Pulping trees for paper could one day take much less energy due to a genetic modification process involving a gene from a Chinese medicinal herb. Source: ABC Science
The gene modifies plant lignin, a type to reinforcement in cell walls, so it is more easily broken down by industrial chemicals, said Professor John Ralph, a biochemist at the US Department of Energy’s Great Lakes Bioenergy Research Center.
Lignin is a polymer that together with cellulose forms the strength of plant cell walls and is responsible for wood’s strength.
When plants are processed to produce paper the lignin must be dissolved away by being broken down into smaller pieces. This leaves the pure cellulose from which paper is made.
However this is currently a very tough job – especially in softwood trees.
“The problem is to cleave the lignin up you have to break some bonds in its backbone,” Ralph said.
“And the easiest bonds to break still take about two hours at 170°C in 1-molar caustic soda, or at 200°C in acid. So it’s very hard to break those bonds.”
In earlier research, Ralph and colleagues showed in-vitro that if you altered the structure of the lignin backbone you could more easily break up the polymer.
“We would like lignin to fall apart chemically really easily so we redesigned the polymer to be more easily degradable,” he said.
To do this the researchers added monomers with ester linkages to the lignin backbone.
“Esters break at room temperature in caustic soda,” Mr Ralph said.
Now, he and colleagues have created actual poplar trees that contain this modified lignin – plants they say are “designed for deconstruction”.
“We have now demonstrated that we can really do it in real plants,” he said.
The first step was to find a gene for an enzyme that would make the new monomers.
Ralph and his colleagues found such an enzyme being produced naturally in the roots of a medicinal herb called Chinese angelica (Angelica sinensis), which is used to treat gynaecological issues.
The enzyme the produces monomers with ester linkages as a defence compound, not for making lignin, said Ralph.
He and colleagues identified the gene responsible for the enzyme and then engineered it into poplar trees.
The researchers used a promoter sequence that ensured the gene would only be expressed in the lignin-producing cells of the plant.
And when the successfully engineered trees were a year old, they were then cut down and tested for the presence of the modified lignin.
The researchers then ground up the wood and found the lignin was easier to break down.
In an industrial setting, this would means less time or lower temperatures, and ultimately, less energy, would be required to make paper, said Ralph.
And having more easily destructible lignin also stands to help the biofuels industry, he added.
Ralph said the modified plants look no different from the wild type and it is unlikely the modification would have an impact on the way microbes break down lignin.
“The way plants degrade naturally is very different to how we degrade them chemically.”
The esters are “biologically stable” even though they fall apart really easily with strong chemicals, so it is also unlikely the modification would impact on the strength of the growing tree.
Field trials would be needed to confirm these assumptions.
The researchers hope to engineer the lignin-modifying gene from Chinese angelica into biofuel crops other than poplars, as well as into other tree species used in paper making, especially softwoods.
