By Loukia Papadopoulos
October 29, 2023

Newly-engineered enzymes can help plants produce more biofuels

In order to improve the efficiency of converting grass plant biomass into biofuels and other bioproducts, plant biologists at the DOE's Brookhaven National Laboratory have created new enzymes that help optimize plants’ ability to develop biomass.

Representational image of biofuels.                                                                                              Scharfsinn86/iStock

Biofuels such as ethanol are sustainable fuels made from biomass. Biomass consists of any biological material formed from recently living species, including plants, animals, and microorganisms.

Because the carbon dioxide (CO2) released during combustion is nearly equal to the CO2 absorbed during the growth of the source material, biofuels are regarded as renewable energy sources and are very high in demand. As such, finding new and efficient ways to produce them is a key endeavor of many researchers.

A new type of enzymes

In order to improve the efficiency of converting grass plant biomass into biofuels and other bioproducts, plant biologists at the DOE's Brookhaven National Laboratory have created a new type of enzymes that optimizes plants’ ability to develop biomass.

These enzymes alter the chemicals that make up plant cell walls to provide access to sugars that ordinarily remain locked within intricate structures. These sugars have unique fuel-making properties.

“The concept of biomass to biofuel seems simple, but it is technically very difficult to release the sugars,” noted Chang-Jun Liu, a senior plant biologist at Brookhaven Lab who led the study.

Liu has been addressing this issue with artificially produced enzymes known as monolignol 4-O-methyltransferases (MOMTs) for almost 15 years. MOMTs have the unique ability to alter monolignols which are the main components of the polymer responsible for a plant’s structure called lignin. Toying with lignin and reducing its presence in plants makes sugars more accessible as the component’s complex structure can impede the efficient extraction of sugars from cellulose and hemicellulose. This extraction is key to biofuel production.

Liu and his team already undertook this process in poplar trees and were focused on doing the same thing with grass plants because they are extremely successful at producing large amounts of biomass.

“The complexity of grass plant cell walls made us curious as to whether our enzymes would improve sugar recovery,” noted Liu. “We wanted to know if MOMTs could modify the grass cell walls in a way that would provide access to the biomass.”

30 percent more sugar

The researchers found that, compared to unmodified plants, they were able to extract up to 30 percent more sugar from plants expressing MOMT because these plants had less lignin in their cell walls. 

“This was quite a difference from what we saw when we expressed the same enzymes in poplar trees,” noted Liu. “The broader effects of expressing the enzymes really surprised us.”

The researchers now hope to determine whether their MOMT enzymes can maximise the yields of sugar from different varieties of grass plants like sorghum and bamboo. The work sheds light on how scientists could best release the sugar contained in cell walls, thereby mitigating some of the waste associated with unmodified biomass crops and resulting in improved biofuel production.

The study is published in the Plant Biotechnology Journal.

Study abstract:

Grass lignocelluloses feature complex compositions and structures. In addition to the presence of conventional lignin units from monolignols, acylated monolignols and flavonoid tricin also incorporate into lignin polymer; moreover, hydroxycinnamates, particularly ferulate, cross-link arabinoxylan chains with each other and/or with lignin polymers. These structural complexities make grass lignocellulosics difficult to optimize for effective agro-industrial applications. In the present study, we assess the applications of two engineered monolignol 4-O-methyltransferases (MOMTs) in modifying rice lignocellulosic properties. Two MOMTs confer regiospecific para-methylation of monolignols but with different catalytic preferences. The expression of MOMTs in rice resulted in differential but drastic suppression of lignin deposition, showing more than 50% decrease in guaiacyl lignin and up to an 90% reduction in syringyl lignin in transgenic lines. Moreover, the levels of arabinoxylan-bound ferulate were reduced by up to 50%, and the levels of tricin in lignin fraction were also substantially reduced. Concomitantly, up to 11 μmol/g of the methanol-extractable 4-O-methylated ferulic acid and 5–7 μmol/g 4-O-methylated sinapic acid were accumulated in MOMT transgenic lines. Both MOMTs in vitro displayed discernible substrate promiscuity towards a range of phenolics in addition to the dominant substrate monolignols, which partially explains their broad effects on grass phenolic biosynthesis. The cell wall structural and compositional changes resulted in up to 30% increase in saccharification yield of the de-starched rice straw biomass after diluted acid-pretreatment. These results demonstrate an effective strategy to tailor complex grass cell walls to generate improved cellulosic feedstocks for the fermentable sugar-based production of biofuel and bio-chemicals.