Lignin is, after cellulose, the second most abundant terrestrial biopolymer, accounting for approximately 30% of the organic carbon in the biosphere. The ability to synthesize lignin has been essential in the evolutionary adaptation of plants from an aquatic environment to land. Lignin is crucial for structural integrity of the cell wall and stiffness and strength of the stem. In addition, lignin waterproofs the cell wall, enabling transport of water and solutes through the vascular system, and plays a role in protecting plants against pathogens. Although researchers have studied lignin for more than a century, many aspects of its biosynthesis remain unresolved.
Source: Lignin Synthesis Annual Review of Plant Biology Vol. 54:519-546
Lignin is a highly complex cross-linked phenolic polymer, which been extensively studied, but the exact chemical structure is still uncertain - and can vary from species to species.
And, as outlined above, the exact chemical polymerisation processes within plants that lead to its 'manufacture' are not yet clear.
The polymerisation step, that is a radical-radical coupling, is catalysed by oxidative enzymes. Both peroxidase and laccase enzymes are present in the plant cell walls, and it is not known whether one or both of these groups participates in the polymerisation. Low molecular weight oxidants might also be involved.