Uranium Toxicity in Plants Traced to Nitrogen and Protein Problems

A new study reveals how uranium poisons plants – findings that could help efforts to clean up uranium-polluted soil using plants. The research by Xi Chen and colleagues, published in Environmental and Experimental Botany, found uranium messes with nitrogen metabolism and protein folding in plant cells, shedding light on why the heavy metal stunts plant growth. Understanding these toxic effects can help identify plants that tolerate uranium better for “phytoremediation” – using plants as living sponges to soak up and remove pollutants from contaminated soil and water.

The researchers examined the effects of uranium on radishes and broad beans. They found that uranium exposure had more severe effects on the growth of broad bean plants compared to radish plants, indicating radish has a higher tolerance to the heavy metal. One reason was due to how the plants handled nitrogen.

The researchers found uranium boosted nitrogen uptake in radish by ramping up the activity of a key enzyme called nitrate reductase that helps plants process nitrogen. But in broad bean, uranium harmed nitrogen processing by depressing the activity of nitrate reductase and another nitrogen-related enzyme named glutamine synthetase.

Uranium also caused stress to an important plant cell structure called the endoplasmic reticulum, which is involved in protein production in both plants. The endoplasmic reticulum is responsible for folding and processing proteins so they have the proper shape and function. Uranium caused proteins in the endoplasmic reticulum to misfold, triggering the plant cells’ quality control systems that try to refold or dispose of defective proteins. However, radish plants could more effectively deal with the uranium-induced protein misfolding by ramping up systems that identify and destroy malformed proteins.

Uranium is a heavy metal that can contaminate soil and groundwater around mining and nuclear sites. Though plants don’t need uranium to grow, some species can absorb and accumulate uranium from contaminated sites through their roots. This has led to interest in using plants to clean up uranium-polluted areas through a process called phytoremediation. However, uranium is toxic to most plants, stunting their growth and thwarting cleanup efforts. By understanding uranium’s toxic effects in plants, scientists hope to identify and develop uranium-tolerant plant species to improve phytoremediation.

To uncover uranium’s toxic effects, the researchers grew radish and broad bean plants hydroponically in nutrient solutions. After allowing the plants to establish growth for 10 days, they exposed them to a uranium concentration of 25 micromoles per litre for 3 days.

The experiments revealed uranium accumulated to higher levels in radish roots but inhibited growth more severely in broad beans in terms of lower biomass and root browning. Analyzing nitrogen content, the team found uranium increased levels of assimilated nitrogen compounds in radishes but decreased them in broad beans. Matching this pattern, the activity of the key nitrogen metabolism enzyme nitrate reductase increased nearly 3-fold in uranium-treated radishes but decreased by over 25% in broad beans.

Looking inside plant cells, the researchers discovered uranium caused misfolding of proteins in the endoplasmic reticulum, an organelle involved in processing proteins. However, radish plants could more effectively deal with these malformed proteins by activating protein disposal systems – twenty-seven related genes were upregulated in radishes, with ten genes downregulated. In comparison, only nine genes were upregulated in broad beans, while twelve were down-regulated. This superior protein quality control in radish likely enables it to handle uranium stress better than broad beans.

Chen and colleagues’ results have shown how uranium inhibits plant growth at the molecular level by interfering with key nitrogen assimilation and protein production processes. These discoveries pave the way for tailoring optimal phytoremediation plants able to withstand uranium’s toxic impacts. Such specially bred “super plants” could provide an eco-friendly remediation solution for cleaning up uranium-polluted soils and groundwater.

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Chen, X., Dang, Y.-X., Li, Q.-L., Li, W., Xie, M.-T., Wang, M.-X., Tao, M.-F., Zhao, S.-S., Lai, J.-L. and Wu, G. (2023) “Uranium affects nitrogen metabolism and endoplasmic reticulum protein homeostasis in plants,” Environmental and Experimental Botany, 213(105444), p. 105444. Available at: https://doi.org/10.1016/j.envexpbot.2023.105444.