Differential regulation of the HAK5 genes encoding the high-affinity K+ transporters of Thellungiella halophila and Arabidopsis thaliana

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 which differ in salt tolerance show differential regulation of the expression of genes encoding K+ transporters.