Scientists develop yellow seed camelina with high oil production

Efforts to achieve net-zero carbon emissions from transportation fuels are increasing demand for oil produced by non-food crops. These plants use sunlight to power the conversion of atmospheric carbon dioxide into oil, which accumulates in seeds. Crop breeders, interested in selecting plants that produce a lot of oil, look for yellow seeds. In oilseed crops such as canola, yellow seed varieties generally produce more oil than their brown seed counterparts. The reason: the protein responsible for the brown seed color – which plants with yellow seeds lack – also plays a key role in oil production.

Now, plant biochemists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory – who are interested in increasing the synthesis of vegetable oil for the sustainable production of biofuels and other bioproducts – have leveraged this knowledge to create a new, high-yield to create an oilseed variety. In an article just published in The Plant Biotechnology Journalthey describe how they used tools of modern genetics to produce a yellow-seed variety of Camelina sativa, a close relative of canola, which collects 21.4% more oil than regular camelina.

“If breeders can achieve a few percent increase in oil production, they consider that significant, because even a small increase in yield can lead to a large increase in oil production when you plant millions of hectares,” says biochemist John Shanklin of Brookhaven Lab. chair of the Lab’s Biology Department and leader of the vegetable oil research program. “Our increase of almost 22% was unexpected and could potentially result in a dramatic increase in production,” he said.

Simple idea, special plant

The idea behind developing this high-yielding camelina variety was simple: mimic what happens in the naturally occurring, high-yielding yellow-seeded canola varieties.

“Breeders had identified plants with more oil, some of which happened to have yellow seeds, and they weren’t really concerned about the mechanism,” Shanklin said. But once scientists discovered the gene responsible for both yellow seed color and increased oil content, they had a way to potentially increase oil yields in other species.

The gene has the instructions for making a protein known as Transparent Testa 8 (TT8), which, among other things, controls the production of compounds that give seeds their brown color. Importantly, TT8 also inhibits some of the genes involved in oil synthesis.

Xiao-Hong Yu, who led this project, hypothesized that getting rid of TT8 in camelina should remove the inhibition of oil synthesis – and free up some carbon that can be channeled into oil production.

Getting rid of a single gene in camelina is quite a challenge, because this plant is unusual among living things. Instead of two sets of chromosomes – that is, two copies of each gene – it has six sets.

“This ‘hexaploid’ genome explains why there are no yellow-seed varieties of camelina in nature,” Yu explained. “It would be very unlikely that mutations would occur simultaneously in all six copies of TT8 and completely disrupt its function.”

Gene editing is a mess

Thanks to the tools of modern genetics, the Brookhaven team was able to eliminate all six examples of the TT8. They used gene editing technology known as CRISPR/Cas9 to target the specific DNA sequences in the TT8 genes. They used the technology to splice the DNA at these locations and then create mutations that deactivated the genes. Yu and the team then performed a series of biochemical and genetic analyzes to monitor the effects of their targeted gene editing.

“The yellow seed phenotype we were looking for was a great visual guide to our search,” said Yu. “This helped us find the seeds we were looking for by screening fewer than 100 plants, among which we identified three independently occurring lineages in which all six genes were disrupted.”

The results: The color of the seed coat changed from brown to yellow only in plants in which all six copies of the TT8 gene were disrupted. The yellow seeds had lower levels of “flavonoid” compounds and “mucilage” – both normally produced by biochemical pathways controlled by TT8 – than brown seeds from strains of camelina with unedited genomes.

In addition, many genes involved in oil synthesis and the production of fatty acids, the building blocks of oil, were highly expressed in seeds of the CRISPR/Cas9-edited plants. This resulted in a dramatic increase in oil accumulation. The modified seeds contained another positive surprise, namely that the levels of protein and starch remained unchanged.

The targeted mutations of TT8 were inherited in subsequent generations of the camelina plants, indicating that the improvements would be stable and long-lasting.

“Our results demonstrate the potential for creating new camelina lines through gene editing, in this case by manipulating TT8 to enhance oil biosynthesis. Understanding further details of how TT8 and other factors control biochemical pathways, may provide additional gene targets for increasing oil yields,” Shanklin said.