Relationships between cell wall digestibility and lignin content as influenced by lignin type and analysis method

Authors: Grabber, John

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2018
Publication Date: 5/16/2019
Citation: Grabber, J.H. 2019. Relationships between cell wall digestibility and lignin content as influenced by lignin type and analysis method. Crop Science. 59:1122-1132. https://doi.org/10.2135/cropsci2018.09.0563.
DOI: https://doi.org/10.2135/cropsci2018.09.0563

Interpretive Summary: Lignin is an indispensable component of cell walls required for normal growth and development of plants, but its presence limits the digestibility and utilization of forage and biomass crops used for livestock and biofuel production. Therefore, a number of plant breeding and bioengineering programs are trying to modify lignin to make it less inhibitory to digestion. Unfortunately, commonly used methods for measuring lignin concentration often give conflicting results, and this has hampered progress in developing forage and biomass crops with improved digestibility. In this study, we used a well-characterized model system to evaluate relationships between cell wall digestibility and lignin content for several types of lignin polymers commonly found in grass crops. This work identified two types of lignin polymers that were less inhibitory to cell wall digestion by microorganisms located in the forestomach (rumen) of cattle. We also identified benefits and shortcomings of two commonly used methods for estimating lignin content. These findings will enable plant breeders and molecular biologists to more efficiently develop grasses with improved digestibility.

Technical Abstract: Difficulties in measuring the lignin content of forage and biomass crops confounds efforts by plant breeders and molecular biologists to grasp how shifts in lignin chemistry affect fiber digestibility. In this study, relationships between digestibility and lignin content were examined by artificially lignifying Zea mays L. cell walls with bulk polymers composed of p-hydroxyphenyl-guaiacyl (HG) and guaiacyl (G) units or with endwise polymers composed of G, guaiacyl-syringyl (GS), and guaiacyl-syringyl-p-coumaric acid (GSpCA) units. Regressions of in vitro rumen gas production or nonfermented polysaccharides (NFP) vs. lignin content accurately measured by an indirect spectrophotometric method revealed that G-bulk and especially syringyl (S)-rich endwise polymers were the least inhibitory to digestion. While providing biased estimates at low lignin contents, the acetyl bromide soluble lignin (ABSL) assay correctly ranked how endwise polymers affected the digestibility of highly lignified cell walls, but the method required unique absorption coefficients to assess relationships for bulk polymers. Conversely, the Klason lignin (KL) method provided more accurate estimates at low lignin contents, but varying recovery of KL among lignin types created bias for highly lignified cell walls. Overall our results indicate S-rich lignins are the least inhibitory to digestion, but detecting desirable shifts in lignin chemistry will require the judicious use of several analytical methods.