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

Laboratoire de Biochimie et Physiologie Moléculaire des Plantes, Unité Mixte de Recherche 5004 Agro-M/Centre National de la Recherche Scientifique/Institut National de la Recherche Agronomique/Université Montpellier II, Montpellier, France

³ To whom correspondence should be addressed. E-mail very{at}ensam.inra.fr; fax 33-467-52-57-37.

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 K_m 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 K_m 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.

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				B. Hertel, F. Horvath, B. Wodala, A. Hurst, A. Moroni, and G. Thiel KAT1 inactivates at sub-millimolar concentrations of external potassium J. Exp. Bot., December 1, 2005; 56(422): 3103 - 3110. [Abstract] [Full Text] [PDF]