Sugarcane Cell Wall Structure and Degradation: From Monosaccharide Analyses to the Glycomic Code

Abstract

Pretreatments and enzymes have been a major hindrance to second-generation (2G) bioethanol production. As a result, most scientists have been focusing on the search for new enzymes and their subsequent characterization. Although this valuable knowledge has significantly improved the field generating initiatives of commercial production of 2G bioethanol, the cell walls themselves have received relatively little attention. In this chapter, we revise the work performed on sugarcane cell wall composition, structure, and architecture. From the status of looking exclusively to monosaccharide composition, research has evolved and several details about sugarcane cell wall polysaccharides and lignin were unrevealed. The studies about cell wall structure led to the proposition of the first model of sugarcane cell wall architecture in which macrofibrils (bundles of microfibrils) of cellulose are likely to be bound together by xyloglucan and arabinoxylans. These macrofibrils are covered with layers of more soluble hemicelluloses such as highly branched arabinoxylans and β-glucan. The lignin seems to be closely associated with the cellulose–hemicellulose domain, which is more hydrophobic than the other cell wall domains. Finally, lignin and cellulose–hemicellulose domains are embedded in a thin layer of pectin matrix. This model led to the proposition of a hypothesis that efficient cell wall degradation in the natural environment could be possible if the glycosyl hydrolases would sequentially degrade each layer at a time inwards towards cellulose microfibrils. This hypothesis was corroborated both during the attack of fungi to sugarcane biomass and during the aerenchyma formation in sugarcane roots. The highly complex sugarcane cell wall is now thought to be a result of a code, which is just starting to be unveiled. We believe that by further understanding the interactions among polymers and how endogenous enzymes attack cell walls, future strategies to induce endogenous biological pretreatments followed by the attack of enzyme consortia might significantly improve industrial processes for 2G bioethanol production from sugarcane.