Abstract
BackgroundMaize brown midrib (bm) mutants associated with impaired lignin biosynthesis are a potential source for the breed of novel germplasms with improved cell wall digestibility. The spontaneous bm5 mutants had been identified since 2008. However, the gene responsible for the bm5 locus, and the comprehensive effects of bm5 mutation on lignin biosynthesis, soluble phenolics accumulation, and cell wall degradation have yet to be elucidated.ResultsThe bm5 locus was identified to encode a major 4-coumarate: coenzyme A ligase (Zm4CL1) through analyzing MutMap-assisted gene mapping data. Two alleles of Zm4CL1 isolated from bm5 mutants contained two transposons inserted in the first exon and the second intron, respectively, and consequently, the activities of 4CLs in the crude enzyme extracts from bm5 midribs were reduced by 51–62% compared with the wild type. Furthermore, five 4CLs were retrieved from maize genome, and Zm4CL1 was the most highly expressed one in the lignified tissues. Mutation of Zm4CL1 mainly impeded the biosynthesis of guaiacyl (G) lignins and increased the level of soluble feruloyl derivatives without impacting maize growth and development. Moreover, both neutral detergent fiber digestibility and saccharification efficiency of cell walls were significantly elevated in the bm5 mutant.ConclusionsZm4CL1 was identified as the Bm5 gene, since two independent alleles of Zm4CL1 were associated with the same mutant phenotype. Mutation of Zm4CL1 mainly affected G lignin biosynthesis and soluble feruloyl derivatives accumulation in maize lignified tissues. The reduced recalcitrance of the bm5 mutant suggests that Zm4CL1 is an elite target for cell wall engineering, and genetic manipulation of this gene will facilitate the utilization of crop straw and stover that have to be dealt with for environmental protection.
Highlights
Plant cell walls are structural supporter and natural barrier in protecting the plants from pathogens and insects, which consist mainly of cellulose, hemicellulose, and lignin [1]
The reduced recalcitrance of the bm5 mutant suggests that Zm4CL1 is an elite target for cell wall engineering, and genetic manipulation of this gene will facilitate the utilization of crop straw and stover that have to be dealt with for environmental protection
We identified Zm4CL1 as the bm5 gene and its mutation impeded G lignin biosynthesis and, increased the level of feruloyl quinic acid (FQA) and feruloyl glycoside (FG) that are derived from ferulate, an intermediate in the lignin biosynthetic pathway in maize
Summary
Plant cell walls are structural supporter and natural barrier in protecting the plants from pathogens and insects, which consist mainly of cellulose, hemicellulose, and lignin [1]. The presence of lignin in cell walls negatively impacts forage digestibility and pulping efficiency [4]. In Arabidopsis, three 4CL isoforms, namely At4CL1, At4CL2, and At4CL4, participate in lignin biosynthesis, and At4CL3 are responsible for flavonoid metabolism [7]. Maize brown midrib (bm) mutants associated with impaired lignin biosynthesis are a potential source for the breed of novel germplasms with improved cell wall digestibility. The gene responsible for the bm locus, and the comprehensive effects of bm mutation on lignin biosynthesis, soluble phenolics accumulation, and cell wall degradation have yet to be elucidated
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