Root colonization by arbuscular mycorrhizal fungi can improve the disease resistance of plants, but the underlying mechanism is not clear. In this study defense responses of four different genotypes of tomato including jasmonic acid (JA) overexpressed transgenic tomato line 35S::PS, JA biosynthetic mutant spr2, JA insensitive mutant jai1, and wide type CM (Solanum lycopersicum Mill.cv Castlemart) plants to infection by early blight pathogen Alternaria solani were investigated after the roots had been colonized by arbuscular mycorrhizal fungus (Funneliformis mosseae, Fm) and the leaves had been treated with 10 mL 0.5 μmol/L methyl jasmonate (MeJA). The results showed that in response to pathogen infection, the activities of peroxidase (POD), polyphenol oxidase (PPO), and lipoxygenase (LOX), as well as transcript levels of allene oxide cyclase (AOC) and CORONATINE INSENSITIVE1 (COI1, JA perception) in the leaves of CM and 35S::PS plants pre-inoculated with arbuscular mycorrhizal fungi (AMF) were significantly higher, and the disease incidence and disease index were significantly lower in relative to sole A. solani inoculation, sole mycorrhizal inoculation, and control without any treatment (CK) at 5 and 10 days after A. solani inoculation. Exogenous spraying of MeJA enhanced the resistance of CM and 35S::PS tomato plants pre-inoculated with Fm to early blight. In the meanwhile, the enzyme activities and transcript levels of the genes were also induced in the spr2 plants in treatment, and the disease incidence and disease index were significantly lower in relative to other four treatments. However, there was no any defense response in jai1 plants after pathogen inoculation and MeJA treatment. Our findings indicate that the induced defense responses and disease resistance by AMF colonization in tomato is mediated by jasmonic acid signaling pathway.