Abstract
BackgroundBiomass pretreatment remains an essential step in lignocellulosic biofuel production, largely to facilitate the efficient removal of lignin and increase enzyme accessibility to the polysaccharides. In recent years, there have been significant efforts in planta to reduce lignin content or modify its composition to overcome the inherent recalcitrance that it imposes on lignocellulosic biomass during processing. Here, transgenic poplar lines in which monolignol ferulate conjugates were synthesized during cell wall development to introduce, during lignification, readily cleavable ester linkages into the lignin polymer backbone (i.e., “zip lignin”), along with wild-type (WT) controls, were pretreated with different ionic liquids (ILs).ResultsThe strategic introduction of ester bonds into the lignin backbone resulted in increased pretreatment efficiency and released more carbohydrates with lower energy input. After pretreatment with any of three different ILs, and after limited saccharification, the transgenic poplars, especially those with relatively higher amounts of incorporated monolignol ferulate conjugates, yielded up to 23% higher sugar levels compared to WT plants.ConclusionOur findings clearly demonstrate that the introduction of ester linkages into the lignin polymer backbone decreases biomass recalcitrance in poplar has the potential to reduce the energy and/or amount of IL required for effective pretreatment, and could enable the development of an economically viable and sustainable biorefinery process.
Highlights
Biomass pretreatment remains an essential step in lignocellulosic biofuel production, largely to facilitate the efficient removal of lignin and increase enzyme accessibility to the polysaccharides
Hybrid poplar transformed with an exogenous feruloyl-CoA monolignol transferase (FMT) that catalyzes the formation of monolignol ferulates that get incorporated into the lignin polymer during lignification were used for all experiments
The amount of monolignol ferulate (ML-FA) conjugate incorporated during poplar lignification can only be crudely estimated from the low release of diagnostic conjugates [4, 27], but digestibility is hypothesized to be associated with the level of ML-FA conjugate in the lignin
Summary
Biomass pretreatment remains an essential step in lignocellulosic biofuel production, largely to facilitate the efficient removal of lignin and increase enzyme accessibility to the polysaccharides. There have been significant efforts in planta to reduce lignin content or modify its composition to overcome the inherent recalcitrance that it imposes on lignocellulosic biomass during processing. Switchgrass recalcitrance has been reduced by downregulating the caffeic acid 3-O-methyltransferase (COMT) gene in the lignin pathway; a reduced lignin content and altered lignin composition led to increases in saccharification efficiency, requiring less severe pretreatment and lower enzyme. With the basic idea of introducing readily cleavable bonds into the lignin backbone to lower the energy requirement for biomass processing, poplar was engineered to produce monolignol ferulate conjugates to augment the monomer pool by introducing an exotic feruloyl-CoA monolignol transferase (FMT) genes [4]. The resultant generation of lignin polymer backbone chains containing ester linkages improved cell wall digestibility, liberated more sugars after pretreatment, and improved saccharification compared to the WT [4]
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