Cloning of Arabidopsis and barley cDNAs encoding HAK potassium transporters in root and shoot cells

Systematic reverse transcription-polymerase chain reaction (RT-PCR) isolati ons of cDNA fragments using specific primers for HAK mRNAs have revealed th at plant K+ transporters are extensively expressed in shoots and roots. At least 13 genes encoding this type of transporter have been identified in ar abidopsis. Apparently, most plant HAK transporters do not show functional e xpression in trk1 trk2 yeast mutants. In one of them, however, a point muta tion increased the V-max of transport approximately 10-fold without affecti ng the K-m or cation selectivity, suggesting that regulatory problems or ta rgeting to the plasma membrane are the cause of failure for functional expr ession of this clone in yeast. The K+:Rb+:Cs+ selectivity of bacterial and eukaryotic Kup-HAK transporters are coincident with the selectivity data gi ven in the literature about alkali cation transport in different plant tiss ues, indicating that K+ transporters may be the most representative plant K + transporters. Phylogenetic analysis of the 19 plant translated sequences that belong to this type of transporter slows that there are 3 different gr oups. In group I and II there are members in which high-affinity K+ or Rbtransport activity has been demonstrated. In other groups this has not been proved. However, present information suggests that all HAK transporters ma y be K+ transporters.