Polystyrene nanoplastics drive regime-dependent molecular non-additivity in heat-stressed rice

Nanoplastics (NPs) and heat stress co-occur in agricultural systems, yet their interactive effects on crops under different heat regimes remain unclear. Here, we investigated rice (Oryza sativa L.) responses to polystyrene NPs under chronic (CH; 36°C, 10 d) and acute (AH; 45°C, 3 h) heat stress using integrated transcriptomics, metabolomics, and alternative splicing analyses. Combined stressors produced additive biomass reductions; however, molecular responses exhibited regime-dependent non-additivity. Under CH, NPs induced widespread antagonistic interactions, suppressing the transcriptional acclimation program by ∼50%. Rather than alleviating stress, this antagonism reflected a systemic failure of heat defense activation, mediated by the disruption of a core regulatory module governing cell wall biosynthesis, phenylpropanoid production, and oxidative stress responses. This resulted in depletion of structural phospholipids and defense compounds (e.g., sakuranetin). Under AH, NPs altered the alternative splicing of circadian clock genes (OsCRY1, OsPRR73, OsGI). These findings reveal that NPs induce regime-specific, non-additive molecular effects despite additive phenotypic responses, demonstrating that traditional organism-level endpoints (e.g., biomass) lack the sensitivity to detect substantial disruption of molecular acclimation programs. This work highlights the need for integrating quantitative interaction modeling and systems-level analyses into multi-stressor risk assessments for agricultural systems facing concurrent pollution and climate stress.