Genetic and physical fine mapping of the novel brown midrib gene bm6 in maize (Zea mays L.) to a 180 kb region on chromosome 2

Brown midrib mutants in maize are known to be associated with reduced lignin content and increased cell wall digestibility, which leads to better forage quality and higher efficiency of cellulosic biomass conversion into ethanol. Four well known brown midrib (bm) mutants, named bm1-4, were identified several decades ago. Additional recessive brown midrib mutants have been identified by allelism tests and designated as bm5 and bm6. In this study, we determined that bm6 increases cell wall digestibility and decreases plant height. bm6 was confirmed onto the short arm of chromosome 2 by a small mapping set with 181 plants from a F(2) segregating population, derived from crossing B73 and a bm6 mutant line. Subsequently, 960 brown midrib individuals were selected from the same but larger F(2) population for genetic and physical mapping. With newly developed markers in the target region, the bm6 gene was assigned to a 180 kb interval flanked by markers SSR_308337 and SSR_488638. In this region, ten gene models are predicted in the maize B73 sequence. Analysis of these ten genes as well as genes in the syntenic rice region revealed that four of them are promising candidate genes for bm6. Our study will facilitate isolation of the underlying gene of bm6 and advance our understanding of brown midrib gene functions.