Responses of antioxidative enzymes and gene expression in Oryza sativa L and Cucumis sativus L seedlings to microcystins stress

Microcystins (MCs) have become an important global environmental issue, causing oxidative stress, which is an important toxic mechanism for MCs in plants. However, the regulating mechanism of antioxidative enzymes in plants in adapting to MCs stress remains unclear. We studied the dynamic effects of MCs at different concentrations (5, 10, 50 and 100 μg/L) in rice and cucumber seedlings on relative growth rate (RGR), and reactive oxygen species and malondialdehyde (MDA) content, and antioxidative enzyme activities, during a stress period (MCs exposed for 1, 3, 5 and 7 d) and recovery period (7 d). During the stress period, MCs at 5 μg/L inhibited RGR in cucumber and promoted RGR in rice. The contents of superoxide anion (O₂·^-), hydrogen peroxide (H₂O₂) and MDA increased and RGR declined in both crops with time and intensity of MCs stress. For cucumber, all these parameters responded earlier to MCs stress, and O₂·^-, MDA and RGR were more responsive to MCs stress than in rice. Moreover, catalase (CAT) and peroxidase (POD), and the relative expressions of CAT genes increased in both crops at 5-100 μg/L MCs, whereas relative expression of POD genes increased only in cucumber. Diversely, superoxide dismutase (SOD) response to MCs in cucumber leaves was later than for rice. MCs at 100 μg/L decreased the relative expression of SOD genes in cucumber but did not change SOD activity. During the recovery period, all the above indicators in both crops were higher than the control and lower than in the stress period. Conversely, RGR was lower than in the control and higher than in the stress period, except for cucumber which was lower, and MDA content higher than the stress period at 100 μg/L MCs. Overall, these results indicated that cucumber was more sensitive to MCs than rice, and SOD, CAT and POD play an important role in plant response to MCs stress.