Summary• Degradation of reactive oxygen species in arbuscular mycorrhizas (AM) may be an efficient mechanism to attenuate the activation of plant defenses. Here, we evaluated the activities of superoxide dismutase (SOD), guaiacol-peroxidase (GPX) and catalase (CAT) in bean ( Phaseolus vulgaris ) mycorrhizal roots at different conditions and stages of symbiosis development.• Bean plants were inoculated with Glomus clarum (Gc) or G. intraradices (Gi), under low (LP) and high P (HP) concentrations, and grown under glasshouse conditions. In a second experiment, bean seeds were treated with formononetin and inoculated with Gc under LP and HP conditions. The activities of SOD, GPX and CAT were evaluated.• SOD was induced only in roots colonized by Gc, at a late stage of the symbiosis development under LP, and at an early stage under HP. GPX was induced in roots colonized by Gc at an early time point and suppressed later under LP. In general, CAT was induced in roots colonized by Gc under LP. CAT activities in roots were dependent on P and formononetin treatment.• The possible roles of SOD, GPX and CAT in AM are discussed.

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