Auxin-induced growth of coleoptiles depends on the presence of potassium and is suppressed by K ⁺ channel blockers. To evaluate the role of K ⁺ channels in auxin-mediated growth, we isolated and functionally expressed ZMK1 and ZMK2 ( Zea mays K ⁺ channel 1 and 2), two potassium channels from maize coleoptiles. In growth experiments, the time course of auxin-induced expression of ZMK1 coincided with the kinetics of coleoptile elongation. Upon gravistimulation of maize seedlings, ZMK1 expression followed the gravitropic-induced auxin redistribution. K ⁺ channel expression increased even before a bending of the coleoptile was observed. The transcript level of ZMK2 , expressed in vascular tissue, was not affected by auxin. In patch-clamp studies on coleoptile protoplasts, auxin increased K ⁺ channel density while leaving channel properties unaffected. Thus, we conclude that coleoptile growth depends on the transcriptional up-regulation of ZMK1, an inwardly rectifying K ⁺ channel expressed in the nonvascular tissue of this organ.