Studies on Arabidopsis athak5, atakt1 double mutants disclose the range of concentrations at which AtHAK5, AtAKT1 and unknown systems mediate K+ uptake

The high‐affinity K⁺ transporter AtHAK5 and the inward‐rectifier K⁺ channel AtAKT1 have been described to contribute to K⁺ uptake in Arabidopsis thaliana. Studies with T‐DNA insertion lines showed that both systems participate in the high‐affinity range of concentrations and only AtAKT1 in the low‐affinity range. However the contribution of other systems could not be excluded with the information and plant material available. The results presented here with a double knock‐out athak5, atakt1 mutant show that AtHAK5 is the only system mediating K⁺ uptake at concentrations below 0.01 mM. In the range between 0.01 and 0.05 mM K⁺ AtHAK5 and AtAKT1 are the only contributors to K⁺ acquisition. At higher K⁺ concentrations, unknown systems come into operation and participate together with AtAKT1 in low‐affinity K⁺ uptake. These systems can supply sufficient K⁺ to promote plant growth even in the absence of AtAKT1 or in the presence of 10 mM K⁺ where AtAKT1 is not essential.