Salinity is one of the environmental problems with greater impact on global food production, especially in semiarid areas such as the Mediterranean region. Previous work has shown that sodium uptake is counteracted by increased potassium transport or decreased H + pumping and that mutants with the latter phenotypes can be obtained by screening for norspermidine (NE) resistance (Alejandro et al, 2007, EMBO J 26: 3203). This polycation enters the cell by some unidentified system strongly dependent on the electrical potential of the plasma membrane. As this biophysical parameter is determined by the relative activities of H + -ATPase and K + transport, mutants tolerant to NE would be more tolerant to Na + and would allow us to identify regulators of H + pumping and K + influx. We have screened mutant lines of Arabidopsis tagged with T-DNA (about seeds) and obtained 40 mutants tolerant to this polycation, confirmed with germination percentages between 2 and 8 times greater than wild type. Identification of Arabidopsis mutants affected in K + homeostasis Martínez-Macías F, Muñoz-Bertomeu J, Serrano R Instituto de Biología Molecular y Celular de Plantas, Universidad Politécnica de Valencia-CSIC, Camino de Vera s/n Valencia, Spain. Figure 2: Putative mutant isolated in the primary screening at 6 days after sowing. Figure 3: Homocygotic phenotype resistent to BASTA/Kanamycin at 7 days after sowing. Figure 4: Heterocygotic phenotype resistent to BASTA/Kanamycin at 7 days after sowing. K. Feldmann T-DNAINRA-Versailles T- DNA D. Weigel T-DNA W. Scheible and C. Somerville T-DNA M Figure 10: Germination percentage for tolerance to BASTA/Kanamycin in confirmed mutants. Figure 12: Southern blot analysis for Scheible and Somerville mutants, isolated in the secondary screening. Genomic DNA was cut with BamHI restriction enzyme, and membrane was hybridaze with 35S probe. Acquire Collection Primary Screening Growth in soil Secondary Screening Confirmed Phenotypes Confirmed Mutants seeds/plate500 seeds/plate mM NE4 mM NE BASTA/Kanamycin Figure 5: Homocygotic phenotype sensitive to BASTA/Kanamycin at 7 days after sowing. Figure 1: Schematic representation of the selection process of mutants tolerant to NE. W. Scheible and C.Somerville T-DNA collection (N31100) Activation tagging lines generated by transformation of Col-2 using pSKI015 Figure 9: Germination in MS medium + 4 mM NE in Scheible and Somerville mutants. Col2 is the control line. Figure 8: Germination in MS medium + 4 mM NE in Weigel mutants. Col-7 is the control line. Figure 7: Germination in MS medium + 4 mM NE in INRA-Versailles mutants. Figure 6: Germination in MS medium + 4 mM NE in Feldmann mutants. WS2 is the control line. D. Weigel T-DNA collection (N21995, N21991, N23153) Activation tagging lines generated by transformation of Col-7 using pSKI015 INRA Versailles T-DNA collection (N5389, N5455, N5600, N5866, N55047) Loss of function lines generated by transformation of WS4 using pGKB5 pGKB5 T-DNA Feldmann T-DNA collection (N3115, N6500, N84442) Loss of function lines generated by transformation of WS2 using p3850:1003 p3850:1003 T-DNA Cosegregation Plasmid Rescue population S population S 1 population S 2 FUNDING: Spanish Government (ADYSARC – EUI ) and Valencian Government (PROMETEO/2010/038). 503 mutants171 mutants40 mutants WS4 is the control line.