By Katherine Wright
Plants bred to retain high levels of sugar in their leaves can produce significantly more oil that wild varieties. The finding, reported by Researchers at Brookhaven National Laboratory in the October issue of the journal Plant Physiology, provides a potential biochemical-genetic route to increasing the oil content in plants grown for biofuels.
“For humans, if you eat a lot of sugar you get fat,” Zhiyang Zhai, one of the study’s researchers, said. “We tried to test whether that is also true in plants.”
To test this hypothesis, the team selectively bred Arabidopsis—a small flowering plant that is used as a model organism by plant biologists—with various genetic mutations to see how the plant’s oil production changed.
The first mutation the team introduced halted the transport of sugar from the plant’s leaves, Zhai said.
“Sugar [in plant leaves] comes from photosynthesis. Once it is generated in the leaves [the plant] has to transport it out of the leaves into other parts of the plant body in order for them to grow their roots and other parts of the plant tissue,” Zhai said. “We blocked the transport so the sugar gets trapped in the leaves.”
The resulting “super sweet” leaves contained 80 times more sugar than the wild type plant without the genetic mutations, according to the research paper.
But just producing a lot of sugar isn’t enough to get high oil content in the leaves, John Shaklin, another of the study’s researchers, said. For that the team had to stop the plant from converting the sugar into starch, which plants use to make new tissues, or store for future energy needs. To do this they bred plants with a second starch-blocking mutation.
These double mutant plants contained 10 times more oil than their wild counterparts, according to the results published in the research paper.
The team was able to increase the plant’s oil content even further by breeding plants with an additional two mutations, including one that acts like a master control for oil synthesis, Shanklin said. The result was a further tripling of oil production in the plant’s leaves. “We’ve rewired the [plant’s] metabolism,” Shaklin said.
Shanklin’s group is not the only one trying to up the oil content of plants using genetic methods, he said, but they have had the most success so far.
“This is the most complex combination of approaches targeting different aspects of lipid and carbohydrate metabolism yielding the highest accumulation of oil accumulation in vegetative tissues to date,” Christoph Benning, a professor at Michigan State University who carries out research on plant metabolism, said in an email. Benning was not involved in this research study.
This research is a promising proof-of-concept for growing high oil content plants for biofuels, Benning said. “The next step will be to take this into a crop plant, a grass species, for example,” he said. “Based on the current work, one can be optimistic that by balancing and refining the approaches chosen, we will be able to produce novel biofuel crops with an enhance bioenergy density.”