Genetic dissection of sorghum traits important for biofuel production

Sorghum is a promising bioenergy feedstock due to its high yield potential and tolerance to stresses. Genetic variants with altered lignin composition and sweet stems are of particular interest, but the genetic and molecular basis for these traits are not well understood and hampers breeding efforts. Efficiency of conversion of lignocellulosic biomass is affected by lignin content and composition. The brown midrib (bmr) mutants of sorghum are characterized by brown vascular tissue and altered lignin content. Genetic and chemical approaches were used to identify four bmr loci, represented by the bmr2, bmr6, bmr12 and bmr19 allelic groups. The changes in subunit lignin composition were characterized for each of the groups to aid in the identification of the genes underlying the mutations. Enzymatic saccharification of stover demonstrated that the mutations in the bmr2, bmr6 and bmr12 groups can increase glucose yields up to 25% compared to wild-type isolines. Lignin subunit composition of the bmr6 group is consistent with reduction of cinnamyl alcohol dehydrogenase (CAD) activity. Analysis of the sorghum genome revealed 14 CAD-like genes. Based on their phylogenetic relationship and the identification of non-conservative mutations in three allelic bmr6 lines, SbCAD2 was identified as the Bmr6 gene. The effects of combining the bmr6 mutation with the sweet stem trait were investigated. The presence of the sweet stem helped reduce the biomass yield penalty associated with the bmr6 mutation. The sweet stem trait and the bmr6 mutation act in an additive manner to reduce lignin content in the stover. This is likely to improve both the saccharification properties of the biomass and forage quality. However, their combination also resulted in increased stalk lodging. Markers associated with agronomic and biomass characteristics were identified to assist in the breeding of bioenergy sorghum lines