The brown midrib3 (bm3) mutation in maize occurs in the gene encoding caffeic acid O-methyltransferase.

Abstract

The brown midrib mutations are among the earliest described in maize. Plants containing a brown midrib mutation exhibit a reddish brown pigmentation of the leaf midrib starting when there are four to six leaves. These mutations are known to alter lignin composition and digestibility of plants and therefore constitute prime candidates in the breeding of silage maize. Here, we show that two independent brown midrib3 (bm3) mutations have resulted from structural changes in the COMT gene, which encodes the enzyme O-methyltransferase (COMT; EC 2.1.1.6), involved in lignin biosynthesis. Our results indicate that the bm3-1 allele (the reference mutant allele) has arisen from an insertional event producing a COMT mRNA altered in both size and amount. By sequencing a COMT cDNA clone obtained from bm3-1 maize, a retrotransposon with homology to the B5 element has been found to be inserted near the junction of the 3' coding region of the COMT gene intron. The second bm3 allele, bm3-2, has resulted from a deletion of part of the COMT gene. These alterations of the COMT gene were confirmed by DNA gel blot and polymerase chain reaction amplification analyses. These results clearly demonstrate that mutations at the COMT gene give a brown midrib3 phenotype. Thus, the gene genetically recognized as bm3 is the same as the one coding for COMT.

Silencing CHALCONE SYNTHASE in Maize Impedes the Incorporation of Tricin into Lignin and Increases Lignin Content

“…In addition, HCT of sorghum (Sorghum bicolor) has been studied via in vitro activity assays (Walker et al, 2013). The FOMT involved in the conversion of luteolin and selgin into chrysoeriol and tricin, respectively, in maize is described by Zhou et al (2008) and is identical to the COMT involved in the conversion of 5-hydroxyconiferaldehyde to sinapaldehyde described by Collazo et al (1992), Vignols et al (1995), andPiquemal et al (2002). Question marks indicate biosynthetic steps that are questionable; the reaction from p-coumaric acid toward caffeic acid via a C3H/C4H complex has so far only been shown by in vitro tests using poplar enzymes .…”

Brown‐midrib maize (bm1) – a mutation affecting the cinnamyl alcohol dehydrogenase gene

“…Maize bm3 is severely deficient in catechol-OMT activity, with only 10% of the activity found in normal plants (Grand et al, 1985). Recent work has confirmed that the OMT gene is indeed the site of the bm3 mutation (Vignols et al, 1995). Biochemical evidence suggests that other brown-midrib plants may also be deficient in lignin biosynthetic enzymes.…”

The maize Brown midrib1 locus affects cell wall composition and plant development in a dose-dependent manner

“…The bm2 mutant has a reduction in guaiacyl residues (Chabbert et al, 1994) and a disturbance in the tissue-specific patterns of lignification (Vermerris and Boon, 2001). The bm3 mutant accumulates 5-hydroxyguaiacyl residues in its lignin (Lapierre, 1993) as a result of a mutation in the gene encoding caffeic acid O-methyltransferase (Vignols et al, 1995). The bm4 mutant is the least characterized brown midrib mutant and little is currently known about its chemical composition.…”