Researchers have tinkered with plant machinery to create crops that can respond to rising levels of atmospheric CO2, photosynthesize more, and produce higher yields. In field experiments, they produced sugarcane and sorghum crops that generated between 15 and 81% more biomass.
The study expands a growing field of work to hack photosynthesis in the attempt to make more productive crops. A lot of this work focuses on Rubisco, an abundant enzyme that helps plants fix carbon into carbohydrates, and so drives the engine of photosynthesis.
Most plants rely on a version of photosynthesis called C3, which is limited by the fact that Rubisco in this scenario has a tendency to fix oxygen molecules as well as carbon. This wastes a lot of the energy that the plant could have used to photosynthesize, and a whole body of research now aims to streamline that process.
But other plants have evolved to develop a new form of photosynthesis called C4, where special leaf structures facilitate the clustering of carbon around Rubisco, limiting its exposure to oxygen. These plants have jumped the oxygen gauntlet—but as some researchers see it, here the challenge is to amp up their production of Rubisco to fix more of the abundant carbon from the air, the one remaining hurdle that limits this type of photosynthesis.
In fact, with global CO2 on the rise, they reason that these C4 crops could deliver heaps more food if they had more of this critical Rubisco machinery in place—thus “future proofing their value under global atmospheric change,” says Coralie Salesse-Smith, a postdoctoral researcher at the University of Illinois, where her work falls under the Realizing Increased Photosynthetic Efficiency (RIPE) project.
To test out the possibility, Salesse-Smith and team focused on two C4 crops: sugarcane, which produces the most harvested biomass of any crop worldwide; and sorghum, the fourth most important cereal in global production. Into these two crops, the researchers bred two engineered genes that are known from other studies to increase the assembly of Rubisco in plants. From this cohort, the researchers grew dozens of new plants, which they planted out in the field (sorghum) and in greenhouses (sugarcane), and grew alongside conventional versions of each crop.
First, studies on the plants themselves showed that compared to the conventional crops, those with the upgraded machinery had increased Rubisco content of between 13 to 25% in sorghum, and a whopping 90% in sugarcane. “By adding more of this enzyme, the plants were able to take up more CO2, resulting in increased photosynthesis and productivity,” says Salesse-Smith. In fact, this translated to 15.5% sorghum biomass increase in the field, and an astonishing 37 to 81% increase in sugarcane production in the greenhouse.
The researchers had been concerned that the plant tissues may not have the available space to store the new deluge of Rubisco. But a separate microscopic study on a sugarcane and sorghum relative called miscanthus had previously revealed that there was potential to double Rubisco content in the tissues of this plant. The researchers say that’s consistent with the increased amount they detected in the crops in their study, suggesting that this tribe of plants may have increased natural storage ability.
There were some caveats to the new research. In sugarcane, the Rubisco-packed plants showed significant increases in the height of the plant, the number of leaves, and the thickness of the stems. But interestingly in sorghum, despite similar increases in biomass, the plants didn’t show increases specifically in the seeds, which is the part that’s harvested for food.
In fact, an analysis of the sorghum seeds showed increased protein, but decreased starch content. Exactly why more Rubisco didn’t lead to more seeds isn’t yet known, but may have something to do with how the plant utilizes and apportions the carbohydrates that it makes, says Salesse-Smith. This will be the focus of future research, the researcher adds.
The findings could have exciting implications for food security under climate change. Compared to C3 crops, C4 plants are in the minority, but many species that fall into this category produce a large share of agricultural output each year. At the same time, yields for sugarcane and sorghum have remained unchanged for at least 20 years in major farming countries like the US.
The researchers believe this picture can change. In previous research using this method, they had already shown similar yield increases in lab-grown corn, another C4 crop. The new research enhances the sense of potential. The next goal is to test out their Rubisco-ramping-up strategy with further multi-years studies on different crops and in new locations, Salesse-Smith says.
“In the bigger picture, I would like to get a better understanding of how plants with increased Rubisco content respond to abiotic stress conditions such as heat, cold and drought in order to further ‘future proof’ these crops.”
Salesse-Smith et. al. “Adapting C4 photosynthesis to atmospheric change and increasing productivity by elevating Rubisco content in sorghum and sugarcane.” PNAS. 2025.
Image: RIPE cassava transformations in growth chambers at the Carl. R. Woese Institute for Genomic Biology. ©Claire Benjamin/RIPE project
