Regulation by External K+ in a Maize Inward Shaker Channel Targets Transport Activity in the High Concentration Range

An inward Shaker K+ channel identified in Zea mays (maize), ZmK2.1, displays strong regulation by external K+ when expressed in Xenopus laevis (African clawed frog) oocytes or COS cells. ZmK2.1 is specifically activated by K+ with an apparent Km close to 15 mM independent of the membrane hyperpolarization level. In the absence of K+, ZmK2.1 appears to enter a nonconducting state. Thus, whatever the membrane potential, this maize channel cannot mediate K+ influx in the submillimolar concentration range, unlike its relatives in Arabidopsis thaliana. Its expression is restricted to the shoots, the strongest signal (RT-PCR) being associated with vascular/bundle sheath strands. Based on sequence and gene structure, the closest relatives of ZmK2.1 in Arabidopsis are K+ Arabidopsis Transporter 1 (KAT1) (expressed in guard cells) and KAT2 (expressed in guard cells and leaf phloem). Patch-clamp analyses of guard cell protoplasts reveal a higher functional diversity of K+ channels in maize than in Arabidopsis. Channels endowed with regulation by external K+ similar to that of ZmK2.1 (channel activity regulated by external K+ with a Km close to 15 mM, regulation independent of external Ca2+) constitute a major component of the maize guard cell inward K+ channel population. The presence of such channels in maize might reflect physiological traits of C4 and/or monocotyledonous plants.