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Abstract Most plants interact with arbuscular mycorrhizal fungi, which enhance disease resistance in the host plant. Because the effects of resistance against bacterial pathogens are poorly understood, we investigated the effects of mycorrhizal colonization on virulent and avirulent pathogens using phytopathological and molecular biology techniques. Tomato plants colonized by Gigaspora margarita acquired resistance not only against the fungal pathogen, Botrytis cinerea, but also against a virulent bacterial pathogen, Pseudomonas syringae pv. tomato DC3000 (Pst). In G. margarita-colonized tomato, salicylic acid (SA)- and jasmonic acid (JA)-related defense genes were expressed more rapidly and strongly compared to those in the control plants when challenged by Pst, indicating that the plant immunity system was primed by mycorrhizal colonization. Gene expression analysis indicated that primed tomato plants responded to the avirulent pathogen, Pseudomonas syringae pv. oryzae, more rapidly and strongly compared to the control plant, where the effect on the JA-mediated signals was stronger than in the case with Pst. We found that the resistance induced by mycorrhizal colonization was effective against both fungal and bacterial pathogens including virulent and avirulent pathogens. Moreover, the activation of both SA- and JA-mediated signaling pathways can be enhanced in the primed plant by mycorrhizal colonization.