The Brown Midrib3 (Bm3) Mutation in Maize Occurs in the Gene Encoding Caffeic Acid O-Methyltransferase

Departament de Genètica Molecular, Centro de Investigación y Desarrollo, Barcelona, Spain.
The Plant Cell (Impact Factor: 9.34). 05/1995; 7(4):407-16. DOI: 10.1105/tpc.7.4.407
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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, 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.

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Available from: Joan Rigau, Sep 25, 2015
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    • "The brown midrib mutants have been used to identify and characterize the genes that encode the major enzymes for specific steps of monolignol biosynthesis for C4 grasses (Figure 1). The maize Bm3 and sorghum Bmr12 genes both encode orthologous caffeic acid Omethyltransferase (COMT), which catalyzes the penultimate step in monolignol biosynthesis (Vignols et al. 1995; Bout and Vermerris 2003). The maize Bm1 and the sorghum Bmr6 genes both encode orthologous cinnamyl alcohol dehydrogenase (CAD) (Saballos et al. 2009; Sattler et al. 2009; Chen et al. 2012), which catalyzes the last step in monolignol biosynthesis. "
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    ABSTRACT: Reducing lignin concentration in lignocellulosic biomass can increase forage digestibility for ruminant livestock and saccharification yields of biomass for bioenergy. In sorghum (Sorghum bicolor (L.) Moench) and several other C4 grasses, brown midrib (bmr) mutants have been shown to reduce lignin concentration. Putative bmr mutants isolated from an EMS-mutagenized population were characterized and classified based upon their leaf midrib phenotype and allelism tests with the previously described sorghum bmr loci bmr2, bmr6 and bmr12. These tests resulted in the identification of additional alleles of bmr2, bmr6 and bmr12, and in addition, six bmr mutants were identified that were not allelic to these previously described loci. Further allelism testing among these six bmr mutants showed that they represented four novel bmr loci. Based on this study the number of bmr loci uncovered in sorghum has doubled. The impact of these lines on agronomic traits and lignocellulosic composition was assessed in a two-year field study. Overall, most of the identified bmr lines showed reduced lignin concentration of their biomass relative to wild-type (WT). Effects of the six new bmr mutants on enzymatic saccharification of lignocellulosic materials were determined, but the amount of glucose released from the stover was similar to WT in all cases. Like bmr2, bmr6 and bmr12, these mutants may affect monolignol biosynthesis, and may be useful for bioenergy and forage improvement when stacked together or in combination with the three previously described bmr alleles.
    G3-Genes Genomes Genetics 09/2014; 4(11). DOI:10.1534/g3.114.014001 · 3.20 Impact Factor
    • "To our knowledge, this is the first reported field study evaluating the potential of transgenic switchgrass with reduced lignin content for use as a bioenergy crop. Brown midrib (bmr) mutant and transgenic grasses with reduced COMT enzyme activity are characterized by significant reductions in S-lignin, resulting in a lower S/G monomer ratio, as well as a decrease in the total lignin content (Chen et al., 2004; Fu et al., 2011a; Jung et al., 2012; Palmer et al., 2008; Piquemal et al., 2002; Tu et al., 2010; Vignols et al., 1995). In agreement with these studies, COMT down-regulation in field-grown switchgrass was associated with a reduction in the S/G ratio and lower lignin levels. "
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    ABSTRACT: Switchgrass (Panicum virgatum L.) is a leading candidate for a dedicated lignocellulosic biofuel feedstock owing to its high biomass production, wide adaptation and low agronomic input requirements. Lignin in cell walls of switchgrass, and other lignocellulosic feedstocks, severely limits the accessibility of cell wall carbohydrates to enzymatic breakdown into fermentable sugars and subsequently biofuels. Low-lignin transgenic switchgrass plants produced by the down-regulation of caffeic acid O-methyltransferase (COMT), a lignin biosynthetic enzyme, were analysed in the field for two growing seasons. COMT transcript abundance, lignin content and the syringyl/guaiacyl lignin monomer ratio were consistently lower in the COMT-down-regulated plants throughout the duration of the field trial. In general, analyses with fully established plants harvested during the second growing season produced results that were similar to those observed in previous greenhouse studies with these plants. Sugar release was improved by up to 34% and ethanol yield by up to 28% in the transgenic lines relative to controls. Additionally, these results were obtained using senesced plant material harvested at the end of the growing season, compared with the young, green tissue that was used in the greenhouse experiments. Another important finding was that transgenic plants were not more susceptible to rust (Puccinia emaculata). The results of this study suggest that lignin down-regulation in switchgrass can confer real-world improvements in biofuel yield without negative consequences to biomass yield or disease susceptibility.
    Plant Biotechnology Journal 04/2014; 12(7). DOI:10.1111/pbi.12195 · 5.75 Impact Factor
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    • "However, evidence from the literature was found for only 42% of the 1985 reactions. Th is model was used to perform fl ux balance analysis for three physiological states (photosynthesis, photorespiration , and respiration) and compared predictions against experimental observations for two naturally occurring maize mutants, bm1 (brown midrib1) (Vignols et al., 1995; Vermerris et al., 2002) and bm3 (brown midrib3) (Vignols et al., 1995) with defi ned defects in cell wall lignin biosynthesis . Because these reports on new models are available only in published article format and/or in supplementary data fi les, they are diffi cult to interact with and necessarily lack visualization mechanisms at the systems level. "
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