BackgroundSoybean (Glycine max (L.) Merr. cv. “Nam Dan”) is one of the most valuable crops in agricultural production in Nghe An province (Vietnam). Our previous study revealed that extract of the cyanobacterium strain Nostoc calcicola HN9 expressed positive effect on growth and development, and raised soybean productivity (Tran et al. in Proceeding of Vietnam national conference of research on biology, Da Nang, 2016). We hypothesized that N. calcicola HN9 would improve the defense responses of G. max cv. “Nam Dan” to cowpea aphid (Aphis craccivora Koch)-a serious pest of leguminous crops.ResultsInfestation of A. craccivora caused oxidative stress in leaves of G. max cv. “Nam Dan”. A strong generation of endogenous reactive oxygen species (ROS) such as superoxide anion radical (O2·−) and hydrogen peroxide (H2O2) resulted in the cellular damages in the aphid-infested leaves through high levels of injury percentage and lipid peroxidation. To protect from aphid attack themselves, soybean plants triggered the antioxidant defense systems, in which, enzymatic antioxidants such as superoxide dismutase (SOD, 1.15.1.1), catalase (CAT, 1.11.1.6) and GPx (EC 1.11.1.9) were strongly accumulated to reduce the toxic effects of ROS. Components of N. calcicola HN9 extract might strengthen the defensive capability of G. max cv. “Nam Dan” to cowpea aphid infestation via establishing the chemical constraints on oxidative stress. Under effect of cyanobacteria extract, generation of O2·− and H2O2 was strictly limited, activities of SOD, CAT and GPx were remarkably accumulated in the aphid-infested leaves leading to a significant reduction of oxidative damages.Conclusions Nostoc calcicola HN9 extract probably not only controlled the generation and effects of O2·− and H2O2 but also augmented the accumulated activity of SOD, CAT and GPx in soybean leaves that allowed them to control oxidative stress, contributed to increase the resistance of G. max cv. “Nam Dan” to A. craccivora. The improvement of cyanobacteria extract on the antioxidative response of soybean “Nam Dan” to cowpea aphid can be a novel aspect to contribute to current knowledge regarding the soybean-aphid interaction.