of cells. Here, we expressed AtMHX in tomato (Lycopersicon esculentum Mill.). The results clarified that the common response of all plant species in which AtMHX was overexpressed thus far was a reduction in plant mass. Transformed tomato plants, in which this reduction was greater compared with tobacco or Arabidopsis thaliana (L.), exhibited reduced cell expansion and a reduction in potassium content. Modifications were also seen in the content of other minerals, including not only metals that can be carried by AtMHX. These changes may thus reflect not only direct metal transport by AtMHX but also the consequences of reduction in cell size. Decreased cell expansion characterises plants with diminished expression of vacuolar proton pumps, presumably due to reduction in the proton-motive force (PMF) necessary to drive solute (mainly potassium) influx into vacuoles and consequently water uptake. This supported a model in which AtMHX-mediated proton efflux from vacuoles affects the PMF, potassium influx, and cell expansion.