A Novel Bmr6 Allele (bmr34) Confers the Brown Midrib Phenotype Without Biomass Penalty in Sorghum

Brown midrib (bmr) mutants are frequently associated with unfavorable agronomic traits. In this study, we identified a novel brown midrib mutant, bmr34, which exhibited distinct brown coloration in roots, stems, and leaf midribs. Although most classic bmr mutants show undesirable agronomic performance, this mutant displays altered lignin accumulation and has important potential for forage quality and biomass utilization, providing a key genetic resource for lignin regulatory research in sorghum. Compared to the wild-type, bmr34 showed no significant differences in five major agronomic traits; however, lignin content was significantly reduced. Bulked segregant analysis (BSA) using an F2 population derived from a cross between bmr34 and the wild type Tx623 mapped the candidate region to chromosome 4. Further sequencing analysis identified a single nucleotide substitution (C → T; reverse strand G → A) at position 5,731,348 within the 5′ splice site of the third intron of Sobic.004G071000 in the mapping interval. KASP marker analysis demonstrated complete co-segregation between the mutation site and the bmr phenotype. Sequence analysis also revealed that this G → A substitution resulted in aberrant splicing and a 33-bp insertion in the third exon, which introduced a premature stop codon. Notably, the normally spliced transcript still accounted for approximately 36.2% of total transcripts in bmr34, indicating partial retention of wild-type transcript processing. These results demonstrate that bmr34 represents a novel weak allele of Bmr6, providing new insights into splice-site mutations and their contribution to lignin biosynthesis regulation in sorghum.