New discoveries in the laboratory of Richard Dixon, Distinguished Research Professor of biological sciences and associate director of UNT’s BioDiscovery Institute, could help develop biomass crops better suited for processing into products such as aviation fuel, plastics and other industrial products.

Dixon and researchers Xin Wang, a visiting scientist, and postdoctoral fellow Chunliu Zhuo recently published their findings in Plant Cell, the nation’s top plant science journal. The research, in collaboration with Oakridge National Laboratory’s Center for Bioenergy Innovation, funded by the U.S. Department Energy, is part of the biotechnology industry’s goal to genetically modify crops to be more efficiently processed into these valuable products. 

Their work involves lignin, the substance that makes plants woody and firm and helps them stand upright. 

“Lignin is sort of the reinforced concrete in the plant cell wall,” Dixon explains. “That’s why it’s in the tree in the first place  — to protect it and help it to stand upright. But lignin is hard to biodegrade and break down into smaller molecules to convert to fuels.”

The wood of a tree or the stalk of a plant is largely composed of lignin and cellulose. When the cellulose is processed for industrial products such as bioethanol, lignin is a byproduct. 

“Currently, lignin is an inconvenient problem that when chemically removed ends up as a gummy byproduct that is then burned,” Dixon says. 

To solve this problem, Dixon says, there are two schools of thought about bioengineering the lignin in a plant. You could reduce the lignin as much as possible, but that would make the tree fall over or become more susceptible to disease. The other solution is to make lignin easier to remove — and then ideally used for other purposes. In 2012, Fang Chen in Dixon’s group discovered C-lignin, found in seeds of certain exotic plants like vanilla orchids and certain species of cactus, which is simpler to process due to its molecular structure.

“Regular lignin is made up of three building blocks and is joined together like chicken wire making it difficult to break down.” Dixon says. “C-lignin, however, is from one building block, and it’s completely linear.” 

His lab has been working on creating plants with higher concentrations of C-lignin. The idea is if scientists could replace a big proportion of the normal lignin with C-lignin, it would still help the plant, but also be easily separated and broken down into simple molecules for making plastics or polymers, rather than being burned or discarded.

More efficient processing of lignin would make biofuels more profitable, increasing the viability of biofuels and other products.

“I predict that 30 years from now the internal combustion engine won’t exist and cars will be driven by electricity,” Dixon says. “But airplanes will continue to run on fuel and not electric batteries. Therefore, aviation fuel is going to be liquid at least for the foreseeable future.” 

Dixon explains that the goal in making huge volumes of aviation fuel is to also have a co-product that's high value in high volume  — something like plastics or carbon fibers from C-lignin, which Dixon’s lab created in collaboration with Nandika D’Souza, professor of materials science and engineering, in 2014, technology that is patented.

The “natural” carbon fiber could replace petroleum-derived fibers in a wide range of goods including parts for cars, aircraft, electronics and sports equipment. The fiber also is stronger and lighter than similar products on the market.  

“You could have an airplane, which is lighter because it's made from carbon fibers, and it's actually running on fuel from the same plant.” Dixon says. “It’s a cool idea.”

Dixon and fellow researchers envision plants such as switchgrass and poplar bioengineered to contain large concentrations of C-lignin and grown as crops that could be harvested and efficiently processed into alternatives to petroleum. They have been looking for a way to turn on the switch in plants that trigger it to build the linear chains of C-lignin. Their new research identifies LACCASE, a class of enzyme, as the catalyst for joining up lignin units into C-lignin. This knowledge will enable plant scientists to genetically modify crops with significant amounts of C-lignin. 

“The overall goal is to engineer this pathway into biocrops,” Zhuo says. “And that will be great for making biomaterials.”

Dixon hopes that combined with alternative technologies for automobiles, renewable plant biomass eventually could completely replace petroleum.

“The ultimate dream would have a tree that’s engineered to be a factory taking sunlight and converting it into building blocks that can be diverted through technology to replace all the things that petroleum provides now,” Dixon says. 

Wang, Zhuo and Dixon outline their research findings in “Substrate-Specificity of LACCASE 8 Facilitates Polymerization of Caffeyl Alcohol for C-Lignin Biosynthesis in the Seed Coat of Cleome hassleriana” published online in October 2020: https://doi.org/10.1105/tpc.20.00598.