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ARS Home Plains Area Lincoln, Nebraska Wheat, Sorghum and Forage Research Research Publications at this Location





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Title:   Author  item
                                            Sattler, Scottitem
                                            Saathoff, Aaronitem
                                            HAAS, ERIC - UNIVERSITY OF CREIGHTONitem
                                            Palmer, Nathan

                                                         - *Nate*item
                                            Funnell-Harris, Deannaitem
                                            Sarath, Gautamitem
                                            Pedersen, Jeffrey
  Submitted to: Plant Physiology
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 3/4/2009
                                    Publication  Date: 6/1/2009
                                    Citation: Sattler, S.E., Saathoff, A.J., Haas, E.J., Palmer, N.A., Funnell-Harris, D.L., Sarath, G., Pedersen, J.F. 2009. A nonsense mutation in a cinnamyl alcohol dehydrogenase gene is responsible for the sorghum brown midrib-6 phenotype. Plant Physiology 150 (2):584-95.Interpretive Summary: Sorghum serves as an important forage crop for US as well as a model for bioenergy crop development.  Brown midrib 6 (bmr-6), which has been commonly used in breeding for improved forage quality, is a mutant that affects lignin production (a cell wall polymer).  In this study, we identified the mutation causing bmr-6 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 completely non-functional in bmr-6 plants.  bmr-6 represents a strategic target to improve forage quality for bioenergy uses.
                                Technical Abstract: Brown midrib 6 (bmr-6) affects phenylpropanoid metabolism resulting in reduced lignin concentrations and altered lignin composition in Sorghum bicolor.  Recently, bmr-6 plants were shown to have very limited cinnamyl alcohol dehydrogenase activity (CAD; EC 1.1.1.195), the enzyme that catalyzes the conversion of phenolic aldehydes (monolignals) to monolignols.  A candidate gene approach was taken to identify Bmr-6.  Two CAD genes (Sb02g024190 and Sb04g005950) were identified in the sorghum genome based on similarity to known CAD genes and through DNA sequencing a nonsense mutation was discovered in Sb04g005950 that results in a truncated protein lacking the NADPH-binding and C-terminal catalytic domains.  Immunoblotting confirmed that the Bmr-6 protein was absent in protein extracts from bmr-6 plants.  Phylogenetic analysis indicated that Bmr-6 is a member of an evolutionarily conserved group of CAD proteins, which function in lignin biosynthesis.  In addition, Bmr-6 is distinct from the other CAD-like proteins in sorghum including SbCAD4 (Sb02g024190).  Although both Bmr-6 and SbCAD4 are expressed in sorghum internodes, an examination of enzymatic activity of recombinant Bmr-6 and SbCAD4 showed that Bmr-6 had 1-2 orders of magnitude greater activity for monolignol substrates.  Modeling of Bmr-6 and SbCAD4 protein structures showed differences in the amino acid composition of the active site that could explain the difference in enzyme activity.  These differences include His57, which is unique to Bmr-6 and other grass CADs.  In summary, Bmr-6 encodes the major CAD protein involved in lignin synthesis in Sorghum bicolor, and the bmr-6 mutant is a null allele.


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