Researchers collaborating through the Center for Bioenergy Innovation (CBI) have discovered important new information about pectin, a starch-like complex sugar that helps hold plant cell walls together. Two recent studies better define the roles of specific pectins and the genes that synthesize them in woody plants. These cellulosic plants are a promising feedstock for biofuel because they are affordable, are abundant, and can grow on marginal lands. However, the pectin helps make cells in their tough stems more difficult for microorganisms to break down into fuel.
“This brings us closer to being able to adjust proteins very specifically to make plants more receptive to bioprocessing and to improve the quality of the biofuel produced,” said Debra Mohnen, senior author of the papers and professor of Biochemistry and Molecular Biology at the University of Georgia Complex Carbohydrate Research Center. “We are going to be able to engineer plants in a much more definitive way because we know exactly how we are changing the pectins.”
Pectins are indispensable for plant growth, structure, and adhesion, said Mohnen, who has been studying these compounds for three decades. The research had already shown that modifying pectin makes plants grow larger and release their sugars more readily for biofuel production. So learning how to manipulate pectin for the best possible balance could provide significant advantages.
In the first study, researchers evaluated genes actively producing proteins that create homogalacturonan, a major pectin in cellulosic feedstocks such as poplar and switchgrass. Each gene makes a protein called an enzyme that triggers a biochemical reaction. In the laboratory, scientists expressed individual plant genes in a cell culture. Because these cell lines do not make proteins that produce pectin, researchers were able to clearly identify the function of each plant gene.
The characteristics of an organism are determined by the function of their proteins. Mohnen said that in prior studies, she and her colleagues had identified the first four genes encoding proteins that make homogalacturonan by themselves. However, based on full gene sequencing of some species, they knew that 11 more candidate genes could make this pectin.
After eliminating several candidate genes, the study confirmed six enzymes known to synthesize homogalacturonan. Some of these enzymes work together in a protein complex, which actively enables pectin to expand into long chains, Mohnen said. Others work more slowly, providing exciting hints at their function in making the diverse and complex pectins in the plant.
“That tells us which genes we should tweak to interfere with the process,” she said. “It will enable us to finesse how we deconstruct or how we modify the cell walls.”
The second study, published in February 2022, enhanced understanding of another related pectin, rhamnogalacturonan I (RG-I).
Scientists at the National Renewable Energy Laboratory started by using a bacteria called Clostridium thermocellum to digest poplar biomass. Collaborators at the University of Georgia then analyzed what was released during that process, as well as any remaining plant cell wall solids the bacteria could not break down. The residues showed a significant increase in RG-I.
“We discovered that the bacteria cannot degrade it,” Mohnen said. “RG-I is not allowing this microbe to effectively use the biomass.”
Significantly, RG-I never exists alone: It is always connected to the homogalacturonan pectin from the first study. That means both are critical for deconstructing woody feedstocks. “This tells you we really have to know how these pectic polymers are all connected,” Mohnen said.
She noted that scientists mostly focus on the more abundant carbohydrates in cellulosic biomass, such as cellulose and hemicellulose. However, their abundance may not indicate their relative importance in strengthening cell walls. In considering lignocellulosic feedstocks for biofuel, “Most people don’t even look at the pectin because in grasses, it is really low in abundance,” Mohnen said. “Yet it’s really critical for holding things together.”
The Center for Bioenergy Innovation at Oak Ridge National Laboratory is one of four US Department of Energy Bioenergy Research Centers focused on advancing biofuels and bioproducts for a vibrant domestic bioeconomy. CBI is accelerating the development of bioenergy-relevant plants and microbes to enable production of drop-in sustainable aviation fuel, bioproducts that sequester carbon, and sustainable replacements for plastics and other environmentally harmful products. CBI research is supported by the Biological and Environmental Research program in DOE’s Office of Science.