Brown midrib mutations and their importance to the utilization of maize, sorghum, and pearl millet lignocellulosic tissues

Brown midrib mutants have been isolated in maize (Zea mays), sorghum (Sorghum bicolor) and pearl millet (Pennisetum glaucum) arising by either spontaneous or chemical mutagenesis. The characteristic brown coloration of the leaf mid veins is associated with reduced lignin content and altered lignin composition, traits useful to improve forage digestibility for livestock. Brown midrib phenotype is correlated with two homologous loci in maize (bm1 and bm3) and sorghum (bmr6 and bmr12), which encode cinnamyl alcohol dehydrogenase (CAD) and a caffeic O-methyl transferase (COMT). These enzymes are involved in the last two steps of monolignol biosynthesis. In maize, bm phenotype is associated with increased livestock digestibility, but at the cost of significantly reduced forage and grain yields. In sorghum, yield reductions were apparent in near isogenic lines, but were ameliorated through construction of hybrids that maintain reduced lignin content and increased digestibility. Near-isogenic sorghum brown midrib lines and hybrids are dispelling old beliefs that brown midrib mutants are significantly more susceptible to plant pathogen attack and to lodging than their non-brown midrib counterparts. Brown midrib mutants from new chemically mutagenized populations hold promise of identifying a non-redundant set of genes involved in lignification of grasses. In addition, early reports indicate brown midrib mutants significantly increase conversion rate in the lignocellulosic bioenergy