Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Biology

Supervisor

Macfie, Sheila M.

Abstract

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.

Summary for Lay Audience

The legume plant family forms symbiotic partnerships with soil bacteria that convert or “fix” atmospheric nitrogen into a form that living things can absorb and use. Soils can be polluted with heavy metals such as copper, which causes oxidative stress in plants. The bacteria-legume partnership is sensitive to oxidative stress, so the bacteria are housed in root organs called nodules that protect them with antioxidant systems. Copper lowers nitrogen fixation in legumes, but how copper does this is unclear. It could be directly toxic to nodule activity or be toxic to the plant overall. This study determines how copper affects the nodules by exposing legumes to different levels of copper and then measuring plant growth, nodule formation, nitrogen conversion, signs of oxidative stress, and antioxidant levels in the nodules and the rest of the plant.

The highest copper treatment lowered the mass of shoots by 40%, roots by 55%, and nodules by 60%, damaging the health of the plant. The number of nodules decreased by 50%, meaning that copper also interrupted the plant-bacteria interactions that start nodule formation. Surprisingly, measurements of copper in the plant tissue revealed that copper concentration did not increase in the nodules. This means that copper likely was not able to act directly on the nodules themselves. Oxidative stress in the nodules did not increase, but copper did cause damage in the roots and influenced activity within the nodule. The total nitrogen fixation happening in each plant dropped by 45%, which might be explained by fewer nodules forming. Antioxidant levels in the nodule also decreased. Less antioxidant activity in the nodule is a sign that the nodule is not supported by the plant. The bacteria in the nodule need the plant’s support to remain active, and they will lose nitrogen fixation activity if the plant cannot protect them. Maintaining nodule activity is very challenging for the plant under stress. The damage copper causes to the roots and shoots likely causes the plant to abandon the nodule and explains the loss of nitrogen fixation activity.

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