Plant roots accumulate potassium from a wide range of soil concentrations, utilizing at least two distinct plasma membrane uptake systems with different affinities for the cation. Although details on the structure and function of these transporters are beginning to emerge many prominent questions remain concerning how these proteins function in plants. Such questions can be addressed through the use of well-defined transport mutants. Csi52, a caesium-insensitive mutant of Arabidopsis thaliana which is defective in potassium transport, is further characterized here using conventional electrophysiology, patch-clamp and radiometric approaches to identify the nature of the potassium transport lesion. Rb+ uptake experiments reveal a reduced uptake in csi52 in both the high-and low-affinity uptake range. Patch-clamp analysis indicates that the activity of the predominant inward rectifying channel observed in wild-type cells is extremely low in root protoplasts isolated from csi52, whereas outward rectifying channel activity is comparable between wildtype and mutant. Rb+ uptake studies show that in both wild-type and csi52 the high-affinity uptake pathway is considerably less sensitive to Cs+ than the low-affinity pathway with K1/2 values for Cs+ of around 1.3 and 0.2 mM, respectively. Furthermore, K+ starvation leads to a larger relative increase in high-affinity K+ uptake in the mutant than the wild-type. The results demonstrate the Cs+ sensitivity of each individual uptake pathway is comparable in wild-type and csi52 but the high-affinity pathway is less Cs+ sensitive (in both wild-type and csi52). Therefore, the larger shift toward high-affinity uptake in the mutant compared with the wild-type under K(+)-starvation conditions will endow the mutant with a higher degree of overall Cs+ resistance. The data supply evidence for the hypothesis that the csi52 mutation lies within a gene that regulates the activity of several potassium transport systems and coordinates their relative contribution to overall root K+ uptake.