ENHANCEMENT OF SORGHUM FOR BIOENERGY, FEED, AND FOOD VALUE
Location: Grain, Forage & Bioenergy Research
Title: Identification and characterization of 4 missense mutations in brown midrib 12 (Bmr12); the caffeic O-methyltranferase (COMT) of sorghum
Submitted to: BioEnergy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 4, 2012
Publication Date: July 10, 2012
Citation: Sattler, S.E., Palmer, N.A., Saballos, A., Greene, A.M., Xin, Z., Sarath, G., Vermerris, W., Pedersen, J.F. 2012. Identification and characterization of 4 missense mutations in brown midrib 12 (Bmr12); the caffeic O-methyltranferase (COMT) of sorghum. BioEnergy Research. 5: 855-865. DOI 10.1007/s12155-012-9197.
Interpretive Summary: In the US, sorghum biomass (stalks and leaves) serves as an important forage crop for livestock. In addition, sorghum is being developed as a bioenergy crop for cellulosic biofuels. Cellulosic biofuels are derived from the breakdown 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. The Bmr12 gene encodes an enzyme in lignin synthesis, and bmr12 mutants reduce lignin content and alter lignin composition of sorghum cell walls. In this study, we identified a series of mutations in bmr12. We characterized effects of this series has on enzyme activity, lignin content and ability to breakdown cell walls into sugars. These mutations had a significant range of effects for these properties we measured. Thus, this set of mutations should allow us to attenuate lignin content and increase the release of sugars from sorghum cell walls without sacrificing the critical functions that lignin serves within the cell wall. These modifications to cell wall composition will be useful improve sorghum for bioenergy.
Modifying lignin content and composition are targets to improve bioenergy crops for cellulosic conversion to biofuels. In sorghum and other C4 grasses, the brown midrib mutants have been shown to reduce lignin content and alter its composition. Bmr12 encodes the sorghum caffeic O-methyltransferase (COMT), which catalyzes the penultimate step in monolignol biosynthesis. From an EMS-mutagenized TILLING population, four bmr12 mutants were isolated. DNA sequencing identified the 4 missense mutations in the Bmr12 coding region, which changed evolutionarily conserved amino acids Ala71Val, Pro150Leu, Gly225Asp and Gly325Ser. The previously characterized bmr12 mutants all contain premature stop codons. These mutants, along with the previously characterized bmr12-ref, represent the first allelic series of COMT mutants available in the same genetic background. The impacts of these mutants have on protein accumulation, COMT enzyme activity, Klason lignin content, lignin subunit composition and saccharification yield were determined. Gly225Asp mutant greatly reduced protein accumulation, and Pro150Leu and Gly325Ser greatly impaired COMT activity compared to wild-type (WT). All four mutants significantly reduced Klason lignin content and altered lignin composition resulting in a significantly reduced S/G ratio relative to WT, but the overall impact of these mutants were less severe than bmr12-ref. Except for Gly325Ser, which is a hypomorphic mutant, all mutants increased the saccharification yield relative to WT. These mutants represent new tools to dampen lignin content and S/G ratio, which were not possible in previous nonsense mutants, leading toward the ability to tailor lignin content and composition in the bioenergy grass sorghum.