Cold Plasma for Inducing Plant Defence Responses against Insect Herbivores

Insect herbivores cause substantial crop losses worldwide, necessitating sustainable alternatives to chemical insecticides. Cold plasma, a non-thermal ionised gas rich in reactive oxygen and nitrogen species (ROS/RNS), has emerged as a promising technology for stimulating plant growth, improving stress tolerance, and activating defence responses without thermal damage. This review summarises the mechanisms by which cold plasma enhances plant resistance against insect herbivores through oxidative signalling, activation of antioxidant systems, regulation of phytohormone-mediated defence pathways, and induction of defence-related metabolites and enzymes, including phenolics, flavonoids, phenylalanine ammonia-lyase, polyphenol oxidase, lipoxygenase, superoxide dismutase, catalase, and peroxidases. It also discusses plasma-induced structural modifications and the limited but growing evidence of reduced insect performance on plasma-treated plants. Although current findings indicate enhanced plant defensive capacity through coordinated physiological and biochemical reprogramming, direct evidence remains limited. Future molecular and field studies are essential to optimise plasma-based sustainable crop protection.