• Transformed aubergine plants constitutively expressing the Dm-AMP1 antimicrobial defensin (from Dahlia merckii ) were generated and characterized. • Transgenic plants were selected on kanamycin and screened by polymerase chain reaction analysis. The expression of Dm-AMP1 in plant tissues and its release in root exudates were detected by Western blot analyses. Dm-AMP1 localization was performed by immunohistochemical experiments. • Dm-AMP1 expression ranged from 0.2% to 0.48% of total soluble proteins in primary transformants and from 0.16% to 0.66% in F 2 plants. Transformed clones showed resistance to the pathogenic fungus Botrytis cinerea , whose development on leaves was reduced by 36–100%, with respect to controls. The protein was released in root exudates of the transformed plants and was active in reducing the growth of the co-cultured pathogenic fungus Verticillium albo-atrum , whereas it did not interfere with recognition responses and symbiosis establishment by the arbuscular mycorrhizal fungus Glomus mosseae . • Dm-AMP1 transformants may represent a useful model to study the interactions between genetically modified plants and pathogenic fungi or beneficial nontarget microorganisms.