A K+ channel from salt-tolerant melon inhibited by Na+.

• The possible roles of K(+) channels in plant adaptation to high Na(+) conditions have not been extensively analyzed. Here, we characterize an inward Shaker K(+) channel, MIRK (melon inward rectifying K(+) channel), cloned in a salt-tolerant melon (Cucumis melo) cultivar, and show that this channel displays an unusual sensitivity to Na(+) . • MIRK expression localization was analyzed by reverse-transcription PCR (RT-PCR). MIRK functional analyses were performed in yeast (growth tests) and Xenopus oocytes (voltage-clamp). MIRK-type activity was revealed in guard cells using the patch-clamp technique. • MIRK is an inwardly rectifying Shaker channel belonging to the 'KAT' subgroup and expressed in melon leaves (especially in guard cells and vasculature), stems, flowers and fruits. Besides having similar features to its close homologs, MIRK displays a unique property: inhibition of K(+) transport by external Na(+) . In Xenopus oocytes, external Na(+) affected both inward and outward MIRK currents in a voltage-independent manner, suggesting a blocking site in the channel external mouth. • The degree of MIRK inhibition by Na(+) , which is dependent on the Na(+) /K(+) concentration ratio, is predicted to have an impact on the control of K(+) transport in planta upon salt stress. Expressed in guard cells, MIRK might control Na(+) arrival to the shoots via regulation of stomatal aperture by Na(+) .