Abstract
Genetic analysis of brown midrib sorghum (Sorghum bicolor) mutant lines assembled in our program has previously shown that the mutations fall into four allelic groups, bmr2, bmr6, bmr12 or bmr19. Causal genes for allelic groups bmr2, bmr6 and bmr12, have since been identified. In this report, we provide evidence for the nature of the bmr19 mutation. This was accomplished by introgressing each of the four bmr alleles into nine different genetic backgrounds. Polymorphisms from four resequenced bulks of sorghum introgression lines containing either mutation, relative to those of a resequenced bulk of the nine normal midrib recurrent parent lines, were used to locate their respective causal mutations. The analysis confirmed the previously reported causal mutations for bmr2 and bmr6 but failed in the case of bmr12-bulk due to a mixture of mutant alleles at the locus among members of that mutant bulk. In the bmr19-bulk, a common G → A mutation was found among all members in Sobic.001G535500. This gene encodes a putative folylpolyglutamate synthase with high homology to maize Bm4. The brown midrib phenotype co-segregated with this point mutation in two separate F2 populations. Furthermore, an additional variant allele at this locus obtained from a TILLING population also showed a brown midrib phenotype, confirming this locus as Bmr19.
Highlights
Donor lines of the brown midrib phenotype carried the bmr2-ref allele (Table 1), but one donor in this group was bmr5, a previous undefined mutant, which contains a C → T mutation at Chr04:5,042,645 resulting in the substitution H251Y called bmr2-5 [14]
Carrying particular brown midrib phenotypes compared to bulked DNA from wildtype parents in the original crosses successfully distinguished the causal mutant and wildtype alleles for these phenotypes when the donor mutant alleles were shared among the members of the bmr bulks
Mixed alleles undermined detection of a candidate Single nucleotide polymorphism (SNP)-index peaks by the analytical protocol employed in the bmr12 bulk because the donor mutant alleles were more evenly mixed with five of the nine members of the bmr12 bulk having the bmr12-18 allele, three with the bmr12-ref allele and Neutral Detergent Fiber
Summary
Lignin is a complex heteropolymer of phenylpropanoid-derived monolignols providing support and water-conducting ability to the cell wall by cross-linking with its polysaccharide components cellulose (polymers of glucose) and hemicellulose (heteropolymers of other monosaccharides including arabinose, glucose, mannose and xylose). Together these cell wall components form the lignocellulosic matrix that constitutes the greatest part of dry plant matter. The cross linking provided by lignin in grass secondary cell walls allows herbaceous panicoids to reach up to several meters in height. The lignin constituent is diverse, involving several possible linkages among the basic monolignol subunits [1]
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