id: MODULE:plant_lignin_monolignol_biosynthesis title: Plant lignin (monolignol) biosynthesis module description: >- 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. status: DRAFT evidence: - source_id: GO:0009809 title: lignin biosynthetic process statement: >- The module is grounded in the GO biological-process term for lignin biosynthesis; H/G/S subprocesses are GO:1901060, GO:1901063, GO:1901066. - source_id: GO:0009699 title: phenylpropanoid biosynthetic process statement: >- The general phenylpropanoid pathway (PAL/C4H/4CL) is the shared upstream trunk feeding lignin, flavonoid, and other phenylpropanoid branches. - source_id: UniProtKB:P35510 title: Phenylalanine ammonia-lyase 1 (AtPAL1) statement: >- Arabidopsis exemplar of the committed entry enzyme deaminating L-phenylalanine to trans-cinnamate (GO:0045548). - source_id: UniProtKB:P92994 title: Trans-cinnamate 4-monooxygenase / C4H (CYP73A5 / REF3, AtC4H) statement: >- Arabidopsis exemplar of the ER-anchored cytochrome P450 hydroxylating cinnamate to p-coumarate (GO:0016710); ref3 mutants have altered lignin. - source_id: UniProtKB:Q42524 title: 4-coumarate--CoA ligase 1 (At4CL1) statement: >- Arabidopsis exemplar activating p-coumarate to p-coumaroyl-CoA (GO:0016207), the branch point into monolignol biosynthesis. - source_id: UniProtKB:Q9FI78 title: Shikimate O-hydroxycinnamoyltransferase / HCT (AtHCT) statement: >- Arabidopsis exemplar transferring the hydroxycinnamoyl group onto shikimate to present it for 3'-hydroxylation (GO:0047172). - source_id: UniProtKB:O22203 title: Cytochrome P450 98A3 / C3'H (REF8, AtCYP98A3) statement: >- Arabidopsis exemplar 3'-hydroxylating p-coumaroyl shikimate to caffeoyl shikimate (GO:0046409); ref8 plants are severely lignin-deficient. - source_id: UniProtKB:O49499 title: Caffeoyl-CoA O-methyltransferase 1 (AtCCoAOMT1) statement: >- Arabidopsis exemplar methylating caffeoyl-CoA to feruloyl-CoA (GO:0042409), the main route to the guaiacyl (G) methoxyl. - source_id: UniProtKB:Q9S9N9 title: Cinnamoyl-CoA reductase 1 (AtCCR1 / IRX4) statement: >- Arabidopsis exemplar reducing hydroxycinnamoyl-CoA thioesters to hydroxycinnamaldehydes (GO:0016621); the first committed monolignol step and the irregular-xylem (irx4) recalcitrance target. - source_id: UniProtKB:Q42600 title: Ferulate 5-hydroxylase / F5H (CYP84A1 / FAH1, AtF5H) statement: >- Arabidopsis exemplar 5-hydroxylating coniferaldehyde/coniferyl alcohol (GO:0046424); the syringyl (S)-lignin-determining step whose expression sets the S/G ratio. - source_id: UniProtKB:Q9FK25 title: Caffeate/5-hydroxyconiferaldehyde O-methyltransferase 1 (AtCOMT / OMT1) statement: >- Arabidopsis exemplar catalyzing the 5-O-methylation en route to sinapyl alcohol (caffeate O-methyltransferase activity, GO:0047763). Note: the UniProt recommended name for Q9FK25 is "Flavone 3'-O-methyltransferase 1" (a substrate-promiscuous OMT), but this is the bona-fide lignin caffeate/5-hydroxyconiferaldehyde COMT (AtCOMT/OMT1), not a mis-picked entry. - source_id: UniProtKB:O49482 title: Cinnamyl alcohol dehydrogenase 5 (AtCAD5 / CAD-D) statement: >- Arabidopsis exemplar reducing hydroxycinnamaldehydes to the monolignol alcohols (GO:0045551), the final soluble step of monolignol supply. - source_id: UniProtKB:Q9FJD5 title: Laccase-17 (AtLAC17) statement: >- Arabidopsis exemplar cell-wall laccase (EC 1.10.3.2) oxidizing monolignols for radical coupling; lac4/lac17 mutants have reduced lignin. - source_id: UniProtKB:Q9FJZ9 title: Peroxidase 72 (AtPRX72 / PER72) statement: >- Arabidopsis exemplar class III secretory peroxidase (GO:0004601) contributing H2O2-dependent monolignol oxidation in the wall. notes: >- 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. module: id: plant_lignin_monolignol_biosynthesis label: Plant lignin (monolignol) biosynthesis module_type: METABOLIC_PATHWAY concepts: - preferred_term: lignin biosynthetic process term: id: GO:0009809 label: lignin biosynthetic process description: >- Synthesis of the phenolic secondary-cell-wall heteropolymer lignin from the monolignols p-coumaryl, coniferyl, and sinapyl alcohol. - preferred_term: phenylpropanoid biosynthetic process term: id: GO:0009699 label: phenylpropanoid biosynthetic process context: taxa: - preferred_term: land plants (angiosperms and gymnosperms) - preferred_term: grasses (with additional wall-bound hydroxycinnamate esters) cellular_components: - preferred_term: cytosol (soluble monolignol pathway enzymes) term: id: GO:0005829 label: cytosol - preferred_term: endoplasmic reticulum membrane (P450 hydroxylases) term: id: GO:0005789 label: endoplasmic reticulum membrane - preferred_term: plasma membrane (monolignol export) term: id: GO:0005886 label: plasma membrane - preferred_term: apoplast (site of oxidative polymerization) term: id: GO:0048046 label: apoplast parts: - order: 1 role: general phenylpropanoid entry (PAL -> C4H -> 4CL) node: id: general_phenylpropanoid label: General phenylpropanoid pathway to p-coumaroyl-CoA module_type: METABOLIC_PATHWAY description: >- The shared trunk that deaminates L-phenylalanine and activates the acid to a CoA thioester, feeding lignin as well as flavonoid and other phenylpropanoid branches. Flux here sets total phenylpropanoid supply. parts: - order: 1 role: phenylalanine deamination node: id: pal_step label: Phenylalanine ammonia-lyase (PAL) module_type: REACTION annotons: - id: pal_activity label: Phenylalanine ammonia-lyase participant: selector_type: FAMILY family: preferred_term: phenylalanine ammonia-lyase (PAL family) representative_members: - preferred_term: Arabidopsis PAL1 term: id: UniProtKB:P35510 label: PAL1_ARATH function: preferred_term: phenylalanine ammonia-lyase activity term: id: GO:0045548 label: phenylalanine ammonia-lyase activity substrates: - preferred_term: L-phenylalanine products: - preferred_term: trans-cinnamate - preferred_term: ammonia locations: - preferred_term: cytosol term: id: GO:0005829 label: cytosol role_description: >- Committed, flux-controlling entry into the phenylpropanoid pathway; non-oxidative deamination of L-phenylalanine. - order: 2 role: cinnamate 4-hydroxylation node: id: c4h_step label: Cinnamate 4-hydroxylase (C4H / CYP73A) module_type: REACTION annotons: - id: c4h_activity label: Trans-cinnamate 4-monooxygenase participant: selector_type: FAMILY family: preferred_term: cinnamate 4-hydroxylase (CYP73A subfamily) representative_members: - preferred_term: Arabidopsis C4H (CYP73A5 / REF3) term: id: UniProtKB:P92994 label: TCMO_ARATH function: preferred_term: trans-cinnamate 4-monooxygenase activity term: id: GO:0016710 label: trans-cinnamate 4-monooxygenase activity substrates: - preferred_term: trans-cinnamate - preferred_term: "O2" - preferred_term: NADPH products: - preferred_term: p-coumarate locations: - preferred_term: endoplasmic reticulum membrane term: id: GO:0005789 label: endoplasmic reticulum membrane role_description: >- ER-anchored cytochrome P450 introducing the 4-hydroxyl; requires cytochrome P450 reductase as electron donor. - order: 3 role: hydroxycinnamate CoA activation node: id: fourcl_step label: 4-coumarate:CoA ligase (4CL) module_type: REACTION annotons: - id: fourcl_activity label: 4-coumarate--CoA ligase participant: selector_type: FAMILY family: preferred_term: 4-coumarate:CoA ligase (4CL family) representative_members: - preferred_term: Arabidopsis 4CL1 term: id: UniProtKB:Q42524 label: 4CL1_ARATH function: preferred_term: 4-coumarate-CoA ligase activity term: id: GO:0016207 label: 4-coumarate-CoA ligase activity substrates: - preferred_term: p-coumarate - preferred_term: ATP - preferred_term: CoA products: - preferred_term: p-coumaroyl-CoA - preferred_term: AMP - preferred_term: diphosphate locations: - preferred_term: cytosol term: id: GO:0005829 label: cytosol role_description: >- Activates hydroxycinnamic acids to CoA thioesters; the branch point channeling flux into monolignol biosynthesis. - order: 2 role: monolignol-specific metabolic grid (to p-coumaryl / coniferyl / sinapyl alcohol) node: id: monolignol_grid label: Monolignol-specific hydroxylation, methylation, and reduction module_type: METABOLIC_PATHWAY description: >- The interconnected set of transferase, P450, O-methyltransferase, and reductase steps that convert p-coumaroyl-CoA into the three monolignols. Modeled as a grid because C3'H acts on shikimate esters presented by HCT, and F5H/COMT can operate at the aldehyde level; the steps below are the dominant in-planta route. parts: - order: 1 role: shikimate ester transfer (HCT) node: id: hct_step label: Hydroxycinnamoyl-CoA:shikimate hydroxycinnamoyltransferase (HCT) module_type: REACTION annotons: - id: hct_activity label: Shikimate O-hydroxycinnamoyltransferase participant: selector_type: FAMILY family: preferred_term: hydroxycinnamoyl-CoA shikimate/quinate transferase (HCT) representative_members: - preferred_term: Arabidopsis HCT term: id: UniProtKB:Q9FI78 label: HST_ARATH function: preferred_term: shikimate O-hydroxycinnamoyltransferase activity term: id: GO:0047172 label: shikimate O-hydroxycinnamoyltransferase activity substrates: - preferred_term: p-coumaroyl-CoA - preferred_term: shikimate products: - preferred_term: p-coumaroyl shikimate - preferred_term: CoA locations: - preferred_term: cytosol term: id: GO:0005829 label: cytosol role_description: >- Forms the shikimate ester that presents the ring for 3'-hydroxylation; acts twice (forward and reverse) in the grid. - order: 2 role: meta (3')-hydroxylation (C3'H) node: id: c3h_step label: p-Coumaroyl shikimate 3'-hydroxylase (C3'H / CYP98A) module_type: REACTION annotons: - id: c3h_activity label: p-coumarate 3-hydroxylase (on shikimate ester) participant: selector_type: FAMILY family: preferred_term: coumaroyl shikimate 3'-hydroxylase (CYP98A subfamily) representative_members: - preferred_term: Arabidopsis C3'H (CYP98A3 / REF8) term: id: UniProtKB:O22203 label: C98A3_ARATH function: preferred_term: p-coumarate 3-hydroxylase activity term: id: GO:0046409 label: p-coumarate 3-hydroxylase activity substrates: - preferred_term: p-coumaroyl shikimate - preferred_term: "O2" products: - preferred_term: caffeoyl shikimate locations: - preferred_term: endoplasmic reticulum membrane term: id: GO:0005789 label: endoplasmic reticulum membrane role_description: >- Introduces the 3-hydroxyl (meta position) that becomes the guaiacyl methoxyl; a near-total lignin block when lost (ref8). - order: 3 role: 3-O-methylation of the CoA ester (CCoAOMT) node: id: ccoaomt_step label: Caffeoyl-CoA O-methyltransferase (CCoAOMT) module_type: REACTION annotons: - id: ccoaomt_activity label: Caffeoyl-CoA O-methyltransferase participant: selector_type: FAMILY family: preferred_term: caffeoyl-CoA O-methyltransferase (CCoAOMT) representative_members: - preferred_term: Arabidopsis CCoAOMT1 term: id: UniProtKB:O49499 label: CAMT4_ARATH function: preferred_term: caffeoyl-CoA O-methyltransferase activity term: id: GO:0042409 label: caffeoyl-CoA O-methyltransferase activity substrates: - preferred_term: caffeoyl-CoA - preferred_term: S-adenosyl-L-methionine products: - preferred_term: feruloyl-CoA - preferred_term: S-adenosyl-L-homocysteine locations: - preferred_term: cytosol term: id: GO:0005829 label: cytosol role_description: >- Installs the 3-O-methyl group generating the feruloyl (G-series) intermediate; the principal route to guaiacyl units. - order: 4 role: CoA-thioester reduction to aldehyde (CCR) node: id: ccr_step label: Cinnamoyl-CoA reductase (CCR) module_type: REACTION annotons: - id: ccr_activity label: Cinnamoyl-CoA reductase participant: selector_type: FAMILY family: preferred_term: cinnamoyl-CoA reductase (CCR) representative_members: - preferred_term: Arabidopsis CCR1 (IRX4) term: id: UniProtKB:Q9S9N9 label: CCR1_ARATH function: preferred_term: cinnamoyl-CoA reductase activity term: id: GO:0016621 label: cinnamoyl-CoA reductase (NADP+) activity substrates: - preferred_term: feruloyl-CoA - preferred_term: NADPH products: - preferred_term: coniferaldehyde - preferred_term: CoA locations: - preferred_term: cytosol term: id: GO:0005829 label: cytosol role_description: >- First committed monolignol-specific reduction; converts hydroxycinnamoyl-CoA thioesters to hydroxycinnamaldehydes. - order: 5 role: aldehyde 5-hydroxylation (F5H) for S lignin optional: true node: id: f5h_step label: Ferulate/coniferaldehyde 5-hydroxylase (F5H / CYP84A) module_type: REACTION annotons: - id: f5h_activity label: Ferulate 5-hydroxylase participant: selector_type: FAMILY family: preferred_term: ferulate 5-hydroxylase (CYP84A subfamily) representative_members: - preferred_term: Arabidopsis F5H (CYP84A1 / FAH1) term: id: UniProtKB:Q42600 label: C84A1_ARATH function: preferred_term: ferulate 5-hydroxylase activity term: id: GO:0046424 label: ferulate 5-hydroxylase activity substrates: - preferred_term: coniferaldehyde - preferred_term: "O2" products: - preferred_term: 5-hydroxyconiferaldehyde locations: - preferred_term: endoplasmic reticulum membrane term: id: GO:0005789 label: endoplasmic reticulum membrane role_description: >- The syringyl-determining step; absent/ineffective in gymnosperms, so its expression level sets the S/G ratio and is a primary recalcitrance-engineering lever. - order: 6 role: 5-O-methylation (COMT) for S lignin optional: true node: id: comt_step label: Caffeate/5-hydroxyconiferaldehyde O-methyltransferase (COMT) module_type: REACTION annotons: - id: comt_activity label: Caffeate O-methyltransferase participant: selector_type: FAMILY family: preferred_term: caffeic acid/5-hydroxyconiferaldehyde O-methyltransferase (COMT) representative_members: - preferred_term: Arabidopsis COMT1 (OMT1) term: id: UniProtKB:Q9FK25 label: OMT1_ARATH function: preferred_term: caffeate O-methyltransferase activity term: id: GO:0047763 label: caffeate O-methyltransferase activity substrates: - preferred_term: 5-hydroxyconiferaldehyde - preferred_term: S-adenosyl-L-methionine products: - preferred_term: sinapaldehyde - preferred_term: S-adenosyl-L-homocysteine locations: - preferred_term: cytosol term: id: GO:0005829 label: cytosol role_description: >- Installs the 5-O-methyl group completing the syringyl substitution pattern. - order: 7 role: aldehyde reduction to monolignol alcohol (CAD) node: id: cad_step label: Cinnamyl alcohol dehydrogenase (CAD) module_type: REACTION annotons: - id: cad_activity label: Cinnamyl-alcohol dehydrogenase participant: selector_type: FAMILY family: preferred_term: cinnamyl alcohol dehydrogenase (CAD) representative_members: - preferred_term: Arabidopsis CAD5 (CAD-D) term: id: UniProtKB:O49482 label: CADH5_ARATH - preferred_term: Maize CAD (Brown midrib 1, BM1) term: id: UniProtKB:O24562 label: CADH_MAIZE ancestral_nodes: - preferred_term: PAINT IBD node PTN000915374 (CAD) in PTHR42683 term: id: PANTHER:PTN000915374 label: PTN000915374 description: >- Local PAINT IBD node in PTHR42683 propagating F:GO:0045551 (cinnamyl-alcohol dehydrogenase activity) and P:GO:0009809 (lignin biosynthetic process); seeds include the exemplar AT4G34230 (CAD5); PAINT date 20260530. evidence: - source_id: GO_REF:0000033 statement: IBD propagation from PAINT node PTN000915374 in PTHR42683. function: preferred_term: cinnamyl-alcohol dehydrogenase activity term: id: GO:0045551 label: cinnamyl-alcohol dehydrogenase activity substrates: - preferred_term: coniferaldehyde / sinapaldehyde / p-coumaraldehyde - preferred_term: NADPH products: - preferred_term: coniferyl / sinapyl / p-coumaryl alcohol (monolignols) locations: - preferred_term: cytosol term: id: GO:0005829 label: cytosol role_description: >- Final soluble step; reduces hydroxycinnamaldehydes to the monolignol alcohols that are exported for polymerization. - order: 3 role: monolignol export to the apoplast optional: true node: id: monolignol_export label: Monolignol transport across the plasma membrane module_type: TRANSPORT_STEP description: >- Monolignols synthesized in the cytosol must reach the wall; ABCG-type transporters have been implicated in p-coumaryl/coniferyl alcohol export, though passive diffusion and other routes may also contribute. annotons: - id: monolignol_abc_transport label: ABCG-mediated monolignol export participant: selector_type: FAMILY family: preferred_term: ABCG monolignol transporter representative_members: - preferred_term: Arabidopsis ABCG29 (PDR1) term: id: UniProtKB:Q94A18 label: AB29G_ARATH function: preferred_term: ABC-type transporter activity term: id: GO:0140359 label: ABC-type transporter activity locations: - preferred_term: plasma membrane term: id: GO:0005886 label: plasma membrane role_description: >- Exports monolignols (notably p-coumaryl alcohol) from the cytosol to the apoplast for oxidative coupling; the mechanism is partly redundant. - order: 4 role: oxidative polymerization of monolignols into lignin node: id: oxidative_polymerization label: Laccase/peroxidase radical coupling of monolignols module_type: METABOLIC_PATHWAY description: >- In the wall, monolignols are oxidized to phenoxy radicals that couple combinatorially (chiefly at 8-O-4, 8-5, and 8-8 linkages) onto the growing polymer. Two oxidase families act, partly redundantly: cell-wall laccases (O2-dependent) and class III secretory peroxidases (H2O2- dependent). The monolignol supply ratio, not the oxidases, sets H/G/S composition. parts: - order: 1 role: laccase-mediated oxidation node: id: laccase_step label: Cell-wall laccase monolignol oxidation module_type: REACTION annotons: - id: laccase_activity label: Laccase (diphenol:O2 oxidoreductase) participant: selector_type: FAMILY family: preferred_term: cell-wall laccase (multicopper oxidase) description: >- Multicopper oxidases (EC 1.10.3.2) oxidizing monolignols with O2 as terminal acceptor; grounded to the valid GO parent because the specific laccase MF term is obsolete. representative_members: - preferred_term: Arabidopsis LAC17 term: id: UniProtKB:Q9FJD5 label: LAC17_ARATH function: preferred_term: oxidoreductase activity, acting on diphenols and related substances as donors, oxygen as acceptor term: id: GO:0016682 label: oxidoreductase activity, acting on diphenols and related substances as donors, oxygen as acceptor substrates: - preferred_term: monolignols (p-coumaryl / coniferyl / sinapyl alcohol) - preferred_term: "O2" products: - preferred_term: monolignol phenoxy radicals - preferred_term: water locations: - preferred_term: apoplast term: id: GO:0048046 label: apoplast role_description: >- O2-dependent single-electron oxidation of monolignols; lac4/lac17 loss strongly reduces stem lignin. - order: 2 role: peroxidase-mediated oxidation node: id: peroxidase_step label: Class III peroxidase monolignol oxidation module_type: REACTION annotons: - id: peroxidase_activity label: Class III secretory peroxidase participant: selector_type: FAMILY family: preferred_term: class III plant peroxidase representative_members: - preferred_term: Arabidopsis PRX72 (PER72) term: id: UniProtKB:Q9FJZ9 label: PER72_ARATH function: preferred_term: peroxidase activity term: id: GO:0004601 label: peroxidase activity substrates: - preferred_term: monolignols (p-coumaryl / coniferyl / sinapyl alcohol) - preferred_term: hydrogen peroxide products: - preferred_term: monolignol phenoxy radicals - preferred_term: water locations: - preferred_term: apoplast term: id: GO:0048046 label: apoplast role_description: >- H2O2-dependent monolignol oxidation, partly redundant with laccases; contributes to lignification of vessels and fibers. variant_sets: - id: lignin_unit_composition label: H / G / S lignin unit output axis: monolignol supplied / lignin unit type selection: ONE_OR_MORE variants: - id: h_lignin label: p-hydroxyphenyl (H) lignin module_type: METABOLIC_PATHWAY annotons: - id: h_unit_polymerization label: p-coumaryl alcohol coupling (H units) participant: selector_type: ANY_PARTICIPANT description: p-coumaryl alcohol phenoxy radicals. function: preferred_term: oxidative radical coupling of p-coumaryl alcohol description: >- Combinatorial coupling of p-coumaryl alcohol radicals; no exact GO molecular-function term is asserted for the non-enzymatic coupling itself. processes: - preferred_term: p-hydroxyphenyl lignin biosynthetic process term: id: GO:1901060 label: p-hydroxyphenyl lignin biosynthetic process role_description: >- Minor unit in most dicots; relatively enriched under stress and in compression/reaction wood contexts. - id: g_lignin label: guaiacyl (G) lignin module_type: METABOLIC_PATHWAY annotons: - id: g_unit_polymerization label: coniferyl alcohol coupling (G units) participant: selector_type: ANY_PARTICIPANT description: coniferyl alcohol phenoxy radicals. function: preferred_term: oxidative radical coupling of coniferyl alcohol description: >- Combinatorial coupling of coniferyl alcohol radicals; no exact GO molecular-function term is asserted here. processes: - preferred_term: guaiacyl lignin biosynthetic process term: id: GO:1901063 label: guaiacyl lignin biosynthetic process role_description: >- Dominant unit of gymnosperm lignin and of vessel walls; the more condensed, recalcitrant unit type. - id: s_lignin label: syringyl (S) lignin module_type: METABOLIC_PATHWAY annotons: - id: s_unit_polymerization label: sinapyl alcohol coupling (S units) participant: selector_type: ANY_PARTICIPANT description: sinapyl alcohol phenoxy radicals. function: preferred_term: oxidative radical coupling of sinapyl alcohol description: >- Combinatorial coupling of sinapyl alcohol radicals; no exact GO molecular-function term is asserted here. processes: - preferred_term: syringal lignin biosynthetic process term: id: GO:1901066 label: syringal lignin biosynthetic process role_description: >- Angiosperm fiber-enriched unit; S-rich lignin couples predominantly at readily cleaved 8-O-4 bonds, so a higher S/G ratio improves pulping and saccharification. connections: - source: general_phenylpropanoid target: monolignol_grid connection_type: PROVIDES_INPUT_FOR description: p-coumaroyl-CoA from the phenylpropanoid trunk feeds the monolignol grid. - source: monolignol_grid target: monolignol_export connection_type: PROVIDES_INPUT_FOR description: Cytosolic monolignols are handed to the export step. - source: monolignol_export target: oxidative_polymerization connection_type: PROVIDES_INPUT_FOR description: Apoplastic monolignols are the substrates for laccase/peroxidase coupling. - source: f5h_step target: comt_step connection_type: PRECEDES chaining_status: VERIFIED chaining_note: >- F5H 5-hydroxylates coniferaldehyde to 5-hydroxyconiferaldehyde, which COMT then 5-O-methylates to sinapaldehyde en route to sinapyl alcohol (S units). - source: ccr_step target: cad_step connection_type: PRECEDES chaining_status: VERIFIED chaining_note: >- CCR reduces hydroxycinnamoyl-CoA to the hydroxycinnamaldehyde that CAD then reduces to the corresponding monolignol alcohol.