Plant lignin (monolignol) biosynthesis module

A taxon-neutral decomposition of plant lignin biosynthesis as a recursively decomposable module. Lignin is a phenolic heteropolymer deposited in secondary cell walls that provides mechanical support and water conduction but is the principal determinant of lignocellulosic biomass recalcitrance. The module separates (1) the general phenylpropanoid entry (PAL -> C4H -> 4CL) that converts L-phenylalanine into p-coumaroyl-CoA, (2) the monolignol-specific "metabolic grid" (HCT, C3'H, CCoAOMT, CCR, F5H, COMT, CAD) that reduces and differentially hydroxylates/methylates hydroxycinnamoyl intermediates into the three canonical monolignols (p-coumaryl, coniferyl, and sinapyl alcohol), (3) export of monolignols across the plasma membrane into the apoplast, and (4) oxidative radical coupling of monolignols by cell-wall laccases and class III peroxidases into the growing lignin polymer, whose p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) unit composition is set by which monolignols are supplied. It is phrased as functions and pathway segments rather than a fixed gene list so it can represent angiosperm, gymnosperm, and grass implementations (gymnosperms make almost no S lignin because they lack F5H/CYP84A activity; grasses additionally incorporate ferulate/coumarate esters, out of scope here). Concrete UniProt members are Arabidopsis exemplars, not species-restricting claims. As a bioenergy module, monolignol supply flux and the S/G ratio are the dominant engineering levers for reducing recalcitrance and improving saccharification and pulping.

Identifiers are grounded only where verified against the local GO term cache or UniProt; descriptors without a `term` (e.g. monolignol intermediates) are deliberate rather than oversights. Laccase activity (EC 1.10.3.2) is grounded to GO:0016682 (oxidoreductase, diphenols as donors, oxygen as acceptor) because the specific "laccase activity" GO term (GO:0008471) is obsolete. The pathway is a metabolic grid rather than a linear chain: C3'H acts on shikimate esters (via HCT), and F5H/COMT can act at the aldehyde level, so the intermediates shown are representative of the dominant in-planta route. Representative UniProt members are concrete Arabidopsis exemplars for orientation, not exhaustive or species-restricting; gymnosperms lack effective F5H and make essentially only G lignin, and grass wall-bound ferulate/p-coumarate esters are a separate module.