Plants Defend Themselves Against Heavy Metals

ABOVE: A type of root metabolite protects maize plants from arsenic toxicity. ©istock, VR19

Farmers rely on crop yields for economic returns, but soil or water contamination with heavy metals like arsenic threatens crop production and human health. However, some plants exhibit tolerance to heavy metal contamination, offering potential interventions to improve farming.

“There was this…observation that maize accumulates less arsenic compared than other plant species,” said Klaus Schläppi, a plant biologist at the University of Basel. “The question was why.”

Four years ago when Schläppi was at the University of Bern, he and his team turned to a group of maize-produced metabolites called benzoxazinoids, which act as insecticides and bind iron for plant use and aluminum to reduce its toxicity to plants.^1-3 In a recent paper, Schläppi and his group demonstrated that benzoxazinoids also bind arsenic, and this improved plant growth in contaminated soil. The team published the findings in Proceedings of the National Academy of Science.⁴ The findings offer potential solutions for farmers in arsenic-contaminated regions.

 “We knew already that [benzoxazinoids] affect the uptake of various metal ions, and so the finding that this also includes arsenic is interesting,” said Georg Jander, a chemical ecologist at the Boyce Thompson Institute who was not affiliated with the study.

To evaluate if benzoxazinoids protected maize from arsenic contamination, the team grew wild type plants and mutated ones that could not produce benzoxazinoids in pots with or without arsenic-laced soil. In the presence of arsenic, wild type plants remained shorter compared to those grown in arsenic-free soil; benzoxazinoid deficiency further reduced plant growth in contaminated soil. Benzoxazinoid-deficient plants also had lower total biomass in the presence of arsenic compared to normal plants. However, supplementing the metabolites through enriched water improved the growth of benzoxazinoid-deficient plants.

Next, the team quantified arsenic in the roots and leaves of plants and found that while the leaves of both types of plants had comparably low levels of arsenic, benzoxazinoid-deficient roots had greater arsenic accumulation compared to wild type plants. Arsenic exists as different compounds depending on its oxidation state. When the researchers investigated the presence of common arsenic species, they observed that benzoxazinoids changed the arsenic species present in the soil.

“How benzoxazinoids change the arsenic species, that will be a follow up thing to solve,” Schläppi said. Jander also considers the fate of the arsenic to be the most important follow up question. “Are we still dealing with a pool of arsenic that might be affecting other things?” he asked.

In a real-world extension, Schläppi’s team identified two fields in an agricultural region of Switzerland, one with higher arsenic levels than the other, to grow their crops. Similar to their potted experiments, arsenic reduced plant height and mass in both genotypes, but the effect was more pronounced in benzoxazinoid-deficient plants.

Finally, the researchers studied the generational effects of the soil metabolites on the growth of plants. While arsenic stunted plant growth, plants grown in fields that previously harbored wild type crops grew taller and bigger than those grown in fields that grew benzoxazinoid-deficient plants.

“This is going to be different in different kinds of soil with different kinds of metal ions present or different kinds of organic matter that might be binding the arsenic,” Jander pointed out. However, he thought that overall, the findings were interesting and added a novel function of the metabolites to the field. “They don't have all the answers, but they do show pretty clearly that benzoxazinoids affect arsenic uptake,” said Jander.

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