A method to produce improved strains, e.g. salt tolerance of a plant, by gene amplification. Random genome fragments are introduced into a vector and grown in a bacterium. The plasmid is then introduced into the original strain at the DNA sequence homologous with the DNA fragment, under conditions that favour tandem repeats. Modified strains are selected first by a resistance marker that was part of the original plasmid, then by cycles of selection under pressure of the desired property, e.g. at high salinity.