low K⁺ signal is required for functional high‐affinity K⁺ uptake through HAK5 transporters

The high‐affinity K⁺ transporter HAK5 is a key system for root K⁺ uptake and, under very low external K⁺, the only one capable of supplying K⁺ to the plant. Functional HAK5‐mediated K⁺ uptake should be tightly regulated for plant adaptation to different environmental conditions. Thus, it has been described that the gene encoding the transporter is transcriptionally regulated, being highly induced under K⁺ limitation. Here we show that environmental conditions, such as the lack of K⁺, NO₃ ⁻ or P, that induced a hyperpolarization of the plasma membrane of root cells, induce HAK5 transcription. However, only the deprivation of K⁺ produces functional HAK5‐mediated K⁺ uptake in the root. These results suggest on the one hand the existence of a posttranscriptional regulation of HAK5 elicited by the low K⁺ signal and on the other that HAK5 may be involved in yet‐unknown functions related to NO₃ ⁻ and P deficiencies. These results have been obtained here with Solanum lycopersicum (cv. Micro‐Tom) as well as Arabidopsis thaliana plants, suggesting that the posttranscriptional regulation of high‐affinity HAK transporters take place in all plant species.