Abstract The transcript level and in turn protein den- sity of the K + -uptake channel ZMK1 in maize (Zea mays) coleoptiles is controlled by the phytohormone auxin. ZMK1 is involved in auxin-regulated coleoptile elongation as well as gravi- and phototropism. To pro- vide unequivocal evidence for the role of ZMK1 in these elementary processes we screened for maize plants containing a Mutator-tagged Zmk1 gene. In a site-se- lected approach, we were able to identify three inde- pendent alleles of Mutator-transposon insertions in Zmk1. zmk1-m1::Mu1 plants were characterised by a Mu1 transposon inside intron 1 of ZMK1. When we analysed the Zmk1-transcript abundance in growing coleoptiles of these homozygous mutants, however, we found the K + -channel allele overexpressed. In conse- quence, elevated levels of K + -channel transcripts resulted in a growth phenotype as expected from more efficient K + -uptake, representing a central factor for turgor formation. Following Zmk1 expression during maize embryogenesis, we found this K + -channel gene constitutively expressed throughout embryo develop- ment and upregulated in late stages. In line with a vital role in embryogenesis, the mutations of exon 2 and in- tron 2 of Zmk1 – zmk1-m2::Mu8 and zmk1-m3::MuA2 – caused a lethal, defective-kernel phenotype. Thus, these results demonstrate the central role of the auxin-regu- lated K + -channel gene Zmk1 in coleoptile growth and embryo development. Keywords Auxin Æ Coleoptile growth Æ Embryo development Æ K + channel Æ Maize Æ Mutator transposons Abbreviations ZMK1 Zea mays K + channel 1 Mutator Mu IAA indole-3-acetic acid TEA tetraethylammonium Introduction The coleoptile of monocotyledonous plants was chosen by Charles and Francis Darwin (Darwin 1880) as a model system to study hormone-induced elongation GenBank accession numbers associated with this paper: AJ937281, X00913, X53604, X62251 K. Philippar Æ K. Bu ¨ chsenschu ¨ tz Æ R. Hedrich (&) Julius-von-Sachs-Institut, Lehrstuhl Molekulare Pflanzenphysiologie und Biophysik, Universita ¨t Wu ¨ rzburg, Julius-von-Sachs-Platz 2, D-97082 Wuerzburg, Germany e-mail: hedrich@botanik.uni-wuerzburg.de D. Edwards Primary Industries Research Victoria, Department of Primary Industries, La Trobe University, Bundoora VIC 3086, Australia J. Lo ¨ ffler Æ H. Lu ¨ then Æ E. Kranz Biozentrum Klein Flottbek und Botanischer Garten, Universita ¨ t Hamburg, Ohnhorststr. 18, D-22609 Hamburg, Germany K. J. Edwards School of Biological Sciences, University of Bristol, Bristol B58 1UG, UK Present Address: K. Philippar Department fu ¨ r Biologie 1, Botanik III, Ludwig- Maximilians-Universita ¨t Mu ¨ nchen, Menzingerstr. 67, D-80638 Munich, Germany Plant Mol Biol (2006) 61:757–768 DOI 10.1007/s11103-006-0047-2 123 The auxin-induced K + channel gene Zmk1 in maize functions in coleoptile growth and is required for embryo development Katrin Philippar Æ Kai Bu ¨ chsenschu ¨ tz Æ David Edwards Æ Julia Lo ¨ ffler Æ Hartwig Lu ¨ then Æ Erhard Kranz Æ Keith J. Edwards Æ Rainer Hedrich Received: 22 December 2005 / Accepted: 16 March 2006 Ó Springer Science+Business Media B.V. 2006