A Ca2+-Sensitive System Mediates Low-Affinity K+ Uptake in the Absence of AKT1 in Arabidopsis Plants

K+ acquisition by Arabidopsis roots is mainly mediated by the high-affinity K+ transporter AtHAK5 and the inward-rectifier K+ channel AtAKT1. This model is probably universal to plants. Mutant plants lacking these two systems (athak5,atakt1) take up K+ and grow when the external K+ concentration is above a certain level, indicating that an additional transport system may compensate for the absence of AtHAK5 and AtAKT1. Here we describe that this alternative system is essential for providing sufficient K+ to sustain growth of athak5,atakt1 plants. This system is especially sensitive to Ca2+, Mg2+, Ba2+ and La3+, it transports Cs+ and its activity is reduced by cyclic nucleotides. These results suggest that a Ca2+-permeable voltage-independent non-selective cation channel, probably belonging to the cyclic nucleotide gated channel (CNGC) family, may provide the pathway for K+ uptake in athak5,atakt1 plants. The genes encoding the two members of the CNGC family that have been described as mediating root K+ uptake, AtCNGC3 and AtCNGC10, are not up-regulated in athak5,atakt1 plants, excluding overexpression of these genes as a compensatory mechanism. On the other hand, an increased driving force for K+ in athak5,atakt1 plants due to a hyperpolarization of the membrane potential of its root cells is also discarded. The identification of this unknown system may provide tools to improve plant K+ nutrition in conditions where AtAKT1 functionality is reduced, such as under salinity. In addition, this system may constitute an important pathway for accumulation of toxic cations such as Cs+ or radiocesium (Cs-137(+)), and could play a role in phytoremediation.