α-Ionone, an Apocarotenoid, Induces Plant Resistance to Western Flower Thrips, Frankliniella occidentalis, Independently of Jasmonic Acid

Apocarotenoids, such as beta-cyclocitral, alpha-ionone, beta-ionone, and loliolide, are derived from carotenes via chemical or enzymatic processes. Recent studies revealed that beta-cyclocitral and loliolide play an important role in various aspects of plant physiology, such as stress responses, plant growth, and herbivore resistance. However, information on the physiological role of alpha-ionone is limited. We herein investigated the effects of alpha-ionone on plant protection against herbivore attacks. The pretreatment of whole tomato (Solanum lycopersicum) plants with alpha-ionone vapor decreased the survival rate of western flower thrips (Frankliniella occidentalis) without exhibiting insecticidal activity. Exogenous alpha-ionone enhanced the expression of defense-related genes, such as basic beta-1,3-glucanase and basic chitinase genes, in tomato leaves, but not that of jasmonic acid (JA)- or loliolide-responsive genes. The pretreatment with alpha-ionone markedly decreased egg deposition by western flower thrips in the JA-insensitive Arabidopsis (Arabidopsis thaliana) mutant coi1-1. We also found that common cutworm (Spodoptera litura) larvae fed on alpha-ionone-treated tomato plants exhibited a reduction in weight. These results suggest that alpha-ionone induces plant resistance to western flower thrips through a different mode of action from that of JA and loliolide.