A dual effect of Se on Cd toxicity: evidence from plant growth, root morphology and responses of the antioxidative systems of paddy rice

It is well established that low levels of selenium (Se) are protective against low levels of cadmium (Cd) toxicity and can significantly reduce Cd uptake in plants. However, our previous study reported that the addition of Se hampered the growth of paddy rice exposed to high levels of Cd and enhanced Cd uptake. The relevant mechanisms underlying the dual effects of Se on Cd uptake and toxicity are unclear. This study attempted to illustrate the potential mechanisms of the effect of selenite on Cd toxicity and uptake in paddy rice using hydroponic culture, mainly focusing on the changes in root morphology and the responses of antioxidative enzymes to low and high levels of Se and Cd. A root image analysis system equipped with WinRHIZO image analysis software was used to analyze the root morphology. When no Cd was added, a level of Se as low as 0.2 mg L-1 decreased the shoot malondialdehyde (MDA) content and enhanced the growth of paddy rice; however, Se levels of up to 0.8 mg L-1 inhibited the growth and increased the MDA content of shoots, demonstrating the dual effect of Se on plants. Superoxide dismutase (SOD) appeared to be activated, especially in the roots, when Se was added to solutions containing a high concentration of Cd; however, single addition of Se or Cd inhibited SOD activity. The addition of Se to a solution containing Cd stimulated only root ascorbate peroxidase (APX) activity. Peroxidase (POD) and catalase (CAT) enzymes appeared to have limited roles in detoxifying Cd when increasing amounts of Se were added. When Se was absent from the solution, Cd levels as low as 1 mg L-1 had beneficial effects on root growth but not on shoot growth; however, these beneficial effects were accompanied by a significant increase in shoot MDA content. Cd levels higher than 4 mg L-1 inhibited the growth of shoots and roots, suggesting that Cd toxicity had occurred in the paddy rice. The addition of Se to the treatments containing Cd significantly reduced the proportion of fine roots and tended to increase the proportion of coarse roots, which might explain the decreases in Cd uptake in this study and the decreases in the uptake of other minerals observed in other studies. The addition of Se could mitigate the toxicity of Cd; however, these protective effects are likely dependent on the doses of Se and Cd. Several Se-mediated mechanisms for the mitigation of Cd toxicity were proposed, including (1) an increase in the proportion of coarse roots to reduce Cd uptake and (2) the activation of certain antioxidative enzymes. When 12 mg L-1 Cd was added to the solution, the addition of Se inhibited plant growth instead of mitigating the toxicity of Cd; this finding was thought to be related to the increased permeability of the root cells to Cd due to the damaged cell membranes produced by Se supplementation.