Sorghum brown midrib (bmr) mutants have reddish-brown vascular tissues in their leaves and stems as a result of changes in lignin content and subunit composition. Past research at Purdue University has generated a set of bmr sorghum mutants via chemical mutagenesis and established some to be allelic to each other. More recently, we identified additional spontaneous mutants in true breeding lines with marked phenotype and a range of agronomic characteristics. One such mutant, bmr-26, is of particular interest because it arose in a drought-tolerant sorghum line. Analysis of testcross hybrids between this spontaneous bmr mutant and the chemically induced mutants, bmr-6 and bmr-12, showed that the bmr-26 allele was allelic to bmr-12 and not to bmr-6. Both the bmr-12 and the bmr-26 mutations significantly reduced lignin content in leaf, blade, sheath, stem, and panicle tissue. The effect of the mutation was relatively more severe in bmr-12 than in bmr-26. The impact of the two mutations on cell wall composition in different tissues varied. The biggest effect of the bmr-12 mutation was in reduction of lignin in the sheath, whereas lignin content in panicles was more affected by the bmr-26 mutation. This suggested an allele-specific effect in tissue lignin reduction of these mutants. Cellulose and hemicellulose concentrations were also significantly higher in certain tissue types for both the induced and spontaneous mutants. Forage quality traits including percent NDF and ADF were significantly increased by both mutations. Improvement in in vitro dry matter digestibility as a result of the bmr-26 mutation was relatively small and was not proportional to the reduction in the lignin content.
Key words: Acid detergent fiber (ADF), bmr, brown midrib, in vitro dry matter digestibility (IVDMD), lignin, neutral detergent fiber (NDF), Sorghum bicolor.
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