Plants Can Defend Themselves Against Heavy Metals

Farmers depend on crop yields for profits. Yet, soil and water contamination with heavy metals like arsenic jeopardizes crop production and human health. Some plants show tolerance to heavy metal contamination, presenting potential methods to enhance farming practices.

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

Four years prior, while at the University of Bern, Schläppi and his team focused on maize-produced metabolites known as benzoxazinoids. These compounds act as insecticides, binding iron for plant use and aluminum to decrease its toxicity. In a recent paper, Schläppi's group revealed that benzoxazinoids also bind arsenic, promoting improved plant growth in contaminated soil. Their findings appear in the Proceedings of the National Academy of Science. This research holds promise for farmers in arsenic-affected areas.

"We already knew that [benzoxazinoids] influence the uptake of various metal ions, so discovering that this applies to arsenic is intriguing," noted Georg Jander, a chemical ecologist at the Boyce Thompson Institute who was not involved in the study.

To assess whether benzoxazinoids shield maize from arsenic contamination, the team grew both wild type plants and mutated variations lacking these metabolites in pots with arsenic-infused soil. The wild type plants, when exposed to arsenic, remained shorter than those in arsenic-free soil. Furthermore, a lack of benzoxazinoids considerably limited plant growth in contaminated conditions. Benzoxazinoid-deficient plants displayed lower total biomass in arsenic-rich soil versus wild type plants. However, adding the metabolites through enriched water enhanced the growth of benzoxazinoid-deficient plants.

The team then measured arsenic levels in the roots and leaves of the plants. Both plant types exhibited similar low arsenic levels in leaves. However, roots from benzoxazinoid-deficient plants accumulated more arsenic than those from wild type plants. Arsenic appears in various forms based on its oxidation state. The researchers found that benzoxazinoids altered the species of arsenic present in the soil.

"How benzoxazinoids change the arsenic species will be crucial for future investigation," Schläppi noted. Jander regards the fates of the arsenic as critical. "Are we still dealing with a pool of arsenic potentially affecting other aspects?" he asked.

In a practical exploration, Schläppi’s team identified two fields in a Swiss agricultural region with differing arsenic levels to cultivate their crops. As in previous experiments, arsenic inhibited plant growth in both genotypes, notably impacting benzoxazinoid-deficient plants more severely.

Lastly, the team analyzed the generational impact of the soil metabolites on plant growth. Although arsenic hindered growth, plants cultivated in fields previously hosting wild type crops outperformed those in fields with benzoxazinoid-deficient plants.

"This situation will vary based on soil types and metal ion composition, as well as organic matter that may bind the arsenic," Jander observed. Despite uncertainties, he emphasized that the findings are significant and provide a new understanding of the metabolites' roles. "They don’t have all the answers but convincingly demonstrate that benzoxazinoids influence arsenic uptake," Jander stated.

Earlier, SSP reported that researchers developed instant turmeric milk.