Transcriptomic and Metabolomic Responses of Botrytis cinerea Under L-Carvone Fumigation

Technical Summary: Transcriptomic and Metabolomic Responses of Botrytis cinerea Under L-Carvone Fumigation

Problem Statement
Botrytis cinerea is a necrotrophic phytopathogen responsible for gray mold, causing approximately USD 100 billion in annual global economic losses. The pathogen's rapid reproduction, massive spore production, and genetic variability have led to severe multidrug resistance against conventional synthetic fungicides (e.g., benzimidazoles and succinate dehydrogenase inhibitors). While plant volatile organic compounds (VOCs), specifically terpenoids like L-carvone, have demonstrated broad-spectrum antifungal activity in physiological studies, the comprehensive molecular mechanisms by which L-carvone suppresses B. cinerea remain unclear. Previous research was limited to superficial indicators such as colony growth and membrane damage, lacking a systems-level understanding of transcriptional and metabolic reprogramming.

Methodology
This study employed an integrated multi-omics approach to dissect the molecular response of B. cinerea mycelia to L-carvone fumigation at the EC₅₀ concentration.

• Experimental Design: B. cinerea was treated with L-carvone (BCEC group) versus sterile water (BCCK control) under dark conditions at 25°C. Three biological replicates were used for transcriptomics, and five for metabolomics.
• Transcriptomics: RNA-seq was performed using Illumina HiSeq 2000 (125/150 bp paired-end reads). Data were processed via Trimmomatic, mapped to the B. cinerea reference genome (B05.10) using HISAT2, and quantified with StringTie. Differentially expressed genes (DEGs) were identified using DESeq with thresholds of |log₂FC| > 1 and FDR < 0.05. Functional enrichment was conducted using GO and KEGG databases.
• Metabolomics: Volatile metabolites were extracted via headspace solid-phase microextraction (HS-SPME) and analyzed using GC-MS (Agilent 8890/7000D). Data were normalized and analyzed via PCA and OPLS-DA. Differentially abundant metabolites (DAMs) were screened based on VIP > 1, |log₂FC| ≥ 1.0, and P-value < 0.05.
• Integration: The study correlated transcriptional changes with metabolic shifts to construct a multi-target antifungal mechanism model.

Key Results

• Transcriptomic Reprogramming: A total of 1,531 DEGs were identified, with a significant bias toward downregulation (1,184 downregulated vs. 347 upregulated).
  • Upregulated Pathways: Genes involved in sulfur metabolism, cysteine/methionine biosynthesis, and transient antioxidant responses (e.g., BcSOD1, BcCAT1, BcGST1, BcatrO) were induced, indicating an emergency response to oxidative stress and xenobiotic efflux.
  • Downregulated Pathways: Critical pathways for vegetative growth and virulence were suppressed. This included carbon energy metabolism (starch/sucrose), glycerolipid biosynthesis, peroxisome biogenesis, and the biosynthesis of the phytotoxin botrydial (genes BcBOT1, BcBOT2, BcBOT6).
  • Virulence Factors: Genes encoding cell wall-degrading enzymes (Bcpg1, Bcpg2, Bccel1) and conidiation regulators (Bccon1, Bccon2) were significantly repressed.
  • Growth Machinery: Ergosterol biosynthesis genes (BcERG7, BcERG12, BcERG27), fatty acid synthase (BcFAS1), and cytoskeletal components (BcactA, BcTUB1, BcCDC42) were uniformly downregulated.
• Metabolomic Shifts: 246 DAMs were identified (166 upregulated, 80 downregulated).
  • Antioxidant Accumulation: Antioxidant terpenoids such as carvacrol, along with membrane repair metabolites like 1-methyl-4-(1-methylethylidene)-cyclohexanol and dipropyl disulfide, accumulated significantly. Conversely, γ-cadinene and camphene were significantly reduced.
  • Toxin and Lipid Depletion: Toxic unsaturated aldehydes (e.g., trans,trans-2,4-decadienal, (E)-2-octenal) were drastically reduced. Benzothiazole, 4-hydroxybenzoic acid, phenylacetic acid, and decanal were also significantly reduced.
  • Compensatory Metabolism: Precursors for cell wall reinforcement and amino acid synthesis accumulated, suggesting a metabolic shift toward stress defense at the expense of growth. Notably, the lipid precursor (1-methylethylidene)cyclohexane was upregulated, while 3,7,7-trimethylbicyclo[4.1.0]heptane was downregulated.
• Mechanistic Insight: The data reveals a dichotomy in the fungal response: a short-term activation of ROS scavenging and efflux pumps, contrasted with the persistent suppression of the long-term glutathione-dependent antioxidant system and ergosterol biosynthesis. This leads to irreversible oxidative damage and membrane failure.

Key Contributions

1. Multi-Omics Elucidation: This study provides the first integrated transcriptomic and metabolomic characterization of B. cinerea under L-carvone stress, moving beyond physiological observations to molecular mechanisms.
2. Multi-Target Mechanism: The research demonstrates that L-carvone acts via a multi-target mode of action, simultaneously blocking:
   • Phytotoxin biosynthesis (botrydial pathway).
   • Cell membrane formation (ergosterol and lipid biosynthesis).
   • Vegetative growth and asexual reproduction (cytoskeleton and conidiation).
   • Host cell wall degradation (enzyme secretion).
3. Resistance Breakthrough: By targeting fundamental metabolic hubs rather than single enzymes, L-carvone overcomes the limitations of conventional fungicides that often select for specific resistance mutations.

Significance and Claims
The authors claim that this study reveals the comprehensive molecular mechanism by which L-carvone suppresses B. cinerea growth and virulence. The findings provide theoretical support for the development of botanical fungicides with novel modes of action. Specifically, the study suggests that L-carvone is a viable candidate for the green control of gray mold, offering a strategy to mitigate economic losses and reduce reliance on synthetic chemicals that drive multidrug resistance. The work establishes a foundation for understanding how natural monoterpenoids disrupt fungal redox homeostasis and membrane integrity, supporting the advancement of eco-friendly postharvest disease management strategies.