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Summary  The midribs of maize  brown midrib  ( bm ) mutants exhibit a reddish‐brown color associated with reductions in lignin concentration and alterations in lignin composition. Here, we report the mapping, cloning, and functional and biochemical analyses of the  bm2  gene. The  bm2  gene was mapped to a small region of chromosome 1 that contains a putative methylenetetrahydrofolate reductase ( MTHFR ) gene, which is down‐regulated in  bm2  mutant plants. Analyses of multiple  Mu ‐induced  bm2‐Mu  mutant alleles confirmed that this constitutively expressed gene is  bm2 . Yeast complementation experiments and a previously published biochemical characterization show that the  bm2  gene encodes a functional  MTHFR . Quantitative  RT  ‐  PCR  analyses demonstrated that the  bm2  mutants accumulate substantially reduced levels of  bm2  transcript. Alteration of  MTHFR  function is expected to influence accumulation of the methyl donor  S‐ adenosyl‐ l  ‐ methionine ( SAM ). Because  SAM  is consumed by two methyltransferases in the lignin pathway (Ye  et al ., [Ye, Z.H., 1994]), the finding that  bm2  encodes a functional  MTHFR  is consistent with its lignin phenotype. Consistent with this functional assignment of  bm2 , the expression patterns of genes in a variety of  SAM ‐dependent or ‐related pathways, including lignin biosynthesis, are altered in the  bm2  mutant. Biochemical assays confirmed that  bm2  mutants accumulate reduced levels of lignin with altered composition compared to wild‐type. Hence, this study demonstrates a role for  MTHFR  in lignin biosynthesis.

The maize brown midrib2 ( bm2 ) gene encodes a methylenetetrahydrofolate reductase that contributes to lignin accumulation