Activation of basal defense mechanisms of rice plants by Glomus intraradices does not affect the arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizal (AM) fungi establish symbiotic associations with a wide range of plant species. AM fungi must then have the ability to suppress, neutralize or evade the plant defense response. We investigated the physiological and molecular responses of rice to inoculation with the AM fungus Glomus intraradices, focusing on the relevance of the plant defense response during the symbiotic mycorrhizal interaction. Defense gene expression analysis and proteomic approaches were used. The impact of defense gene expression on the mycorrhizal process was analyzed using transgenic PRms (Pathogenesis-Related maize seed) rice plants, the PRms plants exhibiting constitutive expression of endogenous defense genes. Inoculation with G. intraradices stimulated growth and biomass production in wild-type and PRms plants. Evidence is presented on the activation of the basal defense response in mycorrhizal rice roots. Analysis of the symbiotic proteome confirmed the accumulation of stress-related proteins in mycorrhizal roots, including PR proteins and antioxidant enzymes. Although constitutive expression of defense genes occurred in the roots of PRms plants, the symbiotic efficiency of G. intraradices in these plants was not affected. These results suggest that AM fungi have evolved the capacity to circumvent defense mechanisms that are controlled by the plant's immune system.