The Effect of Copper-induced Oxidative Stress on Symbiosis Between Model Legume Lotus japonicus and Mesorhizobium loti

Copper can be a soil contaminant at concentrations that are toxic to plants, particularly because of copper-induced oxidative stress. The legume-rhizobia partnership that allows for biological nitrogen fixation is sensitive to oxidative stress, and this study investigates if copper acts directly on the machinery of nitrogen fixation, or indirectly via toxicity to the entire plant. When Lotus japonicus was exposed to 300 or 450 µM of copper, biomass was reduced by 30-40% (shoots), 40-55% (roots), and 40-60% (nodules). Nitrogen fixation and antioxidant activity also decreased (by 40-45% and 40-60%, respectively), which can be indicators of stress-induced nodule senescence. However, the nodule tissue itself did not show elevated copper uptake or malondialdehyde, a marker of oxidative stress. This would suggest that copper-induced reduction in nodule activity is due to indirect stress on the host plant that limits its ability to successfully complete nodule formation and support its symbionts.