When plants are stressed, they accumulate reactive oxygen species (ROS) that strongly inhibit plant growth and lead to about half of all crop yield loss worldwide, explains team leader Juan Pablo Giraldo of the Department of Botany and Plant Sciences at UCR. This is because ROS accumulation leads to decreased photosynthetic activity in plant chloroplasts. Chloroplasts are organelles inside a plant cell that use chlorophyll to capture and store the energy from solar radiation.

Giraldo and postdoctoral researcher Honghong Wu have now found that adding negatively charged spherical ceria particles smaller than 11 nm to chloroplasts in vivo augments ROS scavenging and thus improves photosynthetic activity. Nanoceria are a family of cerium oxide nanoparticles that are between a few nanometres and a few hundred nanometres in size. They can be positively or negatively charged, or neutral, and have been widely studied as antioxidants in biomedical research.

Nanoceria delivered to leaf mesophyll chloroplasts

The UCR team studied Arabidopsis thaliana plants and stressed them by exposing them to very bright light (2000 μmol/m2/s for 1.5 hours), which is about the intensity that can be experienced under full sunlight on a clear day. Plants often encounter light intensities that exceed their photosynthetic capacity and this light induces ROS accumulation, explains Giraldo. Light stress adversely affects plant health, leading to problems such as plant chlorosis, reduced photosynthetic capacity and even death. The researchers also looked at other common environmental stressors such as hot temperatures (35°C for 2.5 hours) and dark chilling (4°C for five days).

They delivered nanoceria to leaf mesophyll chloroplasts by simply infiltrating a solution containing 50mg of nanoparticles per litre through the stomata pores into the leaf lamina. The nanoceria are transported through the lamina into chloroplasts by so-called non-endocytic pathways and helped along by the electrochemical gradient of the cell wall’s plasma membrane potential. Negatively charged nanoceria bind to the outer positively charged side of the plasma membrane and co-localize within chloroplasts by up to two times as much as positively charged ones, say the researchers.

Nanoceria with a low Ce3+/Ce4+ ratio reduce the levels of ROS

“We imaged the nanoparticles' movement in the plants using laser scanning confocal fluorescence microscopy and determined how they affected photosynthetic activity using a portable photosynthesis gas analyser that monitors the key parameters of the light and carbon reactions involved in photosynthesis,” says Giraldo.

“We found that nanoceria with a low Ce3+/Ce4+ ratio reduce the levels of ROS such as hydrogen peroxide, superoxide anions and hydroxyl radicals by 52%, thus protecting the chloroplast photosynthetic machinery from oxidative damage. The decrease in hydroxyl radical levels are particularly interesting, since there is no known naturally occurring plant enzyme to scavenge this ROS.”

Increases in photosystem II quantum yield and carbon assimilation rates

“The nanoceria-infiltrated Arabidopsis plants were also better at tolerating continuous excess light than controls without nanoparticles, and after a day’s exposure showed a higher chlorophyll content index than controls,” he adds.

The UCR researchers also found that plants containing nanoceria that were exposed to abiotic stress showed a 19% increase in quantum yield in photosystem II and a 67% increase in carbon assimilation rates. “The PSII quantum yield is closely related to the photon absorption efficiency and conversion into electron flow, and is routinely used as a tool for investigating photochemical mechanisms underlying photosynthesis,” explains Giraldo.

A valuable tool for improving agricultural production worldwide

“Our study implies that crops might be protected from abiotic stress by applying nanoceria to their leaves,” he tells nanotechweb.org. “This nanobionics approach, as it is known, could help farmers reduce the negative effects of abiotic stress on plant productivity. Indeed, it could become a valuable tool for improving agricultural production worldwide and meeting the food demands of the estimated 9.3 billion people that will be on Earth in 2050.”

The team, reporting its work in ACS Nano, DOI: 10.1021/acsnano.7b05723 says that it will now be working on understanding the underlying mechanisms of how nanoceria interact with crop leaves to develop ways of more efficiently targeting them to specific plant tissues and organelles.