A cDNA encoding a novel inwardly rectifying potassium (K+(in)) channel, LKT1, was cloned from a root-hair-specific cDNA library of tomato (Lycopersicon esculentum Mill.). The LKT1 mRNA was shown to be most strongly expressed in root hairs by Northern blot analysis. The LKT1 channel is a member of the AKT family of K+(in) channels previously identified in Arabidopsis thaliana (L.) Heynh. and potato (Solanum tuberosum L.). Moreover, LKT1 is closely related (97% identical amino acids) to potato SKT1. An electrophysiological comparison of the two channels should therefore assist the identification of possible molecular bases for functional differences. For, this comparison, both channels were functionally expressed and electrophysiologically characterised within the same expression system, i.e. Xenopus laevis oocytes. Voltage-clamp measurements identified LKT1 as a K(+)-selective inward rectifier which activates with slow kinetics upon hyperpolarising voltage pulses to potentials more negative than -50 mV. The activation potential of LKT1 is shifted towards positive potentials with respect to SKT1 which might be due to single amino acid exchanges in the rim of the channel's pore region or in the S4 domain. Like SKT1, LKT1 reversibly activated upon shifting the external pH from 6.6 to 5.5, which indicates a physiological role for pH-dependent regulation of AKT-type K+(in) channels. The pharmacological inhibitor Cs+, applied externally, inhibited K+(in) currents mediated by LKT1 and SKT1 half-maximally with a concentration (IC50) of 21 microM and 17 microM, respectively. In conclusion, LKT1 may serve as a low-affinity influx pathway for K+ into root hair cells. Comparison of homologous K+(in) rectifiers from different plant species expressed in the same heterologous system allows conclusions to be drawn in respect to structure-function relationships.