Investigating plant cell wall components that affect biomass recalcitrance in poplar and switchgrass

One of the key barriers to low cost biological conversion of cellulosic biomass into renewable fuels and chemicals is the recalcitrance of plants to deconstruction by chemical, enzymatic, and/or microbial routes. A deeper understanding of the source of biomass recalcitrance is sorely needed so that specific cell wall chemical and structural features that limit the release of sugars can be identified in different plants. In this study, biomass from two phylogenetically different plants, the monocot switchgrass (Panicum virgatum) and the woody dicot poplar (Populus trichocarpa) were studied. Sets of samples that varied in composition and structure were generated from each native biomass via defined chemical and enzymatic extractions. The two native biomasses, as well as their extracted residues, were characterized, and the enzymatic digestibility of all samples was tested to shed light on substrate-related features that limit sugar release. Based on the results from this study, lignin and hemicellulose were found to influence the enzymatic digestibility of both poplar and switchgrass, but the degree of influence varied significantly. Xylan removal from switchgrass resulted in materials that achieved nearly 100% glucose yields at high enzyme loading in subsequent enzymatic hydrolysis, whereas chlorite extractions that reduced the lignin content had the most beneficial effect in poplar. While lignin content likely plays an important role in biomass recalcitrance particularly in plants such as poplar that contain higher levels of lignin, this work identified subsets of hemicellulose that were key recalcitrance-causing factors in switchgrass. The findings and research approach presented in this study strongly suggest that different strategies will need to be adopted when trying to engineer poplar and switchgrass for reduced recalcitrance or when designing processing conditions to efficiently convert a specific biomass feedstock into sugars.