Plant Bioenergy Modules

Bottom line: biofuels from plants depend on how hard the cell wall is to
digest (set by lignin, xylan acetylation and cellulose crystallinity) and, for
oilseeds, on how much triacylglycerol the seed makes and packages. We curated
three new taxon-neutral ModuleReview modules for these levers (lignin/monolignol
biosynthesis, xylan biosynthesis and seed triacylglycerol biosynthesis, PR #2066)
and grouped them with the existing cellulose and photosynthesis modules. We did
this so that later gene reviews in feedstock species such as poplar and sorghum
have a shared, verified map of the enzymes and steps to anchor to. All five
modules validate and are rendered; each is status DRAFT, and together they
ground 48 distinct UniProt exemplars (mostly Arabidopsis) where the identifiers
were verified. Grass-specific wall modules, suberin/cutin and per-gene reviews of
feedstock orthologs have not started.

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.

Slides

• Slides (Marp source: PLANT_BIOENERGY-slides.md) — AI generated