Location: Grain, Forage, and Bioenergy ResearchTitle: Sorghum Brown midrib 2 (Bmr2) gene encodes the major 4-coumarate Coenzyme A ligase involved in lignin synthesis) Author
Submitted to: Plant Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/27/2012
Publication Date: 4/4/2012
Publication URL: http://handle.nal.usda.gov/10113/54498
Citation: Saballos, A., Sattler, S.E., Sanchez, E., Foster, T., Xin, Z., Kang, C., Pedersen, J.F., Vermerris, W. 2012. Sorghum Brown midrib 2 (Bmr2) gene encodes the major 4-coumarate Coenzyme A ligase involved in lignin synthesis. Plant Journal. 70:818-830. 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 bio-fuels. Cellulosic bio-fuels are derived from the breakdown of cell wall polymers (cellulose and hemicellulose) of the biomass to sugars. A third cell wall polymer, lignin, makes cell walls resistant to breakdown either in livestock digestive systems or in the cellulosic conversion process. Brown midrib 2 (bmr2) is a mutant that reduces lignin content and alters lignin composition of sorghum cell walls. In this study, we identified the mutation causing bmr2 and showed that the mutation affects an enzyme involved in lignin synthesis. We have characterized the function of this enzyme and determined that this enzyme is largely responsible for an initial step in lignin synthesis. The bmr2 gene represents a strategic target to improve biomass for bioenergy and forage uses in sorghum and other bioenergy grasses such as Switchgrass and Miscanthus.
Technical Abstract: Successful modification of plant cell wall composition without compromising plant integrity is dependent on being able to modify the expression of specific genes, but can be very challenging when the target genes are members of multigene families. 4-Coumarate:CoA ligase (4CL) catalyzes the formation of p-coumaroyl-CoA, a precursor of both flavonoids and monolignols, and has been an attractive target for transgenic down-regulation aimed at improving agro-industrial properties. The inconsistent phenotypes of the resulting transgenics has been attributed to variable levels of down-regulation of multiple 4CL genes. Phylogenetic analysis of the sorghum genome revealed 24 4CL(-like) proteins, five of which cluster with bona fide 4CLs from other species. This is the first report of a 4CL mutant, which allowed us to unequivocally establish the role of an individual 4CL gene, and the implications of not having the corresponding 4CL enzyme. Analysis of two sorghum brown midrib2 (bmr2) mutants in which missense mutations result in the near absence of soluble Bmr2 protein and severe reduction in 4CL activity, showed that Bmr2 is the main 4CL involved in lignification in sorghum stems, leaves and roots, both at the seedling stage and in pre-flowering plants. In vitro enzyme assays indicated its preferred substrate is 4-coumarate. The Bmr2 gene appears to be under auto-regulatory control based on the observation that the reduced 4CL activity results in overexpression of the defective gene, along with several other 4CL paralogs, the products of which are likely to compensate for some of the lost activity.