Potassium uptake is one of the most basic processes of plant physiology. However, a comprehensive description is lacking. At a cellular level fungi have provided a helpful but imperfect plant model, which we aim to improve using Physcomitrella patens. Blast searches in expressed sequence tag databases demonstrated that Physcomitrella expresses the same families of K(+) and Na(+) transport systems as flowering plants. We cloned two inward rectifier channels, PpAKT1-2, and four HAK-type transporters (PpHAK1-4). In both types of transport system, phylogenetic analyses revealed that despite their high sequence conservation they could not be included in Arabidopsis or rice (Oryza sativa) clusters. Both inward rectifier channels and one HAK transporter (PpHAK1) were expressed in yeast. PpAKT1 and activated mutants of PpAKT2 and PpHAK1 showed clear functions that were similar to those of homologous systems of flowering plants. A pphak1 null mutant line of Physcomitrella failed to deplete K(+) below 10 mum. Moreover, in a non-K(+)-limiting medium in which wild-type plants grew only as protonema, pphak1-1 plants produced leafy gametophores and contained 60% more K(+). We found that Physcomitrella takes up K(+) through several systems. PpHAK1 is the dominant system in plants that underwent K(+) starvation for long periods but an as-yet unidentified system, which is non-selective for K(+), Rb(+), and Cs(+), dominates in many other conditions. Finally, we discuss that, similar to PpHAK1, one of the functions of AtHAK5 may be to control cellular K(+) content and that a non-selective as-yet unidentified system also exists in Arabidopsis.