Nanoparticles are threatening the stability of plant cells

Nanoparticles – microscopic specks released by cars, factories, forest fires, and volcanoes – float through every ecosystem on Earth.

Scientists are also designing these particles to precisely deliver fertilizers to crop roots, target pests with pesticides, or serve as tiny sensors that detect plant stress.

But new research, led by the University of California, Riverside, shows that once those particles enter a plant cell, they can change in unexpected ways and interfere with the plant’s ability to photosynthesize.

The researchers uncovered a chain reaction: inside the cell, nanoparticles pick up a greasy coating and then cling to RuBisCO, the enzyme that jump-starts photosynthesis. The result is a dramatic slowdown in the plant’s carbon-dioxide intake.

The discovery does not condemn the promise of agricultural nanotechnology, but it does force scientists to rethink how particles are designed.

Unintended consequences of nanoparticles

While many agricultural scientists see nanotechnology as a key to feeding a growing global population, even its strongest proponents admit that today’s fertilizers and pesticides are wildly inefficient.

Juan Pablo Giraldo is the senior author and an associate professor of plant biology at UC Riverside.

“Half of all fertilizer applied on farms is lost in the environment and pollutes groundwater,” he said. “With the most commonly used pesticides, it’s even worse – only five percent may reach their intended targets. There’s a lot of room for improvement.”

Nanoparticles can, in theory, deliver cargo directly to a leaf’s interior and then break down safely. Scientists suspected the traits helping particles enter plants might also cause harmful collisions with sensitive proteins.

Nanoparticles enter plant cells

To test that idea, the researchers exposed Arabidopsis plants – widely used as a model species – to positively charged nanoparticles.

Using super-resolution imaging and spectroscopy, they watched the particles move through cell walls and membranes. Eventually, the particles reached the chloroplasts, where photosynthesis occurs.

“Nanoparticles, both natural and anthropogenic, are prevalent in many of Earth’s ecosystems, but scientists are only recently starting to understand how they interact with different parts of the environment,” said Lin He of the National Science Foundation.

Inside the plant’s watery interior, the particles experienced a lower pH and quickly attracted lipids from nearby membranes. This new greasy shell altered their surface chemistry and made them stick tightly to RuBisCO.

“The lipid coatings of positively charged nanoparticles enhance their binding to RuBisCO, and don’t allow it to do its job very well,” Giraldo explained.

RuBisCO is responsible for capturing carbon dioxide and turning it into sugar. Blocking even a fraction of its activity can leave a plant starved for energy.

The cost to photosynthesis

To quantify that effect, the team measured CO2 uptake in living leaves. Outside the cell, nanoparticles had little impact on RuBisCO activity. Inside the cell – after the particles had acquired lipid coats – RuBisCO’s efficiency fell to roughly one-third of normal.

“This is the first time we’ve been able to compare the effect of a nanoparticle on a protein both outside and inside a living plant cell” Giraldo said.

At first, the scientists assumed the particle’s electrical charge might be responsible. Deeper analysis showed otherwise.

“Now we know that it’s not the charge necessarily, but the transformation they undergo as they enter plants, and potentially, other organisms as well,” Giraldo added.

A greasy molecular puzzle

The team collaborated with chemists at Johns Hopkins University to run molecular simulations of the interaction.

“We each bring in complementary techniques that enable deeper insights into a complex system such as a plant cell,” said co-author Rigoberto Hernandez, a scientist at Johns Hopkins.

His lab’s models showed lipid molecules flocking to the nanoparticle surface and forming a sticky layer that fits RuBisCO almost like Velcro.

“Using computer simulations providing a view of the structure and motion at microscopic scales, we resolved how lipids are acquired by nanoparticles in the presence of RuBisCO,” Hernandez said.

Nanoparticles for a greener future

The researchers emphasize that nanoparticles still hold enormous promise, especially if they can be redesigned to avoid hijacking RuBisCO.

Through the NSF Center for Sustainable Nanotechnology, researchers from various fields of chemistry and environmental science have collaborated closely. Together, they uncovered how nanoparticles transform and interact with a protein involved in plant photosynthesis.

“In so doing, they have laid the groundwork for the safer and more effective use of nanoparticles for agricultural and biotechnology applications,” said Lin He.

Catherine Murphy, a chemistry professor at the University of Illinois and study co-author, put the challenge bluntly: “This landmark study tells us that we have a long way to go to make nanoparticles that are truly beneficial in the environment,” she said.

“But now that we know the mechanism of action, we can retune our methods to solve these problems.”

Next steps include designing coatings that stay inert in cells or steering particles away from RuBisCO-binding areas. With that knowledge, future nanoparticles could nourish crops, fight pests, and measure plant health – without dimming the green engine that feeds the planet.

The study is published in the journal Nature Nanotechnology.

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