2.3 Metabolic Pathways of Sesquiterpenes and Chromones in Agarwood Formation

Agarwood resin is chemically composed of sesquiterpenes (fragrant oils) and chromone derivatives (heavier resin compounds). These molecules are produced through complex plant defense metabolic pathways triggered by wounding, microbial invasion, oxidative stress, or artificial inoculation.

1. Sesquiterpene Biosynthesis Pathways

Sesquiterpenes (C15 molecules) are the primary aromatic compounds in agarwood oil. They come from the isoprenoid biosynthetic pathways:

A. Mevalonate Pathway (MVA) – Cytosolic

This is the major pathway for sesquiterpene formation.

  1. Acetyl-CoA → HMG-CoA → Mevalonic acid
    (via HMG-CoA reductase – the rate-limiting enzyme)
  2. Mevalonic acid → Isopentenyl pyrophosphate (IPP)
    IPP is the universal building block.
  3. IPP ↔ DMAPP
    (isomerization step)
  4. IPP + DMAPP → Farnesyl pyrophosphate (FPP)
    FPP is the precursor for all sesquiterpenes.
  5. FPP → various sesquiterpenes using sesquiterpene synthase (TPS) enzymes.

Common agarwood sesquiterpenes formed from FPP:

  • α-guaiene
  • β-agarofuran
  • selina-3,11-diene
  • aromadendrene
  • dihydroagarofuran
  • nerolidol

Each is formed by a distinct sesquiterpene synthase enzyme activated during stress.

2. Chromone Biosynthesis Pathways

Chromones are the signature markers of high-value agarwood resin, especially 2-(2-phenylethyl)chromones (PECs).

Chromone biosynthesis is less understood but involves:

A. Polyketide Pathway (PKS Pathway)

  1. Starts from malonyl-CoA units
  2. Catalyzed by type III polyketide synthases (PKSs) including PEC synthase
  3. Leads to PEC chromones and their oxidized derivatives

Key steps involve:

  • Polyketide chain elongation
  • Cyclization into the chromone ring
  • Hydroxylation, methylation, and structural diversification by P450 monooxygenases

Major chromones found in agarwood:

  • 2-(2-phenylethyl)chromone
  • 6,7-dihydroxy-2-(2-phenylethyl)chromone
  • Flindersia-type chromones
  • Epoxy-chromones (markers of high-grade resin)

3. Inoculation & Microbial Infection Drives a Metabolic Switch

When agarwood trees are wounded or infected (Fusarium, Lasiodiplodia, etc.):

A. Defensive Signaling Activates

  • Jasmonic acid (JA)
  • Salicylic acid (SA)
  • Reactive oxygen species (ROS)

These compounds signal the tree to:

  • Produce more sesquiterpenes
  • Activate polyketide pathways
  • Strengthen cell walls
  • Encapsulate infection with resin

B. Upregulated Genes Include

  • TPS (terpene synthases)
  • PKS (polyketide synthases)
  • P450 enzyme families
  • Defense transcription factors (WRKY, MYB, bHLH)

This genetic upregulation is what increases:

  • Resin density
  • Resin color
  • Chromone content
  • Aromatic profile

4. Resin Zone Formation (Biochemical + Anatomical Integration)

A. Early Phase

  • Sesquiterpene accumulation
  • Infected or stressed cells produce oils as antimicrobials

B. Later Phase

  • Chromone deposition
  • Darkening of wood
  • Formation of “marbling” and “resin streaks”

C. Mature Resin

A complex mixture of:

  • 50–80% sesquiterpenes
  • 10–30% chromones
  • Phenolics, aldehydes, and protective compounds

This creates the signature scent, varying by:

  • Species
  • Inoculation method
  • Microbial community
  • Environment

5. Summary Diagram (Text Version)

Wound / Microbial Infection
            ↓
   JA / SA / ROS Signals
            ↓
  ↑ TPS Genes        ↑ PKS Genes
(Sesquiterpenes)   (Chromones)
            ↓              ↓
  Terpene Pathway     Polyketide Pathway
            ↓              ↓
       FPP → Sesquiterpenes
            ↓
  Oil Aroma Profile (Top Notes)
                           ↓
              PEC Chromones → Deep Resin Notes
                           ↓
       FULL AROMATIC & RESIN COMPLEX (Agarwood)