The present invention provides a method for producing plants with a desired phenotypic trait which comprises subjecting plants to mutagenesis, screening chimeric progeny for plants having the desired phenotypic trait, and propagating the survivors. In an embodiment, the phenotypic trait comprises an altered amino acid content. Preferably, the technique is used to generate Nicotiana tobacum plant lines having an increase in at least one amino acid. In a preferred embodiment, the invention provides improved Nicotiana tobacum plant lines producing at least 1.35 nmole of threonine per milligram of dry plant weight. These plants are useful for improving the flavor and aroma of the tobacco.