CHARACTERIZATION OF CSI52, A CS-THALIANA ALTERED IN K+ TRANSPORT( RESISTANT MUTANT OF ARABIDOPSIS)

Plant roots accumulate potassium from a wide range of soil concentrati ons, utilizing at least two distinct plasma membrane uptake systems wi th different affinities for the cation. Although details on the struct ure and function of these transporters are beginning to emerge many pr ominent questions remain concerning how these proteins function in pla nta. Such questions can be addressed through the use of well-defined t ransport mutants. Csi52, a caesium-insensitive mutant of Arabidopsis t haliana which is defective in potassium transport, is further characte rized here using conventional electrophysiology, patch-clamp and radio metric approaches to identify the nature of the potassium transport le sion. Rb+ uptake experiments reveal a reduced uptake in csi52 in both the high- and low-affinity uptake range. Patch-clamp analysis indicate s that the activity of the predominant inward rectifying channel obser ved in wild-type cells is extremely low in root protoplasts isolated f rom csi52, whereas outward rectifying channel activity is comparable b etween wildtype and mutant. Rb+ uptake studies show that in both wild- type and csi52 the high-affinity uptake pathway is considerably less s ensitive to Cs+ than the low-affinity pathway with K-1/2 values for Cs + of around 1.3 and 0.2 mM, respectively. Furthermore, K+ starvation l eads to a larger relative increase in high-affinity K+ uptake in the m utant 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-typ e and csi52). Therefore, the larger shift toward high-affinity uptake in the mutant compared with the wild-type under K+-starvation conditio ns will endow the mutant with a higher degree of overall Cs+ resistanc e. The data supply evidence for the hypothesis that the csi52 mutation lies within a gene that regulates the activity of several potassium t ransport systems and coordinates their relative contribution to overal l root K+ uptake.