Inhibitory activity and antioomycete mechanism of citral against Phytophthora capsici

The natural terpenoid citral has antifungal activity against multiple fungi, but its bioactivity against oomycetes is unclear. Therefore, this study investigated the antioomycete activity and mechanism of citral against Phytophthora capsici, a highly destructive invasive oomycete. Results showed that citral not only had a great inhibition on the mycelial growth of P. capsici (EC₅₀ = 94.15 mg/L), but also had a significant inhibition on multiple spores, such as sporangia formation, zoospore discharge and zoospore germination. Citral at 4000 mg/L exhibited favorable protective (73.33%) and curative efficacy (55.11%) against pepper Phytophthora blight. Citral significantly damaged the hyphal morphology, disrupted the cell membrane integrity, increased the permeability of cell membrane, and increased the glycerol content in P. capsici. A total of 250 upregulated and 288 downregulated proteins were identified in iTRAQ-based quantitative proteomic analysis. Downregulated proteins were mostly enriched in pathways of ABC transporters, cyanoamino acid metabolism and starch and sucrose metabolism, suggesting an inhibition of citral on transmembrane transporter (e.g., ABC transporters) and pathogenicity (e.g., β-glucosidases) proteins. Upregulated proteins were enriched in biosynthesis of unsaturated fatty acids, pyruvate metabolism and glycolysis/gluconeogenesis, suggesting an activation of citral on energy generation proteins, including acyl-CoA oxidase, D-lactate dehydrogenase, pyruvate kinase, acetyl-CoA synthetase and phosphoenolpyruvate carboxykinase. Biochemical and iTRAQ analysis suggested that cell membrane may be the target of citral in P. capsici.