Asymmetric exposure to MoS2 nanosheets triggers redox-mediated hormesis effect in plant roots: Insights from in situ chip tracking

The rhizosphere is microscopically heterogeneous, yet the impact of uneven distribution of nanomaterials (NMs) on roots remains unclear. Here, using Arabidopsis thaliana and MoS 2 nanosheets as the model plant and NMs, respectively, we employed a microfluidic root chip housed in a Petri dish with moist dust-free paper to investigate root responses to spatially asymmetric exposure. We compared symmetric exposure (half dose delivered to each root side; a homogeneous scenario) with asymmetric exposure (full dose to one side; a heterogeneous scenario) of MoS 2 nanosheets over a 5-day period. Results showed that this spatial distinction alone is sufficient to elicit divergent growth and physiological responses in seedling roots. Real-time imaging revealed that symmetric exposure induced a classic dose-dependent toxicity from excess MoS 2 nanosheets-induced stress oxidative damage, whereas asymmetric exposure elicited a hormesis effect on root growth, with asymmetric exposure of 50 mg L -1 MoS 2 nanosheets for 64 h increasing root length by 18% compared to symmetric exposure at 25 mg L -1 . Mechanistic investigations, integrating dynamic monitoring and inhibition experiments using fluorescence probes and an auxin reporter, showed that localized stimulation promoted root growth via Fe 2+ -dependent reactive oxygen species signaling with asymmetric exposure increasing Fe 2+ and ROS levels by approximately 1.74-fold and 2.03-fold, respectively, relative to symmetric exposure, leading to auxin accumulation in root tips. Our findings demonstrate that spatial heterogeneity is an essential consideration for evaluating and designing the application of redox-active NMs.