Abstract
Biomass rich in lignocellulose from grasses is a major source for biofuel production and animal feed. However, the presence of lignin in cell walls limits its efficient utilisation such as in its bioconversion to biofuel. Reduction of the lignin content or alteration of its structure in crop plants have been pursued, either by regulating genes encoding enzymes in the lignin biosynthetic pathway using biotechnological techniques or by breeding naturally-occurring low lignin mutant lines. The aim of this review is to provide a summary of these studies, focusing on lignin (monolignol) biosynthesis and composition in grasses and, where possible, the impact on recalcitrance to bioconversion. An overview of transgenic crops of the grass family with regulated gene expression in lignin biosynthesis is presented, including the effect on lignin content and changes in the ratio of p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units. Furthermore, a survey is provided of low-lignin mutants in grasses, including cereals in particular, summarising their origin and phenotypic traits together with genetics and the molecular function of the various genes identified.
Highlights
Cereals are a basic food supply for humans and animals worldwide and include rice, maize, wheat, barley and sorghum
cinnamyl alcohol dehydrogenase (CAD) is responsible for reducing cinnamaldehydes to cinnamyl alcohols, the precursors of the building blocks of lignin, known as monolignols, whereas caffeic acid/5-hydroxyferulic acid O-methyltransferase (COMT) is a multifunctional enzyme, but with a preference for methylations of 5-hydroxyconiferaldehyde to sinapaldehydes and primarily affecting the synthesis of syringyl monolignol [24,25] The genes responsible for the brown midrib phenotype in maize and sorghum, which are known for reduced lignin, have mutations within the CAD and COMT genes affecting their expression
This review focuses on lignin reduction in important cereals for animal feed, with a particular focus on papers published after 2010 and updating an earlier review paper, but still including references to primary papers
Summary
Cereals are a basic food supply for humans and animals worldwide and include rice, maize, wheat, barley and sorghum. CAD is responsible for reducing cinnamaldehydes to cinnamyl alcohols, the precursors of the building blocks of lignin, known as monolignols, whereas COMT is a multifunctional enzyme, but with a preference for methylations of 5-hydroxyconiferaldehyde to sinapaldehydes and primarily affecting the synthesis of syringyl monolignol [24,25] The genes responsible for the brown midrib phenotype in (bm, bm3) maize and (bmr, bmr12) sorghum, which are known for reduced lignin, have mutations within the CAD and COMT genes affecting their expression These naturally-occurring low-lignin mutants are of interest for academia and the fodder industry as an alternative source for animal feed and bioproducts [26]. (2) to provide an overview of recent biotechnology/bioengineering studies targeting genes in the phenylpropanoid and monolignol-specific pathway, and (3) to introduce natural low-lignin mutants with regards to occurrence and phenotypic studies
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