Lignin serves as an essential barrier against pests and diseases.
Lignin is a major secondary metabolite generated by the phenylalanine/tyrosine metabolic pathway in plant cells. It is the second most abundant biopolymer, accounting for 30% of the organic carbon concentration in the biosphere. Lignin biosynthesis is a complicated network comprised of three processes: I lignin monomer production, (ii) transport, and (iii) polymerization. Lignin monomers are synthesised in the cytoplasm and transferred to the apoplast by a sequence of stages that include deamination, hydroxylation, methylation, and reduction.
Finally, peroxidase (POD) and laccase (LAC) in the secondary cell wall polymerize Lignin with three forms of monolignols (sinapyl alcohol, S unit; coniferyl alcohol, G unit; and p-coumaryl alcohol, H unit). Several additional chemicals, such as hydroxycinnamaldehydes, tricin flavones, hydroxystilbenes, and xenobiotics, have also been identified as lignin subunits. When developing Arabidopsis protoxylem tracheary elements, lignin monomers can freely flow in the extracellular environment but are only polymerized in the secondary cell walls.
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