Lignin is an essential aromatic polymer in plant cell walls, contributing to the structural integrity, rigidity, and water resistance of vascular tissues. However, lignin also limits industrial processes by reducing the accessibility of cellulose for monosaccharide extraction. Genetic engineering to reduce lignin content can improve the efficiency of these processes. Previous studies demonstrated that heterologous expression of the coliphage enzyme S-adenosylmethionine hydrolase (AdoMetase) successfully reduced lignin in Arabidopsis. Researchers from Lawrence Berkeley National Laboratory, Joint BioEnergy Institute and colleagues from other institutions examined the impact of AdoMetase expression in sorghum, a bioenergy crop. Results show that AdoMetase expression lowers AdoMet levels, reducing lignin content by up to 19% in sorghum. The AdoMetase lines displayed altered lignin composition, with an increased S/G ratio at maturity, distinct from other lignin mutants like bmr12. The AdoMetase plants also exhibited shorter growth and reduced biomass yields, unlike typical lignin mutants.

Transcriptomic and metabolomic analyses revealed that AdoMetase expression impacts other metabolic processes beyond lignin biosynthesis. Although genes involved in lignin production were not downregulated, changes in AdoMet levels affected histone and DNA methylation, potentially leading to epigenetic modifications. This was accompanied by increased reactive oxygen species markers, possibly indicating a stress response. Metabolomic data highlighted shifts in secondary metabolites, including reduced levels of flavonoids and altered amino acid profiles. These findings suggest that fine-tuning AdoMetase expression, potentially by using developmentally or environmentally regulated promoters, could enhance lignin reduction in bioenergy crops without compromising growth.

Our work on the heterologous expression of AdoMetase in sorghum offers a promising new approach to lignin reduction. This reduction in lignin content results in biomass that is less recalcitrant to enzymatic deconstruction, leading to higher yields of simple fermentable sugars. While our findings are encouraging, we observed developmental delays in the engineered sorghum lines grown in the field. This suggests that further refinement of the approach is needed, specifically through the use of more specific promoters to fine-tune AdoMetase expression. – Eudes

SorghumBase examples:

Figure 1: Pan-genomic distribution of the Caffeic acid O-methyltransferase (COMT) gene family. Presence/absence variation of family members across sorghum varieties can be identified in this visualization.
Figure 2: The Ensembl Variant Image view showing the positions of variants, their predicted consequences, and overlapping Pfam domains in the SbOMT4 gene, a member of the COMT gene family. The red stop gained variants in the first exon have a high minor allele frequency (45%) hinting at widespread adaptation involving this locus.

Reference:

Tian Y, Gao Y, Turumtay H, Turumtay EA, Chai YN, Choudhary H, Park JH, Wu CY, De Ben CM, Dalton J, Louie KB, Harwood T, Chin D, Vuu KM, Bowen BP, Shih PM, Baidoo EEK, Northen TR, Simmons BA, Hutmacher R, Atim J, Putnam DH, Scown CD, Mortimer JC, Scheller HV, Eudes A. Engineered reduction of S-adenosylmethionine alters lignin in sorghum. Biotechnol Biofuels Bioprod. 2024 Oct 15;17(1):128. PMID: 39407217. doi: 10.1186/s13068-024-02572-8. Read more

Related Project Websites: 

Image 1: Field trial of engineered AdoMetase and wildtype sorghum at the University of California-Davis. Photo credit Aymerick Eudes.
Engineering Sorghum with AdoMetase to Reduce Lignin and Enhance Biomass Conversion for Biofuel Production

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