Modifying Lignin Composition and Content of Sorghum Biomass for Improved Bioenergy Conversion

Sorghum (Sorghum bicolor) is a drought tolerant, C₄ grass traditionally used for grain and forage production. Adoption of sorghum as a bioenergy crop has increased because of its high yield potential, and lower nitrogen and water requirements compared to other biomass crops. The cell wall is an interweaving matrix of cellulose, hemicellulose and the phenolic polymer lignin, which provides a rigid, protective barrier that confers both stability and resistance to both biotic and abiotic factors. Consequently, lignin is a key target to improve biomass for both cellulosic and thermal bioenergy conversion. The precursors of lignin are usually derived from the amino acid phenylalanine through the monolignol biosynthetic pathway, making this pathway a central focus for modifications to improve the composition and content of lignin. A series of transgenic events expressing ten genes encoding enzymes involved in the monolignol pathway have been developed in sorghum to examine the consequence of these modifications to biomass and energy content. Over-expression of caffeoyl CoA O-methyltransferase (SbCCoAOMT), an enzyme in the monolignol pathway, resulted in an increase in total energy content (p= 0.02), while agronomical traits and lignin concentration were not significantly different between wild-type and transgenic. Over-expression of SbCCoAOMT stimulates monolignol biosynthesis in sorghum, which leads to changes in cell wall composition that are detectable through histochemical staining and GS-MS analysis. The changes induced by over-expression of SbCCoAOMT could improve sorghum biomass for a range of conversion technologies.