ligninebiosynthese

Lignin biosynthesis is the biochemical pathway that plants use to make lignin, a complex phenolic polymer found in the cell walls of secondary xylem and other tissues. Lignin provides structural support, impermeability, and resistance to microbial degradation. The process begins with the phenylpropanoid pathway, where the amino acid phenylalanine is deaminated by phenylalanine ammonia‑liase (PAL) to form cinnamic acid. Subsequent hydroxylation, methylation and reduction reactions convert cinnamic acid into the monolignol precursors coniferyl alcohol, sinapyl alcohol, and p‑coumaryl alcohol. Key enzymes in this series include cinnamic acid 4‑hydroxylase (C4H), 4‑hydroxy‑cinnamoyl‑CoA ligase (4CL), caffeic acid O‑methyltransferase (COMT), and cinnamyl alcohol dehydrogenase (CAD).

Once monolignols are produced, they are transported across the plasma membrane into the cell wall space where peroxidases and laccases oxidize them to free radicals. Radical coupling reactions between these species create the heterogenic, cross‑linked polymer that constitutes lignin. The ratio of the monolignols determines the S/G/H composition of lignin, influencing its mechanical properties and degradability.

Regulation of lignin biosynthesis involves transcription factors such as MYB and NAC families that modulate the expression of phenylpropanoid genes. Environmental stresses, developmental cues, and hormone signals (e.g., auxin, jasmonate) also affect lignin production. Understanding lignin biosynthesis is crucial for improving biofuel feedstock quality, wood pulp processing, and plant resistance to pathogens.