AimsWe evaluated whether reactive oxygen species (ROS) production and the plant antioxidant system are involved in the symbiotic interaction between bradyrhizobia and legumes infected by crack entry, without intracellular infection threads (IT) formation, such as Arachis hypogaea L. (peanut). The role of bradyrhizobial Nod factors (NF) in modulating the plants' oxidative burst was also analysed. Methods and ResultsHistochemical and quantitative procedures were used to detect ROS levels in inoculated and in NF-treated peanut roots. Increase in root H2O2 production was determined at 10min postinoculation with Bradyrhizobium sp. SEMIA 6144 or after NF addition. ROS production was modulated by NF. From 15 to 30min postinoculation, the compatibility of Bradyrhizobium sp.-peanut interaction depends mostly on the H2O2 detoxification via catalase. ConclusionsWe demonstrated for the first time that the early events of the symbiotic interaction in legumes invaded by crack entry trigger an increase in ROS production (represented exclusively by a higher H2O2 content) in which NADPH-oxidase seems not to be involved. NF modulate this response by enhancing the plant antioxidant machinery, contributing to the creation of adequate conditions for symbiosis development. Significance and Impact of the StudyOur data provide new insights into the mechanism involves in the symbiotic interaction that establish legumes infected by crack entry and suggest that ROS response shows differences compared with legumes invaded by IT formation.