Foliar ZnO NPs trigger transcriptional rewiring of starch and hormone pathways to break the yield-quality trade-off in direct-seeded rice

Background: Direct-seeded rice (DSR) faces a persistent trade-off between grain yield and cooking quality. Conventional zinc fertilization often fails to overcome this limitation due to inefficient translocation and lack of regulatory effects on grain development pathways. Objective: This study aimed to evaluate whether foliar application of zinc oxide nanoparticles (ZnO NPs) could outperform conventional ZnSO 4 in simultaneously enhancing grain zinc biofortification, yield, and cooking quality in DSR, and to elucidate the underlying physiological and transcriptional mechanisms. Methods: A two-season field experiment (Hangzhou summer, Lingshui winter) was conducted using the DSR cultivar ‘Nanjing 46’. Treatments included foliar spraying of deionized water (CK), 10 mg/L ZnSO₄ (T1), and 10 mg/L ZnO NPs (T2) at the booting stage. We assessed zinc distribution, yield components, grain quality traits, antioxidant enzyme activities, endogenous hormone levels, and transcriptomic profiles of developing panicles. Results: ZnO NPs outperformed ZnSO₄ in all key metrics. Compared to CK, T2 significantly increased polished rice zinc concentration by 25.6%, spikelets per panicle by 12.4%, and 1000-grain weight by 4.8%. Cooking quality was improved through reduced amylose and chalkiness. Physiologically, ZnO NPs elevated zeatin and GA₃, boosted SOD, POD, and CAT activities, and reduced oxidative damage (MDA). Transcriptomics revealed that ZnO NPs uniquely upregulated starch-remodeling genes ( OsAmy1C, OsRAmy1A ) and suppressed the cytokinin catabolism gene OsCKX2 , a regulatory pattern not observed with ionic zinc. Conclusions: ZnO NPs function as potent nano-stimulants rather than mere nutrient sources. By orchestrating hormonal balance and reprogramming starch metabolism, ZnO NPs effectively break the yield-quality trade-off in DSR. This nano-enabled strategy provides a sustainable, non-GM approach for high-value rice production under nutrient-limited conditions.