Plant biologists at the U.S. Department of Energy’s Brookhaven National Laboratory have pioneered a breakthrough in the field of biofuels by engineering enzymes that modify grass plants, making their biomass more efficiently convertible into biofuels and other bioproducts. This transformative research, outlined in a recent publication in Plant Biotechnology Journal, revolves around the alteration of molecules within plant cell walls, providing access to the valuable sugars concealed within these complex structures.
Chang-Jun Liu, a senior plant biologist at Brookhaven Lab, who spearheaded this study, acknowledged the technical complexity of converting plant biomass into biofuels. He explained, “The concept of biomass to biofuel seems simple, but it is technically very difficult to release the sugars.”
Plant biomass, brimming with energy-rich complex sugar molecules generated through photosynthesis, is encased within rigid cell walls made up of sugars and lignin. Lignin, a structural support material, poses a challenge in the process of unlocking the sugars, a focal point for research dedicated to using plants for the production of fuels and products traditionally derived from petroleum.
Over nearly 15 years, Liu and his team have tackled this issue using engineered enzymes known as monolignol 4-O-methyltransferases (MOMTs). These enzymes, not naturally occurring, are designed to modify the chemical structure of monolignols, the primary building blocks of lignin. This structural alteration prevents these building blocks from connecting, reducing the lignin content in plants and rendering the sugars more accessible.
Previous work by Liu and his colleagues successfully incorporated MOMTs into poplar trees, leading to a reduction in lignin content and a more abundant sugar yield from these trees. In this latest research, they examined the potential application of MOMT enzymes in grass plants, known for their substantial biomass output.
Grasses, including rice plants, present a unique challenge due to their intricate cell wall structure, comprising not only sugars and lignin but also additional phenolic compounds that “cross-link” cell wall components, further fortifying them.
Despite these complexities, Liu and his team focused on two MOMT enzyme versions, MOMT4 and MOMT9, each designed to modify different lignin subunits. Collaborating with researchers from Kyoto University in Japan, they conducted chemical analyses on rice plants engineered to express either MOMT4 or MOMT9. Their analyses confirmed reduced lignin content in the modified grass plants, consistent with expectations.
Scanning electron microscopy, conducted by collaborators from Appalachian State University in North Carolina, revealed thinner cell walls throughout the stems and, in some instances, deformed or buckled cell walls in the modified plants. With reduced lignin content, the scientists successfully extracted up to 30% more sugar from plants expressing MOMT4 and up to 15% more sugar from plants expressing MOMT9, in comparison to unaltered plants. This sugar can be further converted into biofuels like ethanol through a fermentation process.
A noteworthy revelation in the study was the enzymes’ “promiscuity.” While MOMT4 and MOMT9 were designed to act on monolignols, they unexpectedly altered other cell wall components, such as cross-linking phenolics and a unique grass-specific phenolic called tricin. This additional modification further weakened the cell walls and resulted in an accumulation of modified phenolics in the plant tissue.
The unintended effects included a reduced plant height for MOMT9-expressing plants and the absence of seed production, which is essential for sustainable biofuel sugar sources. To address these challenges, the researchers plan to explore methods to control lignin modification in different parts of the plant, aiming to maximize sugar extraction without affecting plant fertility.
The scientists are also eager to investigate the potential application of their MOMT enzymes to optimize sugar yields in other grass plant species like sorghum and bamboo, further advancing the production of biofuels.
Nidhi Dwivedi, Brookhaven Lab research associate and lead author of the study, emphasized the importance of the research in advancing the biofuels industry. She stated, “Biofuels are a promising alternative to non-renewable energy sources. This study provides insights into how scientists can optimize the release of sugar within cell walls, thus overcoming some of the waste that occurs with unmodified biomass crops.”
This pioneering research at Brookhaven National Laboratory promises to play a pivotal role in advancing the biofuels industry and accelerating the transition to renewable energy sources.