The maize brown midrib4 (bm4) gene encodes a functional folylpolyglutamate synthase

Abstract

<p>Mutations in the <em>brown midrib4 (bm4)</em> gene affect the accumulation and composition of lignin in maize. Fine‐mapping analysis of <em>bm4</em> narrowed the candidate region to an approximately 105 kb interval on chromosome 9 containing six genes. Only one of these six genes, GRMZM2G393334, showed decreased expression in mutants. At least four of 10 <em>Mu</em>‐induced <em>bm4</em> mutant alleles contain a <em>Mu</em> insertion in the GRMZM2G393334 gene. Based on these results, we concluded that GRMZM2G393334 is the <em>bm4</em> gene. GRMZM2G393334 encodes a putative folylpolyglutamate synthase (FPGS), which functions in one‐carbon (C1) metabolism to polyglutamylate substrates of folate‐dependent enzymes. Yeast complementation experiments demonstrated that expression of the maize <em>bm4</em> gene in FPGS‐deficient <em>met7</em> yeast is able to rescue the yeast mutant phenotype, thus demonstrating that <em>bm4</em> encodes a functional FPGS. Consistent with earlier studies, <em>bm4</em> mutants exhibit a modest decrease in lignin concentration and an overall increase in the S:G lignin ratio relative to wild‐type. Orthologs of <em>bm4</em> include at least one paralogous gene in maize and various homologs in other grasses and dicots. Discovery of the gene underlying the <em>bm4</em> maize phenotype illustrates a role for FPGS in lignin biosynthesis.</p>