Field evaluation of sorghum (Sorghum bicolor) lines that overexpress two monolignol-related genes that alter cell wall composition

Authors: Tetreault, Hannah; O`Neill, Patrick; Toy, John; Gries, Tammy; Funnell-Harris, Deanna; Sattler, Scott

Submitted to: BioEnergy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/2020
Publication Date: 11/14/2020
Citation: Tetreault, H.M., Oneill, P.M., Toy, J.J., Gries, T.L., Funnell-Harris, D.L., Sattler, S.E. 2020. Field evaluation of sorghum (Sorghum bicolor) lines that overexpress two monolignol-related genes that alter cell wall composition. BioEnergy Research. 13(4)Available:https://link.springer.com/article/10.1007/s12155-020-10218-4. https://doi.org/10.1007/s12155-020-10218-4.
DOI: https://doi.org/10.1007/s12155-020-10218-4

Interpretive Summary: In the US, sorghum serves as an important forage crop for livestock, and it also is being developed as a bioenergy crop. Energy, biofuels and renewable chemicals can be produced from plant cell walls, which are composed of three main components, cellulose, hemicellulose and lignin. Lignin acts like glue holding the other cell wall components together, which makes the cellulose and the hemicellulose resistant to breakdown into their sugar subunits either in livestock digestive systems or cellulosic biofuel production. The SbMyb60 is a transcription factor that activates lignin synthesis, and caffeoyl CoA O-methyltransferase (SbCCoAOMT) is an enzyme involved in lignin synthesis. USDA-ARS scientists in Lincoln, NE used transgenic overexpression of each gene to alter lignin synthesis and examine the effects on plant growth and cell wall composition under field conditions over two seasons. Field-grown SbMyb60 and SbCCoAOMT overexpression plants had increased energy concentrations and lignin related compounds in their cell walls. Sorghum hybrids with two SbMyb60 lines were also examined as a way to increase yield and improve plant growth. Hybrids improved the yield of one transgenic overexpression line, but not the other. Overall, these results demonstrated that field-grown sorghum overexpressing SbMyb60 or SbCCoAOMT can increase lignin related compounds and energy concentration within cell walls. This study provides a future framework for manipulating lignin in sorghum for emerging renewable chemical applications.

Technical Abstract: Modifying lignin content and composition of cell walls are key targets for bioenergy feedstock improvement for a range of renewable chemical applications. Sorghum (Sorghum bicolor) is a drought tolerant C4 grass being developed as a dedicated bioenergy feedstock. The monolignol biosynthetic pathway produces the hydroxycinnamic subunits of lignin, a major target to increase the energy content of sorghum biomass for thermal bioenergy conversion processes. The SbMyb60 is a transcription factor linked to the activation of the monolignol biosynthetic pathway and caffeoyl CoA O-methyltransferase (SbCCoAOMT) is a S-adenosyl methionine (SAM)-dependent O-methyltransferase responsible for the methylation of caffeoyl-CoA to generate feruloyl-CoA in this pathway. Overexpression of each gene resulted in increased energy content of biomass and increased levels of several aromatic compounds in previous greenhouse experiments. To assess how SbMyb60 and SbCCoAOMT overexpression lines performed under field conditions these lines were planted in field trials over two growing seasons in 2017 and 2018 and the effects on lignin composition and content as well as agronomic traits of the biomass were assessed. Field-grown plants of SbMyb60 and SbCCoAOMT overexpression lines had increased total energy and phenolic compounds. Furthermore, two SbMyb60 transgenic events, SbMyb-10a and SbMyb-15a, with decreased stover yield and reduced seed set were combined with three sorghum lines, N32, N4692 and Tx3118, to investigate whether the yield reduction could be rescued. Hybrids improved the yield of SbMyb60-15a transgenic line; however, hybrids were unable to improve performance for SbMyb60-10a transgenic line. Overall, these results demonstrated that field-grown sorghum overexpressing SbMyb60 or SbCCoAOMT can have biomass with increased phenolic compounds and total energy.