Plant xylan (glucuronoxylan) biosynthesis module

A taxon-neutral decomposition of plant heteroxylan biosynthesis as a recursively decomposable module. Xylan is the dominant hemicellulose of the secondary cell wall: a (1->4)-beta-D-xylan backbone decorated with (alpha-1,2)-linked (4-O-methyl)glucuronic acid and O-acetyl groups in eudicots (glucuronoxylan, GX) and additionally with alpha-arabinofuranosyl and feruloyl-arabinose substitutions in grasses (glucuronoarabinoxylan, GAX). Xylan coats and tethers cellulose microfibrils and is covalently and non-covalently associated with lignin in the lignin-carbohydrate complex, so it is a major contributor to biomass recalcitrance and a key acetyl-group source that inhibits downstream fermentation. The module separates (1) Golgi backbone elongation by the IRX9/IRX10/IRX14 xylan synthase machinery, (2) synthesis of the reducing-end oligosaccharide sequence that may prime/terminate the chain in eudicots, (3) alpha-glucuronosyl sidechain addition (GUX), (4) 4-O-methylation of the glucuronic acid (GXM), and (5) O-acetylation (ESK1/TBL29 with RWA acetyl-CoA supply). It is phrased as functions, complexes, and pathway segments rather than a fixed gene list so it can represent eudicot GX and grass GAX implementations; concrete UniProt members are Arabidopsis exemplars, not species-restricting claims. As a bioenergy module, xylan substitution pattern (acetylation, methyl-GlcA) and quantity are principal engineering targets for reducing recalcitrance and acetate-driven fermentation inhibition.

Identifiers are grounded only where verified against the local GO term cache or UniProt; descriptors without a `term` (e.g. UDP-xylose donor, backbone oligosaccharide intermediates) are deliberate rather than oversights. The xylan backbone synthase is an IRX9/IRX10/IRX14 machine; IRX10 (GT47) is treated as the catalytic xylosyltransferase and IRX9/IRX14 (GT43) as required components. The reducing-end oligosaccharide sequence (IRX7/FRA8, IRX8/GAUT12, PARVUS) is a eudicot feature and is marked optional; grasses lack this reducing-end tetramer. The grass glucuronoarabinoxylan variant (alpha-arabinofuranosyl and feruloyl-arabinose substitutions) is represented as a descriptor-level variant without an asserted UniProt exemplar because the arabinosyl/feruloyl transferase members are grass genes not verified here. Representative UniProt members are concrete Arabidopsis exemplars for orientation, not exhaustive or species-restricting.