Researchers at Penn State University are modifying the structure of lignin—a polymer that is a major component of woody plant material—in poplar trees to facilitate its degradation for the subsequent processing of the woody biomass into liquid fuels. Lignin is woven in with cellulose and provides plants with the strength to withstand strong gusts of wind and microbial attack. However, this protective barrier also limits hydrolytic enzyme access to the cellulose and hemicellulose.
Researchers have previously tried to get around the problem by methods including treatment with lignin-degrading fungi and genetically decreasing the lignin content in plants. The first is at an early stage of development, and the second can lead to a variety of problems such as limp plants unable to stay upright, and plants more susceptible to pests.
|“Trying to engineer trees without lignin is like trying to engineer boneless chicken. It just doesn’t make sense.”|
—Prof. Ming Tien
Instead of decreasing the lignin content in plants, professors John Carlson, Ming Tien and postdoc Haiying Liang are modifying the connections in lignin—without compromising either the biosynthesis of lignin or the structural rigidity of the plant—by introducing in situ peptide crosslinks. Their hypothesis was that the presence of the protein would not change the overall lignin content or distribution in the cell wall, but would make the lignin more susceptible to digestion by protease enzymes.
To test the hypothesis, the Penn State geneticists and biochemists over-expressed a transgene encoding a high tyrosine-content peptide (taken from the common pea) in the lignifying tissues of hybrid poplar.
Now we have a lignin polymer with a protein stuck in between. When that occurs, it creates a type of lignin that is not much different in terms of strength than normal lignin, but we can break open the lignin polymer by using enzymes that attack proteins rather than enzymes that attack lignin.—John Carlson
|Penn State Researchers describe their work on transgenic poplar trees to reduce lignin problems. Credit: Amitabh Avasthi, Penn State|
Fungal enzymes that attack lignin are still in the development stage and are thus not yet widely available, Carlson said, nor are they very efficient in breaking up lignin. By contrast, enzymes targeted at proteins are already used widely in the laundry detergent industry and are commercially readily available, he said. Carlson, Tien and Liang have filed a provisional patent on their approach.
According to a paper published earlier this year in the journal Clean—Soil, Air, Water, the transgenics had no change in total lignin content or overall plant morphology, although the wood in a number of lines had reduced storage modulus. A number of transgenic lines were more susceptible to protease digestion than wildtypes, resulting in a higher polysaccharide release.
Although preliminary, these results suggest that this may be a viable means to facilitate lignin removal and potentially advance the utilization of woody biomass as a biofuel feedstock.—Liang et al. (2008)
The modified plants may also have an unexpected ancillary benefit. Forage crops such as ryegrass and clover have quantities of lignin which can cause ruminants like cows to get sick. Their digestive enzymes go into overdrive to break down the lignin, creating a lot of gas and digestion problems for the animals.
All animals produce enzymes in their digestive process that break down amino acids and small proteins that can be absorbed by the intestine. If this technology were to be transferred to alfalfa or hay or such cattle feed, it might make it easier for the cows to break down the lignin through their own enzymes.—John Carlson
H.-Y. Liang, C. J. Frost, X.-P. Wei, N. R. Brown, J. E. Carlson, M. Tien (2008) Improved Sugar Release from Lignocellulosic Material by Introducing a Tyrosine-rich Cell Wall Peptide Gene in Poplar. Clean 36 (8), 662-668 doi: 10.1002/clen.200800079