Copper indirectly harmed bacteria associated with plant roots

What is it about?

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. The bacteria-legume partnership is sensitive to oxidative stress, so the bacteria are housed in protective nodules on the plant roots. The heavy metal copper induces oxidative stress and lowers nitrogen fixation in legumes, but how copper does this is unclear. It could be directly toxic to bacteria in the nodule or be toxic to the plant overall. In this study, legumes were grown with different levels of copper. At the end of the experiment, plant growth, nodule formation, nitrogen fixation, signs of oxidative stress, and antioxidant levels in the nodules and the rest of the plant were measured. Plants given the highest copper treatment had elevated concentrations of copper in their shoots and roots, and 60% less mass compared to control plants. Copper damaged the health of the plant. The number of nodules on these plants decreased by 50%, meaning that copper also interrupted the plant-bacteria interactions that start nodule formation. Surprisingly, copper did not accumulate in the nodules and oxidative stress in the nodules did not increase. This means that copper did not act directly on the nitrogen-fixing bacteria. However, total nitrogen fixation dropped by 45%. Some but not all of this decrease was explained by fewer nodules forming. Antioxidant levels in the nodule decreased, which is a sign that the nodule has been abandoned 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 caused to the roots and shoots likely caused the plant to abandon the nodule and explains the loss of nitrogen fixation activity.

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Photo by Matthew Simmons on Unsplash

Photo by Matthew Simmons on Unsplash

Why is it important?

We discovered that plants and the bacteria with which they associate are affected differently by copper contamination. While copper can harm isolated bacteria, bacteria inside plant roots were protected from direct contact with copper. However, when plants were damaged by copper-stress, they were unable to continue to protect the bacteria.