Perilla essential oil (PEO) is reported as an aromatic yellowish oily substance with a volatile odor extracted from perilla leaves. It exhibits various biological activities except anti-tobacco mosaic virus (TMV) activity. In this study, we investigated the main components and anti-TMV activity of PEO, identified its primary active components, and examined its mode of action. The results indicated that PEO exhibited anti-TMV activity (65.58%) at 800 μg/mL, with perillaldehyde identified as the main active component. The protective, curative, and inactivation activities of perillaldehyde at 800 μg/mL were 80.41%, 73.42%, and 34.93%, respectively. These values were significantly higher than those of the control drug (commercial chitosan oligosaccharide) and the protective and curative activities were superior to those of ningnanmycin. The results of the mode of action showed that perillaldehyde induced a hypersensitive response (HR) in tobacco. Transmission electron microscope (TEM) observation revealed that perillaldehyde had no direct effect on TMV particles. The treatment of Nicotiana glutinosa with perillaldehyde at 800 μg/mL indicated that perillaldehyde had significant induction activity (58.46%). The expression of three pathogenesis-related tobacco genes (PR genes), including nonexpressor of pathogenesis-related genes 1 (NPR1), pathogenesis-related protein 1 gene (PR1), and pathogenesis-related protein 5 gene (PR5), were induced and upregulated by perillaldehyde treatment. Perillaldehyde also induced the overexpression of the phenylalanine ammonia-lyase gene (PAL), respiratory burst oxidase homolog B gene (RBOHB), and protochlorophyllide oxidoreductase gene 1 (POR1). Furthermore, perillaldehyde increased the salicylic acid (SA) and H₂O₂ contents in tobacco leaves, and enhanced the activities of four defense enzymes:

superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and phenylalanine ammonia-lyase (PAL). N. glutinosa was treated with perillaldehyde at 800 μg/mL for 24 h, and the results showed that the highest SA and H₂O₂ contents (1032.08 pmol/L and 23.40 μmol/g FW, respectively) were obtained in tobacco leaves. Defense enzyme activities also reached a maximum at 800 μg/mL, and the activities of CAT, PAL, POD, and SOD increased by 1.76, 1.95, 2.17, and 3.78 times, respectively, compared to the control. The results of the study showed that perillaldehyde may enhance resistance to pathogen infection by inducing systemic acquired resistance (SAR), which may contribute to the activation of SA signal transduction pathway. Therefore, perillaldehyde has the potential for application in agriculture as a novel antiviral agent and immune inducer.