Volatile 2-phenylethanol and β-cyclocitral reprogram grapevine transcriptome and metabolome in defense against Plasmopara viticola

Plant volatile organic compounds (VOCs) exposure can reprogram the gene expression of plants and activate defense responses against phytopathogens. We investigated the transcriptomic effects of treatments with 2- phenylethanol and β-cyclocitral, two VOCs produced by downy mildewresistant grapevine genotypes, on grapevine leaves inoculated with Plasmopara viticola. Leaf disks of the susceptible cultivar Pinot noir were treated with each VOC before pathogen inoculation, and samples were collected at 1 and 6 days post inoculation for RNA-Seq and metabolomics analyses. Both compounds significantly reduced downy mildew severity and triggered extensive transcriptional and metabolic reprogramming. In particular, differential expression analysis revealed the modulation of over 4,000 transcripts, including those involved in plant defense (e.g., PR proteins, chitinases), oxidative stress responses (e.g., peroxidases, glutathione S-transferases), secondary metabolism (e.g., stilbene and terpene synthases), signal transduction (e.g., receptor kinases, calmodulins), and key transcription factors (e.g., WRKY, MYB, ERF, NAC). Notably, β-cyclocitral elicited a broader and earlier transcriptional response than 2-phenylethanol. Untargeted metabolomics identified increased levels of defense-related compounds such as phenylpropanoids, carboxylic acids, terpenoids, and carbohydrates, including aromadendrin, 3-dehydroquinic acid, camphor, and perillaldehyde in leaf disks treated with the two VOCs. Gene co-expression network analysis highlighted seven modules enriched in defense-related genes correlating with resistance-associated metabolic traits. Our findings suggest that 2-phenylethanol and β-cyclocitral act as resistance inducers through transcriptional and metabolic reprogramming, offering insights into VOC-mediated signaling and its application in grapevine protection.