New sorghum variant outperforms soybeans in oil production

Researchers at the Center for Advanced Bioenergy and Bioproducts Innovation (CABBI) have developed a new sorghum variant that can outperform soybeans in oil production, with great potential as a clean source of renewable fuel.

Scientists have long worked to create new sustainable sources of vegetable oils, known as triacylglycerols (TAG), to meet the growing demand for renewable fuels like sustainable aviation fuel (SAF) and renewable diesel.

Currently, oil palm and oilseeds such as soybeans provide most TAG for renewable fuels, but these sources alone cannot meet future global needs. To address this, researchers have been engineering high-biomass grasses like sorghum to produce oil. These grasses are highly efficient at photosynthesis, produce large amounts of biomass, and can grow in tough climates, making them excellent candidates.

In their new study, published in Plant Biotechnology Journal, CABBI scientists highlight the utility of a lab-to-field pipeline to deliver sorghum that’s high in TAG. Researchers engineered sorghum to accumulate up to 5.5% dry weight TAG in its leaves and 3.5% dry weight in its stems under field conditions—78 times and 58 times more than unmodified sorghum, respectively. This level of production could provide about 1.4 times more oil per hectare than soybeans, making this a promising new feedstock for renewable fuels.

“This work is the culmination of a large team effort that demonstrates how fundamental research can be used to develop new crop feedstocks to address global energy demands,” said Edgar Cahoon, Director of the Center for Plant Science Innovation at University of Nebraska and one of the corresponding authors on the paper. Cahoon worked with Kiyoul Park, Senior Research Associate in the Department of Biochemistry at University of Nebraska and lead author on the paper; and Tom Clemente, Eugene W. Price Distinguished Professor of Biotechnology at the University of Nebraska; along with many other CABBI experts.

In contrast to oil-rich seeds and fruit from plants like oil palm and soybean, TAG typically only accumulates in a plant’s vegetative organs (leaves and stems) as a stress response to membrane damage.

To design sorghum for vegetative oil accumulation, the researchers used a “push-pull-protect” strategy, which CABBI researchers have previously used to increase vegetative oil accumulation in other plants. They introduced genes to “push” more carbon from photosynthesis into oil production, “pull” fatty acids into TAG molecules, and “protect” the stored oil from breaking down. This approach built on previous successes with other crops, focusing in on sorghum for its heat and drought tolerance and well-understood genome.

By using advanced gene transfer methods, CABBI scientists engineered sorghum lines that, when grown in the field at the Eastern Nebraska Research, Extension, and Education Center, not only maintained stable oil production over multiple generations, but also avoided the biomass reductions seen in similar studies with other biomass crops.

“The breadth of expertise in CABBI has allowed us to take a concept from the lab and put it to practice for field production of a new bioenergy and bioproduct feedstock,” Cahoon said.

These newly engineered oil sorghum lines provide potential new sources of feedstocks for renewable diesel and SAF, reducing reliance on traditional oil crops while meeting the growing demand for renewable energy. And this oil sorghum also has the potential to provide new income streams and markets for farmers. Oil sorghum bioprocessing opens up new ways to spur the bioeconomy and support rural vitality.

The research team will continue to study how to further increase oil yields to meet CABBI’s goal of growing crops that are 10% TAG by dry weight.

“The basis for further improvement of TAG yields will depend on in-depth analysis of the effects of the ‘push-pull-protect’ metabolic engineering approach applied in the study,” said Jörg Schwender, Senior Scientist of the Plant Science Group at Brookhaven National Laboratory and another corresponding author on the paper. “For example, in the current study, the team used whole transcriptome shotgun sequencing (or RNA sequencing), a technique that analyzes the activity of thousands of genes at the same time in tissue samples.”

This analysis found that the oil sorghum lines increase production of an enzyme in their leaves that breaks down lipids, and as such likely also attacks TAG. Further analysis of metabolic flux with isotope tracers confirmed that lipids, although being made at a higher rate in the oil sorghum leaves, are degraded faster at the same time. These findings likely can be translated into a refined engineering strategy that further increases oil levels. The research team aims to refine this approach to make sorghum a reliable, sustainable biofuel feedstock.