SbCOMT (Bmr12) is involved in the biosynthesis of tricin-lignin in sorghum

Authors: EUDES, AYMERICK - Joint Bioenergy Institute (JBEI); DUTTA, TANMOY - Joint Bioenergy Institute (JBEI); DENG, KAI - Joint Bioenergy Institute (JBEI); NICOLAS, JACQUET - Joint Bioenergy Institute (JBEI); BENITES, VERONICA - Joint Bioenergy Institute (JBEI); LOGUE, DOMINIQUE - Joint Bioenergy Institute (JBEI); BAIDOO, EDWARD - Joint Bioenergy Institute (JBEI); Sattler, Scott; NORTHEN, TRENT - Joint Bioenergy Institute (JBEI); SINGH, SEEMA - Joint Bioenergy Institute (JBEI); SIMMONS, BLAKE - Joint Bioenergy Institute (JBEI)

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2017
Publication Date: 6/8/2017
Publication URL: https://handle.nal.usda.gov/10113/5801796
Citation: Eudes, A., Dutta, T., Deng, K., Nicolas, J., Benites, V.T., Logue, D., Baidoo, E.E., Sattler, S.E., Northen, T.R., Singh, S., Simmons, B.A. 2017. SbCOMT (Bmr12) is involved in the biosynthesis of tricin-lignin in sorghum. PLoS One. 12(6)e0178160. https://doi.org/10.1371/journal.pone.0178160.
DOI: https://doi.org/10.1371/journal.pone.0178160

Interpretive Summary: In the U.S., sorghum biomass (stalks and leaves) serves as an important forage crop for livestock. In addition, sorghum is being developed as a bioenergy crop. Cellulosic biofuels are derived from the breakdown of the cell wall polymers (cellulose and hemicellulose) of the biomass to sugars and the conversions of these sugars to fuel molecules. A third cell wall polymer, lignin, makes cell walls resistant to breakdown either in livestock digestive systems or in the cellulosic conversion process. Tricin has recently been identified as the compound that starts formation of lignin polymers in grasses. However, the enzymes involved in the final steps of tricin synthesis have not been identified. In this study, the sorghum Caffeic O-methyltransferase (COMT) was investigated as an enzyme involved in tricin synthesis. Previous studies had shown that this enzyme was involved in the synthesis of one type of lignin called syringyl lignin or S-lignin. A mutation in COMT gene caused a 60% reduction in tricin, and the COMT enzyme was able to produce three intermediate compounds in tricin synthesis under laboratory conditions. Collectively, this research gave a new perspective on the functions of COMT in lignin synthesis, and showed its potential role in starting the formation of lignin polymers. This study potentially provides new ways alter cell wall composition of sorghum, which will be useful improve sorghum and other grasses for bioenergy applications.

Technical Abstract: Lignin in plant biomass represents a target for engineering strategies towards the development of a sustainable bioeconomy. In addition to the conventional lignin monomers, namely p-coumaryl, coniferyl and sinapyl alcohols, tricin has been shown to be part of the native lignin polymer in certain monocot species. Because tricin is considered to initiate the polymerization of lignin chains, elucidating its biosynthesis and mechanism of export to the cell wall constitute novel challenges for the engineering of bioenergy crops. Late steps of tricin biosynthesis require two methylation reactions involving the pathway intermediate selgin. It has recently been demonstrated in rice and maize that caffeate O-methyltransferase (COMT) involved in the synthesis syringyl (S) lignin units derived from sinapyl alcohol also participates in the synthesis of tricin in planta. In this work, we validate in sorghum (Sorghum bicolor L.) that the O-methyltransferase responsible for the production of S lignin units (SbCOMT / Bmr12) is also involved in the synthesis of lignin-linked tricin. In particular, we show that biomass from the sorghum bmr12 mutant contains lower level of tricin incorporated into lignin, and that SbCOMT can methylate the tricin precursors luteolin and selgin. Our genetic and biochemical data point toward a general mechanism whereby COMT is involved in the synthesis of both tricin and S lignin units.