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Potato plants (Solanum tuberosum L., var. Desire) were transformed with a pH22Kneo vector harboring the gene ac2, which encodes the fungicidal peptide (defensin) from the seed of amaranth (Amaranthus caudatus L.). The transformation involved cocultivation on a solid MS medium of potato stem explants (excised from aseptically grown plants) and Agrobacterium tumefaciens. The factors affecting in vitro regeneration of the explants and the transformation efficiency were optimized. Regenerated potato plants carrying the amaranth defensin gene were selected by two traits, growth and ability to form roots on a kanamycin-supplemented MS medium. The transgenic state was confirmed by PCR analysis of ac2 in tissues of the kanamycin-resistant plants. The transgenic organisms thus obtained differed from the original plants in their patterns of Ambiol-induced growth and proton translocation across the plasma membrane of the tuber cells.