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A new UC Riverside-led study reveals how common small particles produced by nature as well as human activities can transform upon entering plant cells and weaken plants' ability to turn sunlight into food. The discovery offers a path to control this issue.

Engine combustion and manufacturing, natural processes like forest fires and volcanic eruptions all emit nanoparticles, which are thousands of times smaller than the width of a human hair.

Engineered nanoparticles are also being studied for promising uses in agriculture and biofuels. They can precisely deliver pesticides or nutrients to plants, protect them against drought or heat, and even serve as tiny sensors to monitor plant health.

"Half of all fertilizer applied on farms is lost in the environment and pollutes groundwater. With the most commonly used pesticides, it's even worse—only 5% may reach their intended targets. There's a lot of room for improvement," said Juan Pablo Giraldo, associate professor of plant biology at UCR.

Many scientists argue that nanoparticles are essential for meeting rising global food demands. But there's a catch.

The study in Nature ÌÇÐÄÊÓƵ shows that inside plants, particles can transform in ways that interfere with photosynthesis, the process by which plants turn sunlight into their food.

"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, deputy division director of the U.S. National Science Foundation (NSF) Division of Chemistry.

Led by Giraldo and his Ph.D. student Christopher Castillo, the research team found that while entering plant cells, the nanoparticles undergo changes in pH and pick up lipid coatings from plant membranes. This transformation boosts their ability to bind to RuBisCO, the protein that initiates photosynthesis.

"The lipid-coatings of positively charged nanoparticles enhance their binding to RuBisCO, and don't allow it to do its job very well," Giraldo said.

RuBisCO enables plants and algae to absorb carbon dioxide and convert it into sugars that fuel growth. It is arguably the most abundant protein on Earth and is central to the food chain.

Wanting to understand the impact of nanoparticle transformations on RuBisCO function, the team embarked on a multi-year investigation.

Collaborators came from Santa Clara University, the University of Illinois, Johns Hopkins University, and the Connecticut Agricultural Experiment Station as well as UCR, with support from the NSF Center for Sustainable ÌÇÐÄÊÓƵ.

The team measured how much carbon dioxide is taken up by Arabidopsis leaves, which is an indicator of RuBisCO activity. While the nanoparticles showed little impact in a test tube with RuBisCO, once they undergo transformations in living plant cells they reduced the protein's activity threefold.

"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. "Until now, we didn't have the tools to compare these changes in protein function."

Previously, the group wondered whether the nanoparticles' positive charge might impair protein function. "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 said. The team is the first to make this discovery.

The multi-disciplinary nature of the team was a big asset. "We each bring in complementary techniques that enable deeper insights into a complex system such as a plant cell," said Rigoberto Hernandez, study co-author and Johns Hopkins University chemistry professor.

"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.

Researchers are calling for a better understanding of chemical transformations caused by nanoparticles inside organisms. Nanoparticles remain appealing for agriculture and biofuels because they can be designed to be both biocompatible and biodegradable. Now, they can also be made better for plants.

"Through the NSF Center for Sustainable ÌÇÐÄÊÓƵ, researchers from different branches of chemistry and environmental science have collaborated to produce a new understanding of how nanoparticles transform and interact with a protein involved in plant photosynthesis," NSF's He said. "In so doing, they have laid the groundwork for the safer and more effective use of nanoparticles for agricultural and biotechnology applications."

"This landmark study tells us that we have a long way to go to make nanoparticles that are truly beneficial in the environment," said Catherine Murphy, study co-author and chemistry professor at the University of Illinois Urbana-Champaign.

"But now that we know the mechanism of action, we can re-tune our methods to solve these problems," Murphy said.