α-Linolenic acid in Papilio machaon larvae regurgitant induces a defensive response in Apiaceae

Papilio machaon hippocrates C. Felder et R. Felder, 1864 (Papilionidae) larvae are pests of plants of the family Apiaceae. It is unclear whether Apiaceae plants show induced defensive responses against P. machaon hippocrates larvae, and if so, how these responses are induced. Comparison of the fatty acid (FA) composition of the leaves of host plants and the regurgitant of P. machaon hippocrates larvae by LC-MS revealed a great increase in α-linolenic acid (α-LA) in the regurgitant compared with the FAs contained in the leaves. However, specific FA amino acid conjugates, known as elicitor compounds, such as volicitin, were not detected. Sterile host plants (Saposhnikovia divaricata (Turcz.) Schischk., Apiaceae) were treated with α-LA to mimic the damage made by P. machaon hippocrates larvae. After α-LA treatment to leaves, induced defensive reactions, i.e., release of volatile compounds such as α- and β-pinene and camphene (possible induced indirect defense) and the accumulation of specialized metabolites such as (R)-falcarinol and bergapten (possible induced direct defense) were observed. These findings highlight the role of α-LA in the interaction between P. machaon hippocrates larvae and Apiaceae host plants.