Natural acids in soil could protect rice from toxic nanoparticles

A dose of dirt could defend rice plants from the damaging effects of toxic nanoparticles.

Acids naturally found in the organic matter of soil, collectively called humic acid, can protect rice seedlings from the cell damage and stunted root growth caused by copper oxide nanoparticles, researchers report April 13 in Environmental Toxicology and Chemistry. These toxic nanoparticles are used in products such as paints, electronics and fungicides and can leak into the environment. The new finding, however, suggests that these nanoparticles may not have the same toxic effects in the environment as they do in many lab experiments, which often omit factoring in natural, complex components such as humic acid.

This study demonstrates that simplified lab experiments “can significantly exaggerate the potential for environmental impacts,” says environmental engineer Pedro Alvarez of Rice University in Houston. Past studies have found that certain types of nanoparticles can damage plant DNA, block photosynthesis and cause plants to produce lower-quality food (SN, 10/6/12, p. 18). In particular, copper oxide nanoparticles, which are increasingly used in agricultural products, are toxic to several plants, including maize and radish.

Researchers led by Jiyan Shi of Zhejiang University in China found that rice seedlings exposed to copper oxide nanoparticles had shorter roots. When the researchers looked under a microscope, they found collapsed and dead root cells. The cells’ membranes were battered, allowing their contents, such as proteins, to spill out. The nanoparticles also caused the plants to form high levels of reactive oxygen species, naturally occurring by-products of metabolism that can damage proteins, lipids and genetic material.

When researchers added humic acid, the toxic effects of the nanoparticles were reduced and fewer particles were able to invade the rice plants. The authors speculate that this may be due to how the particles and humic acid interact.  Usually, these copper oxide nanoparticles are positively charged — a characteristic linked to toxicity and plant invasion. Humic acid, on the other hand, includes negatively charged molecules, which may coat the nanoparticles, turning them negative. Their new negative charge may cause the nanoparticles to be repelled by the plants’ negatively charged cell walls, the authors say, preventing damage from occurring.

This is not the first time that components of organic material have been found to protect plants from toxic nanoparticles, says environmental nanoscientist Jorge Gardea-Torresdey of the University of Texas at El Paso. “The uniqueness of this paper is that it’s in rice,” he says. Rice is one of the top crop plants in the world. Understanding how nanoparticles could affect such an important crop is critical for global food production, he says.