Lignin is one of the main components of plant cell wall and it is a natural phenolic polymer with high molecular weight, complex composition and structure. Lignin biosynthesis extensively contributes to plant growth, tissue/organ development, lodging resistance and the responses to a variety of biotic and abiotic stresses.
Lignin is the second largest component of wood. It is a highly polymerised material that makes up
the middle lamella of woody fibres and holds the fibres together. The basic units of the polymeric
structure consist of three aromatic propenyl alcohols (monolignols): coniferyl alcohol (4-(3-hydroxy-
1-propenyl)-2-methoxyphenol); p-coumaryl alcohol (4-[(E)-3-hydroxyprop-1-enyl]phenol); and
sinapyl alcohol (4-hydroxy-3,5-dimethoxycinnamyl alcohol). Coniferyl alcohol represents the
principle unit in lignin.
In the present review, we systematically introduce the biosynthesis of lignin and its regulation by genetic modification and summarize the main biological functions of lignin in plants and their applications. We hope this review will give an in-depth understanding of the important roles of lignin biosynthesis in various plants’ biological processes and provide a theoretical basis for the genetic improvement of lignin content and composition in energy plants and crops.
Lignin is one of the most important secondary metabolite which is produced by the phenylalanine/tyrosine metabolic pathway in plant cells. It is the second most profuse biopolymers that accounts for 30% of the organic carbon content in biosphere.
Lignin biosynthesis is a very complex network that is divided into three processes: (i) biosynthesis of lignin monomers, (ii) transport and (iii) polymerization.After a series of steps involving deamination, hydroxylation, methylation and reduction, lignin monomers are produced in cytoplasm and transported to the apoplast. Finally, lignin is generally polymerized with three main types of monolignols (sinapyl alcohol, S unit; coniferyl alcohol, G unit and p-coumaryl alcohol, H unit) by peroxidase (POD) and laccase (LAC) in secondary cell wall.
In addition, several other compounds including hydroxycinnamaldehydes, tricin flavones, hydroxystilbenes and xenobiotics etc. have also been recognized to be lignin subunits . In differentiating protoxylem tracheary elements of Arabidopsis, lignin monomers can be free to diffuse in the extracellular space but are only polymerized in the secondary cell wal