A putative role for the plasma membrane potential in the control of the expression of the gene encoding the tomato high-affinity potassium transporter HAK5

A chimeric CaHAK1–LeHAK5 transporter with only 15 amino acids of CaHAK1 in the N-terminus mediates high-affinity K⁺ uptake in yeast cells. Kinetic and expression analyses strongly suggest that LeHAK5 mediates a significant proportion of the high-affinity K⁺ uptake shown by K⁺-starved tomato (Solanum lycopersicum) plants. The development of high-affinity K⁺ uptake, putatively mediated by LeHAK5, was correlated with increased LeHAK5 mRNA levels and a more negative electrical potential difference across the plasma membrane of root epidermal and cortical cells. However, this increase in high-affinity K⁺ uptake was not correlated with the root K⁺ content. Thus, (i) growth conditions that result in a hyperpolarized root plasma membrane potential, such as K⁺ starvation or growth in the presence of NH₄ ⁺, but which do not decrease the K⁺ content, lead to increased LeHAK5 expression; (ii) the presence of NaCl in the growth solution, which prevents the hyperpolarization induced by K⁺ starvation, also prevents LeHAK5 expression. Moreover, once the gene is induced, depolarization of the plasma membrane potential then produces a decrease in the LeHAK5 mRNA. On the basis of these results, we propose that the plant membrane electrical potential plays a role in the regulation of the expression of this gene encoding a high-affinity K⁺ transporter.