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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.