Exploration of ZnO-Chitosan Nanoparticles to Enhance Defense Gene Expression in Rice against Brown Planthopper

This study investigated the effects of noncomposite and composite forms of ZnO-chitosan nanoparticles (ZnO-Chi-NPs) on the activation of defense-related genes against brown planthopper (BPH) infestation in rice seedlings. Composites of ZnO-Chi-NPs were prepared by a wet chemical process by combining commercial ZnO powder with chitosan pellets. Field emission scanning electron microscopy analysis revealed that the ZnO-Chi-NPs were uniformly distributed, with particle sizes ranging from 30 to 150 nm, consistent with a specific surface area of 4.97 m²/g obtained from the BET analysis. The synthesized composite ZnO-Chi-NPs did not promote rice growth, resulting in lower Zn accumulation in rice seedlings, and showed reduced activation of defense-related gene expression. In contrast, the noncomposite form of 10 ppm chitosan and 50 ppm ZnO exhibited higher seedling height and root length compared with those treated with the composite formulation. In addition, the noncomposite treatment induced higher expression levels of PR genes (PR2 and PR3) as well as genes involved in jasmonic acid and salicylic acid biosynthesis pathways, including allene oxide synthase and phenylalanine ammonia-lyase genes. Rice seedlings treated with the noncomposite formulation also demonstrated enhanced tolerance to BPH infestation. Furthermore, higher Zn accumulation was detected in rice seedlings grown under the noncomposite ZnO and chitosan treatment. Rice grown with ZnO-Chi-NP composites exhibited higher Zn accumulation at later growth stages, which may result in a slower Zn²⁺ release. These findings suggest that the noncomposite formulation of ZnO and chitosan more effectively activates both SA- and JA-mediated signaling pathways, resulting in the upregulation of PR2 and PR3 defense genes in response to BPH infestation. It is suggested that ZnO and chitosan nanoparticles could be developed as nanofertilizers or integrated with traditional fertilizers to enhance rice resistance against BPH infestation.