Enhancing the value of biofuel crops with plant-based oils

Benning and Ohlrogge
Great Lakes Bioenergy researchers John Ohlrogge (left) and Christoph Benning (center), both at Michigan State University (MSU), work to increase the energy density of cellulosic biofuel crops by producing plant-based oils. Photos by Kurt Stepnitz, MSU. These oils appear as lipid droplets (green) in Brachypodium, a model organism for grass species (right). Photo by Agnieszka Zienkiewicz, MSU.

Ask Christoph Benning what fascinates him about plants and he has an answer at his fingertips: “All life on Earth depends on plants,” he says. “With photosynthesis, they generate the oxygen we breathe and convert sunlight into chemical energy, which we consume as sugars and proteins in our food.”

Director of the Plant Research Lab at Michigan State University, where he is a professor of biochemistry and molecular biology, and a researcher at the Great Lakes Bioenergy Research Center (GLBRC), Benning is working to increase the energy density of cellulosic biofuel crops, such as grasses, woods, and the non-food portion of plants, by boosting the production of plant-based oils. In nature, these oils, or lipid molecules, are found in seeds, where they provide the plant most of the energy it needs to grow to maturity.

But a few years ago, Benning’s lab showed that Arabidopsis, a small flowering plant commonly used as a model organism, can also be coaxed into making oil outside of its seeds.

“We turned on the genetic switch that controls oil production in seeds in vegetative plant tissues, such as stems and leaves,” Benning explains. “By also turning down the conversion of sugars—generated by photosynthesis—into starch, we were able to produce a significant amount of oil, or lipid droplets, in the leaves.”

Since these lipid droplets are the chemical pre-cursors for biodiesel—which can power trucks, some passenger cars, or even the large engines in trains, ships, or airplanes—increasing their volume adds economic value to crops grown primarily for conversion into biofuel.

The added value is substantial: If only 10 percent of a plant’s vegetative tissues produce biodiesel pre-cursors, the energy yield of biofuel crops increases by about 40 percent, compared with yields produced from just fermenting the plant’s sugars to ethanol.

Benning’s GLBRC collaborator John Ohlrogge, professor emeritus of plant biology at Michigan State University, has already taken the group’s basic science results one step closer to applied technology by testing genetically engineered biofuel crops in outdoor field trials.

Ohlrogge was the first to discover the enzyme that makes acetyl-TAGs, a lipid molecule that can directly power large engines without needing additional chemical modifications. He then grew Camelina sativa plants producing this enzyme in order to quantify how much oil the crop plants were able to generate under agricultural conditions. Positive results in hand, Ohlrogge recruited industrial partners for follow-up commercial testing.

Back in the lab, Benning is now turning his attention to a different kind of plant: Brachypodium, a model organism for grass species. If the same genetic switch is capable of producing oil in the leaves and stems of Brachypodium, this tool could eventually be used to make biodiesel from cellulosic grass crops grown on marginal land, such as switchgrass and sorghum.

But it wouldn’t be science if there weren’t bumps in the road.

“We found that Brachypodium behaves somewhat differently than Arabidopsis,” Benning says. “Turning on the switch for oil biosynthesis is more toxic to its leaf because its fatty acid metabolism has diverged from that in Arabidopsis. So our next strategy is to collaborate with GLBRC researchers Federica Brandizzi and Curtis Wilkerson to see if we can produce the oil in Brachypodium’s stems only, thus avoiding detrimental effects in its leaves.”

In the long term, Benning would like to to generate even more value-added compounds inside the lipid droplets, which he calls “a re-designed plant organelle.” Terpenoids, a group of hydrocarbon chemicals that are easily converted into jet fuel, are a good example for such compounds. Collaborating with GLBRC researcher Bjoern Hamberger, Benning hopes to produce terpenoids only inside the lipid droplets—akin to a miniature manufacturing plant—to prevent them from being toxic to the rest of the plant.

Benning credits advanced molecular and genomic technology with the progress his group has already made.

“Thanks to modern DNA sequencing technology, we can now make the transition from model species to actual crop plants much more quickly than in the past,” Benning says. “And being a part of the GLBRC means we can take our findings to commercial applications much faster than in a traditional academic setting. This combination of basic science and applied training prepares our GLBRC students and postdocs well for both university and industry jobs.”

GLBRC is one of three Department of Energy Bioenergy Research Centers created to make transformational breakthroughs and build the foundation of new cellulosic biofuel technology. For more information on GLBRC, visit www.glbrc.org or follow @glbioenergy on Twitter.

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