The Sorghum brown midrib (bmr) Mutants: A Forward Genetics Approach to Lignin

The brown midrib (bmr) mutants of sorghum and other C4 grasses have a visible red to brown colored leaf midrib in contrast to green or white midribs observed in wild-type plants. This phenotype, more importantly, is associated with reduced lignin content and altered lignin composition, which have led to interest in using bmr mutants to improve cellulosic bioenergy conversion and increase forage digestibility for livestock. The sorghum bmr mutants were generated through chemical mutagenesis (EMS and DES), and, through allelism testing eight complementation groups were identified in sorghum.  Three of the bmr loci (bmr2, bmr6 and bmr12) have been identified and shown to encode enzymes in monolignol biosynthesis (4-coumarate-CoA ligase, 4CL; cinnamyl alcohol dehydrogenase, CAD; caffeic acid-O-methyltransferase, COMT), the pathway that synthesizes the subunits of lignin.  The effects of these mutations on agronomic traits, lignin content, lignin composition and enzymatic saccharification were characterized.  Nearly all of mutants had reduced lignin concentrations in their stover (biomass) relative to the wild-type.  However, increased enzymatic saccharification of bmr mutant stover relative to wild-type stover was not universally observed.  For the alleles of the three identified loci, the impact of the mutations of their respective transcript levels, protein abundance and enzymatic activity were also characterized.  Overall, these molecular analyses indicate that while the majority of alleles characterized are likely the result of amorphic (null) mutations, a few of alleles characterized likely represent hypomorphic (weak) mutations. These bmr mutants represent breeding tools for manipulating biomass composition to enhance forage and feedstock quality.