Lignin, a major component of woody plant material, is woven in with cellulose and provides plants with the strength to withstand strong gusts of wind and microbial attack.
However, this protective barrier or "plastic wall" also makes it harder to gain access to the cellulose.
"There is lots of energy-rich cellulose locked away in wood," said John Carlson, professor of molecular genetics, Penn State.
"But separating this energy from the wood to make ethanol is a costly process requiring high amounts of heat and caustic chemicals. Moreover, fungal enzymes that attack lignin are not yet widely available, still in the development stage, and not very efficient in breaking up lignin," he added.
Researchers have previously tried to get around the problem by genetically decreasing the lignin content in plants.
However, this can lead to a variety of problems, like 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," said Ming Tien, professor of biochemistry, Penn State. "It just doesn't make sense," he added.
Now, Carlson, Tien and postdoctoral associate Haiying Liang have used a different genetic approach.
Instead of decreasing the lignin content, they are trying to modify the connections in lignin, without compromising either the biosynthesis of lignin or the structural rigidity of the plant.
The Penn State geneticists and biochemists took a gene from beans and engineered it into a poplar tree. This gene produces a protein that inserts itself between two lignin molecules when the lignin polymer is created.
"Now, we have a lignin polymer with a protein stuck in between," explained Carlson. "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," he added.
The genetic modification does not appear to weaken the plants, and the transformation may have turned them into more efficient sources of ethanol.
"When we looked at the first generation of modified plants we noticed that the lignin content has not changed," said Tien, whose work is funded by the U.S. Department of Energy.
"We haven't done a fitness test yet but we did see an increase in the yield of sugars for converting into ethanol," he added.