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By Kelsey Adkisson
Newswise – Over the past decade, scientists have been striving to understand the impacts of microplastics. With the breaking of plastic bottles, the washing of the seven billion fleece jackets in the world or the microspheres in facial cleansers, microplastics are accumulating. How they affect living things such as plants is still unclear.
In soil, plastic can cause chemical problems. Like a magnetic attraction, contaminants can bind to the plastic, causing a toxic build-up. Contaminants can also obstruct plastic and potentially penetrate plants. But first, researchers need to know whether microplastics – or their even smaller offspring called nanoplastics – can enter plant cells in the first place.
Here’s some good news: They don’t, according to a recent study by the Pacific Northwest National Laboratory (PNNL) and Washington State University (WSU). However, microplastics accumulate at the root tips, which could bode well for the future cleansing of contaminated environments, but not for roots, such as carrots.
Trojan horse for microplastics in plants
Microplastics are a global problem. Particles have been found in all corners of the earth, from remote mountain peaks to ocean depths. Over the past decade, most of the research on microplastics has turned to aquatic environments, which is ironic because more microplastics have been found on earth.
“To understand the problems with nano and microplastics in plants, we need to truly understand what is happening at the chemical and cellular level,” said study co-author Carolyn Pearce, geochemist at PNNL with a joint appointment in the Department of Crops and Soil at WSU Sciences .
Like a toxic Trojan horse, microplastics can act as hot pockets for transporting contaminants. They bind and accumulate soil contaminants, such as long-lived polychlorinated biphenyls (PCBs). PCBs have been linked to cancer: production was banned in 1970, but they still persist in the environment. The result? A potential free ride in organisms and, perhaps, along the food chain.
The first step in testing the toxic Trojan horse theory is to see if microplastics can even get into plant cells in the first place. “We looked at where they could accumulate on plants, what materials stick together and how they concentrate,” Pearce said.
Size matters when it comes to microplastics in plants
Not all microplastics are created equally. They can be as large as a pencil eraser or as small as a bacterium. Nanoplastics are tiny and 100 times smaller than a plant cell. At that size, it’s easy to imagine how plants might absorb the plastic particles, but there are size limits as to what goes through the cell walls.
In general, healthy adult plants only absorb materials 3-4 nanometers in size, which are even smaller than a virus. Some studies have shown that plants can absorb nanoparticles that are 10-12 times larger than that, up to 40-50 nanometers. As small particles pass by, the big question is: plastic?
To test the question, the researchers focused on two types of plants: Arabidopsis and white soft wheat. Arabidopsis he is like the laboratory mouse of the world of plant biology. It is a commonly studied herb related to mustard, with a short life cycle. White soft wheat is grown throughout the Pacific Northwest and is used in Asian pasta and crackers.
The researchers planted seeds on Petri dishes containing agar mixed with two different sizes of microspheres and nanoplastics. One size was the size of a virus, while the other was 25 times larger. After letting the seeds grow for 5-12 days, the researchers used a specialized microscope to acquire cross-sectional images of the plant roots, allowing them to see the root cells from all angles.
“We used a confocal microscope at EMS, the Environmental Molecular Sciences Laboratory, which was used to examine animal tissue, such as lung tissue. I thought it could be used for plants, “said Stephen Taylor, a PNNL soil postdoctoral researcher and lead author of the study. He conducted the research while earning his PhD through the WSU-PNNL Distinguished Graduate Research Program. “As far as we know, this is the first time this technique has been used to look for plastic in plant cells.”
Some good news in 2020
No microplastic granules of either size were absorbed by living tissue cells in any of the plant species.
“We saw accumulations of plastic around the root cap cells and some along the surface to the root. But we haven’t seen any evidence of microplastics within cell structures or between cells, “Taylor said. The cap cells protect the sensitive and growing parts of the roots, are short-lived, and are eliminated often. In conclusion: uptake is not a problem, but root attachment may be.This could potentially be a problem for root crops such as carrots, potatoes or beets.
In addition to helping researchers better understand whether plants absorb plastic particles, the findings also have potential environmental applications.
“Microplastics are a problem that isn’t going away,” Pearce said. Imagining further research, he asked, “If we show that plastic builds up at the root tip, maybe we could use plants to remove plastic in other ecosystems?”
The findings could also have applications for creating more environmentally friendly plastics. “We could also use this information to produce plastics that cannot be absorbed by plants and animals,” he said.
There are advantages to knowing what microplastics do or not are absorbed by living things.
“Ultimately, this will help scientists better understand the tipping point of where there is an impact on plants and ecosystems,” Pearce said.
The uptake of microplastics in plants was the focus of the following study: Taylor, S., Pearce, C., Sanguinet, K., Hu, D., Chrisler, W., Kim, Y., Wang, Z. and Flury, M. 2020. Accumulation of polystyrene nanoparticles and microplastics in Arabidopsis and wheat root cap cells, but no evidence for uptake in roots, which was published in Environmental Sciences: Nano and was rated as HOT because it received particularly high marks in the scientific peer review process.
Funding for this study was provided by the WSU-PNNL Distinguished Graduate Student Fellowship and the U.S. Department of Agriculture’s National Institutes of Food and Agriculture. The initial funds were provided by the seed program for research and development directed by the laboratories of the Energy and Environment Directorate of the GNP. The Environmental Molecular Sciences Laboratory is a user facility of the DOE Office of Science.
Research group: Carolyn Pearce (PNNL / WSU); Stephen Taylor, Dehong Hu, William Chrisler and Yong-Mo Kim (PNNL); Karen Sanguinet and Markus Flury (WSU); and Zhang Wang (Shenyang Agricultural University, China).
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