Scientists Engineer Plant Cell Wall to Improve Sugar Yield for Biofuels

Researchers at the U.S. Department of Energy (DOE)'s Joint BioEnergy Institute (JBEI) used the tools of synthetic biology to reduce the lignin content and to enhance polysaccharide deposition in cell walls of genetically engineered plants. The biomass of the engineered plants can be degraded more easily into fermentable sugars for biofuel production.

The polysaccharide sugars in plant cell walls of cellulosic feedstock like grasses and trees are locked within a tough polymer called lignin which reduces the extractability of these sugars and impedes access to degrading enzymes prior to fermentation into ethanol. In order to liberate these sugars from the lignin cage, expensive pretreatments are used. The high cost of pretreatments is a major obstacle to commercialization of cellulosic biofuels.

Reducing the lignin content in lignocellulosic biomass is not an easy feat because it may reduce biomass yield due to a consequent loss of integrity in vessels, the key tissues that transport and distribute water and nutrient from roots to the above-ground parts. In addressing the lignin problem, JBEI scientists have rewired the secondary cell network in the model plant Arabidopsis thaliana by changing the promoter for a key lignin gene. This modification disconnected the expression of the lignin gene from the fiber regulatory network and rewired the lignin biosynthesis for vessel formation. Through promoter modification the mechanism called artificial positive feedback loop (APFL) was also introduced to increase polysaccharide depositions in fibre cells. The result was a healthy engineered plant that accumulates the good stuff (polysaccharide) and reduces the problematic polymer (lignin). Compared to the non-modified plants, the engineered plants exhibited improved sugar releases from enzymatic breakdown of their biomass.