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Researchers find “zip-lignin” native to multiple plant species; potential for new approaches to degrading lignin for biorefineries

In 2014, researchers from Michigan State University and the University of Wisconsin-Madison and their colleagues successfully engineered poplar trees to produce lignin that degrades more easily, thereby lowering the effort and cost to convert wood to biofuel. (Earlier post.)

Now, in an open-access paper published in Science Advances, some of those same researchers have discovered that various plant species might have naturally convergently evolved to express the same feature natively.

University of Wisconsin–Madison professor of biochemistry John Ralph, Shawn Mansfield, Curtis Wilkerson, and other Great Lakes Bioenergy Research Center (GLBRC) researchers had engineered poplar trees to express a gene from Chinese angelica encoding a feruloyl–coenzyme A (CoA) monolignol transferase (AsFMT). The resulting lignin, dubbed zip-lignin, readily breaks down under simple chemical conditions.

The new GLBRC-led study shows that those poplar trees and many other plants from all over the phylogenetic tree have actually evolved to produce zip-lignin naturally. In other words, not only can we potentially breed for degradability in plants, but humans may have been doing just that—selecting certain plants for easier processing—for thousands of years.

We didn’t know the plants were making the native zip because we couldn’t detect it. When we added the new gene we thought we were adding functionality, but we were actually increasing what was already there.

—Steve Karlen, UW–Madison and the paper’s lead author

Even though the team couldn’t at first detect native zip-lignin in poplar trees or in angelica—the Chinese herb from which the group had taken the gene—its absence did raise some questions. Ralph had long suspected that some plant somewhere was naturally creating zip-lignin. And Karlen wondered how angelica, or any plant for that matter, could be making the molecules that confer weak bonds in lignin but not incorporate them.

Using a method that Ralph’s group had developed decades ago, plus a new and highly sensitive mass spectrometer, Karlen found a way to detect low levels of native zip-lignin in poplar trees. With the help of Phillip Harris, a professor of biological sciences at the University of Auckland in New Zealand, Karlen began a full-scale phylogenetic study, seeking to determine what other plants might contain native zip-lignin.

Examining the more than 60 plant samples brought back to the lab revealed that zip-lignin is found in an exceptionally diverse array of plants—in balsa, in birds of paradise, in all the grasses he sampled, and in about half of the hardwoods, to name just a few.

With collaborator Laura Bartley, an associate professor of microbiology and plant biology at the University of Oklahoma, Karlen also found zip-lignin in rice. Since an entirely different gene was responsible for making this lignin, the team determined that plants have independently evolved to make zip-lignin, essentially developing a common feature though entirely different means.

Although Karlen and his collaborators don’t yet know what the evolutionary advantage of native zip-lignin might be for plants, its widespread presence broadens the scope of their research and holds out the possibility of increasing, either through engineering or breeding, the degradability of a surprisingly vast array of plants.

The convergent evolution and subsequent proliferation of plants that incorporate ML-FA conjugates into their lignins indicate that, potentially, there is a biological advantage for the production of this lignin structure. Regardless of the actual driving forces selecting for them, the diversity and environmental success of plants with native zip-lignins show that they have no apparent general disadvantages in terms of plant defense or structural stability.

… In practical terms, our discovery unveils new approaches to increasing levels of readily cleavable ester bonds in the lignin backbone, either by breeding or by transgenic methods similar to those used to introduce AsFMT into poplar. Further work is also needed to explore the effects of ML-FA–containing lignins on processes such as carbon sequestration and biomass utilization.

—Steve Karlen

GLBRC is one of three Department of Energy Bioenergy Research Centers created to conduct transformational research and build the foundation of new cellulosic biofuels technology.


  • Steven D. Karlen, Chengcheng Zhang, Matthew L. Peck, Rebecca A. Smith, Dharshana Padmakshan, Kate E. Helmich, Heather C. A. Free, Seonghee Lee, Bronwen G. Smith, Fachuang Lu, John C. Sedbrook, Richard Sibout, John H. Grabber, Troy M. Runge, Kirankumar S. Mysore, Philip J. Harris, Laura E. Bartley, John Ralph (2016) “Monolignol ferulate conjugates are naturally incorporated into plant lignins” Science Advances doi: 10.1126/sciadv.1600393


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