Plant Bioenergy Modules

Overview

Plant biomass is the largest renewable feedstock for liquid fuels and
bioproducts. Two broad routes dominate: lignocellulosic conversion of
structural cell walls (cellulose + hemicellulose + lignin) to fermentable sugars,
and oleochemical use of seed storage oil (triacylglycerol) for biodiesel and
oleochemicals. The central technical barrier for lignocellulose is
recalcitrance — the resistance of the wall to enzymatic saccharification —
which is set largely by lignin content/composition, hemicellulose substitution
(especially acetylation), and cellulose crystallinity. For oilseeds the levers are
total oil flux and oil-body packaging capacity.

This project collects the reusable, taxon-neutral biological modules that
underlie these use cases. Each module is a recursively decomposable
ModuleReview document (see modules/README.md) whose
GO and UniProt identifiers are grounded only where verified; concrete members are
Arabidopsis exemplars for orientation, not species-restricting claims.

Modules

Lignocellulosic cell wall (recalcitrance targets)

• Cellulose biosynthesis
  — UDP-glucose supply, glucan polymerization by the primary/secondary cellulose
  synthase complexes (CESA rosettes), microtubule guidance, and accessory factors
  (KORRIGAN, COBRA). Cellulose crystallinity and the secondary-wall CESA set are
  primary digestibility determinants.
  Source: modules/cellulose_biosynthesis.yaml.

• Lignin (monolignol) biosynthesis
  — the general phenylpropanoid entry (PAL/C4H/4CL), the monolignol-specific
  grid (HCT, C3'H, CCoAOMT, CCR, F5H, COMT, CAD), monolignol export, and
  laccase/peroxidase radical coupling into H/G/S lignin. Monolignol flux and the
  S/G ratio are the dominant recalcitrance-engineering levers.
  Source: modules/lignin_monolignol_biosynthesis.yaml.

• Xylan (glucuronoxylan) biosynthesis
  — Golgi backbone elongation (IRX9/IRX10/IRX14), the eudicot reducing-end
  sequence, glucuronosyl substitution (GUX), 4-O-methylation (GXM), and
  O-acetylation (ESK1/TBL29 + RWA). Xylan acetylation and quantity drive both
  recalcitrance and acetate-mediated fermentation inhibition.
  Source: modules/xylan_biosynthesis.yaml.

Storage oil (biodiesel / oleochemical feedstock)

• Seed triacylglycerol (oil) biosynthesis
  — the ER glycerol-phosphate (Kennedy) pathway (GPAT → LPAAT → PAP → DGAT),
  the acyl-CoA-independent PDAT route, and oleosin-mediated oil-body packaging.
  DGAT/PDAT capacity, the acyl-CoA pool, and oleosin dosage set seed-oil yield.
  Source: modules/seed_triacylglycerol_biosynthesis.yaml.

Upstream carbon supply

• Photosynthesis — the light reactions and
  Calvin–Benson–Bassham carbon fixation that ultimately supply the carbon
  skeletons feeding all of the above.
  Source: modules/photosynthesis.yaml.

Why these modules for bioenergy

Use case	Module(s)	Principal engineering lever
Lignocellulosic ethanol / sugars	lignin, xylan, cellulose	lower lignin, raise S/G, reduce xylan acetylation, alter crystallinity
Fermentation inhibitor reduction	xylan	reduce backbone O-acetylation (acetate)
Biodiesel / oleochemicals	seed TAG	increase DGAT/PDAT flux and oil-body capacity
Feedstock productivity	photosynthesis	carbon assimilation upstream of wall and oil

Relevant bioenergy species in this repo

The modules are written taxon-neutrally with Arabidopsis exemplars, but the repo
also contains relevant feedstock species directories, including poplar
(POPTR), sorghum (SORBI), maize (MAIZE), Brachypodium (BRADI),
rice (ORYSJ/ORYSI), soybean (SOYBN), and Chlamydomonas (CHLRE).
Grass-specific wall features (mixed-linkage glucan, glucuronoarabinoxylan
arabinosylation, wall-bound ferulate/coumarate esters) are noted in the module
documents and are candidates for future dedicated modules.

Status / next steps

• [x] Structured modules curated and validated against ModuleReview
  (lignin, xylan, seed TAG; cellulose and photosynthesis pre-existing).
• [ ] Grass-specific modules: mixed-linkage glucan (CSLF/CSLH) and
  glucuronoarabinoxylan arabinosylation/feruloylation.
• [ ] Suberin and cutin biosynthesis (barrier polyesters relevant to some
  feedstocks and to pretreatment).
• [ ] Per-gene reviews for feedstock-species orthologs (poplar/sorghum) of the
  exemplar enzymes above.