Size-Dependent Zinc Delivery and Nanozyme-Mediated Stress Alleviation by ZnO Nanoparticles in Zinc-Deficient Tomato

Zinc (Zn) deficiency severely limits plant growth and yield, highlighting the need for strategies to improve Zn nutrition. Here, Zn oxide nanoparticles (nZnOs) of two sizes (∼8 nm and ∼45 nm, 100 mg·L–1) were evaluated as seed priming agents in tomato grown under Zn-deficient conditions. Both nanoparticle sizes were internalized into embryonic tissues and showed distinct intracellular localization. While 45 nm nZnO and bulk Zn disturbed seed nutrient homeostasis, 8 nm nZnO increased seed Zn content more efficiently, with milder effects on other nutrients. Across developmental stages, nZnO priming improved Zn supply and nutrient distribution more effectively than bulk Zn. At the vegetative stage, 45 nm nZnO enhanced overall biomass, whereas 8 nm nZnO promoted root-specific growth under Zn deficiency. nZnO priming increased root Zn accumulation, particularly with 8 nm nZnO, and alleviated Zn deficiency-induced nonspecific metal accumulation. Zn deficiency-induced disturbances in redox and nitrosative balance were reversed by nZnO priming, accompanied by reduced RNA nitration and a shift toward a catalase-dominated hydrogen peroxide detoxification system. Both nanoparticle sizes exhibited peroxidase-like activity in cell-free assays, indicating nanozyme potential; however, this activity did not translate directly to the in planta responses. Under Zn deficiency, nZnO priming suppressed stress-related metabolic pathways in leaves, while enhancing amino acid, polyamine, and structural carbohydrate metabolism in roots. Zn deficiency-induced transcriptional repression, particularly in leaves, was partially restored, with affected genes enriched in stress-, phytohormone-, and detoxification-related pathways. Overall, size-dependent nZnO seed priming alleviates Zn deficiency in tomato. 45 nm nZnO mainly enhances biomass through improved nutrient homeostasis and redox regulation, while both sizes contribute to restored Zn availability and antioxidant reprogramming under Zn deficiency.