Overexpression of ferulate 5-hydroxylase increases syringyl units in Sorghum bicolor

Authors: Tetreault, Hannah; Gries, Tammy; Palmer, Nathan - Nate; Funnell-Harris, Deanna; Sarath, Gautam; Sattler, Scott; SATO, SHIRLEY - University Of Nebraska; GE, ZHENGXIANG - University Of Nebraska

Submitted to: Plant Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2020
Publication Date: 5/13/2020
Citation: Tetreault, H.M., Gries, T.L., Palmer, N.A., Funnell-Harris, D.L., Sarath, G., Sattler, S.E., Sato, S., Ge, Z. 2020. Overexpression of ferulate 5-hydroxylase increases syringyl units in Sorghum bicolor. Plant Molecular Biology. 103(3):269-285. https://doi.org/10.1007/s11103-020-00991-3.
DOI: https://doi.org/10.1007/s11103-020-00991-3

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 gene ferulate 5-hydroxylase (F5H) encodes an enzyme involved in lignin synthesis. To understand the role of this enzyme in lignin synthesis and its effect on cell walls, levels of the F5H enzyme were increased in sorghum plants using biotechnology. In addition, brown midrib 12 (bmr12) plants are deficient in another enzyme involved in lignin synthesis, caffeic acid O-methyltransferase (COMT). These two traits were combined together in the same plants to alter lignin synthesis in cell walls. The increased levels of the F5H led to changes in the composition of the lignin, but it did not lead to increase total amount of lignin in cell walls. The combination of bmr12 and F5H changed lignin composition and decreased the lignin content within cell walls. All of these changes to cell walls were observable through microscopy. This research demonstrates new ways to change lignin composition of sorghum biomass. Overall this study demonstrates that lignin composition can be altered in sorghum without significantly impacting the growth or biomass yield of the plants, which is a necessary feature for future bioenergy applications.

Technical Abstract: Ferulate 5-hydroxylase (F5H) of the monolignol pathway catalyzes the hydroxylation of coniferyl alcohol, coniferaldehyde and ferulic acid to produce 5-hydroxyconiferyl moieties, which lead to the formation of sinapic acid and syringyl (S) lignin monomers. In contrast, guaiacyl (G) lignin, the other major type of lignin monomer, is derived from polymerization of coniferyl alcohol. In this study, the effects of manipulating S-lignin biosynthesis in sorghum (Sorghum bicolor) were evaluated. Overexpression of sorghum F5H (SbF5H), under the control of the CaMV 35S promoter, increased both S-lignin levels and the ratio of S/G lignin, while plant growth and development remained relatively unaffected. Maüle staining of stalk and leaf midrib sections from SbF5H overexpression lines indicated that the lignin composition was altered. Ectopic expression of SbF5H did not affect the gene expression of other monolignol pathway genes. In addition, brown midrib 12-ref (bmr12-ref), a nonsense mutation in the sorghum caffeic acid O-methyltransferase (COMT) was combined with 35S::SbF5H through cross-pollination to examine effects on lignin synthesis. The stover composition from bmr12 35S::SbF5H plants more closely resembled bmr12 stover than 35S::SbF5H or wild-type stover; S-lignin and total lignin concentrations were decreased relative to wild-type or 35S::SbF5H. Likewise, expression of upstream monolignol biosynthetic genes was increased in both bmr12 and bmr12 35S::SbF5H relative to wild-type or 35S::SbF5H. Overall, these results indicated that overexpression of SbF5H did not compensate for the loss of COMT activity.