The root of microplastics in plants

Over the past decade, scientists have struggled to understand the impacts of microplastics. With the breakdown of plastic bottles, the washing of the world’s seven billion fleece jackets, or microbeads in facial cleansers, microplastics are piling up. How they affect living things like plants is not yet clear.

In the soil, plastics have the potential to cause chemical problems. Much like a magnetic attraction, contaminants can bind to plastics, leading to toxic buildup. Contaminants can also spin free on plastics and potentially enter plants. But first, researchers need to find out if microplastics – or their even smaller offspring called nanoplastics – can get into plant cells in the first place.

Here’s some good news: It’s not, according to a recent study by the Pacific Northwest National Laboratory (PNNL) and Washington State University (WSU). However, microplastics do accumulate at the root tips, which could bode well for future cleaning up of contaminated environments, but not for root crops, like carrots.

Trojan horse for microplastics in plants

Microplastics are a global problem. Particles have been found in all corners of the Earth, from mountain peaks to ocean depths. Over the past decade, most of the research on microplastics has been directed towards aquatic environments, which is ironic as more microplastics have been found on earth.

“To understand the problems with nano- and microplastics in plants, we really need to understand what’s going on at the chemical and cellular level,” said Carolyn Pearce, co-author of the study, a geochemist at PNNL with a joint appointment in the Department of Crops and Soils of the WSU. Sciences.

Like a poisonous Trojan horse, microplastics can act as hot pockets of contaminant transport. They bind to 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 wrinkle in organisms and, perhaps, in the food chain.

The first step in testing the poisonous Trojan horse theory is to see if microplastics can even get into plant cells in the first place. “We looked at where they might collect on plants, what materials attach and how they concentrate,” Pearce said.

Size matters for microplastics in plants

Not all microplastics are created the same. They can be as big as a pencil eraser or as small as a bacteria. Nanoplastics are tiny and are 100 times smaller than a plant cell. At this size, it’s easy to imagine how plants might absorb plastic particles, but there are size limits to what passes through cell walls.

In general, healthy adult plants only take up material of 3 to 4 nanometers, which is even smaller than a virus. Some studies have shown that plants can take up nanoparticles 10 to 12 times larger than that, up to 40 to 50 nanometers. As small particles pass through, the big question is: plastics?

Sow seeds (and some microplastic beads)

To test the question, the researchers focused on two types of plants: Arabidopsis and soft white wheat. Arabidopsis is like the laboratory rat in the world of plant biology. It is a commonly studied weed related to mustard, with a short life cycle. Soft white wheat is grown throughout the Pacific Northwest and is used in Asian noodles and crackers.

The researchers planted seeds on Petri dishes containing agar mixed with two different sizes of microbeads and nanoplastics. One size was the size of a virus, while the other was 25 times the size. After letting the seeds grow for 5 to 12 days, the researchers used a specialized microscope to take cross-sectional images of the roots of the plants, allowing them to see the root cells from all angles.

“We used a confocal microscope at EMSL, the Laboratory for Environmental Molecular Sciences, which was used to examine animal tissue, such as lung tissue. I thought it could be used for plants,” he said. said Stephen Taylor, PNNL postdoctoral soil researcher and lead author of the study. He conducted the research while earning his doctorate. through the WSU-PNNL Distinguished Graduate Research Program. “As far as we know, this is the first time this technique has been used to search for plastics in plant cells.”

Some good news in 2020

No microplastics of any size were taken up by the cells of living tissue in either plant species.

“We saw a buildup of plastic around the root cap cells, and some along the surface to the root. But, we saw no evidence of microplastics inside or between cell structures. cells, ”Taylor said. Cap cells protect sensitive and growing parts of the roots, have a short lifespan, and are often shed. Bottom line, absorption is not a problem, but attachment to the roots can be. This could potentially be a problem for root crops such as carrots, potatoes or beets.

In addition to helping researchers find out whether plants take up plastic particles, the findings also have potential environmental applications.

“Microplastics are a problem that doesn’t go away,” Pearce said. Imagining further research, she asked, “If we show that plastics build up at the tips of roots, maybe we could use plants to remove plastics in other ecosystems?”

The results may also have applications to create more environmentally friendly plastics. “We could also use this information to make plastics that cannot be absorbed by plants and animals,” she said.

There are benefits to knowing what microplastics do or are not absorbed by living things.

“Ultimately, this will help scientists better understand the tipping point from where there is an impact on plants and ecosystems,” Pearce said.

“Accumulation of polystyrene nano- and microplastics in Arabidopsis cells and wheat cap, but no evidence of absorption in the roots” has been published in Environmental Science: Nano.