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Keywords:

  • cell wall;
  • lignin;
  • pathway discovery;
  • phenolic metabolism;
  • phenolic profiling;
  • synthetic biology

Contents

 Summary978
I.Introduction979
II.Phenolic metabolism979
III.Lignin biosynthesis and structure983
IV.Alternative lignin monomers for biofuel applications985
V.Candidate alternative monolignols in biomimetic systems991
VI.From phenolic profiling to lignomics992
VII.Phenolic pathway engineering towards alternative monolignols993
 Acknowledgements994
 References994

Summary

Lignin, a phenolic polymer in the secondary wall, is the major cause of lignocellulosic biomass recalcitrance to efficient industrial processing. From an applications perspective, it is desirable that second-generation bioenergy crops have lignin that is readily degraded by chemical pretreatments but still fulfill its biological role in plants. Because plants can tolerate large variations in lignin composition, often without apparent adverse effects, substitution of some fraction of the traditional monolignols by alternative monomers through genetic engineering is a promising strategy to tailor lignin in bioenergy crops. However, successful engineering of lignin incorporating alternative monomers requires knowledge about phenolic metabolism in plants and about the coupling properties of these alternative monomers. Here, we review the current knowledge about lignin biosynthesis and the pathways towards the main phenolic classes. In addition, the minimal requirements are defined for molecules that, upon incorporation into the lignin polymer, make the latter more susceptible to biomass pretreatment. Numerous metabolites made by plants meet these requirements, and several have already been tested as monolignol substitutes in biomimetic systems. Finally, the status of detection and identification of compounds by phenolic profiling is discussed, as phenolic profiling serves in pathway elucidation and for the detection of incorporation of alternative lignin monomers.