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A Thellungiella halophila cDNA, ThHAK5, encoding a transporter of the KT/HAK/KUP family has been isolated. According to a phylogenetic study, ThHAK5 is located within group I of the family. Expression in yeast demonstrates that it mediates high-affinity K⁺ uptake with similar characteristics to K⁺-starved plants, which show high levels of ThHAK5 transcripts. These results show that, as in glycophytes, HAK1-type transporters are major contributors to high-affinity K⁺ uptake. However, the regulation of this type of gene may differ between glycophytes and halophytes grown under salinity. Whereas ThHAK5 transcripts are detected in K⁺-starved plants grown with NaCl, transcripts of the Arabidopsis thaliana homologue AtHAK5 are absent. As a result, high-affinity K⁺ uptake is reduced to a lesser extent in T. halophila than in A. thaliana when plants are grown with NaCl. The results presented here indicate that plants …